Issue |
A&A
Volume 425, Number 2, October II 2004
|
|
---|---|---|
Page(s) | 489 - 508 | |
Section | Galactic structure, stellar clusters, and populations | |
DOI | https://doi.org/10.1051/0004-6361:20040312 | |
Published online | 21 September 2004 |
A search for late-type supergiants in the inner regions of the Milky Way *,**
1
European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany e-mail: fcomeron@eso.org
2
Departament d'Astronomia i Meteorologia, Universitat de Barcelona, Av. Diagonal 647, 08028 Barcelona, Spain e-mail: jordi@am.ub.es
3
Osservatorio Astronomico di Trieste, Via G.B. Tiepolo 11, 34131 Trieste, Italy e-mail: chiappin@ts.astro.it
4
European Southern Observatory, Alonso de Cordova 3107, Santiago 19, Chile e-mail: vivanov@eso.org
Received:
21
February
2004
Accepted:
2
June
2004
We present the results of a narrow-band infrared imaging
survey of a narrow strip ( wide) around the Galactic equator
between
and
of galactic longitude aimed at
detecting field stars with strong CO absorption, mainly late-type
giants and supergiants. Our observations include follow-up low
resolution spectroscopy (
) of 191 selected candidates in
the H and K bands. Most of these objects have photometric and
spectroscopic characteristics consistent with their being red
giants, and some display broad, strong absorption wings due to
water vapor absorption between the H and K bands. We also
identify in our sample 18 good supergiant candidates characterized
by their lack of noticeable water absorption, strong CO bands in
the H and K windows, and HKS photometry suggestive of high
intrinsic luminosity and extinction reaching up to
mag. Another 9 candidates share the same features except for
weak H2O absorption, which is also observed among some M
supergiants in the solar neighbourhood. Interesting differences
are noticed when comparing our stars with a local sample of
late-type giants and supergiants, as well as with a sample of red
giants in globular clusters of moderately subsolar metallicity and
to a sample of bulge stars. A large fraction of the stars in our
sample have CaI and NaI features markedly stronger than those
typical in the local reference sample (both giants and
supergiants), whereas the equivalent widths of the CO bands are
similar or weaker. In this regard, our stars in the inner Milky
Way disk display differences very similar to those identified by
other authors between cool giants and supergiants near the
galactic center and their counterparts in the solar neighbourhood.
We propose that the systematic spectroscopic differences of our
inner Galaxy stars are due to their higher metallicities that
cause deeper mixing in their mantles, resulting in lower surface
abundances of C and O and higher abundances of CN, which
contribute to the strength of the CaI and NaI features at low
resolution. Our results stress the limitations of using local
stars as templates for the study of composite cool stellar
populations such as central starbursts in galaxies.
Key words: stars: late-type / stars: supergiants / Galaxy: abundances / Galaxy: disk / Galaxy: stellar contents / Galaxy: structure
© ESO, 2004
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