Radiative Hydrodynamic Models of Optical and Ultraviolet Emission from M Dwarf Flares

Abstract

We report on radiative hydrodynamic simulations of M dwarf stellar flares and compare the model predictions to observations of several flares. The flares were simulated by calculating the hydrodynamic response of a model M dwarf atmosphere to a beam of non-thermal electrons. Radiative backwarming through numerous soft X-ray, extreme ultraviolet, and ultraviolet transitions are also included. The equations of radiative transfer and statistical equilibrium are treated in non-LTE for many transitions of hydrogen, helium and the Ca II ion allowing the calculation of detailed line profiles and continuum radiation. Two simulations were carried out, with electron beam fluxes corresponding to moderate and strong beam heating. In both cases we find the dynamics can be naturally divided into two phases: an initial gentle phase in which hydrogen and helium radiate away much of the beam energy, and an explosive phase characterized by large hydrodynamic waves. During the initial phase, lower chromospheric material is evaporated into higher regions of the atmosphere causing many lines and continua to brighten dramatically. The He II 304 line is especially enhanced, becoming the brightest line in the flaring spectrum. The hydrogen Balmer lines also become much brighter and show very broad line widths, in agreement with observations. We compare our predicted Balmer decrements to decrements calculated for several flare observations and find the predictions to be in general agreement with the observations. During the explosive phase both condensation and evaporation waves are produced. The moderate flare simulation predicts a peak evaporation wave of ∼130 km s −1 and a condensation wave of ∼30 km s −1 . The velocity of the condensation wave matches velocities observed in several transition region lines. The optical continuum also greatly intensifies, reaching a peak increase of 130% (at 6000Å) for the strong flare, but does not match observed white light spectra.

A multi-wavelength view of AB Doradus outer atmosphere

“…Asymmetric Stark profiles are expected for moving plasmas, but would have to be calculated with a full 3D hydrodynamic and radiative transfer code, as done by Allred et al (2006) but is beyond the scope of this work. To obtain estimates of the velocities of the plasma movements a Gaussian approximation should be sufficient.…”

Characterizing the Near‐UV Environment of M Dwarfs

“…We compare the near-UV observational data with predictions from the radiative hydrodynamic models of Allred et al (2005Allred et al ( , 2006. The models use solutions to the one-dimensional (1D) equations of radiative hydrodynamics, including non-LTE radiative transfer in H, He, and Ca ii, with flare heating provided by an electron beam.…”

“…A slight offset exists between their fit and the dMe stars in our sample ( filled circles), as Mathioudakis & Doyle (1989) used X-ray fluxes from Einstein and EXOSAT, while we compare with ROSAT, but otherwise our results are in excellent agreement with the fit. used here represents an average midflare atmosphere near a time step of $85 s; see Allred et al 2006). In addition, we used two other preflare models, scaled to be either cooler or hotter at a given column mass than the Allred preflare atmosphere.…”

“…Thus, including Mg and Fe in non-LTE is likely to make little difference in the overall energy balance of the atmosphere. However, since Allred et al (2006) did not predict the Mg ii or Fe ii line profiles that comprise the majority of emission lines in our observed wavelength range, we analyzed our atmosphere models with the ''RH'' non-LTE radiative transfer code described in Uitenbroek (2001). The RH code is based on the multilevel accelerated lambda ( MALI ) formalism of Rybicki & Hummer (1991, 1992, which allows both bound-bound and bound-free radiative transitions to overlap in wavelength.…”

Multi-wavelength observations of a giant flare on CN Leonis

“…coupled radiative and hydro-dynamical models of solar/stellar flares are lacking, although some steps have already been performed in this direction (Allred et al 2005(Allred et al , 2006. Another simpler approach to a first understanding of the flare phenomenon is to use 1D chromospheric models with a semi-empirical or parameterised temperature rise.…”