MODEL ATMOSPHERES OF COOL, LOW-METALLICITY STARS - THE IMPORTANCE OF COLLISION-INDUCED ABSORPTION

We have extended our data base of collision induced absorption (CIA) i n the high-temperature regime applicable to stellar atmospheres. Impro vements of existing data include computation of series of hot-bands an d extension of the spectral wings much further away from the maxima th an hitherto. Computation of new bands includes the first and the secon d overtone bands of H-2-H-2. We apply these data to an extensive grid of oxygen-rich model atmospheres with scaled solar metallicities in or der to investigate for which range of fundamental stellar parameters ( i.e., effective temperature, gravity, and chemical composition) CIA ha s a significant impact on the atmospheric structure. Besides the CIA d ue to H-2-H-2 and H-2-He pairs, our models include complete molecular line data for TiO, H2O, CN, CH, and SiO. For stellar models with low e ffective temperatures, high gravity, and low metallicity, the atmosphe ric structure and the emergent spectrum are completely dominated by th e effects of CIA. For our test-models of lowest effective temperature and lowest metallicity (T-eff = 2800 K and Z = 10(-3) Z.) the effect o f CLA is pronounced even for sub-giant stars with log(g) = 2.0. For dw arf models with Z = 10(-3) Z. and log(g) = 5.0 the effects are visible in the overall flux distribution for effective temperatures as high a s 4000 K, and for models with T-eff = 2800 K and log(g) = 5.0 CIA has effects on the spectrum of stars with metallicities as high as 0.1 Z..