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

Abstract

We compute a sequence of five model atmospheres consisting of the photosphere, chromosphere, and transition region. The models represent the response of the gas in a magnetically confined loop to intense flare energy release. We assume that the energy release is confined to the corona, and include the effects of chromospheric evaporation and indirect heating of the lower atmosphere by X-rays emitted from the coronal plasma. The models are computed in hydrostatic and energetic equilibrium and incorporate a detailed non-LTE solution of the radiative transfer and statistical equilibrium equations for a 6 level plus continuum hydrogen atom, a 5 level plus continuum Ca n ion, and a 3 level plus continuum Mg n ion. Complete tables of the depth-dependent model atmospheres are included in the Appendix. Line and continuum surface fluxes are presented in the wavelength range 1000-9000 Á and are compared with those observed during a giant flare on the M dwarf star AD Leo. Our conclusions are the following : 1. The structure of the flare transition region is consistent with conductive heating balancing optically thin cooling; we also find that some UV line fluxes (e.g., N v, C rv) can be used as a transition-region "pressure gauge" and can provide a constraint on the flare area. 2. Our models predict ratios of Ca n to hydrogen emission which are much greater than those observed; they also predict Balmer line profiles which are much narrower than those observed. This suggests that additional heating is taking place in the upper chromosphere beyond that assumed in the models. 3. The observed flare continuum is much bluer than that computed from the models; the observations fit a blackbody spectrum with T-8500-9500 K. We propose that the flare continuum is formed by photospheric reprocessing of intense ultraviolet to extreme ultraviolet (EUV) line emission from the upper chromosphere. We suggest that if the UV/EUV line emission is formed in response to the deposition of a large flux of nonthermal electrons, the continuum luminosity and color temperature can be used to determine both the energy flux and the flare area being bombarded by energetic electrons. The same reprocessing mechanism may be responsible for some solar "white fight" flares. 4. We use the Ca n and H7 fine fluxes from our chromospheric models to estimate the coronal evolution (temperature and emission measure) in the AD Leo flare. When we compare the result with the coronal evolution predicted from the loop evolution model of Fisher and Hawley, we find good agreement during the first half of the flare but poor agreement toward the end of the flare. This fact, coupled with the large discrepancy between the coverage factors for the fine and the continuum emission, suggests to us that the AD Leo flare evolves in a similar fashion to a solar two-ribbon flare; thus, it is not possible to describe all aspects of the flare using only a single evolving loop.