This paper presents a library of over hundred
m digital spectra of cool, mostly
variable, giant and supergiant stars.
The data set includes multiple observations
of individual variable stars. The wavelength range covered contains most of the
flux emitted by the sample stars. The spectral resolution in the near-IR is sufficient to
identify molecular band substructures and the strongest individual metal lines.
The data clearly
have many applications in the field of purely stellar physics, but in the context
of increasingly detailed studies of external galaxies the importance of this
library for extragalactic work has to be highlighted.
One main aim of this paper is to provide a useful library of spectra of variable AGB stars
for stellar population synthesis. AGB stars are the dominant contributors to the
near-infrared light of an intermediate-age stellar population and
a number of studies have been made of the contribution of AGB stars
to broadband colours of stellar populations (Charlot & Bruzual 1991;
Bressan et al. 1994; Maraston 1998; Lançon 1999;
Lançon et al. 1999).
The importance of the AGB stars to the near infrared light output
of a stellar population is clearly demonstrated by the sudden jump
in the V-K colour from 1 to
3 for a stellar population as
it ages to the point where AGB stars form (Girardi & Bertelli 1998).
Furthermore, at K, the thermally pulsing AGB (TPAGB)
stars alone may contribute up to
80% of the population light,
with the early AGB and other stars contributing the rest.
Observations showing that all TPAGB stars are
variable have recently become very convincing (Eyer & Grenon 1997;
Wood et al. 1999).
Thus the stellar spectra of intermediate-age populations should be
dominated at K by variable AGB stars. Of course, the contribution of
AGB stars to the integrated spectrum of a stellar population decreases
with decreasing wavelength so that by R the contribution of AGB stars
is small.
Among variable AGB stars, those with periods below 500 days provide the most significant contributions to the bolometric and to the near-IR luminosity of a stellar population. Indeed, stars with longer periods have very low effective temperatures and are usually reddened by circumstellar dust, two effects that combine to reduce even the near-IR contribution (Bressan et al. 1998, see also Girardi & Bertelli 1998). The sample of variable stars we have chosen therefore mostly contains optically visible stars with periods no longer than a few 100 days, although a small number of OH/IR stars have also been included. Carbon stars were also observed, because they represent a large fraction of the AGB stars in metal deficient environments (e.g. the Magellanic Clouds) and in various plausible synthetic populations (Marigo et al. 1999; Mouhcine et al., in preparation). Overall, an effort was made to sample the three-dimensional parameter space of spectral type, period and amplitude (Fig. 1), based on the information provided in the General Catalogue of Variable Stars (GCVS, Kholopov et al. 1988).
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Figure 1: Period - amplitude distribution of the variable stars in the sample. Crosses and squares, respectively, represent oxygen and carbon rich objects. Stars with unknown periods or amplitudes (among which are our coolest objects) have not been plotted. The majority of the programme stars are miras and semi-regular variables of the solar neighbourhood. In addition, a small sample of Magellanic Cloud stars and Galactic Bulge stars were included. The Magellanic Cloud LPVs were selected from the brighter O-rich sources of the sample of Wood et al. (1983). The Bulge objects were selected among the SgrI and NGC6522 field LPVs of Wood & Bessell (1983), with the additional criterion that they be red (J-K>2) because such extreme colours were interpreted by the authors as the result of high metallicity. Finally, for comparison purposes a few giants and red supergiants with insignificant variations were included |
Stellar and extragalactic studies are not disconnected. Models designed to predict synthetic spectra for pulsating red giants are progressing rapidly (Bessell et al. 1989b; Hofmann et al. 1998; Höfner et al. 1998; Alvarez & Plez 1998; Loidl et al. 1999; Hauschildt et al. 1999), but are still in their infancy. They are needed as a link between the empirical spectra and the theoretical evolutionary tracks along the AGB. The latter provide the evolution of the global properties (luminosity, effective temperature, surface abundances) of the static so-called parent stars of Mira variables but it is poorly known, for instance, how much lower the effective temperatures become once pulsation has been allowed for. The concept of effective temperature itself becomes unclear due to the extreme extension of the atmospheres of Mira stars, and the relation between photometric/spectroscopic properties and the theoretical effective temperature remains to be understood and calibrated. Comparison of the model predictions with the data in this paper will assess the current reliability of the models. Two features of the present library make it particularly valuable for this comparison: the broad wavelength coverage, which makes the simultaneous study of the global energy distribution and the spectral features possible, and the inclusion of sequences of observations of individual stars at various phases over one or a few pulsation cycles.
The data are described in Sect. 2. Section 3 discusses a selection of properties of the sample of spectra, including effects of variability and of metallicity. Stars of particular interest, deserving further study, are identified in that section and in the Appendix. In Sect. 4, we discuss the evolutionary status of the observed objects, a piece of information relevant both to population synthesis applications and to the comparison of the spectra with stellar model predictions.
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