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.