Metal-rich carbon stars in the Sagittarius Dwarf Spheroidal galaxy

Abstract

We present spectroscopic observations from the {\it Spitzer Space Telescope} of six carbon-rich AGB stars in the Sagittarius Dwarf Spheroidal Galaxy (Sgr dSph) and two foreground Galactic carbon stars. The band strengths of the observed C$_2$H$_2$ and SiC features are very similar to those observed in Galactic AGB stars. The metallicities are estimated from an empirical relation between the acetylene optical depth and the strength of the SiC feature. The metallicities are higher than those of the LMC, and close to Galactic values. While the high metallicity could imply an age of around 1 Gyr, for the dusty AGB stars, the pulsation periods suggest ages in excess of 2 or 3 Gyr. We fit the spectra of the observed stars using the DUSTY radiative transfer model and determine their dust mass-loss rates to be in the range 1.0--3.3$\times 10^{-8} $M$_{\odot}$yr$^{-1}$. The two Galactic foreground carbon-rich AGB stars are located at the far side of the solar circle, beyond the Galactic Centre. One of these two stars show the strongest SiC feature in our present Local Group sample. Comment: 12 pages, 11 figures, accepted for publication in MNRAS

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arXiv:0903.1045v1 [astro-ph.SR] 5 Mar 2009
Mon. Not. R. Astron. Soc. 000, 1–12 (2002) Printed 5 March 2009(MN LATEX style file v2.2)
Metal-rich carbon stars in the Sagittarius Dwarf Spheroidal galaxy
Eric Lagadec1 ⋆, Albert A. Zijlstra1, G.C. Sloan2, Peter R. Wood3, Mikako Matsuura4,5,
Jeronimo Bernard-Salas2, J.A.D.L. Blommaert6, M.-R. L. Cioni7, M.W Feast8,11,
M.A.T. Groenewegen9, Sacha Hony10, J.W. Menzies11, J.Th. van Loon12,
P.A. Whitelock8,11,13
1Jodrell Bank Centre for Astrophysics, The University of Manchester, School of Physics & Astronomy, Manchester M13 9PL, UK
2Department of Astronomy, Cornell University, 108 Space Sciences Building, Ithaca NY 14853-6801, USA
3Research School of Astronomy and Astrophysics, Australian National University, Cotter Road, Weston Creek, ACT 2611, Australia
4Division of Optical and IR Astronomy, National Astronomical Observatory of Japan, Osawa 2-21-1, Mitaka, Tokyo 181-8588, Japan
5Department of Physics and Astronomy, University College London, Gower Street, London WC1E 6BT, UK
6Instituut voor Sterrenkunde, K.U.Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
7Centre for Astrophysics Research, University of Hertfordshire, Hatfield AL10 9AB, UK
8Astronomy Department, University of Cape Town, 7701, Rondebosch, South Africa
9Koninklijke Sterrenwacht van Belgie, Ringlaan 3, 1180 Brussels, Belgium
10CEA, DSM, DAPNIA, Service d’Astrophysique, CEA Saclay, 91191 Gif-sur-Yvette Cedex, France
11South African Astronomical Observatory, PO Box 9, 7935 Observatory, South Africa
12Astrophysics Group, School of Physical & Geographical Sciences, Keele University, Staffordshire ST5 5BG, UK
13National Astrophysics and Space Science Programme, Department of Mathematics and Applied Mathematics
University of Cape Town, 7701 Rondebosch, South Africa
Accepted . Received
ABSTRACT
We present spectroscopic observations from the Spitzer Space Telescope of six carbon-rich
AGB stars in the Sagittarius Dwarf Spheroidal Galaxy (Sgr dSph) and two foregroundGalac-
tic carbon stars. The band strengths of the observed C2H2and SiC features are very sim-
ilar to those observed in Galactic AGB stars. The metallicities are estimated from an em-
pirical relation between the acetylene optical depth and the strength of the SiC feature. The
metallicities are higher than those of the LMC, and close to Galactic values. While the high
metallicity could imply an age of around 1Gyr, for the dusty AGB stars, the pulsation peri-
ods suggest ages in excess of 2 or 3 Gyr. We fit the spectra of the observed stars using the
DUSTY radiative transfer model and determine their dust mass-loss rates to be in the range
1.0–3.3×10−8M⊙yr−1. The two Galactic foreground carbon-rich AGB stars are located at
the far side of the solar circle, beyond the Galactic Centre. One of these two stars show the
strongest SiC feature in our present Local Group sample.
Key words: circumstellar matter — stars: carbon — stars:AGB and post-AGB — stars: mass
loss — galaxies: individual: Sagittarius dwarf Spheroidal — infrared: stars
1 INTRODUCTION
The Asymptotic Giant Branch (AGB) phase occurs during the late
stages of the evolution of low- and intermediate-mass stars. This
phase is characterised by intense mass loss and leads to the forma-
tion of a circumstellar envelope made of gas and dust. The molec-
ular composition of this envelope is dependent on the C/O abun-
dance ratio. The CO molecule is very stable and unreactive, and
if C/O > 1, all the oxygen is trapped in CO. The envelope (and
⋆E-mail: eric.lagadec@manchester.ac.uk
star) is then “carbon-rich”; amorphous carbon dominates the dust,
while (after H2) CO and C2H2dominate its gas. The“oxygen-rich”
AGB stars are characterised by silicate dust and molecules such as
SiO, OH, H2O. Third dredge-up brings the carbon produced by the
triple-α reactions to the surface, increasing the C/O ratio and over
time can change a star from oxygen-rich to carbon-rich.
The mass loss from AGB stars is one of the main agents for
the chemical evolution of galaxies. It expels the products of nuclear
reactions in the core of the star into the interstellar medium. The
mass loss from AGB stars contributes roughly half of all the gas
recycled by stars (Maeder 1992), and up to nearly 90% of the dust

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2
E. Lagadec et al.
(Gehrz 1989). Mass loss from AGB stars is also one of the main
sources of carbon in the universe, together with Wolf-Rayet stars
and supernovae (Dray et al. 2003).
The mechanisms driving this mass-loss process are not fully
understood. It is thought to be a two-step process: pulsations from
thestar leadtotheformationof dust, and thenradiation pressureac-
celerates the dust grains to the escape velocity. The gas is then also
expelled due to friction with the dust grains. The effect of metal-
licity on the mass-loss rates from AGB stars has been discussed at
some length. Some works proposed that mass-loss rates should be
lower in metal-poor environments, as less dust is expected to form,
so that radiation pressure would be less efficient in driving the mass
loss (Bowen & Willson 1991; Zijlstra 2004). This hypothesis on
the metallicity effect on mass loss has recently been tested using
spectroscopy from Spitzer (e.g. Sloan et al. 2006, 2008; Zijlstra
et al. 2006b; Lagadec et al. 2007; Matsuura et al. 2007; hereafter
“SZLM” will refer to all five of these papers). Against early expec-
tations, the dust mass-loss rates of carbon stars inmetal-poor galax-
ies from the Local Group appear to be similar to the ones measured
in the Galaxy. In contrast, oxygen-rich stars in metal-poor environ-
ments do appear to have lower dust mass-loss rates. This suggests
that carbon is important in triggering the superwind (Lagadec &
Zijlstra 2008). Theoretical models confirm that the mass-loss rates
of carbon stars should not depend on metallicity (Wachter et al.
2008). Mattson et al. (2008) argue that the pulsation energy of the
star can drive a strong wind at low metallicity, but the dependence
on chemistry suggests this is not the dominant effect within the ob-
served metallicity range.
The dusty circumstellar envelope surrounding AGB stars ab-
sorbs the light from the central star and re-emits it in the in-
frared. Furthermore, spectral signatures of dust and molecules ap-
pear in this wavelength range. Spectroscopic observations of AGB
stars in the thermal infrared are therefore vital for the study of
the dusty envelopes around these stars. Atmospheric absorption
prevents ground-based mid-infrared observations outside two win-
dows around 10 and 20µm. The Spitzer Space Telescope (Werner
et al. 2004), with its high sensitivity and mid-infrared wavelength
coverage, has proven to be a valuable tool for the study of dusty
envelopes around extragalactic AGB stars.
To study the mass loss from evolved stars at low metallicity,
we are undertaking a Spitzer spectroscopic survey of mass-losing
AGB stars in different Local Group galaxies. We have already
presented results for the Magellanic Clouds and Fornax (SZLM;
Matsuura et al. 2006; Groenewegen et al. 2007). Here we present
Spitzer spectra of eight AGB stars in the direction of the Sagittarius
Dwarf Spheroidal (Sgr dSph) galaxy.1This study aims at studying
the circumstellar properties of these AGB stars.
2TARGET SELECTION
We selected eight AGB candidates in the direction of the Sgr dSph
based on their near-infrared colours. These stars were not spectro-
scopically confirmed carbon stars before this study. The core of
the Sgr dSph has a distance modulus estimated to be 17.02±0.19
(Mateo et al. 1995) and is located behind the Galactic Bulge. It is
being disrupted by the Galaxy so that its tidal tail surrounds the
Milky Way. It contains several stellar populations. The dominant
1This galaxy is also known as the Sagittarius Dwarf Elliptical Galaxy
(SagDEG), but it should not be confused with the Sagittarius Dwarf Irregu-
lar Galaxy (SagDIG).
AGB limit
AGB
Figure 1. The MKvs J − K diagram of the observed sample. Crosses
represent the observed Sgr dSph targets and the large circle the foreground
targets. Points represent the stellar population of Sgr dSph. MKis the abso-
lute magnitude and was calculated assuming a distance modulus estimated
to be 17.02±0.19 (Mateo et al. 1995).
one has a metallicity in the range [Fe/H]=−0.4 to −0.7 and an
age of 8.0±1.5 Gyr (Bellazzini et al. 2006). A second population
is younger and more metal-rich with [Fe/H]=−0.25 for the most
metal-rich objects (Zijlstra et al. 2006a). These populations span
the range of metallicities observed in the Magellanic Clouds.
The eight selected targets are a subsample of the stars pre-
sented by Lagadec et al. (2008). Table1 lists some characteristics
of these stars. For convenience, we will use the names published
previously. The stars were selected to span a wide range in J − K
colour, i.e. a wide range of optical depth due to dust in their en-
velopes.
Fig. 1 displays the eight observed stars in an MK vs J − K
diagram. Asterisks represent the observed stars in the Sgr dSph.
To show that the observed stars are all AGB stars, we overplot the
distribution of Sgr field stars. Lagadec & Zijlstra (2008) explain
the selection of these field stars. To verify that the observed stars
belong to Sgr dSph, we performed radial velocity measurements:
these indicate that six out of the eight observed stars belong to Sgr
dSph but the remaining two are foreground stars (see Sec. 3.3).
3 OBSERVATIONS AND DATA REDUCTION
3.1 Spitzer observations
The observations were made with the InfraRed Spectrograph (IRS,
Houck et al. 2004), on board the Spitzer Space Telescope. We used
the Short-Low (SL) and Long-Low (LL) modules to cover the
wavelength range 5-38µm. The SL and LL modules are each di-
vided in two spectral segments, together known as SL2, SL1 , LL2
and LL1; a “bonus” order covering the overlap between the two
modules is also available. The data reduction is similar to that de-
scribed by Zijlstra et al. (2006b). The raw spectra were processed

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Carbon stars in the Sgr dSph
3
through theSpitzer pipeline S15. Wereplaced the bad pixels by val-
ues estimated from neighbouring pixels. The sky was subtracted by
differencing images aperture by aperture in SL and nod by nod in
LL. We used the software tools available in SPICE (the Spitzer IRS
Custom Extractor) to extract the spectra. The flux calibration made
use of the reference stars HR 6348 (K0 III) in SL and HR 6348,
HD 166780 (K4 III) and HD 173511 (K5 III) in LL. The spec-
tra were individually extracted from the individual images. Both
nods in both apertures were then joined simultaneously, recalculat-
ing the errors in the process by comparing the nods. The different
nods were averaged, using the differences to estimate the errors.
The different spectral segments were combined using scalar mul-
tiplication to eliminate the discontinuities due to flux lost because
of pointing errors. The different segments were also trimmed to re-
move dubious data at their edges. We also retained the data in the
bonus order where it was valid. These steps resulted in a standard
wavelength calibration accuracy of 0.06µm in SL and 0.15µm in
LL.
Fig. 2 presents the spectra of the observed AGB stars, ordered
by their [6.4]-[9.3] colours (see Sec. 5). The molecular bands and
dust features discussed below identify all eight objects as carbon-
rich stars.
3.2Near-infrared photometry
Observations in the near-infrared are important to obtain a reliable
estimateof theluminosity. Duetothevariability, these observations
are best obtained close to the same epoch as the spectra. Multi-
epoch JHKL photometry was obtained at the Australian National
University (ANU) 2.3-m telescope at Siding Spring Observatory
(SSO) in Australia. The filters used were centred at 1.28µm (J),
1.68µm (H), 2.22µm (K) and 3.59µm (L). Groenewegen et al.
(2007) describe the observations. Table1 presents the measured
JHKL magnitudes at the epoch of the Spitzer observations.
The multi-epoch observations, taken before the Spitzer ob-
servations, were used to study the near-infrared variability of the
sources. Light curves were obtained by fitting a sine wave to the
K-band data. Fig.3 displays these light curves.
3.3 Radial velocities
Radial velocities were determined for six of the eight carbon stars
using optical spectra obtained with the Dual Beam Spectrograph
on the 2.3-m SSO telescope. The spectra have a resolution of 0.48
˚ A/pixel for four objects and 3.7 ˚ A/pixel for the remaining two
objects (see Table2). The higher resolution spectra were cross-
correlated with the local carbon star X Vel for which a spectrum
at a resolution of 0.5 ˚ A/pixel was also obtained. A radial veloc-
ity of −5.4kms−1was adopted for X Vel (determined from high-
resolution echelle spectra which were cross correlated against the
radial velocity standard α Cet with a heliocentric radial velocity of
−25.8kms−1). Cross-correlation of thelower-resolution spectraof
the remaining two stars was used to obtain their radial velocities.
Table2 gives the final heliocentric radial velocities and their errors
(as given by the IRAF task FXCOR).
Fig. 4 presents a histogram of the radial velocities of carbon
stars in the direction of Sgr dSph from Ibata et al. (1997). There
are obviously two groups of stars: those with heliocentric radial
velocities of 100 < vhelio(km/s) < 190 which belong to the Sgr
dSph; and those with vhelio(km/s) < 50 which belong to the Milky
Way Galaxy. Two stars in our sample, Sgr02 and Sgr22, clearly
N
(km/s)
This Work
Figure 4. Top panel: Radial velocity distribution of stars in the direction of
the Sgr dSph galaxy (from Ibata et al. 1997). Bottom panel: radial velocity
distribution of six of our targets. Two of these stars, Sgr02 and Sgr22, are
clearly foreground members of the Galaxy.
Table 2. Heliocentric radial velocities for six of the observed targets as
measured from observations at SSO.
Target Resolution
˚ A/pixel
wavelength rangevhelio
km/s
verror
km/s
˚ A
Sgr02
Sgr03
Sgr09
Sgr15
Sgr22
Sgr29
0.48
3.7
0.48
3.7
0.48
0.48
8100-8900
6800-9200
8100-8900
6800-9200
8100-8900
8100-8900
−22.8
128.1
144.9
136.4
-73.1
106.3
1.1
11.1
1.6
12.2
2.1
2.5
belong to the Milky Way Galaxy, while the other stars are members
of the Sgr dSph. The properties of the two foreground stars are
discussed in Sec. 9.3
We used other distance estimates to ascertain the Sgr dSph
membership of the two stars without radial velocity measurements
(see Sec. 7).
4 DESCRIPTION OF THE SPECTRA
The IRS spectra of the eight observed stars (Fig. 2) show both
dust and molecular emission. All the observed stars are carbon-
rich and have spectra typical of AGB stars. The spectra of all stars
clearly show absorption features from C2H2 at 7.5 and 13.7µm
(Fig. 5). SiC dust produces the 11.3µm feature detected in all the
spectra. Some of them (Sgr02, Sgr03, Sgr07 and Sgr18) show a
broad emission feature around 30µm attributed to MgS (Hony et
al., 2002). This feature might also be present in Sgr09 and Sgr22.
A drop is observed for all stars at the blue edge of the spectra. It is
due to several molecules, most notably C3 and CO (Zijlstra et al.
2006b).
Sgr22 displays a very strong SiC emission feature, stronger
than any carbon-rich AGB star of which we are aware, including
the Galactic sample observed by the Infrared Space Observatory
and described by Sloan et al. (2006) or the Local Group galaxies
observed by Spitzer (see upper panel of Fig. 8).
A weak feature attributed to the stretching vibration of a car-
bonyl group (X-CO) has been observed in some LMC AGB stars

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4
E. Lagadec et al.
SiC
MgS
C2H2 C2H2
Figure 2. Spitzer/IRS spectra of the eight observed carbon stars, ordered by dust temperature. The main dust and gas features are labelled on the spectrum of
Sgr 02. The dashed lines represent the SED fits obtained with DUSTY (Sec. 8)

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Carbon stars in the Sgr dSph
5
Figure 3. Light curves of the eight observed carbon stars. These were obtained using a sinusoidal fit to the K-band magnitudes. The vertical lines show the
dates of our Spitzer observations.

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6
E. Lagadec et al.
Table 1. Observed Sgr dSph targets: adopted names, coordinates, photometry and distance. J, H, K and L are taken from near-simultaneous measurements
at SSO. The periods were obtained by sine-fit to the K-band observations. The distance was estimated using the method described in Sec. 7.
Adopted name 2MASS nameIRAS nameRADec
J
mag
H
mag
K
mag
L
mag
P
d
phaseD
kpc(J2000)
Sgr02
Sgr03
Sgr07
Sgr09
Sgr15
Sgr18
Sgr22
Sgr29
18414350 −3307166
18443095 −3037098
18465160 −2845489
18514105 −3003377
18584385 −2956551
19043562 −3112564
19103987 −3228373
19485065 −3058317
18384−3310
18413−3040
18436−2849
18 41 43.50
18 44 30.96
18 46 51.60
18 51 41.05
18 58 43.85
19 04 35.62
19 10 39.87
19 48 50.65
−33 07 16.6
−30 37 09.8
−28 45 48.9
−30 03 37.7
−29 56 55.1
−31 12 56.4
−32 28 37.3
−30 58 31.9
11.095
13.360
15.464
11.753
12.862
15.535
11.315
12.909
9.421
11.436
13.121
10.263
10.921
12.753
9.454
11.014
8.043
9.879
10.944
9.124
9.394
10.734
7.959
9.522
6.501
8.047
8.320
7.913
7.496
8.171
6.196
7.838
301
446
512
370
417
485
370
339
0.65
0.36
0.06
0.40
1.00
0.42
0.02
1.00
17.1
31.6
34.1
30.3
28.0
35.1
16.0
31.5
18555−3001
19013−3117
19074−3233
(Zijlstra et al. 2006b) at 5.8µm. This feature is present in the spec-
tra of Sgr09 and Sgr18, and possibly also in Sgr02, Sgr03, Sgr07
and Sgr15.
5 COLOURS AND BAND STRENGTHS
Todetermine colours of the observed stars, weapplied the so-called
“Manchester System” (Sloan et al. 2006; Zijlstra et al. 2006b), us-
ing four narrow bands selected to represent the continuum at differ-
ent wavelengths. Using this method, we derive two colours. Zijlstra
et al. (2006b) showed that the [6.4]−[9.3] colour is a good esti-
mate of the dust optical depth, while the [16.5]−[21.5] colour pro-
vides an estimate of the dust temperature. The [6.4]−[9.3] colour
shows a linear relation with the measured mass-loss rates (Groe-
newegen et al. 2007; Matsuura et al. 2007; Sloan et al. 2008). Table
3 lists the blackbody temperature derived from this [16.5]−[21.5]
colour. To test that our choice of continuum colours is reasonable,
we plotted [16.5]−[21.5] vs. [6.4]-[9.3] (Fig. 6). For five of the
observed stars, the continua are consistent. But three stars (Sgr02,
Sgr03 and Sgr22) are outliers. Sgr02, Sgr03 are particularly red
at [16.5]−[21.5].
For all the observed stars, we measured the strength of the
main features (Fig. 2). The continuum underlying each feature is
defined using small wavelength ranges on the blue and red sides
of the feature. The continuum is then defined using a straight
line between the blue and red continuum values. Because the red
edge of the MgS feature (Sec. 4) is outside of the IRS spectral
range, we used a blackbody with the temperature derived from the
[16.5]−[21.5] to extrapolate the continuum under this dust feature.
After the definition of the continuum we determined the strengths
of the features in two different ways. For the dust features, we sim-
ply measured the ratio between the integrated feature flux and the
continuum (F/C). For the gas features, seen in absorption, we mea-
sured the equivalent widths. We also measured the central wave-
length of the SiC feature defined as the wavelength at which the
flux on the blue side of the feature equals the flux on the red side.
Table 3 lists the measured strengths and central wavelengths of the
observed features.
6 CIRCUMSTELLAR PROPERTIES
6.1Gas
As mentioned in Sec. 4, the main gas absorption features observed
in our spectra at 7.5µm and 13.7µm are due to C2H2. We have
shown (SZLM, Matsuura et al. 2006) that in metal-poor environ-
ments, the C2H2 absorption becomes stronger. This can be ex-
plained by the fact that at low metallicity, the initial oxygen abun-
dance issmaller thanforGalacticstars,and thedredge-up of carbon
during the AGB leads to higher C/O ratios.
Fig. 7 shows the equivalent width of these features as a func-
tion of the [6.4]−[9.3] colour for the current sample in comparison
to the stars from other Local Group galaxies. The plot relates the
strength of the molecular band to the optical depth (or dust mass-
loss rates) of the envelopes of the observed stars. The C2H2equiv-
alent widths of the Sgr dSph sample are in the lower range of the
observed strengths. They are similar to or a little higher than the
values found in Galactic stars of similar [6.4]−[9.3], but are gener-
ally weaker than found in the SMC, LMC and Fornax.
6.2 Dust
As mentioned in Sec. 4, the main dust features observed in the
spectra are due to SiC and MgS. The featureless continuum arises
primarily from emission from amorphous carbon emission. Fig. 8
shows the strength of the SiC and MgS features as a function of the
[6.4]−[9.3] colour.
The properties of the SiC feature have been extensively stud-
ied previously (e.g. Speck et al. 2005, Leisenring et al. 2008). La-
gadec et al. (2007) have shown that the relationship between the
strength of the SiC feature and the optical depth of the envelope
varies according to the metallicity of the host galaxy, using a sam-
ple of stars in the SMC, LMC and the Galaxy. Fig. 8 illustrates the
trend of increasing SiC feature strength for the Galactic stars when
[6.4]−[9.3]<∼0.5 and a gradual decrease for redder stars. Such a
trend is observed for carbon stars in the LMC, but the inflexion
occurs for redder colours, around ∼1. Very few SMC and Fornax
stars with [6.4]−[9.3]> 1 have been observed, but such stars in
metal-poor galaxies appear to follow the same trend as LMC stars
and have much weaker SiC features at a given optical depth. One
SMC star, GM 780, is an outlier with a very strong SiC emission
feature. The stars from the Sgr dSph are located at similar positions
as the Galactic ones, but with a SiC feature slightly weaker than
those of Galactic stars with the same [6.4]-[9.3] colour. As Si is not

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Carbon stars in the Sgr dSph
7
Table 3. Colours measured using four narrow carbon stars continuum bands, strength of the molecular and dust features, in terms of either the equivalent
width in microns, or the integrated flux-to-continuum ratio, F/C (Sect. 5). The last column gives the continuum (black-body) temperature, derived from the
[16.5]−[21.5] colour listed in Table 1
target [6.4]−[9.3]
(mag)
[16.5]−[21.5]
(mag)
EW (7.5 µm)
(µm)
EW (13.7 µm)
(µm)
F/C(SiC)
λc(SiC)
(µm)
F/C (MgS) T(
(K)
Sgr02
Sgr03
Sgr07
Sgr09
Sgr15
Sgr18
Sgr22
Sgr29
0.375 ±0.012
0.555 ±0.004
0.757 ±0.004
0.319 ±0.016
0.532 ±0.006
0.821 ±0.002
0.636 ±0.005
0.362 ±0.010
0.270 ±0.010
0.292 ±0.009
0.187 ±0.015
0.093 ±0.018
0.152 ±0.014
0.188 ±0.013
0.089 ±0.016
0.105 ±0.017
0.148± 0.002
0.115± 0.005
0.078± 0.005
0.237± 0.009
0.061± 0.004
0.068± 0.004
0.093± 0.002
0.131± 0.004
0.035 ± 0.005
0.020 ± 0.008
0.036 ± 0.002
0.064 ± 0.011
0.018 ± 0.005
0.034 ± 0.003
0.023 ± 0.002
0.044 ± 0.010
0.267± 0.005
0.226± 0.007
0.157± 0.004
0.278± 0.008
0.222± 0.005
0.158± 0.003
0.410± 0.006
0.189± 0.008
11.27 ± 0.03
11.07 ± 0.05
11.17 ± 0.04
11.14 ± 0.04
11.11 ± 0.03
11.18 ± 0.03
11.20 ± 0.02
11.22 ± 0.06
0.639 ± 0.012
0.401 ± 0.012
0.180 ± 0.018
0.050 ± 0.026
0.172 ± 0.018
0.234 ± 0.015
0.055 ± 0.020
0.001 ± 0.021
515.± 16.
480.± 12.
716.± 48.
1385.± 225.
864.± 66.
710.± 41.
1457.± 225.
1230.± 166.
produced in AGB stars, this slight difference appears to indicate
that the Si abundance in the observed Sgr dSph stars is between the
Si abundance of Fornax/SMC/LMC and the Galaxy.
Fig. 9 shows the strength of the MgS feature as a function of
the dust temperature. As already observed for AGB stars in other
Local Group galaxies (Sloan et al. 2006, Zijlstra et al. 2006b, La-
gadec et al. 2007), this feature tends to appear mostly in the en-
velopes of stars with cool dust. In general, the formation process of
MgS begins around 600K and is complete around 300K.
7 DISTANCE ESTIMATES
To check whether the observed stars are members of Sgr dSph, we
estimated their distances using two methods. The distance to Sgr
dSph being well established, our sample allows us to test different
distance estimation methods.
The first one uses the infrared colours of the observed stars,
following the method described by Sloan et al. (2008). They have
shown, from a sample of carbon stars in the Magellanic Clouds,
that:
MK = −9.18 + 0.395(J − K)
(1)
and
M9.3µm =
?
ai([6.4] − [9.3])i
(2)
where a0 = −8.81, a1 = −6.56 and a2 = 2.74. Using these
relations and the infrared colours of the observed carbon stars, we
can obtain two estimates of their distances.
The second method makes use of the period-luminosity rela-
tion for carbon Miras as described by Feast et al. (2006):
Mbol= −2.54 × logP + 2.06
(3)
We converted all of the photometry to the SAAO system and
dereddened the near-infrared data as described by Lagadec et al.
(2008). The reddening was estimated using the extinction maps
by Schlegel et al. (1998). For the observed stars, AJ∼0.1 and
AK∼0.04. Using standard conversion factors, we estimated that
A6µm<0.01 and that the reddening was lower than a percent at
longer wavelengths. We thus did not deredden our spectra. We then
derived the bolometric magnitudes using the equation for bolomet-
ric correction derived by Whitelock et al. (2006):
BCK
=+0.972 + 2.9292 × (J − K) − 1.1144 × (J − K)2
Table 4. Distances estimated from the infrared colours and the period-
luminosity relation.
targetDIR(kpc)DPL(kpc)
Sgr02
Sgr03
Sgr07
Sgr09
Sgr15
Sgr18
Sgr22
Sgr29
17.1
31.6
34.1
30.3
28.0
35.1
16.0
31.5
12.9
33.2
41.4
25.2
25.8
32.7
12.9
25.1
+0.1595 × (J − K)3− 9.5689 103(J − K)4
(4)
Table 4 displays the mean values of the estimated distance
for each star using the dereddened infrared colours and the period-
luminosity relation. This confirms that the stars Sgr07 and Sgr18
belong to Sgr dSph.
Mateo et al. (1995) derived a distance for the central region
of Sgr dSph of 25.35±2.06 kpc. A comparison of our two distance
estimates indicates that the best agreement with the distance to Sgr
dSph is obtained with the period-luminosity relation. Using the in-
frared colours gives distances to the observed stars significantly
higher than the well-determined distance to the galaxy. The dis-
tance estimated using the period-luminosity relation gives results
in perfect agreement for Sgr09, Sgr15 and Sgr29. For the three
other stars, this method gives larger distances. Variability will have
some effect on the derived distances, but given that two of the three
starswith large apparent distances are significantly redder in J−K
than any of the others it would seem quite possible that they are un-
dergoing obscuration events of the type described by Whitelock et
al. (2006) for Galactic Miras and which are also seen in at least one
of the AGB stars in Fornax (Whitelock et al. 2009). Photometric
monitoring over long time scales of these stars would be needed to
confirm that hypothesis, as stars with such behaviours show varia-
tions over periods of a few years.
8 DUST MASS-LOSS RATES
Lagadec et al. (2008) measured the dust mass-loss rates of the stars
presented in this work. These dust mass-loss rates (˙Mcol), mea-

Page 8

8
E. Lagadec et al.
Table 5. Dust mass-loss rates for the observed stars and parameters obtained from our DUSTY models. ˙ Mcoland ˙ Mdustyare the dust mass-loss rates (in
M⊙yr−1) from Lagadec et al. 2008 and our DUSTY models, respectively.
TargetLuminosity
(L⊙)
˙ MDUSTY
(10−8M⊙yr−1)
Teff
(K)
Tin
(K)
SiC/AMC
τ (0.55µm)
˙ Mcol
Vexp
km/s(10−8M⊙yr−1)
Sgr02
Sgr03
Sgr07
Sgr09
Sgr15
Sgr18
Sgr22
Sgr29
11469
4529
4483
6652
8632
5056
13321
9152
2.59
1.29
2.52
1.02
2.73
3.39
3.78
2.04
2800
2800
2800
2800
2800
2800
2800
2800
1200
1200
1000
1200
1200
1000
1200
1500
0.1
0.1
0.08
0.1
0.1
0.08
0.2
0.1
4.67
9.56
9.64
2.22
7.00
13.1
7.11
5.89
0.86
0.84
0.97
0.27
0.93
2.60
1.30
0.96
28
18
14
27
23
13
26
35
sured from near and mid-infrared colours are presented in Table
5.
The Spitzer spectra, together with simultaneous near-infrared
photometric measurements give us an opportunity to better model
the dust emission from these stars. We modeled the spectral energy
distributions (SEDs), defined by the JHKL flux and the Spitzer
spectra, using the radiative transfer code DUSTY (Ivezi´ c & Elitzur
1997). Our primary objective is to fit the dust continuum in order to
estimate the optical depths and the mass-loss rates. DUSTY solves
the 1-D problem of radiation transport in a dusty environment. For
all our models, we assume that the irradiation comes from a point
source (the central star) at the centre of s spherical dusty envelope.
The circumstellar envelope is filled withmaterial from a radiatively
driven wind. All the stars are carbon-rich and display SiC in emis-
sion. We thus assumed that the dust composition of the envelope
is a mixture of amorphous carbon and SiC. Optical properties for
these dust grains are taken the work of Hanner (1988) and Pegouri´ e
(1988) for amorphous carbon and SiC, respectively. The grain-size
distribution is assumed to be a typical MRN distribution, with a
grain size a varying from 0.0005 to 0.25µm distributed according
to a power law with n(a)∝a−qwithq=3.5 (Mathis et al. 1977). The
outer radius of the dust shell was set to 103times the inner radius;
this parameter has a negligible effect on our models.
To model the emission from the central star, we used a hy-
drostatic model including molecular opacities (Loidl et al. 2001,
Groenewegen et al. 2007). We used those hydrostatic models as an
illuminating source only, we did not attempt to model the molecu-
lar absorption features. The C/O ratio is assumed to be 1.1 for the
hydrostatic models, which isatypical value for GalacticAGB stars.
This ratio is not well known in the observed stars: the C/O ratio of
AGB stars appears to be higher in metal-poor environments than in
the Galaxy (Matsuura et al. 2002, 2005). But varying the C/O ratio
mainly affects the depth of the molecular features (such as CO and
C2H2). The radiative transfer model fits the dust emission, and the
C/O ratio has little effect on the model. We also performed some
model calculations assuming that the central star emits as a black-
body, but these did not provide satisfactory fit to the near-infrared
data, due to the absence of molecular absorption. The free parame-
ters of the models are the dust temperature at the inner radius, the
mass ratio of SiC to amorphous carbon dust and the central star
effective temperature, Teff. As one output, DUSTY provides an es-
timate of the terminal outflow velocity and the mass-loss rate for a
luminosity of 104L⊙. Asthe luminositycan be determined byscal-
ing the emerging spectrum, we can then recalibrate the mass-loss
rates accordingly, assuming a distance of 25 kpc for the six stars in
Sgr dSph, and 17.1 and 16.0 kpc for Sgr02 and Sgr22 respectively.
DUSTYgives total (gas+dust) mass-loss ratesassuming agas-
to-dust ratio of 200, but the dust is the dominant constraint on the
fitting to the SED. Therefore, we have divided the resulting mass-
loss rates by the gas-to-dust ratio and report only the dust mass-
loss rate in Table 5. This assumes that the gas and dust expansion
velocities are the same. Note that we had to make a number of
assumptions in our models. For example, we assume that the enve-
lope and the dust grains are spherical. This cannot be the case in
reality, so our fits cannot be perfect. However, the models achieve
our primary aim, estimating the dust mass-loss rates. These values
are of the same order of magnitude as those derived by Lagadec et
al. (2008). The values found by Lagadec et al. (2008) are generally
a factor of two or three smaller than the present ones.
For all the stars, the best fit is obtained with Teff=2800K. The
best model for most of the starswas obtained using atemperature at
the inner radius of 1200K and a mixture of 10% of SiC and 90% of
amorphous carbon. For two stars, we needed Tin= 1000K to model
the red part of the SED, and for another one we used Tin= 1500K.
The very strong SiC feature observed in the spectrum of Sgr22 is
very well fitted using 20% of SiC. Sgr07 and Sgr18 have weaker
SiC features, which can be fitted using a SiC mass fraction of 8%.
We confirm the general correlation found by Groenewegen
et al. (2007) and Matsuura et al. (2007) between the mass-loss
rates and the [6.4]−[9.3] (Fig10). But the stars we observed in
the Galaxy and in Sgr dSph are on average offset towards higher
mass-loss rates for the same colour, by about a factor of two. The
two stars in Fornax show a (smaller) offset in the same direction.
All of these mass-loss rates of these stars were estimated from our
DUSTY models, while the ones in the Magellanic Cloud stars were
estimated using another radiative transfer model (Groenewegen et
al.,2007). Themodels by Groenewegen et al. (2007) were made as-
suming a constant expansion velocity of 10 km s−1while DUSTY
calculates an expansion velocity for each model. These calculated
velocitiesaregenerally afactor of 2-3higher than theones assumed
by Groenewegen et al., suggesting that the difference in mass-loss
rates arise from different assumptions made by the models, rather
than an intrinsic differences between the stars. The dust mass-loss
rates determined by Lagadec et al. (2008) for the Sgr dSph stars
nicely follow the relation with [6.4]−[9.3] colour found in previ-
ous work.

Page 9

Carbon stars in the Sgr dSph
9
Figure 5. Spitzer spectra around 13.7µm of the observed stars, showing
the C2H2absorption. The two grey bands represent the continuum used to
measure the band strengths.
9 DISCUSSION
9.1Estimating metallicities
The survey of carbon-rich AGB stars in the Local Group has
yielded Spitzer IRS spectra of a large sample of stars in galaxies
covering a range of metallicities.
The metallicity of extragalactic AGB stars is normally taken
0.3
0.1
0.01
1.0
3
10
31
Figure 6. [16.5]−[21.5] vs. [6.4]−[9.3]colour diagram. Asterisks and open
circles represent the stars we observed. The solid line and filled squares
represent a series of DUSTY models, as described by Zijlstra et al. (2006a).
The optical depth at 1µm is indicated for each model
as that of the underlying dominant stellar population of the galaxy.
Individual metallicities of carbon stars are especially difficult to
determine. Generally, stellar metallicities are determined from the
strengths of fine-structure lines from heavy elements in optical or
near-infrared spectra. The continua of AGB stars are very uncer-
tain, due to the presence of multiple, overlapping molecular bands
in their envelopes, e.g. CO and CN. To model this continuum, a
hydrostatic stellar atmosphere model is required, with a known sur-
face gravity, C/O abundance ratio, and effective temperature. Such
modelshavebeenmade todeterminethemetallicityof LocalGroup
AGB stars, but are very time-consuming, and have been applied to
only a small number of stars (de Laverny et al. 2006).
The SiC feature is expected to depend on metallicity, through
the Si abundance. It may be possible to use the strength of this fea-
ture in our Spitzer spectra to estimate the metallicity of the stars
in our sample. Simultaneously, the C2H2 abundance depends on
the amount of free carbon, [(C−O)/O] (Lagadec & Zijlstra 2008),
which is expected to increase (on average) with decreasing metal-
licity (e.g. Matsuura et al. 2005). Thus the Spitzer spectra provide
two means of determining metallicity.
Fig. 11 shows theequivalent widthof theC2H27.5µm feature
versus the SiC flux-over-continuum ratio, for all the Local Group
carbon stars we observed. A clear separation is seen between the
stars from different galaxies, i.e. different metallicities. The Galac-
tic stars, which are expected to be the most metal-rich, have the
largest SiC/C ratio for a given C2H27.5µm equivalent width. The
LMC stars are spread out between the SMC and Galactic stars. The
diagram shows a good (average) separation between the galaxies,
in order of expected metallicity. Stars from each individual galaxy
show a significant spread, which may be due to a spread in metal-
licity within a galaxy, or due to an evolutionary spread, as the stars
willshow anincreasing carbon dredge-up over time.But for thefull
carbon-star population, this diagram appears to provide a metallic-
ity indicator.
We can apply this diagram to the stars observed in this paper.
The two galactic foreground stars fall within the general population
of Galactic stars, with one star (Sgr22) showing an unusually high
SiC/C ratio which can be interpreted as arising from a high metal-

Page 10

10
E. Lagadec et al.
Figure 7. The equivalent width of the C2H2 features at 13.7µm (upper
panel) and 7.5µm (lower panel) as a function of [6.4]−[9.3] colour. The
plot combines stars from the present sample and stars from previous Lo-
cal Group observations: open triangles represent the current Sgr dSph sam-
ple, filled squares the SMC stars from Sloan et al. (2006) and Lagadec et
al. (2007) samples, open circles the ISO Galactic sample defined by Sloan
et al. (2006), filled circles the LMC sample of Zijlstra et al. (2006b) and
Leisenring et al. (2008), and triangles stars from the Matsuura et al. (2007)
Fornax sample.
licity. Surprisingly, the stars in the Sgr dSph are found between the
Galactic and LMC stars, with an indicated metallicity higher than
that of theLMC, although stillsub-solar. Basedon thisdiagram, the
observed Sgr dSph carbon stars are the most metal-rich population
after the Galactic stars.
9.2Metal-rich carbon stars in the Sgr dSph
Thedominant giant branchpopulation intheSgrdSphisbelievedto
have a metallicity ([Fe/H]) ∼−0.55 or less (Whitelock et al. 1996,
Dudziak et al. 2000, Carraro et al. 2007) which is lower than that
of the LMC. The observed high metallicity implied for the dusty
carbon stars in our Sgr dSph sample is therefore unexpected.
Evidence for a more metal-rich stellar population in the Sgr
dSph was first detected by Bonifacio et al. (2004), who, based on
spectroscopy of ten giants, found the dominant population to have
[Fe/H]= −0.25, much higher than what is found from typical
AGB stars. Zijlstra et al. (2006a) and Kniazev et al. (2008) show
that one of the four planetary nebulae in the galaxy has the same
high metallicity, confirming the existence of this population even if
it does not dominate. Chou et al. (2007) find a median metallicity
Figure 8. The strength of the SiC features as a function of the [6.4]−[9.3]
colour. L/C(SiC) is the integrated-flux-to-continuum ratio of the SiC feature
(Sec. 5). Comparison data are from the literature are as in Fig.7
Figure 9. Strength of the MgS feature as a function of dust temperature
estimated from the [16.5]−[21.5] colour. Literature data as in Fig.7
in the core of the Sgr dSph of [Fe/H]= −0.4, similar to the LMC
and again a little higher than typical AGB stars.
The dusty carbon stars studied in this paper appear to be com-
paratively metal-rich. It is therefore natural to suggest that they
could be an AGB counterpart of the population whose age has been
estimated at 1Gyr (Bonifacio et al. 2004), the formation of which
mayhavebeentriggeredbythepassage ofthedwarf galaxythrough
the Galactic disc. However, their pulsation periods, which range
from 310 to 512 days, imply considerably older ages. For example
there are three carbon Miras which have periods around 490 days
(Nishida et al. 2000) in Magellanic Cloud Clusters which have ages
about 1.6Gyr (Mucciarelli et al. 2007a,b; Glatt et al. 2008), whilst
van Loon et al. (2003) suggest that the cluster KMHK 1603, which
contains acarbon-rich Mirawitha period of 680 days, hasan age of
0.9-1.0 Gyr. The kinematics of Galactic carbon-rich Miras (Feast et

Page 11

Carbon stars in the Sgr dSph
11
Figure 10. Dust mass-loss rates as a function of the [6.4]−[9.3] colour. The
comparison data are described in Fig. 7
al. 2006) is also consistent with increasing periods going with de-
creasing age. The periods of the Sgr dSph Miras would therefore
imply ages of 2 to 3 Gyr, or perhaps a little greater. More work is
required to reconcile these observations.
One can wonder why a relatively small galaxy would show
the most metal-rich stellar population in the Local Group after the
two main spirals (e.g. Zijlstra et al. 2006a). In general the dwarf
spheroidals show a much steeper age-metallicity relation than do
dwarf irregulars of the same luminosity. Dwarf spheroidals are
found around large galaxies while the dwarf irregulars are more
isolated systems, showing the effect of environment on enrichment
history. It is also of interest that of the satellites of the Milky Way,
only the Sgr dSph has reached such high metallicity. Fornax, other-
wise a very similar system, contains carbon stars with more metal-
poor characteristics (Matsuura et al. 2007). As the Sgr dSph is the
nearest of thesatellites(Fornaxisaround 130 kpc away), theeffects
of the Galaxy seem important.
There aretwo possible explanations for thevery high metallic-
ities. First, the stripping of the interstellar gas by a passage through
the Milky Way may have allowed fast chemical evolution through
stellarmasslossand supernovae. Second, thesystemmay havecap-
tured gas from the Milky Way. The second option may have some
support from the fact that the high metallicity is the same as that of
the Galactic disk at the orbital distance of the Sgr dSph.
9.3 Two foreground carbon stars
Asdiscussed inSec. 3.3, twostarsin our sample, Sgr02 and Sgr22,
belong to the Milky Way Galaxy while the other stars are members
of the Sgr dSph. These stars have been selected through their J
and K colours and were not spectroscopically confirmed carbon
stars before these observations. The Spitzer spectra clearly show
that these stars are carbon-rich AGB stars. Their distance is esti-
mated to be 17.1 kpc and 16.0 kpc for Sgr02 and Sgr22, respec-
tively, from their infrared colours (see Sec. 7). Sgr22 displays the
strongest SiC feature among all the stars from our present Local
Group sample (including the Galaxy). The other foreground star,
Sgr02, has a weaker SiC feature than Sgr22, but Fig. 8 indicates
Figure 11. Strength of the C2H27.5µm feature as function of the strength
of the SiC feature. The comparison data are as described in Fig.7
that its SiC feature strength is similar to the ones observed in the
Galaxy rather than in more metal-poor galaxies.
The two Galactic stars are on line of sight to the Sgr dSph,
and they are located on the far side of the Galaxy, close to the solar
circle. There are very few carbon stars known in the inner Galaxy:
carbon star distribution begins around the solar circle and extends
outwards (LeBertreet al. 2001). Thetwo starsare thereforelocated
around the inner edge of the carbon star distribution and should be
similar to carbon stars in the Solar neighbourhood. However, their
height above theGalactic plane is 3.6and 4.9 kpc respectively. This
is well beyond what is expected from the disc stars. There is no
clear explanation, and it would be very interesting to establish the
metallicity of other halo carbon stars, some of which are known to
be Mira variables (e.g. Totten et al. 2000; Mauron 2008). The high
metallicity suggests the stars escaped from the disc. Whether the
escape process was triggered by the Sgr dSph passage is not clear.
10 CONCLUSIONS
Wehavepresented aSpitzer spectroscopic survey ofsixcarbon-rich
AGB stars in Sgr dSph and two serendipitously discovered Galac-
ticcarbon stars. Weused the low-resolution short-low and long-low
mode of the IRS spectrometer to obtain mid-infrared spectra cov-
ering the range 5-38µm. These observations enabled us to study
dust and gas features from the circumstellar envelopes around these
stars. We observed molecular absorption bands due to C2H2at 7.5
and 13.7µm. A weak absorption feature at 14.3µm, due to HCN, is
observed in two stars. A weak absorption feature at 5.8µm due to
a carbonyl group (X-CO) is observed around two stars and might
be present in the spectra of three others. The continuum of all the
observed stars is due to emission from amorphous carbon, which
does not display any spectral features. All the stars show a SiC fea-
ture at 11.3µm. A broad dust emission emission due to MgS is also
observed for six of the stars around 30µm.
Radial velocity measurements show that two of our sample
are actually carbon stars. The remaining six stars in our sample
are members, as confirmed with radial velocities, infrared colour-
magnitude relations, and/or period-luminosity relations. One of the

Page 12

12
E. Lagadec et al.
two Galactic carbon stars displays the strongest SiC feature ever
observed. Both Galactic stars are certainly carbon stars located at
the far side of the solar circle, but they have unexpectedly large
distances from the Galactic plane.
We fitted the SEDs of all the observed spectra using the radia-
tive transfer code DUSTY. The estimated dust mass-loss rates are
found to be in the range 1.0-3.3×10−8M⊙yr−1.
The observed strengths of C2H2 and SiC are very similar to
the ones observed for Galactic carbon stars. Stronger C2H2 and
weaker SiC feature were expected in this metal-poor dwarf galaxy.
We have shown that the strength of these features depends on the
metallicity and that the observed stars have metallicities close to
Galactic values. This result is unexpected, as the usual measure-
ments of the metallicity ([Fe/H]) of the Sgr dSph range from −0.4
to −0.7. The enhanced metallicity of the observed carbon stars
indicates that the interstellar medium in the Sgr dSph has been
strongly enriched.
ACKNOWLEDGMENTS
EL acknowledges support from a STFC rolling grant. EL thanks
Xander Tielens and Kevin Volk for very helpful discussions dur-
ing the preparation of this paper. These observations were made
with the Spitzer Space Telescope, which is operated by JPL, Cal-
ifornia Institute of Technology under NASA contract 1407. This
research has made use of the SIMBAD and VIZIER databases, op-
erated at the Centre de Donn´ ees astronomiques de Strasbourg, and
the Infrared Science Archive at the Infrared Processing and Analy-
sis Center, which is operated by JPL.
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