Continuum and line emission of flares on red dwarf stars

Abstract

The emission spectrum has been calculated of a homogeneous pure hydrogen layer, which parameters are typical for a flare on a red dwarf. The ionization and excitation states were determined by the solution of steady-state equations taking into account the continuum and all discrete hydrogen levels. We consider the following elementary processes: electron-impact transitions, spontaneous and induced radiative transitions, and ionization by the bremsstrahlung and recombination radiation of the layer itself. The BibermanHolstein approximation was used to calculate the scattering of line radiation. Asymptotic formulae for the escape probability are obtained for a symmetric line profile taking into account the Stark and Doppler effects. The approximation for the core of the H−α line by a gaussian curve has been substantiated.The spectral intensity of the continuous spectrum, the intensity of the lines of the Balmer series and the magnitude of the Balmer jump have been calculated. The conditions have been determined for which the Balmer jump and the emission line intensities above the continuum decrease to such low values that the emission spectrum can be assumed to be continuum as well as the conditions at which the emission spectrum becomes close to the blackbody.

Origin of the Blue Continuum Radiation in the Flare Spectra of dMe Stars

“…The corresponding layer thickness, L, was introduced according to this condition (p. 2 and Eq. (53) in [9]). The transition from a transparent gas to a gas whose continuum emission is close to the Planck function was, however, subsequently examined for the layer of a fixed thickness L (see the first paragraph in section 7 in [9]).…”

“…(53) in [9]). The transition from a transparent gas to a gas whose continuum emission is close to the Planck function was, however, subsequently examined for the layer of a fixed thickness L (see the first paragraph in section 7 in [9]). We have also assumed (the last paragraph in section 8) that radiative cooling of the partially ionized gas heated at the front of a stationary shock wave can create a zone that is responsible for the blue component of the optical continuum of stellar flares.…”

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Calculations of the emission spectrum of a homogeneous plane layer of pure hydrogen plasma taking into account nonlinear effects (the influence of bremsstrahlung and recombination radiation of the layer itself on its Menzel factors) [9] show that the blue component of the optical continuum during the impulsive phase of large flares on dMe stars originates from the near-photospheric layers [1]. The gas behind the front of a stationary radiative shock wave propagating in the red dwarf chromosphere toward the photosphere is not capable of generating the blackbody radiation observed at the maximum brightness of the flares.

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“…In [9] the emission spectrum of a two-temperature (6 eV ≤ T ai ≤ 12 eV, 0.8 eV ≤ T e ≤ 1.5 eV) motionless homogeneous plane layer of pure hydrogen plasma (3 · 10 14 cm −3 ≤ N H ≤ 3 · 10 16 cm −3 ) has been calculated taking into account the influence of bremsstrahlung and recombination radiation of the layer itself on its Menzel factors. This layer is located behind the front of a stationary plane-parallel radiative shock wave (we use a frame of reference in which the shock front is at rest).…”

“…This layer is located behind the front of a stationary plane-parallel radiative shock wave (we use a frame of reference in which the shock front is at rest). In [9] the value of N H at which the intensity of the continuum spectrum approaches the Planck function was determined.…”