Far-Ultraviolet Continuum of G-Type Stars: A Signature of the Temperature Minimum Region

Abstract

The main results of a program of systematic comparison between observed and computed UV spectral energy distributions of field G-type stars are illustrated. We constructed the UV observed energy distributions for 53 G stars, starting from the IUE Uniform Low Dispersion Archive (ULDA) and computed the corresponding theoretical fluxes by using the atmospheric parameters from the Catalogue of [Fe/H] Determinations (1996 edition) and a Kurucz grid of model fluxes. From the comparison between observations and classical models, a UV excess shortward of 2000 Å is evident for all the program stars. The UV continuum in the region 1600-2000 Å can be described by synthetic fluxes computed from semiempirical models based on the temperature minimum concept. Values for the T_min/T_eff ratio on the order of 0.80 are suitable for the interpretation of the observed fluxes. The residual discrepancies shortward of 1600 Å are suggested to be effects of the chromosphere, on the basis of a comparison with the Maltby et al. semiempirical model of the Sun.

Based on data dearchived from ULDA at the Italian National Centre and on observations made with the NASA/ESA Hubble Space Telescope, obtained from the data archive at the Space Telescope Science Institute. STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.