A&A 446, 1107-1118 (2006)
DOI: 10.1051/0004-6361:20053423
P. de Laverny1 - C. Abia2 - I. Domínguez2 - B. Plez3 - O. Straniero4 - R. Wahlin5 - K. Eriksson5 - U. G. Jørgensen6
1 - Observatoire de la Côte d'Azur, Dpt. Cassiopée UMR6 202, 06 304 Nice Cedex 4, France
2 -
Dpto. Física Teórica y del Cosmos, Universidad de Granada, 18 071 Granada, Spain
3 -
GRAAL, UMR5024, Université de Montpellier II, 34 095 Montpellier Cedex 5, France
4 -
INAF - Osservatorio di Collurania, 64 100 Teramo, Italy
5 -
Department of Astronomy and Space Physics, Box 515, 75120
Uppsala, Sweden
6 -
Niels Bohr Institute, Astronomical Observatory, Juliane Maries vej 30, 2 100 Copenhagen, Denmark
Received 12 May 2005 / Accepted 12 September 2005
Abstract
We present the first results of our ongoing chemical study
of carbon stars in the Local Group of galaxies. We used spectra
obtained with UVES at the 8.2 m Kueyen-VLT telescope and a new grid of
spherical model atmospheres for cool carbon-rich stars which include
polyatomic opacities, to perform a full chemical analysis of one
carbon star, BMB-B 30, in the Small Magellanic Cloud (SMC) and two,
IGI95-C1 and IGI95-C3, in the Sagittarius Dwarf Spheroidal (Sgr dSph)
galaxy. Our main goal is to test the dependence on the stellar
metallicity of the s-process nucleosynthesis and mixing mechanism
occurring in AGB stars. For these three stars, we find important s-element enhancements with
respect to the mean metallicity ([M/H]), namely
,
similar to the figure found in galactic AGB stars
of similar metallicity. The abundance ratios derived between elements
belonging to the first and second s-process abundance peaks,
corresponding to nuclei
with a magic number of neutrons N=50 (88Sr, 89Y, 90Zr) and N=82(138Ba, 139La, 140Ce, 141Pr), agree
remarkably well with the theoretical predictions of low mass
metal-poor AGB nucleosynthesis models where the
main source of neutrons is the 13C
O reaction activated during the long interpulse
phase, in a small pocket located within the He-rich intershell. The derived C/O and 12C/13C ratios
are, however, more difficult to reconcile with theoretical expectations.
Possible explanations, like the extrinsic origin of the composition of these
carbon stars or the operation of a non-standard mixing process during the
AGB phase (such as the cool bottom process), are discussed on the basis
of the collected observational constraints.
Key words: stars: abundances - stars: carbon - nuclear reactions, nucleosynthesis, abundances - galaxies: Local Group
Asymptotic giant branch (AGB) stars are believed to be the main producers of s-elements in the Universe. Indeed, the s-process is activated during the late AGB phase, within the intershell region, when the He-burning shell suffers recurrent thermal instabilities (thermal pulses or TPs). After each TP, the convective envelope can penetrate inward in mass, dredging up the material enriched with the ashes of He-burning (third dredge up or TDU), mainly carbon and s-elements. Later on, this material is ejected through stellar winds, modifying the chemical composition of the interstellar medium (cf. Iben & Renzini 1983; Busso et al. 1999).
In intermediate mass AGB stars (
,
IMS),
free neutrons can be released at the base of
the convective zone
generated by a thermal pulse through the 22Ne(
)25Mg
reaction (see e.g. Iben & Renzini 1983). Indeed,
in the He-rich intershell of these massive AGB,
the 22Ne is exposed to a temperature as high as
K,
which corresponds to
an equilibrium density of about 1011 neutrons per cm-3.
However, due to such a high neutron density,
the resulting nucleosynthesis is characterized by the activation of
several branchings
along the s-process path, leading to an elemental distribution of the
heavy nuclei that is rather different from
those observed in the majority of the galactic MS, S and C (N type) stars
(see e.g. Lambert et al. 1995; and
Abia et al. 2000) and to an isotopic composition that is in conflict
with those found
in meteoritic SiC grains, which are presolar condensates formed
in the outflows of carbon-rich AGB stars (e.g. Zinner 1998).
On the contrary, in low mass AGB stars (
,
LMS),
the temperature within the He-rich intershell barely attains
K
and the 22Ne(
)25Mg is only marginally activated.
Nowadays, it is well accepted that the 13C
O reaction
is the main neutron source acting in LMS.
It only requires
K to be activated, a
temperature usually attained at the top layer of the He-rich
intershell during the interpulse periods. The extant theoretical
models (Straniero et al. 1995; Straniero et al. 1997; Herwig et al. 1997; Gallino et al. 1998; Goriely & Siess 2001) assume that a
small amount of hydrogen is injected from the convective envelope into
the intershell region during the TDU. At hydrogen reignition,
owing to proton captures on 12C, a 13C pocket forms within
the intershell. Then, the 13C is fully consumed by
captures during the interpulse phase, and then leads to
a substantial s-process nucleosynthesis with a
peak neutron density never exceeding 107 cm-3. The
freshly synthesized s-elements are finally engulfed by the convective shell
generated by the next TP. The marginal activation of the 22Ne
neutron source may slightly modify the s-element distribution within the
He-rich intershell. The enhancement of the s-elements revealed by
spectroscopic studies of MS, S, SC and C-stars confirms the occurrence of
repeated TDU episodes.
One of the most important theoretical results concerning this new s-process nucleosynthesis paradigm is its critical dependence on the stellar metallicity and mass. Current models (e.g. Busso et al. 1999; Goriely & Siess 2001) show that the predicted relative abundances of the s-peak nuclei (Zr, Ba and Pb) vary according to the stellar metallicity. At low metallicity, the flow along the s-path drains the Zr and Ba peaks and builds an excess at the doubly magic 208Pb, which is at the termination of the s-path. Thus, as the metallicity of the AGB star decreases, models predict larger Pb/Ba/Zr ratios. However, for a given metallicity, a spread in the Pb/Ba/Zr ratios indicates a spread in the amount of 13C, which drives the neutron production and the subsequent s-process nucleosynthesis in a LMS (Gallino et al. 1998; Delaude et al. 2004). The s-elements abundance pattern found in the metal-poor Pb-rich stars (Van Eck et al. 2001, 2003; Aoki et al. 2002) and, at higher metallicities, in post-AGB stars (Reyniers et al. 2004) points to the existence of such a spread.
A comfortable agreement between nucleosynthesis calculations and
s-element abundance patterns derived in AGB stars of different types
in the solar neighborhood has been found (e.g. Busso et al. 2001).
However, such comparisons mainly concern extrinsic stars
(i.e. stars that owe their chemical peculiarities probably
to mass transfer in a binary system) belonging to the disk of
our Galaxy. Only in a few
cases has the predicted chemical pattern
in intrinsic AGB or post-AGB stars (i.e. stars that owe their chemical peculiarities to
an in situ nucleosynthesis and mixing processes)
in a range of metallicity been checked (Reyniers et al. 2004;
Domínguez et al. 2004; Abia et al. 2002).
Note that metal-poor
AGB belonging to the old galactic halo
have such small masses (0.6
on the average) that the TDU cannot
take place (Straniero et al. 2003).
Due to their different chemical evolution histories, the satellite
galaxies have stellar populations with metallicities covering a wide
range (
,
see e.g. Groenewegen 2004; Shetrone
et al. 1998; Bonifacio et al. 2004a). Therefore, the study of AGB
stars in the Local Group of galaxies provides an alternative sample of
intrinsic metal-poor and relatively young AGB stars. Most importantly,
since the distance of these external stellar systems is well
known, a more accurate determination of the stellar luminosity
can be estimated. Furthermore, there is
observational evidence that in many of these galaxies the star
formation histories extended at least until a few Gyr ago
(e.g. Demers et al. 2003; Battinelli et al. 2003;
Arimoto et al. 2004; Domínguez et al. 2004; Rizzi et al. 2004). This
increases the probability of observing in
these stellar systems metal-poor thermally pulsing AGB (TP-AGB) stars, whose
abundance pattern is being modified by the occurrence of the TDUs
(intrinsic AGBs). Last but not least, the knowledge of the
contribution of these low metallicity AGB stars to the chemical
evolution of their parent galaxies provides new hints to discriminate
between alternative scenarios of galactic formation (Venn et al. 2004): were the satellite galaxies the basic building blocks of
our own Galaxy? (e.g. Bullock et al. 2001), or are they debris of
larger systems whose structure and evolution have been altered by
their proximity to our Galaxy (e.g. Grebel et al. 2003)?
In this paper we report the first chemical analysis of extragalactic low-metalicity carbon stars found in the Small Magellanic Cloud and the Sagittarius dwarf spheroidal galaxy. The selected stars are presented in Sect. 2 together with the observations. We describe in Sect. 3 the chemical analysis performed. Then, by analyzing the derived abundances and in particular those of the s-elements versus metallicity, we provide constraints to the evolutionary status of the studied stars and discuss our results in the framework of the current AGB nucleosynthesis models at low metallicity.
Table 1: Carbon stars observations log and properties.
From the available catalogs of carbon-rich stars in external galaxies (e.g. Groenewegen 2002), we first selected the brightest candidates that are spectroscopically confirmed as carbon-rich and as members of their host galaxy. Host galaxies have been selected to span a large range of metallicities, in order to provide better constraints to the nucleosynthesis models of AGB stars. In this first study we present the spectroscopic results for three carbon stars, one belonging to the Small Magellanic Cloud (SMC) and two stars to the Sagittarius dwarf spheroidal galaxy (Sgr dSph). Table 1 shows the photometric properties of our programme stars.
The SMC is an irregular galaxy at a distance of about 63 kpc (Cioni et al. 2000) and suffering an interstellar extinction
(NASA/IPAC database, Schlegel et al.
1998). Its mean metallicity is around
(Luck et al. 1998) although lower metallicities (e.g. -1.05) are also
reported (Rolleston et al. 1999). The selected star,
BMB-B 30 (B, standing for the bar of the SMC), was first identified as
carbon-rich by Blanco et al. (1980). Its carbon-rich nature
was later confirmed by Rebeirot et al. (1993) and, since
the survey of Smith et al. (1995), it was also known to be
non-enhanced in lithium.
The Sgr dSph galaxy was discovered ten years ago by Ibata et al. (1995). It lies at about 26 kpc from the Sun (Monaco et al. 2004) and is characterized by a large spread in
metallicity,
(see e.g. Bonifacio et al. 2004b), and recent observations reveal the existence of stars with
metallicity as low as
(Bonifacio et al. 2004a). The adopted interstellar extinction is
(NASA/IPAC database, Schlegel et al. 1998). The two selected stars were confirmed to be
carbon-rich and members of the Sgr dSph galaxy by Ibata et al. (1995). According to Whitelock et al. (1996), IGI95-C3, is
one of the brightest star in Sgr dSph, with extremely red colors
suggesting a large mass-loss rate and/or
a low effective temperature (see below the problems encountered during
its chemical analysis).
To date, the spectral type
of the three carbon stars is unknown since their full spectrum has
never been observed. We therefore cannot use common indicators as the relative intensity of
molecular bands of C2 and CN to classify them (such indicators lie
outside our observed spectral ranges).
We have estimated their bolometric luminosity using the
calibrations by Alksnis et al. (1998) from MK (see Table 3).
These calibrations are obtained from studies of galactic carbon stars
with known distances from Hipparcos parallax measurements. It is not
clear whether these calibrations can be safely applied to metal-poor
carbon (C) stars such as those studied here. Nevertheless, using the bolometric
corrections of MK from the (J-K)0 index for carbon stars
in the SMC by Wood et al. (1983), we obtain very similar
luminosities (see Table 3). The bolometric magnitudes obtained
for BMB-B 30 and IGI95-C3 are compatible with the hypothesis that they
are TP-AGB stars undergoing TDU. IGI95-C1 seems too faint to be an intrinsic
C-star, even considering that the minimum luminosity at which an AGB
star becomes carbon-rich is lower at low metallicity (Straniero et al. 2003). In fact, by using a simple core mass-luminosity relation to
constraint the initial luminosity of a thermally pulsing AGB phase
(Paczynski 1970), and adopting
for the mass of
the H-exhausted core, one gets a minimum bolometric magnitude of
about -4. In any case, the bolometric magnitudes shown in Table 3
are affected by large uncertainties. As pointed out by Guandalini et al. (2006),
with an effective temperature of
K, most of
the energy radiated by a C-star is in the mid infrared wavelength
range. By using IR data,
these authors conclude that the galactic carbon stars are, in many
cases, 1 or 2 bolometric magnitudes brighter than reported in
the available catalogs (Groenewegen 2002).
The three selected targets were observed in service mode with the UVES
spectrograph attached to the second VLT unit (Kueyen telescope) in
June and July 2003. The spectra have been collected using the UVES
standard settings
centered at 4370 Å and
centered at 8600 Å, leading to observed spectral domains
from
to
Å and from
to
Å. The slit width was
,
corresponding to
a resolving power of about 40 000. The UVES Data Reduction Standard
Pipeline was used for the reduction of the spectra. Then, for each
star and spectral range, the spectra were first averaged and then
binned by three pixels as well as corrected to the local standard of
rest. Finally, all the spectra were normalized to a local continuum by
fitting a polynomial connecting the higher flux points in the spectral
regions studied. For this procedure and, as a guide, we used the continuum location
that may be inferred looking at the spectral atlas of carbon stars of different types
by Barnbaum et al. (1996) and, in particular, in the 8000 Å region
the theoretically expected continuum windows identified by Wyller (1966) were used
as reference points. Due to the huge number of atomic and molecular lines
used for the synthetic spectra calculation, it is reasonable to think
that the theoretical continuum points are not too far from the
true continuum. Indeed, our theoretical spectra show maximum flux points
at these wavelengths. It was never necessary to modify the initial
placement of the continuum by more than
5%. Errors introduced by this
uncertainty in the continuum position were taken into account. However, systematic errors
due to a larger uncertainty in the continuum location cannot be completely
discarded (see below).
We have used the method of spectral synthesis in LTE to derive the
chemical abundances of the sample stars with a special emphasis on
specific spectral regions. In particular, the selected regions were: (i)
between 4750-4950 Å mainly for s-elements and the mean
metallicity; (ii) 6700-6730 Å for Li; (iii)
7050-7080 Å for some Ti lines and one Sr line;
(iv) 7780-7820 Å for Rb and two
Ni lines and 7990-8040 Å to derive the carbon isotopic
ratio. The adopted atomic line list is basically that used in Abia et al. (2001, 2002). We have added some lines taken from the atomic data
bases of Kurucz (CD-ROM No. 13) and VALD (Kupka et al. 1999).
Some revisions have been made using solar gf-values derived by
Thévenin (1989, 1990), and the theoretical estimates by Xu et al.
(2003) and Den Hartog et al. (2003) for the Nd II lines, and the
DREAM data base (http://w3.umh.ac.be/~astro/dream.shtml) for Sm and Ce
singly ionized lines. With respect to Abia et al. (2001, 2002), we
have identified additional spectral features corresponding to elements
whose abundances are derived here (see Table 2).
Table 2: Spectroscopic parameters of the new identified lines.
The molecular line list includes CN, C2, CH and MgH lines with the
corresponding isotopic variations. C2 lines are from Querci et al.
(1971). CN and CH lists were assembled from the best available data and
are described in Hill et al. (2002) and Cayrel et al. (2004). MgH lines
come from Kurucz (CD-Rom No. 13). In particular, the MgH lines, which were
not included in Abia et al. (2001, 2002),
act like a pseudo-continuum below 4800 Å. This might be
significant if the star under analysis is found to be
-enhanced (
), as one would expect for metal-poor
stars according to the trend found in our Galaxy. However, as far as
we know, all the chemical analysis so far performed in the SMC and the
Sgr dSph stellar populations do not reveal a clear
-enhancement trend. Thus, we have adopted in
the analysis no enhancement, i.e.
.
The ratio [Ca/M] derived in two stars (see
Table 4) is compatible with this figure although we note that our Ca
abundance estimation is made only from one line, CaI 6717 Å.
We checked our line list by comparing theoretical
with observed spectra of the Sun and Arcturus. The
comparison with Arcturus allowed us to check some molecular lines
whereas most of these features are not visible in the Solar
spectrum. For the Sun, we used the semi-empirical model atmosphere by
Holweger & Müller (1974) with abundances from Grevesse & Sauval
(1998). For Arcturus we computed a model atmosphere with main
parameters according to Decin et al. (2000) and
abundances from Peterson et al. (1993). Theoretical spectra were
computed with the turbospectrum code (Alvarez & Plez 1998, and further improvements by Plez)
in spherical geometry. The comparisons showed an
excellent agreement with the solar spectrum in all the spectral ranges
studied, although the comparison was not as good in the case of
Arcturus in the 4750-4950 Å region. This indicates that
our atomic and molecular line list is still not complete in this
spectral range (despite the fact that we included about 100 000 lines
in this region) and/or the line data of some molecules are partially
erroneous. Most of the spectral features used in the chemical analysis
of our stars, nevertheless, are well fitted in the Arcturus
spectrum.
The effective temperatures were first estimated from the
calibrations of the (V-K)0 and (J-K)0 color indexes by Aaronson
& Mould (1985). However, the estimated values are just used as a
starting point, the final values adopted in Table 3 being obtained
through an iterative process by comparing observed and theoretical
spectra computed with different effective temperatures. It is well
known that AGB stars are variable and, hence, their
may
change during the pulsation.
The photometric calibration
used to estimate the effective temperature indicates that the
uncertainty in our
is certainly not lower than
250 K. We also note that effective temperature
estimates from (V-K) may be affected by stellar variability
since the measurements in the two bands were obtained on different occasions.
Nevertheless, the final adopted
values agree quite well with those derived from the photometric
calibrations within
100 K.
We set the gravity at log g = 0.0 for all the stars,
following Lambert et al. (1986) who to a considerable
degree based their choice on the luminosities and masses of
carbon stars in the Magellanic Clouds. For the microturbulence,
we adopted
km s-1, which is a value suitable for
AGB stars (Lambert et al. 1986).
Theoretical spectra were convolved using Gaussian functions with a
FWHM value according to the instrumental profile (
Å) and macroturbulence
velocities ranging from 4 to 7 km s-1. Uncertainties in the abundances
derived due to changes in gravity, microturbulence and macroturbulence within
0.5 dex,
0.20 km s-1 and
1 km s-1, respectively,
were considered.
Table 3:
Main characteristics of the extragalactic C-stars.
We give the stellar name, the effective temperature, the mean
metallicity [M/H], the carbon over oxygen abundances ratio (C/O),
the corresponding difference between carbon and oxygen
abundances (
), the carbon isotopic ratio
and the range of estimated bolometric absolute magnitudes.
Apart from
,
the main atmospheric parameter affecting
theoretical spectra
of carbon-rich stars is
the C/O ratio. We estimated this ratio through an
iterative process, by comparing observed and theoretical spectra
computed for different values of the carbon abundance, while keeping
the other atmospheric parameters constant. For that purpose, we mainly
considered the region near 8000 Å since it is the most
sensitive to changes in the C/O ratio. Indeed, the spectrum of C-stars in this region is
mostly affected by CN and much weaker C2 absorptions.
We could not derive the carbon abundance directly from C2 lines since (i) our linelist is not enough accurate around the 8000 Å
region (see Plez & Cohen 2005);
(ii) the C2 Swan (0,0) band around
5160 Å was not observed; and (iii) the collected spectra in the blue part
become too noisy
to consider the C2 Swan (1, 0) band around 4735 Å.
We thus derive the carbon abundance from the CN lines
assuming that
.
We checked in previous works
(Abia & Isern 1996) that the C/O ratio derived from the 8000 Å range is almost
insensitive to the assumed abundance of nitrogen within
0.3 dex.
However, the derived C/O ratios have an additional uncertainty due to the
adopted O abundance which we cannot determine independently. This is because theoretical
spectra are almost insensitive to a large variation in the O abundance provided that the
difference
(O) is kept constant.
Indeed, when adding equal amounts of carbon and oxygen to the atmosphere,
the sole effect in the outer layers is to increase the abundance of
the CO molecule
which has a negligible effect on the atmospheric structure.
Therefore, this ambiguity allows a
range of oxygen abundances and C/O ratios giving almost identical synthetic spectra.
Nevertheless, even considering an uncertainty of a factor three in the oxygen abundance,
the C/O ratios derived in our stars are found in the range
.
On the other hand, and as already found by Abia et al. (2002), the derived C/O ratio slightly decreases with the
wavelength of the spectral region fitted. For instance, in BMB-B 30,
we found a best value of
at
Å, but
at
Å. Similar differences are obtained for the
other two program stars. The reason of this discrepancy is
unknown. It may be related to the continuous opacity of the model
atmosphere or to an incomplete or erroneous molecular line list.
Note that a modification of the continuum location by
in the
4750-4950 Å range would increase our derived C/O ratio in this region
by only a few hundredth of dex. Thus, the discrepancy would remain.
The range in the C/O ratios derived for each star is given in Table 3 as well as the
corresponding abundance difference
.
Table 4:
Summary of the abundances derived in the programme stars.
N, indicates the number of lines
utilized for a specific element if more than one line was used (we then
give the mean abundance together with its dispersion).
[el/M] is the abundance
ratio with respect to the mean derived [M/H] (see text and Table 3) and is only quoted for
elements with .
Regarding the model atmospheres, we used a new grid of models for cool carbon-rich stars.
The spherically symmetric model atmospheres were calculated with
the MARCS code, using opacity sampling in 11 000 frequency points. Atomic,
diatomic and polyatomic (C2H2, HCN and C3) absorptions were included.
The microturbulence parameter was set to 2-3 km s-1. Turbulence pressures were
neglected. Convection was included according to the local mixing-length recipe
and found to be insignificant. The masses of the models were set to
,
following Lambert et al. (1986). The abundance difference
and the overall metallicity were varied.
The adopted solar abundances for C, N and O in the models are 8.41, 7.80 and 8.67
according to the recent revision by Asplund et al. (2005). As mentioned
previously, the models were found to be quite insensitive
to
as long as
was unchanged. The typical
extension (from optical depths 10-4 to 102 in
)
was 7% of the total stellar radius. The temperature structures for the models
used for the three stars are presented in Fig. 1. Further details will be
given in forthcoming papers by Gustafsson et al.
(2005) and Jørgensen et al. (2005).
For each star we chose from the grid a specific
model atmosphere with a given value of
,
C/O and the
metallicity and proceeded iteratively by
changing these parameters until a good fit (by eye) in all the
spectral regions was found. Then, the abundances of the
other chemical species (s-elements, Li, etc.) were changed to fit
specific spectral features.
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Figure 1:
Temperature structures of the model atmospheres computed with
the stellar parameters corresponding to the three stars studied
in this work. The points where the optical depth
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A full discussion of the sources of error in the derived absolute
abundances and element ratios due to uncertainties in the
atmospheric parameters, continuum location (5%) and random errors when a
given element is represented by a few lines, as in our case, can be
found in Abia & Isern (1996) and Abia et al. (2001, 2002) and will not be repeated
here. These errors range from
0.2 dex for Ba to
0.45 dex
for Ce. We estimate a typical uncertainty in the mean metallicity, [M/H],
of
0.30 dex. The typical error for the elemental ratios
with respect to the metallicity
([X/M]) range between 0.30-0.40 dex, since some of the uncertainties
cancel out when deriving this ratio. For the same reason, the error
in the abundance ratio between elements ([X/Y]) is somewhat lower,
0.30 dex.
For Nb and Gd, the identified spectral features are very
weak and blended, thus in some cases we can only establish upper
limits. We estimate an uncertainty in the carbon isotopic ratio of
9 (see
Abia & Isern 1996). These numbers do not include possible systematic errors
as N-LTE effects or an uncertainty larger than
in the continuum location
(see below).
The abundances derived in IGI95-C3 from the 4850-4950 Å region, merit a
special note of caution. We found
many difficulties to fit this spectral region. In fact, in this region
several choices of
,
C/O ratio and metallicity can lead
to a similar match of the observed spectrum. Finally we adopt the
parameters shown in Table 3 as the best ones for IGI95-C3 (with
)
but, in any case, we cannot obtain as good a fit as that
obtained for BMB-B 30 and IGI95-C1 in this spectral region. We believe
that this may be caused by the presence of Merrill-Sanford bands
(SiC2) in this star. These molecular bands are identified between
4100-5500 Å in carbon stars (e.g. Sarre et al. 2000;
Yamashita & Utsumi 1968; and McKellar 1947). In fact we detect
extra-absorptions around 4867 and 4906 Å, not present in the other
stars of the sample, which coincides with the position of some
band heads of this molecule. Merrill-Sanford bands are indeed detected in this
spectral region in some galactic C-stars with
K
(Bergeat et al. 2001; Morgan et al.
2004), and our estimated effective temperature for this star
(
K, see Table 3) agrees with this figure. This
means that the derived abundances in IGI95-C3 from the region
4850-4950 Å have to be considered with caution. Obviously,
Merrill-Sanford bands do not affect the abundances derived in this
star from the other spectral ranges.
![]() |
Figure 2: Synthetic fits to the spectrum of the star IGI95-C1 in the region around 4825 Å. From this region we derived most of the s-elements abundances and the mean metallicity of the stars. Some atomic lines, main contributors to the specific spectral feature, are marked. Black dots represent the observed spectrum. Lines are synthetic spectra: only molecules and metals (dashed line), and including s-elements (best fit, continuous line). |
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Figure 3:
Synthetic fit to the spectrum of IGI95-C1 in the region of the Ba II line
at
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Figures 2 and 3 show examples of theoretical fits to spectral regions from where most s-element abundances are derived. The fits are reasonably good despite the line lists used are likely incomplete. We are able to obtain a reasonable reproduction of some specific spectral features representative of single heavy elements. As noted above, for several elements the abundance is derived from just one line. Table 4 summarizes the abundances derived for individual elements in the programme stars. In the case that more than one line was used in the analysis we derived the mean value and its dispersion.
The three stars have low Li abundances, similar to that typically
found in large Li abundance surveys of galactic C-stars (Denn et al. 1991; Abia
et al. 1993). The Li content in BMB-B 30 was already studied by Smith
et al. (1995) and their estimate agrees with the value given in Table 4.
On the other hand, the mean metallicity derived in the stars, are compatible with the
typical metallicity ranges derived from the stellar populations of these satellite
galaxies in other observational studies (see references in Sect. 1). In particular, the metallicity
derived for BMB-B 30, belonging to the SMC, confirms the low values previously reported
by Rolleston et al. (1999) as the mean metallicity for this galaxy. On the
basis of the derived
Ca and Ti abundances, and considering the error bars, we do not find evidence of -enhancement
in any of the stars studied.
It should be remarked that this is the first time that species like Zn, Ru, Hf
and W are detected and measured in giant carbon stars. We find a moderate overabundance
of Zn in all the studied stars. Zn abundance is derived from the only accessible line
(Zn I at 4810 Å) which seems to be more sensitive to errors in the atmospheric
parameters and damping constant than other Zn lines (see discussion in Chen
et al. 2004).
In any case, we estimate a formal error in [Zn/M] of 0.25 dex. Keeping in mind this
uncertainty, let us recall that AGB stars do not produce a sizable
amount of Zn (Bisterzo et al. 2004; Travaglio et al. 2004), so we can directly compare
the abundances measured in our sample of extragalactic carbon stars with that in unevolved galactic
stars. For [Fe/H] between -1 and -0.5, unevolved galactic stars show,
on the average,
,
with a rather large spread
(Mishenina et al. 2002; Bihain et al. 2004; Cayrel et al. 2004; Travaglio et al. 2004). Taking into account this spread and the quoted error bar,
we can conclude that the overabundance of Zn we find in Sagittarius
dSph and SMC C-stars does not significantly depart from the figure found
for galactic stars of similar metallicity. Note
that Reyniers et al. (2004) found
in the galactic post-AGB star
IRAS 08143-4406 with
.
The average abundances of the light s-elements (ls: Sr, Y and Zr, corresponding to
the first peak of the main component) and that of the heavier s-elements (hs: Ba, La, Nd and Sm,
corresponding to the second peak) are reported in Table 5. Ce is not included in
the hs definition, because there is no single feature in the spectral ranges studied whose
main contributor is Ce and, our derivation of its abundance is quite uncertain.
In addition, we do not include Sm in the case of IGI95-C3 because it was not measured.
The abundance ratios in Table 5 would be not significantly modified if a
different continuum location would have been adopted.
For instance, we checked
if a veil of moderate intensity of some unknown molecular
absorption is contributing in this region.
It has been found that, considering a
continuum placement
higher, the mean metallicity
should be increased by +0.25 dex, the [s/M] ratio by +0.30 dex, +0.25 dex
for the [ls/M] ratio, +0.38 dex for the [hs/M] ratio, but only by +0.13 dex
in the [hs/ls] ratio. In this case, however, the global fit obtained to the
observed spectra in the 4750-4950 Å region is considerably worse.
The same test shows that the C/O and 12C/13C ratios would be altered
by +0.02 dex and -5, respectively.
Table 5:
Metallicity and the s-process indices of the extragalactic C-stars
studied. The typical error in
and
is
0.3 dex and the average error on the other indices is
0.35 dex.
Table 5 shows that the three stars have moderate s-element enhancements,
.
This level of enhancement agrees with the figure found in galactic extrinsic
(MS, S and CH types with no Tc) and intrinsic (halo) AGB stars of similar metallicity
(see Busso et al. 2001, and references therein). The same happens when comparing the intrinsic index [hs/ls],
used to characterize the neutron capture process. Hence, independently of their extrinsic or
intrinsic nature (see below), the extragalactic C-stars studied here are similar to
their galactic counterparts as far as the s-element enhancements
are concerned. On the other hand, comparisons with galactic
post-AGB stars of low metallicity (Zacs et al. 1995; Reddy et al. 1999; Van Winckel & Reyniers 2000; Reyniers et al.
2004) show a s-element enhancement typically lower by one order of magnitude.
![]() |
Figure 4:
Synthetic fit to the spectrum of the star IGI95-C1 in the region around 8025 Å
from which we derive the 12C/13C ratio and a first estimate of the C/O ratio (see text).
The 13C abundance is estimated from the weaker 13CN features at
![]() ![]() ![]() |
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Figure 4 shows a theoretical fit of the observed spectrum for the star
IGI95-C1 in the spectral region around 8025 Å. This
spectral region is dominated by CN absorptions and it has been used to
derive the C/O and (mainly) the 12C/13C ratios. The values
obtained for our stars are reported in Table 3. Similarly to the figure found in most galactic
N-type C-stars (Lambert et al. 1986; Eglitis & Eglite 1995, 1997; Abia et al. 2002), the derived C/O ratios are only slightly larger than
1. Even considering the uncertainty in the model atmosphere
parameters, we do not believe that the spectra of
our stars could be reasonably fitted with a
,
or larger as it is
clearly seen in Fig. 4.
At solar metallicity, several TP and TDU
episodes are required to transform an oxygen-rich M giant
into a carbon star, because of the relatively large O content in the envelope. When the
metallicity is reduced down to 1/10 of the solar value,
is
attained after 1 or 2 dredge-up episodes, only. More dredge-up episodes
may be required if the original composition was enhanced in
oxygen, a possibility that we cannot exclude given the low metallicity
of the stars
.
It is often claimed that the reason for the recurrence of
in the majority of galactic giant carbon stars may be
an observational bias: an excess of carbon in the envelope
is immediately translated into a copious production of carbon-rich dust
that induces the formation of a thick circumstellar shell obscuring
the photosphere at optical wavelengths. In this case, the mass loss
rate can reach up to
/yr and the AGB star rapidly
looses its H-rich envelope. Then, visible C-stars represent a
short stage of AGB evolution.
We also note that even without the star being obscured by dust, the rapid
increase in visible and IR gas opacity when C/O passes unity
might in itself create a
strong levitation which will facilitate the (wind driven) mass loss
and quickly bring the stellar life time to an end.
Numerical calculations (Jørgensen & Johnson 1992)
give some support to this theory.
However, exceptions seem to exist: Draco 461 is an intrinsic metal-poor C-star
belonging to the Draco Spheroidal galaxy which shows
(Domínguez et al. 2004). Therefore, if we apply the same reasoning to our sample stars,
we should conclude that they have suffered a limited number of
TDUs.
This might be in contrast with the evidence that also for
metal-poor stars, numerous TDU episodes are required to obtain
s-element enhancements at the level found in the stars analyzed here
(see Table 5).
In any case, this figure merits further
observational studies in a larger sample of metal-poor C-stars
.
Another possible explanation of the low C/O may be a partial conversion of
the C dredged up into 13C and 14N at the bottom of the
convective envelope. Intermediate mass AGBs are known to experience a hot bottom
burning (e.g. Lattanzio & Forestini 1999), but in low mass AGBs the
temperature at the base of the convective envelope is too low to activate
the CN cycle. Some low mass AGB, however, show evidences of a cool bottom process
(Abia et al. 2002; Nollet et al. 2003), i.e. a slow mixing process acting
below the convective envelope that may be capable of reducing the C/O and the
12C/13C ratios. An alternative solution for the low C/O ratio would arise
in the case that our stars were extrinsic C-stars. Abia et al. (2002) showed
that, under different assumptions for the dilution factor the secondary star cannot appear as a carbon
star (
)
if the metallicity exceeds
.
This
also implies that extrinsic C-stars with a metallicity slightly lower
than this limit should have C/O ratios not too much larger than unity, in
agreement with our finding. This is not a proof of the extrinsic
nature of our sample stars, but simply indicates that this hypothesis
is compatible with the measured C/O ratios. We will come back to this point later, but
note that according to the estimations by Abia et al. (2002) (see their Table 6),
this hypothesis requires that the C/O ratio in the primary star at the mass-transfer
epoch should have been larger than about 5 (the exact value depends on the metallicity).
As far as we know, only a few planetary nebulae has already been
observed with such a large C/O ratio.
A second remarkable result, which is related to the C/O ratio,
concerns the 12C/13C ratio. In two of the stars studied here
we find relatively low carbon isotopic ratios (20-40), similar
to those found in many galactic carbon stars of nearly solar
metallicity (see e.g. Abia et al. 2003, and references
therein). The observed 12C/13C ratio in galactic RGB stars of low metallicity
is near 7 (Gilroy & Brown 1991; Gratton et al. 2000). As a consequence of the TDU, the carbon
isotopic ratio is expected to increase during the TP-AGB phase. Then,
the values measured in the case of IGI95-C1 and IGI95-C3, namely about 5 times larger than the typical value found in bright red giant stars,
are compatible with the 12C enhancement implied by their
.
This is also valid in the case of mass transfer and
dilution (extrinsic C-stars).
In contrast to this, the very high carbon isotopic ratio found in
BMB-B 30 (>300)
would require a much higher C/O ratio than the one we derive.
There are no obvious solutions for
this problem, namely how to keep the C/O ratio close to unity without
decreasing the 12C/13C ratio. A possibility would be
the existence of a deep enough convective intershell region during the thermal
pulse. This would mix O (in addition to C) up to the top of the intershell.
Then, the subsequent normal TDU may bring C and O to the surface, reducing
the increase of the C/O ratio. However, there are at least two important consequences
of this. First, because the temperature at the base of the convective TP would be
as large as
K, then one would expect an s-element pattern altered
according to the activation of the 22Ne neutron source. Second, since the TDU of
carbon would be very strong, the 12C/13C ratio should be very large (
).
Leaving apart that this possibility may have also important consequences for the subsequent
evolution of the star, the s-element pattern found in this star (see Tables 4, 5
and Fig. 5)
is at odds with this possibility.
From the previous discussion, it is seen that the C-N-O abundances are key to understanding AGB evolution and nucleosynthesis. Additional observational studies in a large sample of stars are required, preferably with different indicators, such as the CNO-bearing molecular bands in the infrared, which seem to be less prone to line formation problems like saturation or blends.
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Figure 5:
Comparison of the observed [Rb/Sr] ratio vs. [M/H] with the
theoretical predictions for different choices of the 13C-pocket (see text)
for a 1.5 ![]() ![]() |
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![]() |
Figure 6:
Comparison of the observed mean of heavy (Ba, La, Nd and Sm) to
light-mass (Sr, Y and Zr) s-element [hs/ls] enhancement (signature of the
neutron exposure) against metallicity with theoretical prediction for
a 1.5 ![]() |
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The relative abundances of s-elements that follow the 85Kr
reaction branching (Rb, Sr, Zr and Y) along the s-process path
yield information on the neutron density prevailing during the
neutron capture processes occurring in the He intershell (Beer &
Macklin 1989; Lambert et al. 1995; Abia et al. 2001). In particular,
the [Rb/Sr] helps to discriminate
between the two main neutron sources, the
13C
O and 22Ne
Mg, operating
in TP-AGB stars. The former
dominates the s-process nucleosynthesis in low mass AGBs
(
)
for which
is expected.
Figure 5 shows the comparison of the [Rb/Sr] ratios derived
in BMB-B 30 and IGI95-C1 (no detection for IGI95-C3)
with s-process model predictions for a
1.5
AGB star at
(Gallino et al. 1998; Busso
et al. 1999, 2004). The same ratios derived in
galactic N-type C-stars are also shown for comparison. The
lines in Fig. 5
correspond to a specific assumption about the
13C abundance in the 13C-pocket. We show the case
defined as standard (ST) by Gallino et al. (1998), corresponding to
of 13C
. The derived ratios are in good
agreement with the model predictions and are similar to those obtained
for galactic N-type C-stars. Comparison of the derived [Rb/Zr,Y] ratios with
model predictions leads to the same conclusion.
Figure 6 reports the intrinsic index [hs/ls] ratio in our stars
in comparison with model predictions for a 1.5
AGB star at the
beginning of the TP-AGB phase (the same models as Fig. 5).
The solid line
represents the standard choice for the amount of 13C in the 13C-pocket (ST case). Similar
abundance ratios found in N-type galactic C-stars (Abia et al. 2002) are
also plotted. At high (nearly solar) metallicity the
light-s (ls) are slightly overproduced with respect to the Ba
group elements (hs). The [hs/ls] ratio increases with decreasing metallicity
as more neutrons are available per seed. A maximum is reached around
,
a metallicity within the range spanned by our sample of
stars. Then, at very low metallicity (
)
and at the end of the
TP-AGB phase, the [hs/ls] tends toward a constant value, since most of the neutrons
are spent to produce lead (see e.g. Busso et al. 1999).
The three studied stars seem to follow
the variation with the metallicity of the standard choice of the
13C-pocket (ST case). Obviously, the large uncertainty in the
abundance ratios and the scarcity of data at low metallicity prevent us
from concluding whether
there is a preferred average abundance of 13C (or spread)
with a different effect at different metallicities.
To return to the question of the extrinsic or intrinsic nature
of our stars, as it is well known, the detection of Tc is an
undeniable evidence that a star, with s-element enhanced composition,
is currently undergoing TDU. The only Tc isotope with a
sufficiently long life to be observable is indeed 99Tc, whose
decay lifetime is comparable to the time elapsed between two
subsequent TPs in low mass AGB stars. This Tc isotope is produced by
the s-process and it is expected to appear at the surface of a
TP-AGB star undergoing TDU. Unfortunately, our
instrument set-up did not include the adequate spectral regions for
Tc detection. However, an alternative test of the intrinsic nature of a
C-star is provided by the measurement of Nb abundance, a chemical element with
only one stable isotope, 93Nb, which can be produced by
93Zr decay
. The 93Zr lifetime is comparable with the duration of the
whole AGB phase and, thus, the comparison of the abundance of Nb and
that of its neighbors (e.g. Zr, Y) would tell us if the s-process
enhancement is intrinsic or due to mass transfer.
S-process nucleosynthesis calculations
predict that the Zr isotopes and, in particular 93Zr, increase
during the AGB phase, while Nb remains almost unchanged (Straniero et al. 2005). As a consequence,
when the star attains the C-rich star stage,
,
almost
independently of its metallicity. After a few million years, however, as a consequence
of the 93Zr decay, this ratio rises to the scaled solar value.
In other words, for an extrinsic C-star [Nb/Zr] should be
.
From our spectra we were only able to derive upper limits,
,
for all the three studied C-stars. This figure would
hence indicate that they are probably intrinsic C-stars. A better
determination of the Nb abundance is however needed to give a
definitive answer.
Acknowledgements
We warmly thank B. Gustafsson for his careful reading of the manuscript and his valuable suggestions that considerably improved this work. The referee, H. Van Winckel, is thanked for his useful comments. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation. Part of this work was supported by the Spanish grant AYA2002-04094-C03-03 from the MCyT.