The energy distribution of 21 Comae Berenices (A3p): comparison to standard stars of similar spectral types and modeling.

Abstract

The energy distribution (ED) of 21 Com is constructed from 1200 to 22000 Å using UV and optical spectrophotometry and recent IR photometry (Fernley 1990). The comparison of the ED with those of standard A3 stars confirms the A3 spectral type ascribed to 21 Com by Cowley et al. (1969) and reveals many similarities with the A3 IV standard τ Vir. The effective temperature derived using the Infrared Flux Method (Blackwell and Shallis 1977) is 8830±200K, in agreement with Stepien and Dominiczak (1989). A radius close to 2.5 R_sun is derived from the angular diameter and the distance to the Coma cluster. Model atmospheres have been calculated using Kurucz's ATLAS8 for a grid of effective temperatures around 8850K, surface gravities from 3.5 to 4.5 and three different chemical compositions. The best fit to the entire ED, searched using a χ²-technique, is not unique: the models of parameters (Teff = 8700K, log g = 4.0) and (Teff = 8700K, log g = 3.75), both having 3 times the solar abundances minimize the χ². None of these models reproduce the far-UV energy distribution. To improve the fits in the far-UV, the authors have introduced the continuous opacity of Fe I in ATLAS8. The effect of this absorber remains however modest for the overabundance considered here. The effective temperature of 21 Com is very similar to those of HD 102647 and HD 216956, the two A3V standards. Its surface gravity, not properly constrained by the fitting of the energy distribution, should be derived independently.