2,105 research outputs found
Near-infrared spectroscopy of AGB star candidates in Fornax, Sculptor and NGC 6822
Context: The Asymptotic Giant Branch (AGB) phase is characterised by
substantial mass loss that is accompanied by the formation of dust. In extreme
cases this will make the star no longer visible in the optical. For a better
understanding of AGB evolution it is important to identify and characterise
these very red AGB stars.
Aims: The first aim of this article is to improve the census of red AGB stars
in three Local Group galaxies, based on near-IR spectroscopic observations of
new candidates with red 2MASS (J-K) colours. The opportunity is taken to
compare the near-IR spectra with those of Milky Way stars.
Methods: We used ISAAC on the ESO VLT to take J and H-band spectra of 36
targets in Fornax, Sculptor and NGC 6822.
Results: Twelve new C-stars are found in Fornax, and one is confirmed in
Sculptor. All C-stars have (J-K) > 1.6, and are brighter than -3.55 in
bolometric magnitude. Ten new oxygen-rich late-type giant stars are identified
in Fornax, but none is extremely red or very luminous. Five luminous O-rich AGB
stars are identified in NGC 6822, of which 3 show water absorption, indicative
of spectral type M. Again, none is as red as Milky Way OH/IR stars, but in this
galaxy the list of candidate AGB stars is biased against very red objects. In
some C-stars with (J-K)>2 an extremely strong 1.53 m absorption band is
found. These stars are probably all Mira variables and the feature is related
to the low temperature, high density chemistry that is a first step towards
dust formation and mass loss.Comment: A&A accepte
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Optical and near-infrared spectrophotometric properties of Long Period Variables and other luminous red stars
Based on a new and large sample of optical and near-infrared spectra obtained
at the Mount Stromlo and Siding Spring Observatories (Lancon & Wood 1998;
Lancon & Wood, in preparation), spectrophotometric properties of cool oxygen-
and carbon-rich Long Period Variables and supergiants are presented.
Temperatures of oxygen-rich stars are assigned by comparison with synthetic
spectra computed from up-to-date oxygen-rich model atmosphere grids. The
existence of reliable optical and near-infrared temperature indicators is
investigated. A narrow relation between the bolometric correction BC(I) and the
broad-band colour I-J is obtained for oxygen-rich cool stars. The ability of
specific near-infrared indices to separate luminosity classes, atmospheric
chemistry or variability subtypes is discussed. Some comments are also given on
extinction effects, water band strengths in Long Period Variables and the
evaluation of 12CO/13CO ratio in red giants.Comment: 14 pages, 21 figures, Latex, accepted for publication in Astronomy
and Astrophysics main journal. Also available at
http://www-astro.ulb.ac.be/~ralvarez
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