Contents of: VI/111/./abstract/SDEGUCHI_DEGUCHI4.abs

The following document lists the file abstract/SDEGUCHI_DEGUCHI4.abs from catalogue VI/111.
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 The Asymptotic-Giant-Branch (AGB) stars in the Galactic halo occupy
a special status from a view point of Galactic structure. The
halo stars are supposed to be metal poor. They belong to old
population and are supposed to be formed about 15 Gyr ago, i.e.,
at the time of formation of our Galaxy. A recent discovery of MACHOs
(Massive Compact Halo Objects) by gravitational lens experiments brought
myriads of questions on a formation of low mass stars and a dark
matter in our Galaxy. For the future survey and identification of
MACHOs in various frequencies, especially in the mid- and far-infrared
wavelengths, the AGB stars in the halo must be potential contaminators
to the infared spectra of MACHOs. Because the surface temperature of
the MACHOs, about 300-1000 K, is very similar to the temperature of
dust envelopes of AGB stars, infrared radiation from more luminous
AGB stars would hide the light of MACHOs. For the identification of
MACHOs, it is very important to have reasonably high-resolution spectra
of halo AGB stars. The difference of the molecular-band spectra between
MACHOs and AGB stars would be used for the discrimination. For this
purpose we have selected about 20 AGB stars at high galactic latitudes
with the distance from the galactic plane larger than 5 kpc. These
stars have IRAS 12 micron intensities of about a few Jy. The infrared
spectra of these stars can be used as templates of halo AGB stars.
The identification of oxygen-rich or carbon-rich stars can be made
based on 10 and 18 micron features (for o-rich stars) and 11 micron
feature (for c-rich stars). One of interesting aspects of this
spectral survey of halo AGB stars is a determination of the number
ratio of c- to o-rich stars in the halo. Our past SiO maser
survey at 43 GHz for the Galactic disk and bulge IRAS sources showed
that the ratio of C- to O-rich stars increases with the distance
from the galactic center. We intend to check this ratio also with
observing the spectra of halo AGB stars with ISO SWS.

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