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Near- and mid-IR studies of galaxies in or near the Local Group
The availability of sensitive arrays of detectors for the near-infrared (1 to 3 micron) and mid-infrared (10 to 20 micron) attached to favorably located telescopes on the ground and the availability of space observatories (ISO, Spitzer/SIRTF) for detections at wavelengths between 20 and 40 $\mu$m, has made it possible to detect cool stars, especially RGB and AGB stars plus a few red supergiants in all galaxies of the Local Group, and even in galaxies beyond that. Large-scale surveys (DENIS, 2MASS, ISOGAL, MSX) have covered the Milky Way Galaxy (MWG) and make it possible to tie the properties of the AGB population in other galaxies to those in the MWG population. Apart from the interest in these objects themselves studies of RGB and AGB stars give information on the stellar population of these galaxies beyond what is detectable at visual wavelengths. Studies in the infrared will improve significantly our understanding of the star formation history of these galaxies over look-back times between 1 and 10 Gyr, a time that is inadequately covered by the luminous blue stars or by the very old stars (e.g. RR Lyrae) that can be detected at visual wavelengths. Because the AGB stars divide into stars with a carbon-dominated chemistry and with an oxygen-dominated chemistry and because this division depends on the abundances of heavy elements (Ca, Fe), we may expect to derive an estimate of these abundances from the properties of AGB stars.
Consider first the observations. Luminous AGB stars (i.e. infrared stars with a luminosity exceeding that of the tip of the Red Giant Branch) have been detected in almost all Local Group galaxies, and the hunt for such stars is still continuing. But the mere detection of AGB stars is not enough when one needs measurement of such fundamental properties as luminosity, age and elemental abundances. Follow-up studies are needed. Three observable properties are especially important:
(i) the total luminosity,
(ii) the possible variability (and if the stars are variable: the period and the amplitude of the variations) and
(iii) whether the stars have carbon-dominated or oxygen-dominated atmospheres.
These properties are often not measured well-enough in the detection surveys and then there is a need for follow-up studies. On the theoretical side, the stellar models appear to have advanced as far as could reasonably be expected and now include the detailed account of atomic abundances on e.g. the stellar atmosphere, on nuclear processes and on the dredge-up that follows these processes. The models of these stars now also include the superwind that curtails the existence of the stars. The availability of ''synthetic models'' that allow to interpolate between a smaller number of fully calculated models, is a major step forward. All this also implies that the comparison of models and observations have to be made in a more and more sophisticated manner.