X-Ray--heated Models of Stellar Flare Atmospheres: Theory and Comparison with Observations

Abstract

A sequence of five model atmospheres consisting of the photosphere, chromosphere, and transition region is computed. The models represent the response of the gas in a magnetically confined loop to intense flare energy release. It is concluded that the structure of the flare transition region is consistent with the conductive heating balancing optically thin cooling; some UV line fluxes can be used as a transition-region 'pressure gauge' and can provide a constraint on the flare area. These models predict ratios of Ca II to hydrogen emission which are much greater than those observed; they also predict Balmer line profiles which are much narrower than those observed. It is inferred that additional heating is taking place in the upper chromosphere beyond that assumed in the models. The observed flare continuum is much bluer than that computed from the models; the observations fit a blackbody spectrum with T approximately equal to 8500-9500 K. It is proposed that the flare continuum is formed by photospheric reprocessing of intense UV to the EUV line emission from the upper chromosphere.