A Likely Close-in Low-mass Stellar Companion to the Transitional Disk Star HD 142527

Abstract

With the uniquely high contrast within 0.1" (\Delta mag(L') = 5-6.5
magnitudes) available using Sparse Aperture Masking (SAM) with NACO at VLT, we
detected asymmetry in the flux from the Herbig Fe star HD 142527 with a
barycenter emission situated at a projected separation of 88+/-5 mas
(12.8+/-1.5 AU at 145 pc) and flux ratios in H, K, and L' of 0.016+/-0.007,
0.012+/-0.008, 0.0086+/-0.0011 respectively (3-\sigma errors) relative to the
primary star and disk. After extensive closure-phase modeling, we interpret
this detection as a close-in, low-mass stellar companion with an estimated mass
of ~0.1-0.4 M_Sun. HD 142527 has a complex disk structure, with an inner gap
imaged in both the near and mid-IR as well as a spiral feature in the outer
disk in the near-IR. This newly detected low-mass stellar companion may provide
a critical explanation of the observed disk structure.

Full text

The Astrophysical Journal Letters, 753:L38 (5pp), 2012 July 10 doi:10.1088/2041-8205/753/2/L38
C
2012. The American Astronomical Society. All rights reserved. Printed in the U.S.A.
A LIKELY CLOSE-IN LOW-MASS STELLAR COMPANION TO THE TRANSITIONAL DISK STAR HD 142527
Beth Biller1, Sylvestre Lacour2, Attila Juh ´
asz3, Myriam Benisty1, Gael Chauvin1,4, Johan Olofsson1,
J¨
org-Uwe Pott1,Andr
´
eM
¨
uller1, Aurora Sicilia-Aguilar5, Micka¨
el Bonnefoy1, Peter Tuthill6, Philippe Thebault2,
Thomas Henning1, and Aurelien Crida7
1Max-Planck-Institut f¨
ur Astronomie, K¨
onigstuhl 17, 69117 Heidelberg, Germany; biller@mpia.de
2LESIA, CNRS/UMR-8109, Observatoire de Paris, UPMC, Universit´
e Paris Diderot, 5 place Jules Janssen, 92195 Meudon, France
3Leiden Observatory, Leiden University, P.O. Box 9513, 2300 RA Leiden, The Netherlands
4UJF-Grenoble 1/CNRS-INSU, Institut de Plan´
etologie et dAstrophysique de Grenoble (IPAG) UMR 5274, Grenoble, F-38041, France
5Departamento de F´
ısica Te´
orica, Facultad de Ciencias, Universidad Aut´
onoma de Madrid, 28049 Cantoblanco, Madrid, Spain
6School of Physics, University of Sydney, NSW 2006, Australia
7Universit´
e de Nice - Sophia antipolis /C.N.R.S. /Observatoire de la Cˆ
ote d’Azur, Laboratoire Lagrange (UMR 7293), Boulevard de l’Observatoire,
B.P. 4229 06304 NICE cedex 04, France
Received 2012 April 27; accepted 2012 June 6; published 2012 June 26
ABSTRACT
With the uniquely high contrast within 0.
1(Δmag(L)=5–6.5 mag) available using Sparse Aperture Masking with
NACO at Very Large Telescope, we detected asymmetry in the flux from the Herbig Fe star HD 142527 with a
barycenter emission situated at a projected separation of 88 ±5 mas (12.8 ±1.5 AU at 145 pc) and flux ratios
in H,K, and Lof 0.016 ±0.007, 0.012 ±0.008, and 0.0086 ±0.0011, respectively (3σerrors), relative to the
primary star and disk. After extensive closure-phase modeling, we interpret this detection as a close-in, low-mass
stellar companion with an estimated mass of ∼0.1–0.4 M. HD 142527 has a complex disk structure, with an inner
gap imaged in both the near and mid-IR as well as a spiral feature in the outer disk in the near-IR. This newly
detected low-mass stellar companion may provide a critical explanation of the observed disk structure.
Key words: brown dwarfs – circumstellar matter – planetary systems – stars: emission-line, Be – stars: low-mass
Online-only material: color figures
1. INTRODUCTION
Transition disks may trace a key step in the process of forming
planets and dissipating primordial stellar disks. Transition disks
are primordial disks characterized by weak mid-IR emission
(at ∼15 μm) relative to the Taurus median spectral energy
distribution (i.e., the median SED of primordial disks in the
young (<2 Myr) Taurus star-forming region; Najita et al. 2007).
A number of transition disks possess gaps either posited from
SED studies or directly imaged (Andrews et al. 2011; Fukagawa
et al. 2006; Pott et al. 2010; Brown et al. 2009) which may be due
to clearing of dust by a forming planet or brown dwarf and thus
may produce the observed mid-IR deficit. Transition disks have
therefore been popular targets for high-contrast high-resolution
imaging planet searches.
While transition disks have previously been searched for plan-
ets using high contrast imaging techniques (e.g., adaptive optics
and coronagraphy), most of these techniques currently do not
extend to the inner 0.
1, which corresponds to the crucial planet-
forming regions (∼10 AU) of the disk for objects at distances
>100 pc. Only interferometric techniques possess the resolu-
tion to reveal these inner regions (see, e.g., Pott et al. 2010). The
interferometric technique of Sparse Aperture Masking (SAM)
uniquely allows us to both probe the inner 0.
1 of transitional
disks and reach high contrasts of Δmag(L)=5–6.5 mag. Prob-
ing this region enables us to understand how planets form in
their native disk and how they impact the surrounding material.
Indeed, observations using SAM have already yielded two plan-
etary or brown dwarf candidate companions to transition disk
stars (Hu´
elamo et al. 2011; Kraus & Ireland 2012).
The transition disk star HD 142527 is a Herbig Fe star with
spectral type of F6 IIIe (Houk 1978; Henize 1976; Waelkens
et al. 1996). HD 142527 notably possesses a very complex and
interesting disk that has long been posited as a possible site
of planet formation. Recent SED modeling and VISIR imaging
suggests a disk gap from 30 to 130 AU (Verhoeff et al. 2011).
The outer edge of the gap as well as a spiral feature in the outer
disk have been imaged in the near-IR (Fukagawa et al. 2006).
Fukagawa et al. (2006) also find an offset of 20 AU between
the star center and disk center, which they posit is caused by an
unseen eccentric binary companion. Baines et al. (2006) note
this system as a possible (but unconfirmed) binary detection
from spectroastrometry. The disk of HD 142527 also possesses
an extremely high fraction of crystalline silicates, possibly
formed by a massive companion inducing spiral density waves
in the disk material (van Boekel et al. 2004). Here we report
the discovery of a likely close-in, low-mass stellar companion
(12.8 ±1.5 AU at 145 pc and flux ratios in H,K, and Lof
0.016 ±0.007, 0.012 ±0.008, and 0.0086 ±0.0011, respec-
tively) to this star. This is the first confirmation of the binarity
of HD 142527 and may provide a critical explanation of the
observed disk structure.
2. STELLAR PARAMETERS
Hipparcos measurements for HD 142527 yield a distance
of 230+70
−40 pc (van Leeuwen 2007). Alternately, HD 142527 has
been associated with both the Sco OB-2 association (Acke & van
den Ancker 2004) and Upper Centaurus Lupus (de Zeeuw et al.
1999; Teixeira et al. 2000). Membership in either association
(indeed, both are part of the larger Sco-Cen association) place
HD 142527 at a distance of 140–145 pc (de Zeeuw et al. 1999)
and an age of 2–10 Myr. We consider the evidence of association
within Sco-Cen to be very strong and thus adopt a distance of
145 ±15 pc, which still lies within 2σof the rather uncertain
Hipparcos measurement.
1

The Astrophysical Journal Letters, 753:L38 (5pp), 2012 July 10 Biller et al.
Tab l e 1
Properties of the HD 142527 AB System
Primary + Disk Secondary
Distance 145 ±15 pca
Age 5+8
−3Myra
Proper motion (μα,μδ)(−11.19 ±0.93, −24.46 ±0.79) mas yr−1b
Separation: UT 2012 March 11 88 ±5 mas (12.8 ±1.5 AU)
Position angle: UT 2012 March 11 133.3 ±2.
◦5
Flux ratio in H... 0.016 ±0.007
Flux ratio in Ks .. . 0.012 ±0.008
Flux ratio in L... 0.0086 ±0.0011
H(mag) 5.94c10.5 ±0.2
Ks (mag) 5.20c10.0 ±0.3
L(mag) 3.89c9.1 ±0.1
H−Ks (mag) 0.74 0.5 ±0.4
Ks −L(mag) 1.31 0.9 ±0.3
MH(mag) 0.2 ±0.2 4.8 ±0.3
MKs(mag) −0.6 ±0.2 4.2 ±0.3
ML(mag) −1.9 ±0.2 3.3 ±0.2
Spectral type F6IIIe ...
Estimated mass 2.2 ±0.3 Ma0.1–0.4 M
Notes.
aVerhoeff et al. (2011).
bvan Leeuwen (2007).
cMalfait et al. (1998).
Verhoeff et al. (2011) obtain a stellar luminosity of 15 ±
2Lfrom comparison to Kurucz (1991) photospheric models
for the object spectral type of F6 III. They correct the stellar
luminosity to 20 ±2Lafter taking into account a model-
dependent gray extinction component of the disk and halo and
obtain updated stellar parameters by comparing the position of
this star in the Hertzsprung–Russell diagram to the pre-main-
sequence evolution tracks of Siess et al. (2000). Here, we adopt
the stellar parameters from Verhoeff et al. (2011)—specifically,
a stellar mass of 2.2 ±0.3 Mand an age of 5+8
−3Myr consistent
with membership in the Sco OB-2 association (Table 1).
3. OBSERVATIONS AND DATA REDUCTION
HD 142527 was observed on 2012 March 10 with Very
Large Telescope (VLT) NACO.8Observations were taken in
the H,K, and Lbands (λL=3.80 ±0.31 μm) using the “7
holes” aperture mask and the IR wavefront sensor (WFS). The
observing log is presented in Table 2. The target was observed
in each band for 30 minutes to 2 hr.
The use of the “7 holes” (C7-892; Tuthill et al. 2010) aperture
mask transforms the telescope into a Fizeau interferometer. The
point-spread function is a complex superposition of fringes at
given spatial frequencies. In specific cases, pupil-masking can
outperform more traditional differential imaging for a number of
reasons (Tuthill et al. 2006; Lacour et al. 2011b). First, the masks
are designed to have nonredundant array configurations that
permit phase deconvolution; slowly moving optical aberrations
not corrected by the AO can be accurately calibrated. Second,
the mask primarily rejects baselines with low spatial frequency
and passes proportionately far more baselines with higher
λ/B(where B is baseline length) resolution than does an
orthodox fully filled pupil. Third, high-fidelity recovery of phase
information allows “super resolution,” with a marginal loss of
dynamic range up to λ/2D(where Dis the mirror diameter). The
principal drawback is a loss in throughput so that photon and
8Program ID: 088.C-0691(A)
Tab l e 2
HD 142527 2012-03-11 (UT) Observation log
Target UT time Band DIT NDIT
(ms)
HD 142527 2012-03-11T06:02:25.1402 L120 330
HD 142695 2012-03-11T06:37:33.3990 L120 330
HD 142527 2012-03-11T06:46:55.5591 L120 330
HD 142384 2012-03-11T06:57:19.0879 L120 330
HD 142527 2012-03-11T07:07:34.3389 L120 330
HD 144350 2012-03-11T07:18:46.5699 L120 330
HD 142527 2012-03-11T07:32:05.3348 L120 330
HD 142695 2012-03-11T07:41:37.5524 L120 330
HD 142527 2012-03-11T07:51:14.5767 L120 330
HD 142527 2012-03-11T08:13:19.5884 Ks 109 360
HD 142384 2012-03-11T08:27:00.5782 Ks 109 360
HD 142527 2012-03-11T08:42:30.3669 Ks 109 360
HD 142695 2012-03-11T08:53:16.9986 Ks 109 360
HD 142527 2012-03-11T09:03:59.4995 Ks 109 360
HD 142527 2012-03-11T09:23:19.1784 H100 360
HD 142384 2012-03-11T09:34:21.3448 H100 360
HD 142527 2012-03-11T09:44:44.1641 H100 360
detector noise can affect the signal-to-noise ratio even where
targets are reasonably bright for the AO system. The effective
field of view of SAM is determined by the shortest baseline
so that the technique is not competitive at separations that are
greater than several times the formal diffraction limit. For SAM
observations with the “7 holes” mask with VLT NACO, the field
of view is 300 mas in the Hband, 400 mas in the Kband, and
600 mas in the Lband. For more details on the SAM mode, see,
e.g., Lacour et al. (2011a) and Tuthill et al. (2010).
The HD 142527 observations were processed with both the
Sydney FFT and Observatoire de Paris SAMP pipelines (Tuthill
et al. 2000; Lacour et al. 2011a). Both the χ2map and the phase
in the UV plane (extracted from the closure phases; see Hu´
elamo
et al. 2011) show an asymmetry typical of point sources (at the
2

The Astrophysical Journal Letters, 753:L38 (5pp), 2012 July 10 Biller et al.
−1 deg −0.5 deg 0.5 deg 1 deg
−5 0 5
−5
0
5
u (meters)
v (meters)
Δmag=5.2
PA=133deg
Sep=88 mas
−400−200 0 200 400
−400
−200
0
200
400
RA [mas]
dec [mas]
(a)
−1.5
−1.0
−0.5
0.0
0.5
1.0
1.5 (b)
(c)
−1.5
−1.0
−0.5
0.0
0.5
1.0
1.5 (d)
(e)
−45 −40 −35 −30 −25 −20
−1.5
−1.0
−0.5
0.0
0.5
1.0
1.5 (f)
−45 −40 −35 −30 −25 −20
Parallactic angle (deg)
Closure Phase (deg)
Figure 1. Left panel: L-band UV coverage on HD 142527. The size and colors of the markers are relative to the phase measured. The larger the size, the higher
the value of the phase. The colors denote the sign of the phase (red are negatives values). The plot shows diagonal stripes orthogonal to the direction of the binary
companion (indicated by the arrow). Middle panel: L-band χ2surface as a function of R.A. and decl. obtained from the best-fit binary model to the closure phases.
A clear minimum indicates the position of the stellar companion, coherent with the orientation of the stripes in the Fourier domain. The red contours correspond to
3σand 5σerror bars in the detection. Right panel: L-band closure phase as a function of parallactic angle for the six largest three-hole triangles. Calibrator data are
represented as colored squares (red HD 142695, green HD 144350, and blue HD 142384), while HD 142527 data are plotted as black squares. The solid line is the
closure phase predicted by the best-fitting binary system model.
(A color version of this figure is available in the online journal.)
0 2 4 6
wavelength (μm)
10−11
10−10
10−9
10−8
10−7
λFλ (erg/s/cm2)
Primary + Disk
Companion
Figure 2. SED for HD 142527 as well as companion fluxes in the same
bandpasses. SED data points are drawn from the photometry of Malfait et al.
(1998).
(A color version of this figure is available in the online journal.)
resolution of the telescope; see left panel of Figure 1). Therefore,
we fitted the closure phases with a model of two point-like
objects (the star and a companion of lesser flux). In all three
bands, the best-fit model to the closure phases shows a point-
like asymmetry at 88 ±5 mas from the central star with flux
ratios in H,K, and Lof 0.016 ±0.007, 0.012 ±0.008, and
0.0086 ±0.0011, respectively (3σerrors), relative to the
primary star and disk. The χ2map of the L-band data is shown
in the right panel of Figure 1. The fit on several of the 35 closure
phases (triangle) is plotted in Figure 2(L-band data).
4. RESULTS
4.1. Photometry
We adopt the HKL magnitudes from Malfait et al. (1998),
since no L-band data are available from the Two Micron All
Sky Survey (2MASS). We note, however, that there is a sig-
nificant divergence between the reported 2MASS photometry
and the Malfait et al. (1998) photometry. The 2MASS photom-
etry is brighter than the Malfait et al. (1998) photometry by
0.1–0.3 mag, which may suggest variability for this system. No
errors are provided for the Malfait et al. (1998) photometry; we
assume error bars are similar to the 2MASS photometry.
Raw photometry for this system is comprised of light from
three components—primary star, secondary companion, and
disk. Some portion of the disk (the “outer disk”) lies outside of
the SAM field of view. From the SED model of Verhoeff et al.
(2011) we estimate that the outer disk comprises ∼10% or less
of the total system flux at HKL
. Thus, we do not correct the raw
photometry to remove the outer disk component. Magnitudes in
H,K, and Lfor the star+disk are presented in Table 1.
The detected companion has flux ratios in H,K, and L
of 0.016 ±0.007, 0.012 ±0.008, and 0.0086 ±0.0011,
respectively (3σerrors). To account for errors in the initial
photometry as well as our measured SAM flux ratio, we adopt
a Monte Carlo approach. We simulated an ensemble of 106
observations per band, with photometry and flux ratios drawn
from Gaussian distributions centered on the measured values
and with σdrawn from the reported errors. The apparent
magnitude in each band is given as the median of this ensemble,
with error bars drawn from the standard deviation of the same
ensemble. Thus, the measured flux ratios and primary star
photometry correspond to apparent magnitudes of 10.5 ±0.2,
10.0 ±0.3, and 9.1 ±0.1 in H,K, and L(Table 1,in
CIT bandpasses; Elias et al. 1982). The companion appears
anomalously bright in L. While the H−Kcolors are similar to
what would be expected for a young red companion, K−L
∼0.9 mag, diverging significantly from the expected value of
∼0.4 mag (Baraffe et al. 1998). Companion fluxes in these
bandpasses, along with the full SED for the system are plotted
in Figure 2.
We employed a similar Monte Carlo approach in converting
from apparent to absolute magnitudes. For our ensemble of 106
simulated objects, we simulate corresponding distances drawn
from a Gaussian centered at 145 pc and with σof 15 pc. Absolute
magnitudes are also reported in Table 1.
4.2. Probability of Chance Alignment
We estimated the likelihood that this companion is an un-
related background or foreground object using source counts
3

The Astrophysical Journal Letters, 753:L38 (5pp), 2012 July 10 Biller et al.
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
Mass (Solar Masses)
0
5.0•104
1.0•105
1.5•105
N
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
Mass (Solar Masses)
0
5.0•104
1.0•105
1.5•105
N
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
0
5.0•104
1.0•105
1.5•105
0.0 0.2 0.4 0.6 0.8 1.0 1.2 1.4
0
5.0•104
1.0•105
1.5•105
H
K
L
0.000 0.005 0.010 0.015
age (Gyr)
6.0
5.5
5.0
4.5
4.0
3.5
3.0
2.5
Mh (mag)
0.100 Msun
0.110 Msun
0.130 Msun
0.150 Msun
0.175 Msun
0.200 Msun
0.250 Msun
0.300 Msun
0.350 Msun
0.400 Msun
0.450 Msun
0.500 Msun
0.570 Msun
Figure 3. Left: mass estimate histograms for HD 142527B. We adopt Monte Carlo methods to account for the range of possible ages for this system. An ensemble
of 106possible ages are drawn from a Gaussian in log space, centered on log(age) =6.7 and with σ(log(age)) =0.4. We then interpolate with age and single-band
absolute magnitude to find the best mass for the companion from the models of Baraffe et al. (1998). We plot here the resulting mass distributions from single-band
absolute magnitudes in H,K,andL. It is instantly apparent that the mass estimate for the companion is not well constrained at these ages but is most likely to lie in
the range from 0.1 to 0.4 M. Right: age vs. absolute magnitude in the Hband. We plot isomass contours (from the models of Baraffe et al. 1998) as a function of
age and absolute magnitude, as well as the assumed age and absolute magnitude of the companion. The models themselves are highly dependent on age; the region
constrained by the observations (yellow rectangle) is consistent with companion masses from 0.1 to 0.4 M.
(A color version of this figure is available in the online journal.)
fromthe2MASSsurvey.Withina1degradiusoftheprimary,
2MASS detects 1918 objects with Hof 10.7 mag or brighter
and 1505 objects with Ks of 10.0 mag or brighter. Thus, adopt-
ing the approach of Brandner et al. (2000), in particular their
Equation (1), we estimate the probability of finding an unrelated
source at least as bright as the observed companion within 0.
088
of the primary to be ∼1.1×10−6in the Hband and ∼8.3×10−7
in Ks. We also considered simulated stellar populations along
this line of sight (Galactic latitude and longitude of 335.
◦6549,
+08.
◦4804) using the Besan¸con Galactic population synthesis
models (Robin et al. 2003). This line of sight is directly into the
Galactic bulge, so the models yield 882 background sources per
square degree that are brighter than K=10.5 mag. However,
the chances of finding one of these within 0.
088 of the primary
are still vanishingly small—∼1.6×10−6—and these objects are
predominantly M giant stars, with considerably bluer expected
colors (for an M5III star, H−K=0.29 mag and K−L=
0.22 mag; Tokunaga 2000) than measured for the detected com-
panion. It is therefore extraordinarily unlikely that the compan-
ion is unrelated to the primary, although proper motion confir-
mation in a year will be necessary to finally determine this.
4.3. Mass Estimate
Estimated masses for both system components are highly
dependent on adopted age. Certainly the HD 142527 system is
quite young, but whether it is 1 Myr or 10 Myr makes a critical
difference in the mass estimate for the faint companion. Here,
we adopt a similar age range as Verhoeff et al. (2011), which is
dependent on membership in the Sco OB-2 association.
We again adopt Monte Carlo methods to account for the range
of possible ages for this system. An ensemble of 106possible
ages are drawn from a Gaussian in log space, centered on
log(age) =6.7 and with σ(log(age)) =0.4. We then interpolate
with age and single-band absolute magnitude to find the best
mass for the companion from the models of Baraffe et al. (1998).
The resulting mass distributions are presented as histograms in
Figure 3. It is apparent that the mass estimate for the companion
is not well constrained at these ages but is most likely to lie
in the range from 0.1 to 0.4 M. We estimate a best mass
estimate for each band of 0.28 ±0.15 M,0.34±0.19 M, and
0.60 ±0.29 Min H,K, and Lrespectively. However, the
mass distributions from our Monte Carlo simulations are highly
non-Gaussian, with significant probability to find considerably
higher companion masses. All mass estimates are within 2σof
each other, but the L-band mass estimate is particularly high and
we note that the companion appears anomalously bright in L.
While the H−Kcolors are similar to what would be expected
for a young red companion, K−L∼0.9 mag, considerably
divergent from the expected value of 0.4 mag. We thus do not
attempt to estimate spectral type using H−Kand K−Lcolors.
The models of Siess et al. (2000) yield a similar mass range for
the companion of 0.1–0.4 Mfor ages of 2–12 Myr. The
models themselves are highly dependent on age; to illustrate
this, we plot isomass contours (from the models of Baraffe
et al. 1998) as a function of absolute Hmagnitude and age in
Figure 3.
4.4. Constraints on the Orbit
We estimate the semimajor axis of HD 142527B’s orbit
from its observed separation. Assuming a uniform eccentricity
distribution between 0 <e<1 and random viewing angles,
Dupuy et al. (2010) compute a median correction factor between
projected separation and semimajor axis of 1.10+0.91
−0.36 (68.3%
confidence limits). Using this, we derive a semimajor axis of
14+12
−5AU. While the mass estimate for the companion is quite
uncertain, we adopt the K-band value as characteristic and adopt
a total system mass of 2.54 ±0.35 M. Our derived semimajor
axis estimate corresponds to an orbital period estimate of
33+42
−18 years.
After a year, we expect up to 20 mas of orbital motion on the
sky for the companion, which is easily detectable with SAM.
The degree of motion observed will put important constraints
on the mass of the companion (after adopting an estimate of
the mass of the primary) and will provide a key data point
for an eventual dynamical mass determination for this system.
Assuming a coverage of one third of an orbit is necessary
for a good orbital determination (Dupuy et al. 2010); such
a determination may be possible in 10 year timescales for
4

The Astrophysical Journal Letters, 753:L38 (5pp), 2012 July 10 Biller et al.
this system. HD 142527A has a measured proper motion of
−11.19 ±0.93 mas in R.A. and −24.46 ±0.79 in decl.—about
∼20 mas on-sky motion in a year, with 3σastrometric errors
of ∼5 mas. Thus, we will also be able to completely rule out
the extremely unlikely case that the companion is an unrelated
background object.
5. DISCUSSION
The existence of an inner binary has been predicted for
HD 142527 (Fukagawa et al. 2006; Baines et al. 2006)but
this is the first confirmation of the binary companion. The inner
binary likely explains the 20 AU offset observed by Fukagawa
et al. (2006) between the primary and the disk center (Pichardo
et al. 2008;Nelson2003).
The discovery of the inner binary provides an important
update for modeling efforts of the structure of the inner disk.
The modeling efforts of Verhoeff et al. (2011) find a flat, dusty
inner disk from 0.3 to 30 AU (the outer radius supported as
well by marginally resolved VISIR imaging), an optically thin
halo from 0.3 to 30 AU, and an outer disk starting at 130 AU.
The current projected separation for the companion, ∼13 AU,
places it right inside the modeled inner disk! Thus, it is likely
that the companion may produce a cleared ring within the inner
disk. Models of the physical properties of disks around eccentric
stellar binaries often show entirely cleared inner disks (Pichardo
et al. 2008), which may not be the case here.
The HD 142527 disk is notable for being comparably bright
or even brighter than the primary star at infrared wavelengths. At
the Hand Kbands, the star is as bright or brighter than the disk,
whereas at the Lband, the disk is considerably brighter than the
star. We consider all components of the Verhoeff et al. (2011)
SED model in deriving photometry for the companion. However,
if a portion of the inner disk does not contribute to the brightness
of the central source, this may be the reason why our L-band
flux measurement for the companion is anomalously bright.
Alternatively, the very bright measured L-band magnitude may
suggest that the secondary has its own small circumstellar disk
(with possible accretion onto the secondary) or that the relatively
massive secondary may produce local disk heating. Transient
heating caused by the secondary inducing spiral shock waves in
the disk material could also be the source of the high fraction of
crystalline silicates found at large radii in this disk (van Boekel
et al. 2004).
The large cavity observed in the HD 142527 disk may be
the signature of an unseen companion interacting with the inner
binary. A binary + planet system can open a much larger gap
than can be formed by the binary by itself (Nelson 2003;Kley
et al. 2008; Kley & Nelson 2008,2012). Nelson (2003) note that
in the case of an eccentric inner binary system, the circumbinary
disk itself can become eccentric, ending the inner migration of
the planet and producing a stable orbital configuration. This
seems a likely explanation of the observed disk structure in the
HD 142527 system, especially the wide gap within 130 AU, but
must be confirmed by continued orbital monitoring of the binary
system to confirm that it is indeed in an eccentric orbit. Follow-
up SAM observations are thus absolutely critical for this system
and are still possible within the lifespan of NACO. Eventually,
a dynamical mass can be determined for this system, perhaps in
10 year timescales and monitoring on 1–2 year timescales may
help confirm the eccentricity of the orbit (see, e.g., Biller et al.
2010).
6. CONCLUSIONS
We detect a likely close companion to HD 142527 with
separation of 88 ±5 mas (12.8 ±1.5 AU at 145 pc) and
flux ratios in H,K, and Lof 0.016 ±0.007, 0.012 ±0.008,
and 0.0086 ±0.0011, respectively (3σerrors), relative to the
primary star and inner disk. The companion is consistent with
mass estimates of 0.1–0.4 Mfrom the models of Baraffe
et al. (1998). However, continued orbital monitoring will be
necessary to provide more accurate mass estimates, as model
masses contain significant uncertainties at these young ages.
The inner binary likely explains the 20 AU offset observed by
Fukagawa et al. (2006) between the primary and the disk center
(Pichardo et al. 2008;Nelson2003). Additionally, the large
cavity observed in the HD 142527 may be the signature of an
unseen planet interacting with the inner binary.
Based on observations made with ESO Telescopes at the La
Silla Paranal Observatory under programme ID 088.C-0691. We
gratefully acknowledge the support of the ESO VLT UT4 staff
as well as useful conversations with Roy van Boekel and Jeroen
Bouwman.
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