COOL STARS - SPECTRAL ENERGY-DISTRIBUTIONS AND MODEL ATMOSPHERE FLUXES

The main results of a program of systematic comparison between observe d and computed spectral energy distributions of late G and K type sola r chemical composition stars are illustrated. We built the observed en ergy distributions of eleven representative stars, from the ultraviole t to the infrared, starting from IUE archive data, supplemented by our own IUE observations, and data from the literature. Broad-band Johnso n and DDO color indices, together with suitable calibrations, were use d for estimating the basic stellar atmospheric parameters, i.e., effec tive temperature and surface gravity. Theoretical fluxes were computed for the appropriate parameters of each star, starting from a recent g rid of atmosphere models. While very good consistency between data and model predictions has been obtained in the optical and infrared, sign ificant discrepancies were found in the ultraviolet. We present semi-e mpirical models, based on the minimum temperature concept, obtained by modifying the temperature (versus optical depth) structure so as to m imic the effect of non-radiative heating in the upper photospheres of cool stars. The fluxes predicted by these models provide a fairly good description of the data not only in the visible and IR regions, but a lso in the ultraviolet region. Our results indicate that different val ues for the T(min)/T(eff) ratio, ranging from 0.76 (as in the Sun) up to 0.85 (theta Boo), are required to reproduce the observed ultraviole t fluxes of different stars.