Issue |
A&A
Volume 393, Number 1, October I 2002
|
|
---|---|---|
Page(s) | 167 - 181 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361:20020585 | |
Published online | 18 September 2002 |
The modelling of intermediate-age stellar populations
II. Average spectra for upper AGB stars, and their use
Observatoire Astronomique, Université L. Pasteur & CNRS: UMR 7550, 11 rue de l'Université, 67000 Strasbourg, France
Corresponding author: A. Lançon, lancon@astro.u-strasbg.fr
Received:
19
June
2001
Accepted:
15
April
2002
The upper Asymptotic Giant Branch (AGB) is populated with oxygen rich and
carbon rich Long Period Variables (LPVs). These stars are essential contributors
to the near-IR light of intermediate age stellar populations.
Individual observed spectra of LPVs are so diverse that they cannot be used
directly in the synthesis of galaxy spectra. In this paper, the library of
individual spectra of Lançon & Wood ([CITE]) is used to construct averages
that can be incorporated conveniently in population synthesis
work. The connection between such spectra and stellar evolution tracks
is discussed.
In order to select a sorting criterion and to define averaging bins for the
LPV spectra, correlations between their spectrophotometric properties
are reexamined. While optical properties and
broad baseline colours such as () are well correlated,
a large dispersion is observed when these indices are plotted
against near-IR ones. This is partly due to the intrinsic width
of the upper AGB, which is illustrated by locating each of the multiple
observations of individual LPVs on the HR diagram. It is argued that
broad baseline colour-temperatures are the most sensible sorting
criteria. The properties of the resulting sequence of average
spectra indeed vary regularly.
We further address: (i) the bolometric corrections and temperature
scales needed to associate a spectrum with a given point on
a theoretical stellar evolution track (or isochrone), (ii) the
simplifying assumptions that will be implicitely made when using
the average spectra, (iii) potential biases in the sample
of Lançon & Wood and their effects, (iv) the small contribution
of LPVs to the interstellar hydrogen emission lines in galaxies. It
is emphasized that an a posteriori calibration of the effective
temperature scale remains necessary, until consistent models for the
evolution, the pulsation and the spectral appearance of LPVs become available.
We suggest a recipe for the use of the average spectra at various metallicities.
Key words: stars: AGB and post-AGB / stars: late-type / stars: variable: general / Hertzsprung-Russell diagram / infrared: stars / galaxies: stellar content
© ESO, 2002
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