A large and homogeneous sample of SX Phe stars in the Fornax dwarf spheroidal galaxy .

Abstract

We report on the detection of sixty-one SX Phe stars in the Fornax dwarf
spheroidal galaxy using the Wide-Field Imager at the 2.2m ESO-MPI
telescope. In spite of their extreme faintness (22.0

Full text

Mem. S.A.It. Vol. 77, 219
c
SAIt 2006 Memorie della
A large and homogeneous sample of SX Phe
stars in the Fornax dwarf spheroidal galaxy
E. Poretti1, L. Dell’Arciprete1, G. Clementini2, E. V. Held3, C. Greco2,4,
M. Gullieuszik3,5, M. Maio4, L. Rizzi6, M. Catelan7, H.A. Smith8, B. Pritzl9,
N. De Lee8, and A. Rest10
1INAF – Osservatorio Astronomico di Brera, Via Bianchi 46, I-23807 Merate, Italy
2INAF – Osservatorio Astronomico di Bologna, Via Ranzani 1, I-40127 Bologna, Italy
3INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, I-35122
Padova, Italy
4Dipartimento di Astronomia, Universit`
a di Bologna, Via Ranzani 1, I-40127 Bologna,
Italy
5Dipartimento di Astronomia, Universit`
a di Padova, Vicolo dell’Osservatorio 2, I-35122
Padova, Italy
6Institute for Astronomy, University of Hawaii, 2680 Woodlawn Drive, Honolulu, HI,
USA
7Departamento de Astronom´
ıa y Astrof´
ısica, Pontificia Universidad Cat´
olica de Chile,
Avenida Vicuna Mackenna 4860, 782-0436 Macul, Santiago, Chile
8Department of Physics and Astronomy, Michigan State University, East Lansing, MI
48824-2320, USA
9Macalester College, 1600 Grand Avenue, Saint Paul, MN 55105, USA
10 Cerro Tololo Inter-American Observatory, Casilla 603, La Serena, Chile
Abstract. We report on the detection of sixty–one SX Phe stars in the Fornax dwarf
spheroidal galaxy using the Wide–Field Imager at the 2.2m ESO–MPI telescope. In spite of
their extreme faintness (22.0<B<24.0), the light curves of these short–period pulsators
have been clearly detected. We compare the Period–Luminosity relations in the Fornax
galaxy and in the Milky Way. The light curves of a candidate SX Phe star in the For 5
globular cluster is also presented (Magellan/Clay observations).
Key words. Stars: Population II – Stars: variables : δSct – Galaxies : Fornax – Stars:
oscillations – Galaxies: distances
1. Introduction on the nomenclature
of short–period pulsating stars
The nomenclature of the short–period
(P<0.20 d) pulsating stars located below the
Send offprint requests to: E. Poretti
Horizontal Branch in the classical instability
strip is a bit confusing. In the Milky Way there
is a physical distinction between δSct and
SX Phe stars: the former are Pop. I stars, the
latter are Pop. II stars. Among Pop. I stars,
the amplitude can vary from a few 0.001 mag

220 Poretti et al.: SX Phe stars in Fornax
to several 0.1 mag. Even if high amplitude δ
Sct (HADS) stars are likely radial pulsators,
there are several examples of the excitation of
nonradial modes (Poretti 2003). In a similar
way, SX Phe stars show a mixture of modes,
especially in globular clusters. Therefore, the
separation between HADS and SX Phe stars is
not an easy task in the Milky Way, if no details
about the metallicity are known. The study of
double–mode pulsators has introduced a new
tool, since the ratio between the fundamental
and the first overtone radial modes is sensitive
to metallicity effects (Poretti et al. 2005).
In the past, it was quite common to use
the term “Dwarf Cepheids” when these short–
period variables were observed in globular
clusters. This originates from the fact that they
can be used as distance indicators since they
follow a P−Lrelation similar to the more
evolved Classical Cepheids. However, the term
has an unclear physical meaning in the astro-
physical context, since the two groups of stars
are quite different. It is more appropriate to use
the SX Phe term to identify the short–period
variables in the globular clusters, since they are
most likely old, metal–poor stars. On the other
hand, it sounds inappropriate for the variables
in the galaxies, since they could have different
ages and metallicities; hence, we should ap-
ply the same distinction as in the Milky Way,
i.e., SX Phe and HADS stars. Such a distinc-
tion may seem a bit pedantic, so the old term
“Dwarf Cepheids” still survives in the current
literature.
The use of pulsating stars with period
shorter than 0.20 d as distance indicators has
made some progress in the last years, partic-
ularly in the case of globular clusters (e.g.,
Pych et al 2001, Mazur et al. 2003). On the
other hand, due to their faintness, not many
of these pulsating stars have been discovered
so far in other galaxies: Mateo et al. (1998)
and Poretti (1999) describe the first results ob-
tained on the Carina dwarf spheroidal (dSph)
galaxy. However, it appeared possible to ex-
ploit in a more complete way their potential-
ities by performing more intensive and deep
surveys. Therefore, we were strongly moti-
vated to start an observational project on the
Fornax dSph galaxy with the twofold goal to
reduce the gap between knowledge of SX Phe
stars in the Milky Way and those in other
galaxies and to test them as distance indicators.
2. Observations and data reduction
The feasibility of our project to detect SX Phe
stars in external galaxies was demonstrated by
the first observations of the northern part of the
Fornax dSph galaxy in November 2001. The
Wide–Field Imager (WFI) at the 2.2 m ESO–
MPI telescope was kept fixed on the same po-
sition on three consecutive nights. Since the
expected periods were very short, we took
three consecutive images in B–light (700 sec
each), followed by a single exposure in V–light
(1000 sec). This strategy ensured the dense
Btimeseries necessary to perform a reliable
frequency analysis and allowed us to obtain
the mean brightness and amplitude values in
a two–colour system. Photometric reductions
were carried out with the packages DAOPHOT
ALLSTAR-II (Stetson 1996) and ALLFRAME
(Stetson 1994). The variable stars were identi-
fied using the Image Subtraction Technique as
performed within the package ISIS 2.1 (Alard
2000). We have fully reduced four chips of the
WFI mosaic, detecting 61 short–period stars
and hundreds of RR Lyr variables. For the lat-
ter stars, see Clementini et al. (these proceed-
ings) and Greco et al. (2005).
The Fornax galaxy is known to host “a mix
of old and intermediate–age stars with different
metal abundances” (Held 2005) and therefore
we should distinguish between HADS and SX
Phe stars. However, at this stage, we consider
them all as SX Phe stars, i.e., stars belonging
to the older population. This seems justified by
the very short periods we found and by the av-
erage low metallicity of the Fornax galaxy. We
also stress that such a sample is very large and,
being comprised of similar stars at the same
distance, quite unique for number and homo-
geneity.
3. The detection of SX Phe stars
The intranight light curves are accurate enough
to exhibit the rapid variability of the SX Phe

Poretti et al.: SX Phe stars in Fornax 221
Fig. 1. The intranight light curves of three SX Phe
stars in the Fornax galaxy. Variability is discernible
on these curves even at the shortest periods (upper
row). Note the faintness of the variables.
Fig. 2. Folded light curves of SX Phe stars in the
Fornax galaxy. The Bmean luminosity increases by
1.7 mag from the shortest to longest periods.
stars (Fig. 1), in spite of their extreme faint-
ness (22.0<B<24.0). The timeseries just
sketch the light curves for the shortest peri-
ods (upper row, P=92 min; each exposure cov-
ers 0.13 p). They define the light curve shape
in a clearer way when the period lengthens
(middle row, P=114 min; the exposure cov-
ers 0.10 p) and, finally, they show the regular
light curve for the longest periods (lower row,
P=180 min; 0.06 p). When performing the fre-
Fig. 3. Folded light curves and power spectrum of
the candidate SX Phe star in the For 5 globular clus-
ter (Magellan/Clay observations).
quency analysis, the Bdata covering three con-
secutive nights ensured a high level of detec-
tion. Figure 2 shows the folded light curves in
B–light (61 datapoints); the Vdata (17 points)
have also been folded using the period val-
ues obtained from the more numerous Bdata.
Figure 2 is a P−Lrelation by itself: we can
see how the mean Bmagnitude brightens from
24.0 to 22.0 with increasing period.
In a follow–up of the project, we obtained
specific observations of the globular clusters in
Fornax using the Magellan telescope and the
Clay camera. Figure 3 shows the Band Vlight
curves (the Vfilter has been used to obtain the
denser timeseries here) and the power spec-

222 Poretti et al.: SX Phe stars in Fornax
Fig. 4. The solid line (least–squares fit) is the P−L
relation as derived from Fornax SX Phe variables
(filled circles); the dashed line is the P−Lrelation as
obtained from SX Phe and HADS stars in the Milky
Way assuming (m−M)V=20.72 for Fornax.
trum of the only candidate we found in For 5.
The period is 0.0495 d (i.e., 71 min) and the
full amplitude is 0.21 mag in V–light. We also
note that two SX Phe stars were found in the
outskirts of For 3 on the WFI images.
4. The P−Lrelation and the distance
modulus
We used the periods and the mVapparent mag-
nitudes to determine the slope of the Period–
Luminosity (P−L) relation in Fornax (Fig. 4,
solid line). The result, −3.02 ±0.44, is smaller
than that observed in the Milky Way (–3.725,
McNamara 1997). We stress that the range
covered by the Fornax stars in log P(from
−1.35 to −0.90) is narrower than that covered
by the Milky Way stars (from −1.40 to −0.55).
The different distribution of periods probably
reflects the differences in the stellar content:
the Milky Way contains many more Pop. I
HADS stars than Fornax and in turn these stars
have longer periods than Pop. II SX Phe stars.
The P−Lrelation given by McNamara
(1997) can also be drawn in the log P−mV
plane by assuming a distance modulus of (m−
M)V=20.72 for Fornax (dashed line), as
derived from RR Lyr variables (Greco et al.
2005). As can be seen (Fig. 4), the two lines
are nearly coincident in the period range of the
Fornax variables. We can also make the reverse
exercise to calculate the distance modulus of
Fornax assuming the McNamara P−Lrelation.
In such a case we get (m−M)V=20.71 ±0.04,
a value in excellent agreement with the previ-
ous ones (e.g., Buonanno et al. 1999, Saviane
et al. 2000, Greco et al. 2005).
5. Conclusions
The large sample of SX Phe variables in the
Fornax galaxy shows a well–defined P−L
relation, which is in good agreement with
the one found in the Milky Way. The results
briefly presented here seem to confirm that
these short–period stars can be used as stellar
candles in the Local Group and that they can
provide an independent method to measure dis-
tances.
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