Cool stars : spectral energy distributions and model atmosphere fluxes.

Abstract

The main results of a program of systematic comparison between observed and computed spectral energy distributions of late G and K type solar chemical composition stars are illustrated. We built the observed energy distributions of eleven representative stars, from the ultraviolet to the infrared, starting from lUE archive data, supplemented by our own lUE observations, and data from the literature. Broad-band Johnson and DDO color indices, together with suitable calibrations, were used for estimating the basic stellar atmospheric parameters, i.e., effective temperature and surface gravity. Theoretical fluxes were computed for the appropriate parameters of each star, starting from a recent grid of atmosphere models.

While very good consistency between data and model predictions has been obtained in the optical and infrared, significant discrepancies were found in the ultraviolet.

We present semi-empirical models, based on the minimum temperature concept, obtained by modifying the temperature (versus optical depth) structure so as to mimic 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 also in the ultraviolet region. Our results indicate that different values for the T_min/T_eff ratio, ranging from 0.76 (as in the Sun) up to 0.85 (θ Boo), are required to reproduce the observed ultraviolet fluxes of different stars.