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arXiv:1302.2926v2 [astro-ph.CO] 15 May 2013
Draft version May 16, 2013
Preprint typeset using L
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SPECTROSCOPIC OBSERVATIONS OF SN 2012FR: A LUMINOUS, NORMAL TYPE IA SUPERNOVA WITH
EARLY HIGH-VELOCITY FEATURES AND A LATE VELOCITY PLATEAU
M. J. Childress
1,2,3
, R. A. Scalzo
1,2
, S. A. Sim
1,2,4
, B. E. Tucker
1
, F. Yuan
1,2
, B. P. Schmidt
1,2
, S. B. Cenko
5
,
J. M. Silverman
6
, C. Contreras
7
, E. Y. Hsiao
7
, M. Phillips
7
, N. Morrell
7
, S. W. Jha
8
, C. McCully
8
,
A. V. Filippenko
5
, J. P. Anderson
9
, S. Benetti
10
, F. Bufano
11
, T. de Jaeger
9
, F. Forster
9
, A. Gal-Yam
12
,
L. Le Guillou
13
, K. Maguire
14
, J. Maund
4
, P. A. Mazzali
15,10,16
, G. Pignata
11
, S. Smartt
4
, J. Spyromilio
17
,
M. Sullivan
18
, F. Taddia
19
, S. Valenti
20,21
, D. D. R. Bayliss
1
, M. Bessell
1
, G. A. Blanc
22
, D. J. Carson
23
,
K. I. Clubb
5
, C. de Burgh-Day
24
, T. D. Desjardins
25
, J. J. Fang
26
, O. D. Fox
5
, E. L. Gates
26
, I-T. Ho
27
, S. Keller
1
,
P. L. Kelly
5
, C. Lidman
28
, N. S. Loaring
29
, J. R. Mould
30
, M. Owers
28
, S. Ozbilgen
24
, L. Pei
23
, T. Pickering
29
,
M. B. Pracy
31
, J. A. Rich
22
, B. E. Schaefer
32
, N. Scott
30
, M. Stritzinger
33
, F. P. A. Vogt
1
, G. Zhou
1
Draft version May 16, 2013
ABSTRACT
We present 65 optical spectra of the Type Ia SN 2012fr, of which 33 were obtained before maxi-
mum light. At early times SN 2012fr shows clear ev ide nc e of a high-velocity feature (HVF) in the
Si ii λ6355 line which can be cleanly decoupled from the lower velocity “photospheric” component.
This Si ii λ6355 HVF fades by phase −5; subsequently, the photospheric component exhibits a very
narrow velocity width and remains at a nearly c onstant velocity of ∼12,00 0 km s
−1
until at least 5
weeks after maximum brightness. The Ca ii infr ared (IR) triplet exhibits similar evidence for both
a photospheric component at v ≈ 12,000 km s
−1
with narrow line width and long velocity plateau,
as well as a high-velocity component beginning at v ≈ 31,000 km s
−1
two weeks before maximum.
SN 2012fr resides on the border be tween the “shallow silicon” and “core-normal” subcla sses in the
Branch et al. (2009) classification scheme, and on the border between normal and “high-velocity”
SNe Ia in the Wang et al. (2009a) system. Though it is a clea r member of the “low velocity gradi-
ent” (LVG; Benetti et al. 2 005) group of SNe Ia and exhibits a very slow light-curve decline, it shows
key dissimilarities with the overluminous SN 1991T or SN 1999aa subclasses of SNe Ia. SN 2012fr
represents a well-observed SN Ia at the luminous end of the normal SN Ia distribution, and a key
transitional event between nominal spectroscopic subclasses of SNe Ia.
Subject headings: supernovae: individual: SN 2012fr — supernovae: general — galaxies: individual:
NGC 1365
1
Research School of Astronomy and Astrophysics, Australian
National University, Canberra, ACT 2611, Australia.
2
ARC Centre of Excellence for All-sky Astrophysics (CAAS-
TRO).
3
E-mail: mjc@mso.anu.edu.au .
4
Astrophysics Research Centre, School of Mathematics and
Physics, Queen’s University Belfast, Belfast BT7 1NN, UK.
5
Department of Astronomy, University of California, Berke-
ley, CA 94720-3411, USA.
6
Department of Astronomy, University of Texas, Austin, TX
78712-0259, USA.
7
Las Campanas Observatory, Carnegie Observatories, Casilla
601, La Serena, Chile.
8
Department of Physics and Astronomy, Rutgers, the State
University of New Jersey, 136 Frelinghuysen Road, Piscataway,
NJ 08854, USA.
9
Departamento de Astronom´ıa, Uni versidad de Chile, Casilla
36-D, Santiago, Chile.
10
INAF Osservatorio Astronomico di Padova, Vicolo
dell’Osservatorio 5, 35122 Padova, Italy.
11
Departamento de Ciencias Fisicas, Universidad Andres
Bello, Avda. Republica 252, Santiago, Chile.
12
Department of Particle Physics and Astrophysics, The
Weizmann Institute of Science, Rehovot 76100, Israel.
13
UPMC Univ. Paris 06, UMR 7585, Laboratoire de
Physique Nucleaire et des Hautes Energies (LPNHE), 75005
Paris, France.
14
Department of Physics (Astrophysics), University of Ox-
ford, DWB, Keble Road, Oxfor d OX1 3RH, UK.
15
Astrophysics Research Institute, Liverpool John Moores
University, Egerton Wharf, Birkenhead, CH41 1LD, UK.
16
Max-Planck-Institut f¨ur Astrophysik, Karl-Schwarzschild
str. 1, 85748 Garching, Germany.
17
European Southern Observatory, Karl-Schwarzschild-
Strasse 2, Garching D-85748, Germany.
18
School of Physics and Astronomy, University of Southamp-
ton, Southampton SO17 1BJ, UK.
19
The Oskar Klein Centre, Department of Astronomy, Al -
baNova, Stockholm University, 10691 Stockholm, Sweden.
20
Las C umbres Observatory Global Telescope Network, 6740
Cortona Dr., Suite 102, Goleta, C A 93117, USA.
21
Department of Physics, University of California, Broi da
Hall, Mail Code 9530, Santa Barbara, CA 93106-9530, USA.
22
Observatories of the Carnegie Institution of Washington,
813 Santa Barbara St., Pasadena, CA 91101, USA.
23
Department of Physics and Astronomy, University of Cal-
ifornia, Irvine, CA 92697-4575, USA.
24
School of Physics, University of Melbourne, Parkvil le, VIC
3010, Australia.
25
Department of Physics and Astronomy, The University of
Western Ontario, London, ON N6A 3K7, Canada.
26
University of California Observatories/Lick Obser vatory,
University of California, Santa Cruz, CA 95064, USA.
27
Institute for A stronomy, University of Hawaii, 2680 Wood-
lawn Drive, Honolulu, HI 96822, USA.
28
Australian Astronomical O bservatory, PO Box 915, North
Ryde, NSW 1670, Australia.
29
South A frican Astronomical Observatory (SAAO), P.O.
Box 9, Observatory 7935, South Africa.
30
Centre for Astrophysics & Supercomputing, Swinburne
University of Technology, PO Box 218, Hawthorn, VIC 3122,
Australia.
31
Sydney Institute for Astronomy, School of Physics, Univer-
sity of Sydney, NSW 2006, Australia.
32
Department of Physics and Astronomy, Louisiana State
University, Baton Rouge, LA 70803, USA.
33
Department of Physics and Astronomy, Aarhus University,
Ny Munkegade 120, DK-8000 Aarhus C, Denmark.
2
3
1. INTRODUCTION
Type Ia supernovae (SNe Ia) are critical cosmolog-
ical tools for measuring the expansion his tory of the
Universe (Riess et al. 1998; Perlmutter et al. 1999), yet
much remains unknown about the natur e of these en-
lightening explosions. Their luminosities show low in-
trinsic dispersion (∼ 0.35 mag) and they generally obey
a scaling of their absolute luminosity with the width
of their optical light curve (Phillips 1993; P hillips et al.
1999), about which the brightnes s dispersion is even
lower. The width-luminosity relationship appears to
be driven by the amount of ra dioactive
56
Ni produced
in the explosion and the opacity (Hoeflich & Khokhlov
1996; Pinto & Eastman 2000; Mazz ali et al. 2001, 2007),
but the progenitor mechanism driving these proper-
ties remains uncertain. While it is generally accepted
that SNe Ia arise from the thermonuclear disruption
of a car bon-oxygen (C-O) white dwarf (WD) in a bi-
nary system (Hoyle & Fowler 1960), scenarios in which
the companion is a main sequence or red giant s tar
(the single-degenerate scenario; Whelan & I ben 1973)
and thos e in which the companion is another WD
(the double-degenerate scenario; Tutukov & Iungelson
1976; Tutukov & Yungelson 1979; Iben & Tutukov 1984;
Webbink 1984) have both proven consistent with some
observational features of SNe Ia. Whether SNe Ia rep-
resent a unified class o f objects with a common physical
origin or result from multiple progenitor channels has
yet to be determined, and is a critical question for the
continued use of SNe Ia in cosmology.
Optical s pectroscopy of SNe Ia can provide vital in-
sight into the question of SN Ia diversity. Large sam-
ples of SN Ia spectra have been made publicly avail-
able (e.g., Matheson et al. 2008; Blondin et al. 2012;
Silverman et al. 2012a; Yaro n & Gal-Yam 2012), and in-
vestigations of spectroscopic subclassification of SNe Ia
have been a vigorous area of study (Benetti et al. 2005;
Branch et al. 2009; Wang et al. 2009a; Silverman et al.
2012b). While a rigorous acc ounting of the diversity of
SNe Ia is crucial for understanding the source of their
luminosity dispers ion, individual cas es of well-studied
SNe Ia (e.g., Stanishev et al. 20 07; Wang et al. 20 09b;
Foley et al. 2012; Silverman et al. 2012c) can yield key
insights into the nature of the explosions themselves.
In this work we focus on optical spectroscopy of
SN 2012fr, a SN Ia which was discovered on 2012 Oct. 27
in the nearby barred spiral galaxy NGC 1365. Shortly af-
ter its discovery we initiated a rigorous photometric and
spectroscopic follow-up prog ram for SN 2012 fr. Optical
photometry will be presented by Contreras et al. (2013,
hereafter Paper II), where we show that SN 2 012fr ha s
a norma l light curve for an SN Ia. This paper presents
optical spec tra o f SN 2012fr, while Hsiao et al. (2013,
Paper III) will pr esent near-infr ared (NIR) spectr a and
Tucker et al. (2013, Paper IV) will analyze constraints
on the progenitor from pre-explos ion imaging a nd very
early photometry.
This paper is organized as follows. In § 2 we present the
observational data. Sec tion 3 focuses on the Si ii λ6 355
line, and characterizes both the high-velocity features ob-
served at early times and the long velocity plateau ob-
served at late times. Other absorption features of par-
ticular note for SN 2012fr – including narrow Na i D,
unburned C, the Ca ii IR triplet, and Fe-group elements
– are inspected in § 4. We addres s the spectroscopic
“subclassification” of SN 2012fr in the context of mod-
ern classification schemes in § 5. We then discuss im-
plications of our observational results in § 6 and present
concluding remarks in § 7.
2. SPECTROSCOPIC OBSERVATIONS
SN 2012fr was discovered on 2012 Oct. 27 (UT dates
are used throughout this paper ) by Klotz et al. (201 2)
at α = 03
h
33
m
36.274
s
, δ = −36
◦
07
′
34.46
′′
(J2000 ) in
the nearby barred spiral galaxy NGC 1365, and shortly
thereafter classified as a SN Ia (Childress et al. 2012;
Buil 2012). Extensive photometric c overage presented
in Paper II shows that SN 2012fr reached a peak bright-
ness of m
B
= 12.0 mag on 2012 Nov. 12.04 with a 15-
day decline of ∆m
15
(B) = 0.80 mag. Given the nom-
inal distance modulus to NGC 1365 of µ = 31.3 mag
(Silbermann et al. 1999; Freedman et al. 2001), this im-
plies a peak luminosity of M
B
= −19.3 mag, placing it
in firm agreement with the Phillips (1993) relation.
Spec tra of SN 2012fr were collected at multiple loca-
tions. The two main sources were the Wide Field Spec-
trograph (WiFeS; Dopita et al. 2007, 2010) on the Aus-
tralian National University (ANU) 2.3 m telescop e at
Siding Spring Observatory in northern New South Wales,
Australia, and the Public ESO Spectroscopic Survey of
Transient Objects (PESSTO) utilizing the 3.6 m New
Techno logy Telescope (NTT) at La Silla, Chile.
WiFeS sp ectra were obtained using the B3000 and
R3000 gratings , providing wavelength c overage from
3500
˚
A to 9 600
˚
A with a resolution of 1.5
˚
A and 2.5
˚
A (all reported instrument resolutions are full width at
half-maximum intensity, FWHM) in the blue and red
channels, respectively. Data cub e s for WiFeS observa-
tions were produced using the PyWiFeS software
34
(Chil-
dress et al. 2013, in prep.). Spectra of the SN were ex-
tracted from final data cubes using a point-spread func-
tion (PSF) weighted extrac tion technique with a simple
symmetric Gaussian PSF, and the width of this Gaussian
was measured directly from the data cube. We found this
method to produce flux measurements consistent with a
simple aperture extraction method, but with improved
signal-to-noise ratio . Background subtraction was per-
formed by calculating the median background spectrum
across all spaxels outside a dis tance from the SN equal
to ab out thre e times the seeing disk (which was typically
1.5–2
′′
FWHM). Due to the negligible galaxy background
and good spatial flatfielding from the Py WiFeS pipeline,
this technique produced favorable subtraction of the sky
background from the WiFeS spectra of SN 2012fr.
A major component of our obser ving campaign was
a s eries of optical and NIR spectra obta ine d as part of
the PESSTO (Smartt et al. 2013, in prep.)
35
survey
using the NTT-3.6 m telesc op e in La Silla, Chile . Opti-
cal spectra from PESSTO were obtaine d with EFOSC2
(Buzzoni et a l. 1984) using the Gr11 and Gr16 g risms,
which both have a resolution of 16
˚
A. NIR spectra were
obtained with SOFI (Moorwood et al. 199 8) using the
GB and GR grisms, which give respective resolutions of
34
http://www.mso.anu.edu.au/pywifes/ .
35
http://www.pessto.org .
4
14
˚
A and 21
˚
A, with observations dithered to facilitate sky
background subtraction. SOFI spe c tra will be released
as part of the PESSTO data products for SN 2 012fr, and
will constitute a portion of the NIR spectra of SN 2012fr
analyzed in Paper I II. Both EFOSC and SOFI spectra
were re duce d using the PESSTO pipeline developed by
S. Vale nti, which is a custom-built python/pyraf pack-
age that performs all standard spectros copic reduction
steps including preprocessing, wavelength and flux cali-
bration, spec trum extraction, and removal of telluric fea-
tures measured from long exposure standard star spectra.
Continued observations of SN 201 2fr in 2013 are ongoing
as part of the PESSTO operations and will be presented
in a future PE SSTO paper.
Additional spectra of SN 2012fr were obtained
with the Robert Stobie Spectrograph on the South
African La rge Telescope (SALT), the Grating Spectro-
graph on the South African Astronomical Observatory
(SAAO) 1.9 m telescope, the Kast Double Spectro graph
(Miller & Stone 1993) on the Shane 3 m telescope at
Lick Observatory, the Wide Field Reimaging C C D Cam-
era (WFCCD) on the 2.5 m Ir´en´ee du Pont telescop e at
Las Campanas Obser vatory, the Inamo ri-Magella n Are al
Camera and Spectrograph (IMACS; Dressler et al. 201 1)
on the 6 m Magellan-Baade telescope at La s Campanas,
and the Andalucia Faint Object Spectrograph and Cam-
era (ALFOSC) on the 2.5 m Nordic Optical Telescope
(NOT) on La Palma.
SALT/RSS observations were obtained with a 900
l mm
−1
VPH grating at three tilt angles to cover the
range 3480–9030
˚
A. The 1.5
′′
wide slit yielded a resolu-
tion of ∼ 6
˚
A. Initia l processing of the SALT data utilized
the SALT scie nc e pipeline PySALT
36
(Crawford et al.
2010). SAAO-1.9 m observations used the 300 l mm
−1
grating (#7) at an angle of 17.5
◦
, corresponding to a
central wavelength of 5400
˚
A, a wavelength range of
∼ 3500–7300
˚
A, and a r e solution of 5
˚
A. Lick/Kast ob-
servations employed the 600 l mm
−1
grating on the blue
arm, blazed at 4310
˚
A, and provides wavelength cover-
age of 3500–5600
˚
A with a resolution of 6–7
˚
A. Different
observers use d different gratings on the Kast r ed arm,
including the 300 l mm
−1
grating blazed at 7500
˚
A and
covering 5500–10,300
˚
A with a resolution of 11
˚
A, the
600 l mm
−1
grating blaze d a t 7000
˚
A and covering 5600–
8200
˚
A with 5.5
˚
A resolution, and the 8 30 l mm
−1
grat-
ing blazed at 6500
˚
A and covering 5600–7440
˚
A with 4
˚
A
resolution. WFCCD observations were obtained with the
400 l mm
−1
grism y ie lding 8
˚
A re solution, and data were
reduced following the proc e dures described in detail by
Hamuy et al. (2006). The IMACS spectrum employed
the 300 l mm
−1
grating and 0.9
′′
slit yielding a resolu-
tion of 2.7
˚
A.
All long- slit low-resolution spectra were reduced us-
ing standard techniques (e.g., Foley et al. 2003). Routine
CCD proces sing and spectrum extraction were completed
with IRAF. We obtained the wavelength scale from low-
order polynomial fits to calibratio n-lamp spectra. Also,
we fit a spectrophotometric standard-star spectrum to
the data in order to flux calibrate the SN and to remove
telluric absorption lines.
36
http://pysalt.salt.ac.za .
We obtained a hig h-resolution optical spe ctrum of
SN 2012fr with the High Resolution Echelle Spectrometer
(HIRES; Vogt et al. 1994) on the 10 m Keck I telescope
with the blue cross -disperser (“HIRESb”) on 2012 Oct.
29.45. We us e d the C2 decker (i.e., the 1.15
′′
slit), provid-
ing coverage from the atmospheric cutoff to λ = 5 960
˚
A
with a resolution of 37,000.
A full table of our optical spectra is given in Table 1,
and a representative plo t of our spectral time series is
shown in Figure 1. At the earliest epochs of SN 2012fr,
our observing strategy was to request spectra from mul-
tiple sources worldwide. This resulted in several spectra
during the same night (often separated by 0.3–0.5 day) on
some occasions, but consistently re sulted in at least one
spectrum every night until nearly two weeks after maxi-
mum light. On nights with extremely poor seeing (>3
′′
)
at Siding Spring, some WiFeS observers cho se to observe
SN 2012fr multiple times due to the inability to observe
their own fainter targe ts . Upon publicatio n of this paper
we will make all of our optical spectra publicly available
via the WISEREP (Yaron & Gal-Yam 2012) SN spec-
troscopy r epository.
3. EVOLUTION OF THE Si ii λ6355 LINE
The Si ii λ63 55 line is a mong the most prominent fea-
tures in SN Ia spectra. Both its characteristics at max-
imum light (e.g., Nugent et al. 1995) and its evolution
in time (e.g., Benetti et al. 2005) have been key tools in
characterizing SN Ia diversity. Additionally, the prop-
erties of this line in combination with behavior of other
lines (“spectral indicators”) have been used to identify
potential subclasses of SNe Ia (e.g., Benetti et al. 2005;
Bongar d et al. 20 06; Branch et al. 200 9; Silverman et al.
2012b; Blo ndin et al. 2012).
For SN 2 012fr, the evolution of the Si ii λ6355 line has
three key features of note: (1) the velocity width of the
line (and indeed other lines; see § 4) is extremely nar-
row, starting about a week after maximum brightness;
(2) early-time spectra show clear signatures of a two-
component Si ii λ6355, indicating a layer of ejected ma-
terial at higher velocities than the nominal photospheric
layer; and (3) the velocity of the photospheric component
remains constant (to within ∼ 200 km s
−1
) until at least
∼ 40 days after maximum.
The clear detections of both the high-velocity layer and
the constant photospheric velocity are facilitated by the
extremely narr ow velo city width of the photospheric ab-
sorption lines in SN 2012fr. In Figure 2 we show the
Si ii λ6355 width of SN 2012fr viewed in velocity space,
compared to that o f SN 2005hj (Quimby e t al. 200 7),
SN 1994D (from Blondin et al. 2012), and SN 2002bo
(Benetti et al. 2004). SN 2 012fr has narrower Si ii λ6355
than the other SNe Ia, exc e pt fo r perhaps SN 2005hj
whose similarly narrow line width was highlighted by
Quimby et al. (2007). One can even visibly identify a
flattening at the base of this feature due to the doublet
nature of the line. We measure the obse rved line width
(FWHM) to be ∼ 3400 km s
−1
; if we account for the 14
˚
A
separatio n of the doublet lines, this implies an intrinsic
line width of ∼ 3000 km s
−1
.
3.1. High-Velocity Si ii λ6355 in Early-Time Spectra
At two weeks before maximum light, Si ii λ6355 ap-
pears to be composed of a single broad, high-velocity
5
Table 1
Optical Spectroscopy Observation Log
UT Date Phase
a
Tel escope Exposure Wavelength Observers
b
(days) / Instrument Time (s) Range (
˚
A)
2012-Oct-28.53 -14.51 ANU-2.3m / WiFeS 900 3500-9550 GZ, DB
2012-Oct-28.87 -14.17 SALT / RSS 1200 3480-9030 SJ, CM1
2012-Oct-29.45 -13.59 Keck-I / HIRES 1200 3500-5960 BZ, MJ, SX, BK
2012-Oct-30.38 -12.66 Lick-3m / Kast 2000 3500-7440 CM2, BZ
2012-Oct-30.51 -12.53 ANU-2.3m / WiFeS 1200 3500-9550 GZ, DB
2012-Oct-31.53 -11.51 ANU-2.3m / WiFeS 1200 3500-9550 GZ, DB
2012-Nov-01.59 -10.45 ANU-2.3m / WiFeS 1200 3500-5700
c
MB, SK
2012-Nov-01.99 -10.05 SAAO-1.9m / GS 900 3500-7150 NSL
2012-Nov-02.48 -9.56 ANU-2.3m / WiFeS 900 3500-9550 MB, SK
2012-Nov-02.69 -9.35 ANU-2.3m / WiFeS 900 3500-9550 MB, SK
2012-Nov-03.05 -8.99 SAAO-1.9m / GS 900 3500-7150 NSL
2012-Nov-03.57 -8.47 ANU-2.3m / WiFeS 900 3500-9550 MB, SK
2012-Nov-03.72 -8.32 ANU-2.3m / WiFeS 900 3500-9550 MB, SK
2012-Nov-04.07 -7.97 SAAO-1.9m / GS 900 3500-7150 NSL
2012-Nov-04.34 -7.70 Lick-3m / Kast 600 3500-8220 EG
2012-Nov-04.50 -7.54 ANU-2.3m / WiFeS 900 3500-9550 MB, SK
2012-Nov-05.22 -6.82 NTT-3.6m / EFOSC 100 3360-10000 PESSTO
2012-Nov-05.37 -6.67 Lick-3m / Kast 600 3500-8220 EG
2012-Nov-05.61 -6.43 ANU-2.3m / WiFeS 900 3500-9550 MB, SK
2012-Nov-06.38 -5.66 Lick-3m / Kast 180 3500-10300 SBC, PK
2012-Nov-07.27 -4.77 NTT-3.6m / EFOSC 100 3360-10000 PESSTO
2012-Nov-07.28 -4.76 du Pont / WFCCD 60 3500-9600 NM, BM
2012-Nov-07.34 -4.70 Lick-3m / Kast 180 3500-8180 LP, DC
2012-Nov-08.24 -3.80 NTT-3.6m / EFOSC 100 3360-10000 PESSTO
2012-Nov-08.27 -3.77 du Pont / WFCCD 60 3500-9600 NM, BM
2012-Nov-08.34 -3.70 Lick-3m / Kast 180 3500-8180 LP, DC
2012-Nov-09.52 -2.52 ANU-2.3m / WiFeS 900 3500-9550 FV
2012-Nov-09.62 -2.42 ANU-2.3m / WiFeS 900 3500-9550 FV
2012-Nov-10.26 -1.78 du Pont / WFCCD 80 3500-9600 NM, BM
2012-Nov-10.57 -1.47 ANU-2.3m / WiFeS 700 3500-9550 FV
2012-Nov-10.70 -1.34 ANU-2.3m / WiFeS 900 3500-9550 FV
2012-Nov-11.26 -0.78 du Pont / WFCCD 90 3500-9600 NM, BM
2012-Nov-11.67 -0.37 ANU-2.3m / WiFeS 900 3500-9550 FV
2012-Nov-12.38 +0.34 Lick-3m / Kast 180 3500-8220 TD, JF
2012-Nov-12.74 +0.70 ANU-2.3m / WiFeS 600 3500-9550 MO, MP
2012-Nov-13.24 +1.20 du Pont / WFCCD 270 3500-9600 JR, GB
2012-Nov-13.26 +1.22 NTT-3.6m / EFOSC 100 3360-10000 PES STO
2012-Nov-13.75 +1.71 ANU-2.3m / WiFeS 600 3500-9550 MO, MP
2012-Nov-14.25 +2.21 du Pont / WFCCD 270 3500-9600 JR, GB
2012-Nov-14.32 +2.28 Lick-3m / Kast 300 3500-10300 KC, OF
2012-Nov-15.22 +3.18 NTT-3.6m / EFOSC 100 3360-10000 PES STO
2012-Nov-15.24 +3.20 du Pont / WFCCD 270 3500-9600 JR, GB
2012-Nov-16.24 +4.20 du Pont / WFCCD 270 3500-9600 JR, GB
2012-Nov-16.51 +4.47 ANU-2.3m / WiFeS 600 3500-9550 NS
2012-Nov-17.22 +5.18 du Pont / WFCCD 270 3500-9600 JR, GB
2012-Nov-17.46 +5.42 ANU-2.3m / WiFeS 600 3500-9550 NS
2012-Nov-18.24 +6.20 du Pont / WFCCD 270 3500-9600 JR, GB
2012-Nov-18.42 +6.38 ANU-2.3m / WiFeS 600 3500-9550 NS
2012-Nov-19.16 +7.12 du Pont / WFCCD 100 3500-9600 JR, GB
2012-Nov-19.56 +7.52 ANU-2.3m / WiFeS 900 3500-9550 MC
2012-Nov-20.15 +8.11 du Pont / WFCCD 100 3500-9600 NM
2012-Nov-20.30 +8.26 Lick-3m / Kast 360 3500-10300 SBC, OF
2012-Nov-20.48 +8.44 ANU-2.3m / WiFeS 900 3500-9550 MC
2012-Nov-21.13 +9.09 du Pont / WFCCD 300 3500-9600 NM
2012-Nov-21.24 +9.20 NTT-3.6m / EFOSC 100 3360-10000 PES STO
2012-Nov-21.68 +9.64 ANU-2.3m / WiFeS 900 3500-9550 MC
2012-Nov-23.25 +11.21 NTT-3.6m / EFOSC 100 3360-10000 PESSTO
2012-Nov-29.42 +17.38 ANU-2.3m / WiFeS 900 3500-9550 CL, BS
2012-Nov-30.08 +18.04 Baade / IMACS 900 3400-9600 DO
2012-Dec-04.21 +22.17 NTT-3.6m / EFOSC 300 3360-10000 PESSTO
2012-Dec-08.50 +26.46 ANU-2.3m / WiFeS 900 3500-9550 ITH
2012-Dec-12.22 +30.18 NTT-3.6m / EFOSC 600 3360-10000 PESSTO
2012-Dec-16.92 +34.88 NOT / ALFOSC 300 3300-9100 MS
2012-Dec-17.41 +35.37 ANU-2.3m / WiFeS 900 3500-9550 JM , CD, SO
2012-Dec-21.22 +39.18 NTT-3.6m / EFOSC 900 3360-10000 PESSTO
a
With respec t to B-band maximum brightness on 2012 Nov. 12.04.
b
BK, Beth Klein; BM, Barry Madore; BS, Brad Schaefer; BZ, Ben Zuckerman; CD, Catherine de Burgh-Day; CL, Chris Lidman; CM1, Curtis
McCully; CM2, Carl Melis; DB, Daniel Bayliss; DC, Dan Carson; DO, David Osip; EG, Elinor Gates; FV, Fr´ed´eric Vogt; GB, Guillermo Blanc; GZ,
George Zhou; ITH, I-Ting Ho; JF, Jerome Fang; JM, Jeremy Mould, JR, Jeff Rich; KC, Kelsey Clubb; LP, Liuyi Pei; MB, Mi ke Bessell; MC, Mike
Childress; MJ, Michael Jura; M O , Matt Ower s; MP, Mike Pracy; MS, Max Stritzinger; NM, Nidia Morrell; NS, Nic Scott; NSL, Nicola S. Loaring;
OF, Ori Fox; PK, Pat Kellyl SBC, S. Bradley Cenko; SJ, Saurabh Jha; SK, S tefan Keller; SO, Sinem Ozbilgen; SX, Siyi Xu; TD, Tyler Desjardins.
c
WiFeS red channel cryo pump failure.
6
Figure 1. Representative sample of SN 2012fr spectra, labeled by
phase with respective to B-band maximum light.
component, but beginning around −12 days a second
distinct component at lower velocities begins to develop.
By −9 days the HVF and the lower velocity component
exhibit equal strength, but by −5 days the HVF becomes
difficult to distinguish visually.
While such Si ii λ63 55 HVFs have been observed in
other SNe Ia, nota bly SN 2005cf (Wang e t al. 2009b)
and SN 2009 ig (Foley et al. 2012; Marion et al. 2013)
(see also § 6.1), the distinction between HVF and pho-
tospheric compo nents is cleaner in SN 2012fr tha n ever
seen before. In this section we follow the evolution of
the two components in a quantitative way by fitting the
Si ii λ6355 line as a simple double-Gaussian profile.
We show in Figure 3 some example fits of the
Si ii λ6355 line at several epochs. We first begin by
defining regio ns of the blue and red pseudo-continuum,
then perform a simple linear fit between the two re-
gions. The flux in the line region is next divided by the
pseudo-continuum, and the normalized absorption pro-
Figure 2. The Si ii λ6355 feature of SN 2012fr (thick blue line)
at +8 days in velocity space centered at the velocity minimum.
Plotted for comparison are SN 2005hj (Quimby et al. 2007) at +9
days in red, SN 1994D at +11 days (from Blondin et al. 2012) in
magenta, and SN 2002bo (Benetti et al. 2004) at +5 days in green,
in order of thickest to thinnest lines.
file is fitted with two Gaussians. The fit parameters are
the center, width, and depth of each component, and the
only constraints imposed are that the HVF component be
above 14,000 km s
−1
and the low-velocity photospheric
component be below that same threshold. This threshold
between the two fitted velocity components was chosen
because it is higher than the velocities obs e rved in most
SNe Ia, and provided favorable separation between the
two velocity components.
In Table 2 we present the fitted parameters for our
two-component Si ii λ6355 fits including velocity center
(v), velocity width (∆v; i.e., FWHM), and calculated
pseudo-equivalent width (pEW). In Figure 4 we s how
the velocity evolution of the two components compared
to the v
Si
evolution of other SNe Ia from Benetti et a l.
(2005). The HVF component shows a strong velocity
gradient ( ˙v
Si
= 353 km s
−1
day
−1
) but at velocities much
higher than those seen in most SNe Ia, even at early
times. T he photospheric comp onent, on the other hand,
remains virtually constant in velocity even to late times
(see § 3.2), except for tentative evidence fo r higher veloc-
ity at the ea rliest epochs. However, we caution that the
photospheric component is much weaker than the HVF
component at those phases, so the velocity is mo re un-
certain.
The relative strength of the two compo ne nts is most
clearly captured by examining the absorption strength
of each component as quantified by the pEW. T his can
be trivially calculated as the ar ea of the normalized ab-
sorption profile. We show in Figure 5 the pEW of the two
fitted components from the earliest epoch (−13 days) to
the latest epoch (−1 days) at which both features have
a significant detection, and for the full Si ii λ6355 profile
for all epochs be fore +10 days. As previously noted, the
strength of the HVF fades very quickly while that of the
photospheric component slowly rises, with the equality
point occurring between −10 and −9 days. The total
pEW of the Si ii λ6355 line declines until a few days
before maximum light, when it remains nearly cons tant
at around 65
˚
A. As we discuss below (§ 5), the pEW of
7
Figure 3. Left: Evoluti on of the Si ii λ6355 line of SN 2012fr in
velocity space. Right: Several representative examples of the two-
component Gaussian fits. Data are in blue, regions of the pseudo-
continuum fit are denoted by the vertical dotted black lines, the
fitted pseudo-continuum is the dashed green line, the fully fitted
profile is s hown as the smooth red line, and the two individual
components are the dotted cyan curves.
Figure 4. Velocity evolution of the Si ii λ6355 feature of SN 2012fr
compared to that of a number of SNe Ia from the Benetti et al.
(2005) sample. The HVF component of SN 2012fr is shown as the
large black triangles, while the lower velocity photospheric compo-
nent is shown as large black circles.
Table 2
Si ii λ6355 Fit Results
HVF Photospheric
Phase v ∆v pEW v ∆v pEW
(days) (km/s) (km/s) (
˚
A) (km/s) (km/s) (
˚
A)
-14.51 22704 11829 164.2 – – –
-14.17 22233 11587 171.6 – – –
-12.66 21525 9046 125.4 13444 6589 24.3
-12.53 21468 8898 120.2 13444 6294 23.0
-11.51 21223 7665 82.0 13430 6473 30.5
-10.05 20798 6315 43.5 13095 6547 38.0
-9.56 20374 6642 41.8 12685 6262 40.4
-9.35 20416 6326 37.6 12755 6431 43.0
-8.99 20275 6188 33.8 12595 6241 41.0
-8.47 20431 5767 29.5 12656 6347 46.8
-8.32 20327 5735 28.2 12571 6241 46.9
-7.97 20147 5503 22.6 12538 6041 46.2
-7.70 19902 6304 26.2 12057 6072 48.7
-7.54 20105 5735 24.5 12397 6167 51.3
-6.82 19501 5334 16.2 12284 5999 52.1
-6.67 19463 5545 16.2 11977 5978 53.6
-6.43 19817 4892 15.9 12251 6231 58.5
-5.66 19902 5651 16.1 12203 5841 55.5
-4.77 18713 5071 10.0 12104 5535 56.9
-4.70 18657 5345 9.8 11892 5619 58.3
-3.80 18397 5197 8.8 12048 5450 59.6
-3.70 18572 4892 5.5 11821 5398 58.4
-2.52 17242 4934 7.4 11944 5250 61.5
-2.42 18478 5693 8.0 12010 5377 63.9
-1.47 18006 4322 4.2 12057 5155 63.9
-0.37 – – – 12034 4965 63.4
+0.34 – – – 11821 4776 61.7
+0.70 – – – 12180 4755 62.9
+1.22 – – – 12175 4818 64.0
+1.71 – – – 12095 4617 62.8
+3.18 – – – 12156 4533 64.0
+4.47 – – – 12180 4270 62.3
+5.42 – – – 12109 4185 62.2
+6.38 – – – 12123 4132 62.4
+7.52 – – – 12227 3995 61.4
+8.44 – – – 12104 3953 61.1
+9.20 – – – 12118 3953 60.7
+9.64 – – – 12071 3953 61.3
+11.21 – – – 12006 – –
+17.38 – – – 11732 – –
+22.17 – – – 11423 – –
+26.46 – – – 11794 – –
+30.18 – – – 11829 – –
+35.37 – – – 11771 – –
+39.18 – – – 11716 – –
this line is lower in SN 20 12fr than in ma ny other nor-
mal SNe Ia as measured in the Berkeley SN Ia Program
(BSNIP) sample (Silverman et a l. 2012a,b).
3.2. Full Velocity Evolution of Si ii λ6355
After the HVF Si ii λ6355 feature fades , the main pho-
tospheric c omponent is well fit by a single Gaussian until
about two weeks after maximum light. At that time, Fe
lines to the red and blue of Si ii λ6355 begin to develop
significant opacity and make it impo ssible to correctly
determine the pseudo-continuum of the Si ii λ6355 line.
Thus, for the epochs +17 days and later, we fit the ve-
locity minimum of Si ii λ6355 by fitting a simple Gaus-
sian profile only in a region of width 30
˚
A centered on
the Si ii λ6355 minimum. Rema rkably, the velocity
8
Figure 5. Pseudo-equivalent width (pEW) of the Si ii λ6355 com-
ponents in SN 2012fr as a function of phase. When both the HVF
and photospheric components are clearly detected, they are shown
as open blue triangles and open green circles, respectively. The
total pEW of the Si ii λ6355 li ne is shown as filled red squares. For
references, the BSNIP sample (Silverman et al. 2012a,b) is show as
small black points.
of the line minimum remains nearly consta nt at about
11,800 km s
−1
even out to phas e +39 d. This is confirmed
by visual inspection of the Si ii λ6355 region as plotted in
Figure 6. We do note that at such late epochs, emission
becomes increasingly important (see, e.g., van Rossum
2012) in the line profiles, so it is possible that the flux
minimum may no t necessary trace the true τ = 1 surface.
We see once again that the narrow width of the pho-
tospheric lines provides a n advantage in following the
Si ii λ6355 velocity reliably to very late epochs. The Fe
lines to the red and blue of this feature are clearly dis-
tinguished in SN 2012fr, enabling an accurate isolation
of Si ii λ6355 and a reliable measurement of its velocity.
In most other SNe Ia, the broade r line widths result in
a blend of the Si ii λ635 5 with its neighboring Fe lines,
making velocity measurements difficult at late epochs.
We will return to this point in § 6.1.
4. ADDITIONAL ATOMIC SPECIES IN SN 2012FR
The narrow velocity width of the photospheric
Si ii λ6355 line noted in § 3 holds true for nearly all
absorption features in the optical spec tra of SN 2012fr
starting about a week after maximum light. This pro-
vides a unique advanta ge in line identifications in the
SN spectra, as blending of neighboring lines is less pro-
nounced in SN 201 2fr than in many other SNe Ia.
In this section we focus on four element gr oups of par-
ticular interest. We begin by inspecting narrow Na i D
absorption in § 4.1, and show that SN 2012fr shows no
detectable a bsorption in this line. We briefly present
in § 4.2 our search for signatures of unburned C, which
showed no clear detection. In § 4.3 we inspect the Ca ii
IR triplet, which exhibits behavior simila r to that of
Si ii λ6355, and then brie fly examine the more complex
Ca ii H&K line at maximum light. Finally, in § 4.4, we
examine the velocities of Fe-group elements in SN 2012fr.
4.1. Na i D Narrow Absorption
Narrow absorption in the Na i D line in SN Ia spec-
tra is commonly used to quantify the amount of fore-
Figure 6. Late-time evolution of the Si ii λ6355 line in SN 2012fr,
shown in velocity space. The wavelengths corresp onding with
10,000 km s
−1
and 15,000 km s
−1
are shown as thick blue lines,
with 1,000 km s
−1
intervals denoted by thin blue lines. The veloc-
ity plateau at late times appears around 11,800–12,000 km s
−1
.
ground dust that reddens a SN Ia (dust is associated
with the detected gas). Na i D absorption at the redshift
of the SN host galaxy presumably arises from either fore-
ground interstellar gas (e.g., Poznanski et al. 2011, 2012)
or very nearby circumstellar material shed from the SN
progenitor system prior to explosion (Patat et al. 2007;
Simon et al. 2009; Sternberg et al. 2011; Dilday et al.
2012).
SN 2012fr shows no detectable narrow Na i D abs orp-
tion in the HIRES spectrum taken at phase −13.6 days.
In Figure 7 we show the regions of the HIRES spectrum
corres po nding to the Na i D line as well as the Ca ii H&K
lines, both at the redshift of the host galaxy NGC 1365
(v = 1636 km s
−1
; Bureau et al. 1996) and at zero red-
shift. Meas urements of the line detectio ns, or 3σ upper
limits, are presented in Table 3.
Narrow absorption features from Milky Way gas are
clearly detected at 2.6σ and 6.0σ in the D1 and D2 line s
of Na i, and at very high significanc e (> 10σ) in the
H&K lines of Ca ii. Using the empirical scaling rela-
9
Figure 7. Keck HIRES observations of SN 2012fr. Top: N arrow
Na i D and Ca ii H&K at rest velocity, arising from Milky Way gas.
Bottom: Narrow Na i D and Ca ii H&K at the recession velocity
of NGC 1365.
Table 3
HIRES Absorption Equivalent Widths
Line Milky Way NGC 1365
(m
˚
A) (m
˚
A)
Ca ii λ3934.777 129.3 ± 3.6 107.3 ± 7.0
Ca ii λ3969.591 66.0 ± 4.0 57.3 ± 6.0
Na i λ5891.5833 82.9 ± 13.8 < 42.0 (3σ)
Na i λ5897.5581 35.4 ± 13.4 < 40.8 (3σ)
tions of Poznanski et al. (2012) to convert Na i D ab-
sorption into the reddening E(B − V ), the measured
D1 and D2 absorption strengths imply r eddenings of
E(B − V ) = 0.0 186 and E(B − V ) = 0.021 3 mag,
respectively. These are in excellent agree ment with
the measured value of E(B − V ) = 0.018 mag from
Schlafly & Finkbeiner (2011).
The only possible absorption features at the reds hift
of NGC 1365 appear in the Ca ii H&K lines at v ≈
−100 km s
−1
from the rest redshift of NGC 1365. Given
that these features lack corr esponding ones in Na i D, in
addition to the facts that NGC 1365 is a nearly face-on
barred spiral and SN 2012fr is very close to the cen-
ter of the galaxy and thus well away from the dusty
spiral arms, it appears unlikely that this fea ture near
Ca ii H&K is truly caused by interstellar gas. Thus,
we detect no significant narrow absorption features in
SN 2012fr. Given the traditional correlation of these
narrow absorptio n features with reddening by foreground
dust, our observations are consistent with SN 2012fr hav-
ing no obsc uration by foreground dust. The strongest
constraint arises from the D2 line of Na i, which places
a 3σ uppe r limit of E(B − V ) < 0.015 mag (again us-
ing Poznanski et al. 2012) for the reddening of SN 2012fr
from within NGC 1365.
4.2. C ii
We comment briefly in this se c tion on the search
for unburned C features in spectra of SN 2 012fr.
Such signatures typically manifest themselves as weak
C ii absorption lines in optical spectra of SNe Ia ,
and have been of particular interest in recent years
(Thomas et al. 2011b; Parrent et al. 2011; Folatelli et al.
2012; Silverman & Filippenko 2012; Blondin et al. 2012).
The strong e st C feature in the optical is typically the
C ii λ6580 line, and the slightly weaker λ7234, λ4745,
and λ4267 lines of C ii are also sometimes visible (e.g.,
Mazzali 2001; Thomas et a l. 2007).
The redshift of NGC 1365 places the likely location
of the blueshifted absorption minimum of C ii λ6580
coincident with a weak telluric absorption feature at
λ = 6280
˚
A (corresponding to v ≈ 13,700 km s
−1
for C ii
λ6580). This telluric line is of comparable strength to the
typical C ii absorption, but it was entirely or largely r e -
moved during the reduction process. We cannot identify
any obvious signature of C ii λ6580 absorption at its po-
sition; nor do we detec t the other major o ptical C ii lines
even at early epochs.
This is de monstrated directly in Fig ure 8, where we
plot regions aro und the four C ii lines in velocity spac e
for the −14.17 day spectrum from SALT. No clea r sig-
nature of C ii seems visible in any of these lines, even
for C ii λ6580 after removal of the telluric featur e. C i
features in the NIR may provide better detec tion of un-
burned material in SN 2012fr (see, e.g., SN 2011fe in
Hsiao et al. 2013), and will be investiga ted with detailed
spectroscopic fitting in Paper III (Hsiao et al. 2013, in
prep.).
4.3. Ca ii
4.3.1. Ca ii Infrared Triplet
The Ca ii IR tr iple t in SN 2012fr begins at very high
velocities two weeks before maximum light, with the blue
edge of the absorption in the first spectrum reaching rel-
ativistic velocities of nearly 50,000 k m s
−1
(v ≈ 0.17c).
The line complex gradually recedes in velocity and ex-
hibits complex structure roughly a week before maxi-
mum light, and by two weeks after maximum it appears
dominated by a single narrow component.
The complex structure of the Ca ii IR triplet ap-
pears to be indica tive of multiple components in veloc-
ity space, similar to that seen in the Si ii λ6355 line
(see § 3.1). Mo de ling this line multiplet is more com-
plex than the simple two-c omponent Gauss ian fits em-
ployed for Si ii λ6355. Instead, each component of the
Ca ii IR triplet in velocity space must be modeled as a
triplet of Gaussian profile s with common velocity width,
separatio n in velocity space as dictated by the line r est
10
Figure 8. Sections of the −14.17 day SALT spectrum of SN 2012fr
corresponding to the strongest typical C ii features in SNe Ia, pl ot-
ted in terms of velocity of the respective lines.
wavelengths, and with relative absorption depths appro-
priately constrained.
For epochs where spectral coverage extended to suf-
ficiently red wavelengths to cover the Ca ii IR triplet,
we fit the absorption profile as a two-component model
after normalizing to a fitted pseudo-co ntinuum. Each
absorption component is described by a central velocity,
velocity width, and absorptio n depth (here we set relative
absorption depths of the triplet lines to be equal, assum-
ing the optically thick regime). As with Si ii λ6355, the
only constraint applied here was to force the two compo-
nents to occupy different regions of velocity s pace split
at 14,000 km s
−1
. We show the spec tral evolution of the
Ca ii IR triplet a s well as some represe ntative profile fits
in Figure 9. As w ith the Si ii λ6355 fits, the pseudo-
continuum s hape begins to be poo rly represented by a
simple linear fit at late times. Thus we employ a s imi-
lar Gaussian line minimum fitting technique as that for
Si ii λ6355 at epochs after +8 days, here meas uring the
minimum of the cleanly separated 8662
˚
A line.
We report results in Table 4, w ith fit parameters la-
beled similarly as in Table 2. The high-velocity com-
ponent consistently exhibits a broad veloc ity width (>
5000 km s
−1
), producing a broad c omponent where all
lines in the triplet a re blended. The low-velocity compo-
nent exhibits the same narrow veloc ity width as observed
in the Si ii λ6355 line, making it possible to distinguish
the 8662
˚
A line from the blended 8498
˚
A and 8542
˚
A lines.
This is illustrated directly in Figure 10, w here we show
the Ca ii IR triplet evolution at late times and demon-
strate the ability to both resolve the triplet lines and
observe their consistent velocity at v ≈ 12,000 km s
−1
.
In Fig ure 11 we plot the velo city evolution of the fitted
Ca ii IR triplet components compared to the analogous
Figure 9. Same as Figure 3 but for the Ca ii IR triplet. Here
velocities are pl otted with respect to the reddest line in the Ca ii
IR triplet at 8662
˚
A.
Table 4
Ca ii IR triplet Fit Results
HVF Photospheric
Phase v ∆v pEW v ∆v pEW
(days) (km/s) (km/s) (
˚
A) (km/s) (km/s) (
˚
A)
-14.51 31087 21853 615.5 – – –
-11.51 28640 15311 364.8 – – –
-9.35 25571 9581 146.9 – – –
-7.54 24126 5507 94.5 11221 1629 4.1
-4.77 23043 5808 99.7 11259 2407 13.2
-2.52 21994 6226 96.1 11219 2697 29.9
-0.37 20989 6662 90.0 11357 3151 49.5
+1.22 20131 6935 82.9 11461 3187 60.3
+3.18 19133 7368 78.2 11564 3207 70.8
+5.42 15913 10582 121.5 11690 2769 52.9
+8.44 – – – 12206 – –
+11.21 – – – 12240 – –
+17.38 – – – 12092 – –
+22.17 – – – 12121 – –
+26.46 – – – 11976 – –
+30.18 – – – 12082 – –
+35.37 – – – 11937 – –
+39.18 – – – 12052 – –
components in the Si ii λ6355 line (§ 3). The HVF Ca ii
IR triplet component is consistently at higher velocities
than the Si ii λ6355 HVF, has a steeper velocity gra-
dient ( ˙v
Ca
= 6 86 km s
−1
day
−1
), and is visible to later
epochs than the Si ii λ6355 HVF (we note that these
11
Figure 10. Late-epoch evolution of the Ca ii IR triplet in
SN 2012fr, showing the narrow velocity width and constant ve-
lo ci ty of the three lines comprising the triplet. For reference, we
mark the wavelengths of the three tr iplet lines at a velocity of
v = 12,000 km s
−1
, showing that the reddest line of the triplet at
8662
˚
A is cleanly resolved from the bl uer two lines.
characteristics were also observed in the Ca ii IR triplet
feature of SN 2009ig; Marion et al. 2013). In SN 2012fr
the late-time velocity of the Ca ii IR triplet plateaus at
v ≈ 12,000 km s
−1
, consistent with the velocity plateau
observed in the Si ii λ6355 line.
Finally, we comment on the dependence of our results
on the assumption of optical thickness in the Ca ii IR
triplet. We rep e ated the above fits under the assumption
of the optica lly thin regime, where the relative absorp-
tion depths of the triplet lines are proportional to their
Einstein B (absorption) values (1.71 × 10
9
cm
2
s
−1
erg
−1
for λ8498, 1.03 × 10
10
cm
2
s
−1
erg
−1
for λ8542, 8.66 ×
10
9
cm
2
s
−1
erg
−1
for λ8662; Wiese et al. 1969) and the
relevant statis tical weights, and found that the quali-
tative behavior of the velocity evolution was consistent
with that found in our fiducial fits. The velocities of
the HVF Ca ii IR triplet component were consis tently
higher due to the shift in weighted mean wavelength of
the triplet, but the late-time velocity was still cons istent
Figure 11. Velocity evolution of the high-velocity (triangles) and
photospheric (circles) components of the Ca ii IR triplet (open red
symbols) in SN 2012fr compared to that of the Si ii λ6355 line
(filled blue symbols ).
with the v ≈ 12,000 km s
−1
velocity plateau due to the
8662
˚
A line being distinguishable from the bluer line s in
the triplet. Interestingly, the photospheric component
appears to be better fit (and yields a velocity consis-
tent with the velocity plateau) in the optically thin a s-
sumption at ea rly ep ochs, and then transitions to being
optically thick aro und the time when the HVF compo-
nent fades. The true absorption strengths of the lines
in the Ca ii IR triplet likely fall somewhere between the
optically thin and optically thick regimes, but we have
confirmed that our res ults are consistent in both extre me
cases.
4.3.2. Ca ii H&K
The Ca ii H&K doublet in SNe Ia is a line co m-
plex o f keen interest, as its behavior at maximum light
may be an indicator of intrinsic SN Ia color (Foley et al.
2011; Chotard e t al. 2011; Blondin et al. 2012; Foley
2012). Additionally, recent work by Mag uire et al.
(2012) showed that the Ca ii H&K velocity of the mean
rapidly declining (low “stretch”) SN Ia spectrum at max-
imum light is lower than that of the mean slowly declin-
ing (high “str etch”) SN Ia spectrum. Thus, the Ca ii
H&K line seems promising for helping to unravel SN I a
diversity.
Interpreting Ca ii velocities with this line complex is
difficult, however, due to the pr e sence of the ne arby
Si ii λ3858 line (see the thorough discussion in Foley
2012), as well as the complex underlying pseudo-
continuum. We therefore chose to examine this line com-
plex only at maximum light for SN 2012fr. We employed
a fitting procedure simila r to that of the Ca ii IR triplet,
but with fit parameter s informed and tightly constrained
by results o f the Ca ii IR triplet and Si ii λ6355 fits.
We model absorption in the Ca ii H&K line with mul-
tiple velocity components, where each comp onent is a
doublet profile with relative depths set to unity (i.e.,
the optically thick regime). In addition to the HVF
and photo spheric components of the Ca ii H&K line,
we a dd a single Gaussian absorption profile to model
Si ii λ3858. We note that fits assuming the optically
thin regime (with line depths proportional to the E in-
12
Figure 12. Fit to the Ca ii H&K line profile in the maximum-light
(WiFeS Nov. 11.67) spectrum of SN 2012fr. Line colors and styles
are the same as in Figure 3, but with the Si ii λ3858 line profile
plotted as the dashed-dotted black line. The top panel shows a fit
without the Si ii λ3858 line, while the bottom panel shows a fit
including this line.
stein B values of 4.50 × 10
10
cm
2
s
−1
erg
−1
for λ3934 and
2.20 × 10
10
cm
2
s
−1
erg
−1
for λ3968; Wiese et al. 1969)
produced poor fits to the Ca ii H&K line, especially the
photospheric component.
To enforce consistency with the other lines measured
for Ca and Si, we constrained the velocity center and
velocity widths of the two Ca ii H&K components to
be within 20% of their values fitted for the Ca ii IR
triplet, but left the absorption depths as free parame-
ters. Similarly, we forced the velocity width and center
of the Si ii λ3858 line to be within 20% of the Si ii λ6355
line. To investigate the importance of the Si ii λ3858 line
in this complex, we pe rformed fits both with and without
the Si ii λ3858 line included, and our best fits for each
case are shown in Figure 12.
In Table 5 we summarize the main results of our Ca ii
H&K line fits, fo r the cases with and without Si ii λ3858,
as well as the Ca ii IR triplet and Si ii λ6355 results for
reference. Bo th Ca ii H&K fits yield Ca and Si velo cities
within 1000 k m s
−1
of their red counterparts, but the fit
with Si ii λ3858 included shows a more favorable ratio
of pE W for the photospheric and HVF Ca components,
as well as a much better fit to the overall line profile.
Our fits indicate that both Ca ii H&K and Si ii λ3858
are needed to explain the absorption profile of the Ca ii
H&K line complex in SN 2012fr, with the HV Ca ii H&K
being dominant over the Si ii λ3858.
We note, however, that the quantitative details of these
results depend on how tightly we constrain the velocity
center and width of the HV Ca ii H&K component. If we
loosened the constraints to be within 30% of the Ca ii
IR triplet velocity and width, it changes the pEW of
the HV Ca ii H&K and Si ii λ3858 lines to be 68
˚
A and
35
˚
A, respectively, from 80
˚
A and 24
˚
A when cons trained
to 20%. This is b e cause the high velocities of the HV
Ca ii H&K component place it nearly coincident with
the wavelength of the lower velocity Si ii λ3858 line. We
therefore cannot say conclusively what the a bsorption
ratio of thes e two lines is in this line complex, but we did
find consistently that both were needed to adequately fit
the absorption profile.
We found here that decoupling the Si ii λ3858 line from
the Ca ii H&K line is a no ntrivial procedure. While it is
difficult to derive precise quantitative results, two genera l
qualitative re sults ar e clear. The first is tha t both the
Ca ii H&K line and the Si ii λ3858 line are ope rative
in this line complex in SN 2012fr, and that high-velocity
Ca ii H&K can be nearly degenerate with lower velocity
Si ii λ3858. The second conclusion for SN 2012fr is that
regardless of the details of how much the line velo cities
and velocity widths are allowed to vary, high-velocity
Ca ii H&K appears to be the dominant contributor to
absorption in this line region.
4.4. Fe-Group Elements
The relatively narrow line widths of SN 2012fr are
useful for identifying absorption features which are
typically blended in other SNe Ia, and are particu-
larly advantageous for identifying Fe lines. In Fig-
ure 13 we illustrate this principle with the +8 day spec-
trum of SN 2012fr compared to two other “normal”
SNe Ia, SN 2005cf (Wang et al. 20 09b) and SN 2003du
(Stanishev et al. 2007), as well as to the broad-lined
SN 200 2b o (Benetti et al. 2004). Of particular note is
the line complex at ∼ 4700
˚
A, comprising several Fe ii
lines and the Si ii λ5054 line, which shows cleane r sepa-
ration in SN 2012fr than the other SNe Ia.
In the +8 day spectrum, three major Fe ii lines (λ4924,
λ5018, and λ5169) all have velocity minima consistent
with the velocity plateau (v ≈ 12,000 km s
−1
) identified
in Si ii λ6355. We illustrate this in Figur e 14, where we
plot the post-maximum spectra of SN 2012 fr in the rele-
vant wavelength region and highlight the wavelengths as-
sociated with each of these lines at v =12 ,000 km s
−1
. Af-
ter this epoch, the observed velocity minima of these Fe
lines decrease with time. We also tentatively identify two
features which may be ass ociated with Cr ii lines (λ4876
and λ5310), though their velocity evolution is more dif-
ficult to follow due to the strongly evolving shape of the
underlying pseudo-co ntinuum. We note here that these
lines which we attribute to Fe-group elements show no
clear signature of high-velocity features in the early-time
spectra of SN 2012fr (some Fe HVFs have been possi-
bly identified in several SNe Ia; e.g., Branch et al. 2004;
Mazzali et al. 2005a; Marion et a l. 2013), tho ugh further
modeling of the spectra may reveal more subtle insights.
The velo city behavior of the Fe group lines indicates
13
Table 5
Ca ii H&K Fit Results
Ca HVF Ca Photospheric Si Photospheric
v ∆v pEW v ∆v pEW v ∆v pEW
(km s
−1
) (km s
−1
) (
˚
A) (km s
−1
) (km s
−1
) (
˚
A) (km s
−1
) (km s
−1
) (
˚
A)
Ca NIR + Si ii λ6355 20989 6662 96.1 11357 3151 49.5 12034 4965 –
Ca H&K + Si ii λ3858 22545 6633 79.9 11775 2675 23.6 10749 4707 25.4
Ca H&K only 21216 7994 103.3 11787 3665 23.8 – – –
Figure 13. Line identifications in SN 2012fr (blue) for the +8
day spectrum, compared to the spectra of other SNe Ia at a similar
epoch. SN 2005cf (Wang et al. 2009b), SN 2003du (Stanishev et al.
2007), and SN 2002bo (Benetti et al. 2004) are s hown as the green,
red, and magenta spectra, respectively.
that the τ = 1 surface of the photosphere is at v ≈
12,000 km s
−1
one week after ma ximum brightness, and
recedes inward after that time. From this we can con-
clude two important results: (i) the Si and C a velocity
plateaus imply that the layer of intermediate-mass el-
ements (IMEs) is unlikely to extend deeper than v ≈
12,000 km s
−1
since the Fe-group elements are detected
at lower velocities at the same epochs; and (ii) the layer
of Fe-group elements extends deeper in the ejecta than
the IMEs, indicating a likely s tratification of the ejecta.
While these preliminary line identifications are in-
teresting, a full accounting of all elements and their
velocities would require a spectrum-synthesis fit (see,
e.g., Thomas et al. 2011 a) such as that undertaken by
Parrent et al. (2012) for SN 2011fe (Nugent et al. 2011;
Li et al. 2011), or a modeling of abundance stratification
in the ejecta such as that underta ken by Stehle et al.
(2005). Analyses of this mag nitude are beyond the in-
tended scope of this paper, but we believe that the sp ec-
tra rele ased here will be invaluable for such efforts, and
Figure 14. Post-maximum spectra of SN 2012fr highlighting a
selected region having significant features of Fe-group elements.
Vertical lines correspond to wavelengths of labeled features at v =
12,000 km s
−1
, the measured velocity of Fe ii features in the +8
day spectrum.
we strongly encourage future modeling of the SN 2012fr
spectra.
5. SPECTROSCOPIC SUBCLASSIFICATION OF
SN 2012 FR
In recent years much effort has been focused o n cat-
egorizing the observed diversity of SNe Ia by means of
quantitative metrics measured from their optical spec-
tra. In this section we will examine SN 2012fr in the
context of these classification schemes. T he spectral in-
dicator values of SN 2012fr employed for that purpose
14
Table 6
Spectral Indicators for SN 2012fr
Quantity Value Unit
∆m
15
(B) 0.80 ± 0.01 mag
pEW(5972) 3.9 ± 5.0
˚
A
pEW(6355) 66.5 ± 15.5
˚
A
v
Si
(max) 12037 ± 200 km s
−1
˙v
Si
0.3 ± 10.0 km s
−1
day
−1
are pre sented in Table 6. These include several quanti-
ties (pEW(5972), pEW(6355), v
Si
) calculated from the
maximum-light spectrum (the Nov. 11.67 WiFeS spec-
trum), the veloc ity gradient of the Si ii λ6355 line ˙v
Si
(measured from the abs olute decline between phases 0
and +10 days), and the light-curve decline ∆m
15
(B)
(from Pape r I I ). We note that the pEW values used here
are calculated from direct integration of the line profile
and differ insignificantly from the Gaussian area reported
in § 3.1.
Branch et al. (2009) prop osed that SNe Ia can be split
into four broa d categories based on their location in the
parameter space defined by the pEW of the Si ii λ5972
and λ6355 lines. We show SN 2012fr on this “Branch
diagram” in the uppe r-left panel of Figure 15, along with
a set of SNe Ia combining the samples of Blondin et a l.
(2012) and Silverman et al. (2012b). SN 2012fr falls on
the boundary between the “shallow silico n” class and
the “core normal” class. As previously noted by both
Silverman et al. (20 12b) and Blondin et al. (2012), these
Branch classes do not represent disjoint samples with
distinct features, but instea d regions within a co ntinuum
of SN Ia characteristics. SN 2012fr appear s to be a c lear
example of a transition-like event that bridges the gap
between two subclasses.
Wang et al. (2009a) s howed that a subset of SNe Ia dis-
play high velocities in the Si ii λ6355 line, and this “HV”
subset exhibits different co lor behavior than SNe Ia with
normal velocities. In the upper-right panel of Figur e 15
we show SN 2012fr on the “Wang diagram” which plots
pEW(6355) vs. v
Si
at maximum light, as well as the
same comparison s ample of SNe Ia from the Branch dia-
gram. Once again, SN 2012fr has a v
Si
value which is just
slightly ab ove the establishe d boundary separating HV
from normal SNe Ia, marking it as a transition-like event
between velocity classes. Its location in this diagram is
noteworthy because it exhibits a lower pEW(6355) than
any of the other HV SNe Ia, and its velocity is higher
than that of other SNe Ia having weak Si ii λ635 5 ab-
sorption.
Benetti et al. (2005) examined subclasses of SNe Ia
based on the velocity e volution of the Si ii λ6355 line.
They found that rapidly declining SNe Ia tended to
have consistently high velocity gradients (dubbed the
“faint” subclass), while slowly declining SNe Ia appeared
to occur in two classes with either high or low veloc-
ity gradients (“HVG” and “LVG,” respectively). In the
bottom-left panel of Figure 15, we show SN 2012fr on
this “Benetti diagram” along with the subset of SNe Ia
from the other panels with sufficient data to measure a
velocity gradient. SN 201 2fr clearly resides in the LVG
region of this diagram, as expected due to the observed
Si ii λ6355 velocity plateau (see § 3.2).
Finally, in the bottom-right pane l of Figure 15 we show
the velocity gradient vs. velocity at maximum light. As
has been previously noted, the HV objects appear to
show a correlation between their velocity gradient and
velocity at maximum light. SN 2012fr has a lower veloc-
ity gradient than any of the HV members, and instead
appears to reside at the edg e of the cloud of p oints pop-
ulated by normal-veloc ity SNe Ia.
SN 2012fr e xhibits much of the spectroscopic and pho-
tometric be havior of the more lumino us SNe Ia such as
SN 1991T (Phillips et al. 1992; Filippenko et al. 1992) or
SN 1999aa (Li et al. 2001; Garavini et al. 20 04). It has a
slow light-curve decline rate, r elatively shallow Si ii ab-
sorption at maximum light, and a very low velocity gra-
dient in the Si ii λ6355 line. However, two key features
of SN 2012fr are not observed in SN 1999aa-like or SN
1991T-like SNe Ia: the high velocity of the Si ii λ6355
line (both a t maximum lig ht in the photospheric com-
ponent and in the early HVF compo ne nt), and strong
absorption in the Si ii λ6355 line at early epochs (phase
about −10 days).
In Figur e 16 we compare the spectra of SN 2012fr at
−12 days and at ma ximum lig ht to comparable spectra
from the SN 1991T-like SN 1998es (from Blondin et al.
2012), SN 1999aa (from Blondin et al. 2012), and the
normal SNe Ia SN 2005c f (Wang et al. 2009b) and
SN 2003du (Stanishev et al. 2007). We see that at max-
imum light SN 2 012fr shows slightly weaker Si ii λ5972
and Si ii λ6355 than the normal SNe Ia, but higher ve-
locity and stronger Ca H&K absorption tha n SN 1999aa
and the SN 1991T-like SN. At very early epochs (−12
days) SN 2012fr is very dissimilar to the extremely slow
decliners, as it displays stronger absorption in Si ii λ6355
and the “sulfur W,” and it lacks the characteristic strong
Fe absorption found in SN 1999aa/SN 1991T-like SNe Ia.
The stronger absorption and higher velocities of the pho-
tospheric components of the Si ii λ6355 line and the lack
of strong Fe absorption also argue against the possibility
of SN 2012fr being a SN 1999aa/SN 1991T-like SN Ia
with HVFs superposed on its spectrum. Thus, while
SN 2012fr shares a number of characteristics with these
very slowly declining SNe Ia, it does not exhibit sufficient
spectroscopic similar to be clas sified as a member of this
peculiar SN Ia subclass.
6. DISCUSSION
The spectra of SN 2012fr exhibit several noteworthy
characteristics: (1) a very narrow velocity width in the
photospheric absorption lines for phases later than one
week after maximum light; (2) high-velocity Si ii λ6355
and Ca ii IR triplet features which could be cle anly de-
coupled from the lower velocity photospheric component;
(3) a clear plateau in the Si ii λ6355 and the Ca ii IR
triplet veloc ities, extending out until +39 days; (4) Si
absorption-line strengths plac ing it on the borderline be-
tween “shallow silicon” and “core normal” classes; and
(5) Si velocity placing it on the borderline between “nor-
mal” and “hig h-velocity” SNe Ia.
In this section we first re fle c t on what the Si ii λ63 55
velocity behavior of SN 2012fr can reveal a bo ut interpre-
tation of this velocity evolution in other SNe Ia (§ 6.1).
We then consider how the aforementioned observationa l
characteristics of SN 2012fr inform us a bout the proba-
ble nature of its explosion (§ 6.2). Finally, we consider
15
Figure 15. Spectral indicators from SN 2012fr compared to those of other SNe Ia as measured by Blondin et al. (2012) and Silverman et al.
(2012b). Top Left: pEW(5972) vs. pEW(6355) at maximum light with s pectroscopic subclasses as defined by Branch et al . (2009). Top
Right: pEW(6355) vs. v
Si
at maximum light with velocity-based subclasses as defined by Wang et al. (2009a). Bottom Lef t: Velocity
gradient of the Si ii λ6355 l ine ˙v
Si
, as measured from the absolute decline between phases 0 and +10 days, vs. light-curve decline ∆m
15
(B),
as previously inspected by Benetti et al. (2005). Bottom Right: Velocity gradient vs. v
Si
at maximum light.
Figure 16. Spectrum of SN 2012fr compared to other SNe Ia at ∼ 12 days before maximum light (left) and at maximum light (right).
Comparison SNe Ia are the SN 1991T-like SN 1998es (from Blondin et al. 2012), SN 1999aa (from Bl ondin et al. 2012), and the normal
SNe Ia SN 2005cf (Wang et al. 2009b) and SN 2003du (Stanishev et al. 2007).
16
Figure 17. Si ii λ6355 feature in the −10 day spectrum of
SN 2012fr (blue) vs. early spectra of other SNe Ia with likely
Si ii λ6355 HVFs: SN 2009ig (Foley et al. 2012; Marion et al.
2013), SN 2007le (from Blondin et al. 2012), and SN 2005cf
(Wang et al. 2009b).
the viability of SN 2012fr as a fundamental calibrator for
measuring the Hubble constant (§ 6.3).
6.1. SN 2012fr and Velocity Evolution of Other SNe Ia
One of the most noteworthy features of the spectra of
SN 2 012fr was the extremely cle ar distinction between
the low-velocity photospheric Si ii λ6355 and an HVF.
Clear identification of two absorption minima in the
Si ii λ6355 line has only been prev iously observed con-
vincingly in SN 2009ig (Marion et al. 2013), but HVFs
may manifest themselves mor e subtly in the line profiles
of other SNe Ia. This may then have an impact on the
measurement of the Si ii λ6355 line velocity.
To inspect how common early HVF behavior is in
SNe Ia, we searched for SNe Ia having spectra simi-
lar to the −10 day spectrum of SN 2012fr by employ-
ing the SN identification (SNID; Blondin & Tonry 2007)
code. The top matches were, as expected, spectra of
SNe Ia at about 10 days before maximum light, many
of which exhibited a br oad boxy absorption profile in
the Si ii λ6355 line. In Figure 17 we plot the Si ii λ6355
profile of three no table matches – SN 2009ig (Fole y et al.
2012; Marion et al. 2013), SN 2007le (from Blondin et al.
2012), and SN 2005cf (Wang et al. 2009b) – all of which
exhibit an absorption-line profile which seems difficult
to explain with a single component, either Gaussian or
P-Cygni. The Si ii λ6 355 shape is best explaine d by a
two-component profile like that observed in SN 2012 fr,
and this probability has been noted by previous authors
(Mazzali 2001; Wang et al. 2009b; Foley et al. 2012).
A possible cons equence of early high-velocity features
in SNe Ia could be an overestimate of the velocity gra-
dient in the Si ii λ6355 line. To test this possibility, we
convolved our spectra of SN 2012fr with a Gaussian fil-
ter of width σ = 3500 km s
−1
(FWHM ≈ 6700 km s
−1
)
and measured the velocity minimum of the Si ii λ6355
line. Several examples of the broadened spectra, along
with the velocity evolution measured from these s pectra,
Figure 18. Top panels: Observed spectra (blue) of SN 2012fr at
−10 and +17 days, compared to spectra broadened by 3500 km
−1
(red). B ottom: Same as Figure 4, but with velocity measurements
of the broadened SN 2012fr spectra shown as open red diamonds.
are shown in Figure 18. During the early epochs when
the Si ii λ6355 HVF is clearly distinct in the observed
spectra, our convo lved spectr a show a single bro ad ab-
sorption featur e whose velocity declines smoothly from
∼22,000 km s
−1
to ∼12,000 km s
−1
over about 6 days
(velocities were almost exactly the pEW-weighted mean
of the values from Table 2).
Though this implied velocity gra die nt is much higher
than that observed in any other SNe I a, it illustrates the
fact that multiple distinct co mponents blended in veloc-
ity space could masquerade as a single rapidly evolving
component. Inspection of the shape of the Si ii λ6355
absorption profile in HVG SNe Ia may provide insight
into the possible impact of high-velocity feature s on the
measured velocity gradients of these SNe.
Finally, we note that the long velocity plateau obse rved
in SN 2012fr may no t be unique, but instead may have
been missed in other SNe Ia where the Si ii λ6355 line
is blended with the neighboring Fe lines at late times.
Insp ection of the full line complex in spectra later than
+20 days (see Figure 1) shows that the mean wavelength
of the absorption lines near Si ii λ6355 becomes red-
der, due to multiple Fe lines appearing to the red of
Si ii λ6355, while the individual lines remain a t con-
stant velocity. This behavior may be hidden in other
SNe Ia having broader lines, or it may be misinterpreted
as a decrease in the line velocity. Indeed, our test of
the velocity-broadened s pectra of SN 2012fr revealed a
slight decrease in the broadened velocity minimum start-
ing at about +10 days. Future modeling of the SN 2012fr
spectra rele ased he re may be a ble to address this ques-
17
tion in more detail by identifying the lines neighboring
Si ii λ6355 and how their relative strengths evolve with
time.
6.2. The Nature of the SN 2012fr Explosion
The key observational features of SN 2012fr – long Si
and Ca velocity plateau, narrow absorption features, and
early HVFs – provide critical clues to the nature of its
explosion. We will argue here that these observations
indicate a thin region of partial burning products which
produce the observed narrow line widths and IME veloc-
ity plateau, and which may be indicative o f stratification
in the ejecta. We then speculate on the origin of the
HVFs, particularly in the context of recent proposa ls for
surface He-shell burning.
6.2.1. Ejecta Stratification
The narrow velocity widths of the absorption profiles
in SN 20 12fr have enabled us to unambiguously track the
velocity of the Si ii λ6355 line to very late times, reveal-
ing a velocity plateau at ∼12,000 km s
−1
until at le ast
the final epoch of observation at +39 days. SNe Ia with
such low velocity gradients have been observed before
(e.g., the “LVG” subclass from Benetti et al. 2005). No-
tably, several SNe Ia have been found to exhibit a short-
lived (i.e., few weeks duration) veloc ity plateau, such
as the super-Chandras ekhar candidates presented by
Scalzo et al. (2012), as well as SN 2005hj (Quimby et al.
2007) which also demonstrated narrow velocity width.
The velocity plateaus for these other SNe Ia were ex-
plained in in terms of explosion models featuring density
enhancements at a particular velocity, which form at the
reverse shock of an interaction between the SN ejecta and
overlying material. E xamples include “tamped detona-
tions” and “pulsating delayed detonations,” such as the
DET2ENV and PDD series models of Khokhlov et al.
(1993). In a tamped detonation, the SN ejecta inter-
act with a compact envelope of H-poor material, such
as the C- O envelope which mig ht remain after a do uble-
degenerate merger (Fryer et al. 20 10; Shen et al. 2012);
the interaction freezes out quickly a nd the shock struc-
ture expands homologously thereafter. In a pulsating de-
layed detonation, the initial deflagration phase quenches
and the WD progenitor undergoes a strong pulsation,
causing the outer layers to contract. The co ntraction
then reignites C and eventually results in a tr ansition to
a detonation, in which a shock forms a t the interface be-
tween the expa nding inner layers and contr acting outer
layers.
Regardless of how it forms, the dense layer in these
models remains optically thick for some time, resulting in
a plateau in the Si ii velocity. Our analysis of Fe ii lines in
SN 2012fr, however, suggests that Si ii detaches from the
photosphere between day +8 and day +11, disfavoring a
pure density enhancement as the cause of the Si ii plateau
in SN 2 012fr. Additionally, in a tamped detonation one
would expect that material above the density-enhanced
layer would correspond primarily to the unburned en-
velope; thus the Si ii HVF we observe in SN 2012fr is
difficult to e xplain in this scenario.
Rather than indicating a pure density enhancement,
the velocity plateau in Si ii λ6355 may be evidence of Si ii
being confined to a narrow region in velocity space. This
possibility is reinforced by the analogous velocity plateau
in the Ca ii IR triplet (§ 4.3.1) as well as the slightly lower
velocity of the Fe ii lines at late times (§ 4.4). This layer-
ing, along with the narrowness of the absorption features,
indicates a proba ble stratification of the progress to com-
plete nuclear burning in the ejecta. These properties
are co mmon in scenarios where the IME s are produced
mainly in a detonation phase (see , e.g., the recent work
by Seitenzahl et al. 2013) as compared to deflagration
scenarios, which result in significantly increased mixing
in the ejecta.
Corro bo ration of stra tification in the ejecta of
SN 2012 fr will be aided by nebular spectr oscopy. Specif-
ically, if the IMEs are distributed in a spherically sym-
metric shell, these should manifest themselves as double-
peaked nebular lines such as the argon lines astutely
observed by Gerardy et al. (2007) in the mid-IR neb-
ular spectrum of SN 2005df. Conversely, if the IMEs
are distributed asymmetrically, as discerned from spec-
tropolarimetric observations by Maund et al. (2013), this
may be revealed in asymmetry of the nebular IME line
profile (unless, of cour se, the asymmetric geometry is
not oriented favorably ). However, IME nebular lines are
challenging to observe, and mos t lines in nebular spec-
tra arise from material burned to full nuclear statistical
equilibrium (Fe and Ni). As Maund et al. (2013) note, a
global asymmetry of SN 2012fr would produce velocity
shifts of the nebular Fe lines, following the prediction of
Maeda et al. (2010).
6.2.2. High-Velocity Features
The HVFs in both Si and Ca also shed lig ht on
the nature of the SN 2012fr explosio n. HVFs in the
Ca ii IR triplet appear to be a very common, perhaps
even ubiquitous, feature in early-time SN Ia spectra
(e.g., Maz zali et al. 2005b). HVFs in the Si ii λ6355
line have also be en observed in many SNe Ia (e.g.,
Wang et al. 200 9b; Foley et al. 2012; Blondin et al. 2012;
Silverman et al. 2012b; Marion et al. 2013). Positive
identifica tion of HVFs in other lines has been more
challenging (though see Marion et al. 2013, for a thor-
ough inspection of HVFs in multiple atomic species in
SN 2009ig), so much effort has b een focused on investi-
gating the Ca ii IR triplet and Si ii λ6355 HVFs. Both
the geometric distribution and physical origin of the ma-
terial responsible for these HVFs are active areas of in-
vestigation.
Tanaka et al. (2006) posited that the HVFs could be
due to pa tches of material outside the nominal photo-
sphere, and the relative strength of the photospheric and
HVFs is due to the relative “covering fraction” of the
outer layer of material. If the HVFs origina ted from
patchy layers of materia l, they would likely lack spher-
ical symmetry. Maund et al. (2013) recently presented
spectropolarimetry of SN 2012fr and argued that the rel-
atively high degree of polarization in the HVFs is in-
consistent with spherically symmetric geometry. Such
asymmetry in HVFs has been inferred from spectropo-
larimetry of other SNe Ia as well (for a review, see
Wang & Wheeler 2008).
The physical origin of HVF material remains un-
clear. It has been suggested that HVFs could be due
to circum-stellar material (CSM; Gerardy e t al. 2004;
Mazzali et al. 2005a), and for some SNe Ia CSM models
18
have yielded favorable agreement with early-time SN Ia
spectra with HVFs (Altavilla et al. 2007; Tanaka et al.
2008). In this scenario, absorption by Si ii a nd Ca ii may
be enhanced if the CSM is partially enriched in H (CSM
with X(H) < 0.3 would not produce a detectable Hα fea-
ture; Tanaka et al. 2008), there by favoring a lower ioniza-
tion state for these ions. However, significant absorption
by Ca and Si across a range of velocities in SN 2012fr is
indicative of these features being produced by material in
(or on) the WD that undergoes partial nuclear burning
during (or pr ior to) the SN explosion.
A possible explanation for the HVFs in SN 201 2fr may
be the detonation of He-rich material at the surface of
the exploding WD. If a He layer of sufficiently low den-
sity is c onsumed by a detonation, the low density pr e-
vents burning to Fe-group ele ments but can be sufficient
to produce Si and Ca (K. Nomo to 2012, private com-
munica tion). In the single-degenerate Chandrasekhar-
mass scenario , a He layer may be present following
accretion from a binary companion. Explosive burn-
ing in a surface He layer is also a gene ric feature in
the double-detonation scenario (Fink et al. 2007, 2010;
Kromer et al. 2010; Sim et al. 2 010, 2012), where the
detonation o f the He layer induces a detonation near the
core of the WD pro genitor, and even in merg er scenarios
in which the CO WDs retain a small He-rich atmosphere
(R. Pakmor 2012, private communication). Recent work
by Townsley et al. (2012) showed that He-shell burning
could produce significant amounts of Ca, especially if
the surface layer dredges up C from the WD surface
through convection. However, one drawback of the He-
shell scenario is that significant amounts of unburned
He remain in most simula tions, and while the remain-
ing He should produce clear signature s in SN Ia spectra
(Mazzali & Lucy 1998), these features are not actually
observed in SNe Ia.
Another possibility is that Si and Ca abundances a re
enhanced in the outer layers of a WD due to surface He
burning that occurred prior to the SN event. This might
arise during a surface He fla sh (e.g., Nomoto et al. 2013)
that manifests itself as a recurrent nova. T his He-shell
flash could produce relatively high Si and Ca abundances
in the outer layer s of the WD, or it might also ejecta par-
tial burning products. These could either fall back onto
the surface of the WD (again providing enrichment of the
outer WD layers prior to e xplosion), or perhaps enrich
part of the CSM into which the SN ejecta subsequently
expand, and in principle the ejected material might be
directly detected as CSM interaction of the SN (e.g., as
in PTF11kx; Dilday et al. 2012).
Thus, it is difficult at this time to constrain the exact
origin of HVFs in SNe Ia, including SN 2012fr, but our
data provide important observational constra ints. HVFs
in both Si ii λ6355 and the Ca ii IR triplet were ob-
served from two weeks before maximum light until they
faded to obscurity at roughly −2 and +10 days, respec-
tively. These HVFs were cleanly distinguished from the
thin photospheric shell, and showed s trong velocity gra-
dients c onsistent with a receding photosphere. The ve-
locity and absorption-strength evolution of these HVFs,
along with the geometry implied from spectropolarime-
try (Maund et al. 2013), provide important constraints
for any model w hich posits an explanation for these fea-
tures.
6.3. SN 2012fr and the Hubble Constant
SN 2012fr occurred in NGC 1365, one of the galaxies
included in the HST Key Project on the extragalactic
distance scale (Freedma n et al. 2001; Silbermann et al.
1999). This fortuitous situation has made it a lead-
ing candidate to contribute towa rd mea surement of the
peak luminosity of SNe Ia. Such information is critical
for determining the Hubble constant, and in the recent
H
0
measurement by Ries s et al. (2011) only eight SNe Ia
with good light c urves had independent distance mea-
surements from observations of Cepheid variable stars in
their host galaxies. Due to the existing C epheid data for
NGC 1365, SN 2012fr stands poised to be added to this
sample. Because much of the effort in studying poten-
tial spectroscopic subclasses of SNe Ia has been aimed a t
improving the standardization of their luminosities, it is
valuable to investigate where the fundamental calibrator
SNe Ia sit in this context. Thus, in this section we in-
spect where the existing and probable future members of
the Hubble-constant sample, including SN 2012fr, reside
in the new SN Ia spectroscopic classification schemes.
In Table 7 we summa rize the light-curve decline rate
and spectroscopic indicator data for the current sam-
ple of H
0
calibrators fr om Riess et al. (2011), as well as
a sample of seven additional SNe Ia which are strong
candidates to be added to the H
0
sample, along with
SN 2012fr . Selection criteria for the “likely” sample in-
clude (i) low reddening of the SN from its host, i.e.,
A
V
. 0.5 mag, (ii) high sampling of the SN light curve
including pre-maximum data, (iii) no spectroscopic pe-
culiarity of the SN, (iv) distance mo dulus of the host
of µ . 32.8 mag, and (v) sufficiently low inclination
to avo id crowding of Cepheids (L. Macri & A. Riess
2012, private communication). The seven new SNe Ia
which likely satisfy the above criteria are SN 1998dh,
SN 2001el, SN 2003du, SN 2005cf, SN 2006D, SN 2009ig,
and SN 2011fe.
We note here that for our selection criteria, spectro -
scopic peculiarity is explicitly defined as simila rity to
the peculiar SN Ia subclasses defined by their respec-
tive pr ototypes SN 1991bg, SN 1991 T , SN 19 99aa, and
SN 2002cx. While SN 20 12fr exhibits some rare be-
haviors, such as very narrow lines and a late velocity
plateau in the IME s, these characteristics are all seen
in other SNe Ia (though not previously in this combina-
tion). Most importantly, the composition of SN 2012fr
inferred from its spectral features is consis tent with that
of normal SNe Ia (see Figure 16), and the more subtle
characteristics of its absorption features fall within the
range of typical SN Ia b ehavior.
In Figure 19 we plot the location of existing and likely
future H
0
calibrator SNe Ia along with SN 2012fr and the
same comparison sample from Figure 15. The H
0
calibra-
tor sample is not composed exclusively of SNe Ia falling
in the “core normal” sp ectroscopic class of Branch et al.
(2009) o r the “normal” velocity class of Wang et al.
(2009a). SN 2012fr would have the slowest light-curve
decline rate and shallowest Si absorption of the H
0
sam-
ple, but not the highest velocity. This prompts the ques-
tion of what degree of “normality” is required for a SN Ia
to be used for cosmology. The answer will likely require
further study of SN Ia spectroscopic diversity and its im-
pact on SN Ia luminosities.
19
Table 7
Spectroscopic Classes of H
0
Fundamental Calibrators
SN Host d
a
∆m
15
(B) pEW(5972) pEW(6355) v
Si
Branch Wang References
(Mpc) (mag) (
˚
A) (
˚
A) (km s
−1
) Class Class
SN1981B NGC 4536 14.8 1.07 ± 0.09 20.0 129.0 13754 BL HV 1,2,3
SN1990N NGC 4639 21.6 1.00 ± 0.03 10.9 87.1 9352 C N N 4,5
SN1994ae NGC 3370 26.6 0.96 ± 0.04 7.4 81.6 10979 CN N 5
SN1995al NGC 3021 30.5 0.87 ± 0.04 15.0 112.9 12149 BL HV 5
SN1998aq NGC 3982 22.5 1.11 ± 0.04 11.1 77.1 10796 CN N 5
SN2002fk NGC 1309 32.5 1.13 ± 0.03 10.3 75.7 10057 CN N 5
SN2007af NGC 5584 22.4 1.04 ± 0.01 17.0 105.2 10969 BL N 5
SN2007sr NGC 4038 21.7 1.13 ± 0.06 · · · · · · · · · · · · · · · 6
SN1998dh NGC 7541 36.7 1.17 ± 0.06 10.1 121.8 12091 BL HV 5
SN2001el NGC 1448 15.9 1.13 ± 0.04 12.0 93.0 11321 CN N 7,8
SN2003du UGC 9391 26.1 1.07 ± 0.06 1.9 88.8 10527 CN N 5,9
SN2005cf MCG -01-39-3
b
26.4 1.05 ± 0.03 6.5 99.1 10352 CN N 10,5
SN2006D MCG -01-33-34 34.9 1.35 ± 0.05 21.4 94.4 10833 CN N 5
SN2009ig NGC 1015 35.9 0.89 ± 0.02 4.7 79.9 13400 CN HV 11, 12, 5
SN2011fe M101 6.7 1.07 ± 0.06 15.1 101.4 10331 CN N 13, 14
SN2012fr NGC 1365 17.9 0.80 ± 0.01 3.9 66.5 12037 SS/CN HV/N –
a
Distances from Cepheids for the Riess et al. (2011) sample, Kennicutt et al. (1998) and Freedman et al. (2001) for M101, Silbermann et al.
(1999) and Freedman et al. (2001 ) for NGC 1365, and redshif t using H
0
= 73.8 km s
−1
Mpc
−1
for others.
b
Tidal bridge between MCG –01-39-3 and MCG –01-39-2 (NGC 5917); see Wang et al. (2009b).
References: (1) Schaefer (1995), (2) Branch et al. (1983), (3) Branch et al. (2009), (4) Ganeshalingam et al. (2010), (5) Blondin et al.
(2012), (6) Hicken et al. (2012), (7) Krisciunas et al. (2003), (8) Wang et al. (2003), (9) Stanishev et al. (2007), (10) Wang et al. (2009b),
(11) Foley et al. (2012), (12) Marion et al. (2013), (13) Vink´o et al. (2012), (14) Parrent et al. (2012).
7. CONCLUSIONS
We present optical spectra of SN 2012fr, a relatively
normal Type Ia supernova which exhibits several inter-
esting features. These include the distinct presence of
high-velocity features in both the Si ii λ6355 line and
Ca ii IR triplet, as well as a long-lived velocity plateau
at late epochs in both lines. The se behaviors were made
more clear by the extremely narrow velocity width of
the photospheric absorption-line profiles. We s how that
SN 2012fr has Si ii velocities and abs orption strengths
which place it near the b oundary between the “shal-
low silicon” and “core normal” spectroscopic classes de-
fined by Branch et al. (2009), and on the boundary be-
tween nor mal and “high-velocity” SNe Ia as defined by
Wang et al. (2009 a).
SN 2012fr exhibits a very slow decline rate
(∆m
15
(B) = 0.80 ± 0.01 mag; see Paper II), relatively
shallow Si a bsorption at maximum light (pEW(6355)
= 66.5 ± 15.5
˚
A), a nd a very low velocity gradient
( ˙v = 0.3 ± 10.0 km s
−1
day
−1
). All of these character-
istics are common in the very luminous SN 199 1T-like
and SN 1999aa-like SN Ia subclasses, but SN 201 2fr has
higher ejecta velocities and much stronger Si and Ca ab-
sorption a t early epochs than most SNe Ia in these classes
(see Figure 16). Thus, SN 2012fr likely represents the
most luminous end of the normal SN Ia spectrum, and
it may be a transitional object between normal and very
luminous events.
In the modern era only three well-observed normal
SNe Ia (SN 1981B, SN 2001el, SN 2011fe) have occurred
in galaxie s that are both suitable for Cepheid distances
and closer than the ho st galaxy of SN 2012 fr, NGC 1365.
Furthermore, this galaxy has been extensively studied
and was part of the HST Key Project on the extragalac-
tic distance scale. This makes SN 2012fr a prime ca n-
didate for measuring the pe ak luminosity of SNe Ia and
thereby constraining the Hubble constant, and we show
that SN 2012fr should provide an excelle nt complement
to the existing and likely future sample of H
0
SNe Ia.
Individual SNe Ia such as SN 2012fr with e xtensive ob-
servational datasets can prove invaluable for better un-
derstanding the nature of SN Ia explosions. Here we al-
ready identify a number of tantalizing observational char-
acteristics of this spectra l time series, and we expec t that
future detailed modeling could reveal additional subtle
insights.
Facilities: ANU:2.3m (WiFeS), NTT (EFOSC2), NTT
(SOFI), SALT (RSS), SAAO:1.9m (Grating Spectro-
graph), Shane (Kast Double spectrograph), Keck:I
(HIRES)
Acknowledgments: We are very grateful to the staff of RSAA and
Siding S pring Observatory for their rapid replacement of a broken
cooling pump for the WiFeS red channel on November 1, particularly
Graeme Blackman, Gabe Bloxham, Donna Burton, Harvey Butcher,
Mike Ellis, Mike Fowler, Mike Petkovic, Annino Vaccarella, and Peter
Verwayen. We thank Bruce Bassett for arranging observations with
the SAAO 1.9 m telescope, as well as Carl Melis, Michael Jura, Siyi
Xu, Beth Klein, David Osip, Ben Zuckerman, and Barry Madore for
contributing data. We are grateful to Stephane Blondin for providing
his velocity-gradient values from th e CfA SN Ia sample, and to Ken
Nomoto, Rudiger Pakmor, Lucas Macri, and Adam Riess for helpful
discussions. We also thank Howie Marion for providing an advance
copy of his paper on SN 2009ig, and for very helpfu l discussions.
This research was conducted by the Austral ian Research Council
Centre of Excellence for All-sky Astrophysics (CAASTRO), through
project number CE110001020. Chris Lidman is t he recipient of
an Australian Research Council Future Fellowship (program num-
ber FT0992259). J.A. acknowledges support by CONICYT through
FONDECYT grant 3110142, and by the Millennium Center for Super-
nova Science (P10-064-F), with input from ’Fondo de Innovaci´on para
la Competitividad, del Ministerio de Econom´ıa, Fomento y Turismo
de Chile’. A.G.-Y. is supported by the EU/FP7 via an ERC grant.
F.B. acknowledges su pport from FONDECYT through Postdoctoral
grant 3120227. F.B. and G.P. thank the Millennium Center for Su-
pernova Science for grant P10-064-F (funded by “Programa Bicente-
nario de Cienci a y Tecnolog´ıa de CONICYT” and “Programa Ini cia-
tiva Cient´ıfica Milenio de MIDEPLAN”). S.B. is partially supported by
20
Figure 19. Current members of the H
0
fundamental calibrator
sample (cyan pentagons) and likely new additions to the sample
(yellow hexagons), along with SN 2012fr (purple star), compared
to other SNe Ia on the Branch et al. (2009) diagram (top) and the
Wang et al. (2009a) diagram (b ottom). Other SN Ia subclasses are
denoted as in Figure 15.
the PRIN-INAF 2011 with the project “Transient Universe: from ESO
Large to PESSTO”. Support f or this research at Rutgers University
was provided in part by NSF CAREER award AST-0847157 to S.W.J.
M.D.S. and F.T. acknowledge the generous support provided by the
Danish Agency for Science and Technology and Innovation through a
Sapere Aude Level 2 grant. E.Y.H. is supported by the NSF under
grant AST-1008343. A.V.F.’s group at U.C. Berkeley is su pported by
Gary and Cynthia Bengier, the Richard and Rhoda Goldman Fund, the
Christopher R. Red lich Fund, the TABASGO Foundation, and NSF
grant AST-1211916.
This work is based in part on observations collected at the European
Organisation for Astronomical Research in the Southern Hemisphere,
Chile, as part of PESSTO (the Public ESO Spectr oscopic Survey for
Transient Objects) ESO programs 188.D-3003 and 089.D-0305. This
paper also uses data obtained at the South African Astronomical O b-
servatory (SAAO). Some observations were taken with the Southern
African Large Telescope (SALT) as part of proposal ID 2012-1-RU-005
(PI: Jha). Some of the data presented herein were obtained at the
W. M. Keck Observatory, which is operated as a scientific partnership
among the California Institute of Technology, the University of Cali-
fornia, and NASA; the observatory was made possible by the generous
financial sup port of the W. M. Keck Foundation. Based in part on
observations made with the Nordic Optical Telescope, operated on the
island of La Palma jointly by Denmark, Finland, Icelan d, Norway, and
Sweden, in the Spanish Observatorio del Roque de los Muchachos of t he
Instituto de Astrofisica de Canarias. For their exce llent assistance, we
are grateful to the staffs of the many observatories where we collected
data.
This research has made prodigious use of the NASA/IPAC Extra-
galactic Database (NED), which is operated by the Jet Propulsion Lab-
oratory, California Institute of Technology, under contract with NASA.
It has also made use of NASA’s Astrophysics Data System (ADS), the
CfA Supe r nova Archive (funded in part by NSF grant AST-0907903),
and the Central Bureau for Astronomical Telegrams (CBAT) list of
SNe (http://www.cbat.eps.harvard.edu/lists/Supe rnovae.html).
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