Cool stars and their spectra.
Optical and near-IR spectra of O-rich Mira
variables: A comparison between models and
A. Tej, A. Lançon, M. Scholz, P.R. Wood
A&A 412, 481 (2003)
Pulsation models are crucial for the interpretation of spectrophotometric
and interferometric observations of Mira variables. Comparing predicted
and observed spectra is one way of establishing the validity of such
models. In this paper, we focus on the models published between 1996
and 1998 by Bessell, Hofmann, Scholz and Wood. A few new model
spectra are added, to improve available phase coverage. We compare the
synthetic spectra with observed low resolution spectra of optically
selected oxygen-rich Miras, over a range of optical and near-IR
wavelengths that encompasses most of the stellar energy output. We
investigate the overall energy distributions, and specific spectral features
in the near-IR wavelength range. The agreement between the observed
and the model-predicted properties is found to be reasonably good.
However, there are discrepancies seen especially in various molecular
bands. We find that different combinations of stellar parameters and
pulsation phases often result in very similar model spectra. Therefore the
problem of deriving parameters of a Mira variable from its spectrum has
no unique solution. More advanced models than presently available,
providing even better fits to the data and covering a wider range of
parameters, would be needed to achieve better discrimination.
The structure of H2O shells in Mira atmospheres.
Correlation with disk brightness distributions
and a spectrophotometric signature.
A. Tej, A. Lançon, M. Scholz
A&A 401, 347 (2003)
Dynamic models of M-type Mira variables predict the occurrence of
water ``shells", i.e. of zones of high H2O density and high H2O absorption
inside the stellar atmosphere. The density, position and width of these
shells is closely correlated with different types of two-component shapes
of the intensity distribution on the disk in the H, K and L near-continuum
bandpasses. We investigate these correlations and highlight the role of a
spectrophotometric H2O index that warns against serious complications in
diameter measurements in the case of substantial water contamination of
the bandpass of observation. Simultaneous spectrophotometric and
interferometric measurements may allow observers to estimate real
continuum diameters more precisely.
- Spectra of carbon-rich
asymptotic giant branch stars between 0.5 and 2.5 micron:
theory meets observations.
R. Loidl(-Gautschy), A. Lançon, U.G. Jorgensen, 2001
A&A 371, 1065 (2001)
A hydrostatic analysis of five carbon rich stars, BH Cru, T Cae, S Cen,
RU Pup and Y Hya in the wavelength range between 0.5 and 2.5 micron is
presented. All except BH Cru, which is a Mira star, show only modest
variability. We identify the absorption features of the molecules CO, CN
and C2. The overall energy distribution, which is very sensitive to the
effective temperature in the investigated wavelength range, as well as
the bands of these molecules put strict limits on the possible values of
effective temperature and C/O. We show that our model atmospheres and
corresponding synthetic spectra are able to reproduce the observed
spectra quite accurately from about 0.7 to 2.5 mu m. The discrepancies
are mainly due to uncertainties in the molecular input data.
We discuss briefly why the variations of the molecular features are small
and why dynamic phenomena do not play a very important role in this
wavelength range. We identify colour indices based on commonly available
filters and potentially suitable for the empirical determination of
fundamental parameters of carbon stars.
- A Library of 0.5-2.5 micron
Spectra of Luminous Cool Stars.
A. Lançon, P.R. Wood, 2000
Astronomy and Astrophysics Suppl. Ser. 146, 217
This paper presents a library of digital spectra of cool, mostly
variable, giant and supergiant stars, which should be useful for spectral
synthesis applications as well as for comparison with theoretical model
atmosphere calculations. The library includes Miras and semi-regular
asymptotic giant branch variables, carbon stars, Galactic
Bulge and Magellanic Cloud stars, and OH/IR stars.
The stars observed cover a range of metallicities and
masses. Most variable objects were observed several times, at different phases
of their pulsation cycle. We discuss the effects of various fundamental
parameters and of variability on the spectra.
Based on period-luminosity relations and stellar evolutionary tracks,
we tentatively assign initial masses to each object.
- Optical and near-infrared
spectrophotometric properties of Long Period Variables and
other luminous red stars
R. Alvarez, A. Lançon, B. Plez, P.R. Wood, 2000
Astronomy and Astrophysics 353, 322
This paper presents selected results of a systematic study of the
spectrophotometric properties of static and variable cool stars.
Indices that discriminate between surface temperatures, luminosity
classes, surface chemistries and variability subtypes are discussed.
Comments are also made on extinction properties, and the
strengths of molecular bands.
- The Near-IR/Optical
spectra of AGB stars and their variations
A. Lançon, P.R. Wood
Poster proceedings of IAU Symposium 189 on Fundamental
Stellar Properties (January 1997), Editor: T.R. Bedding,
Publisher: School of Physics, Univ. of Sydney
This poster paper presents first results of an extensive observing campaign undertaken by the authors in order to obtain reference data both for population synthesis and for calibration of stellar atmosphere and pulsation models.
High quality spectra were obtained that cover the optical and near-IR range (0.5-2.5 microns)
quasi-simultaneously. The near-IR spectra were taken with the infrared
spectrograph and imager CASPIR, on the 2.3m telescope at Siding Spring.
Individual stars are observed at various phases in their pulsation cycle.
- Suivi spectroscopique d'étoiles
variables de la branche asymptotique et applications \`a la
synth\`ese de populations
Journées Miras, Montpellier, Octobre 1997