Origin of the optical continuum of flares of red dwarfs

Katsova, M. M.; Kosovichev, A. G.; Livshits, M. A.

doi:10.1007/bf01005196

Cited by 11 publications

(60 citation statements)

References 7 publications

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“…The first calculations of explosive evaporation in the atmosphere of red dwarfs were carried out by Livshits et al (1981) and Katsova et al (1981). Their main conclusion was that the low-temperature condensation formed in this process during large flares should emit in optical continuum.…”

Section: Accelerated Particles In Impulsive Solar Flaresmentioning

confidence: 99%

Physical Processes During Impulsive Solar and Stellar Flares

Katsova

Livshits

1995

International Astronomical Union Colloquium

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Investigations of impulsive flares on both the Sun and red dwarf stais during more than 30 years allow us to arrive at quite definite conclusions. Here we will consider impulsive events; on the Sun the impulsive phase of a flare is observed as a hard X-ray burst with the emission of photons with energies E > 30 keV up to the γ-ray range. At the same time microwave radio bursts, and sometimes UV and optical continuum bursts are registered. Typical durations of these processes are ∼l-3 min. In this time interval other kinds of flare emission like soft X-ray (2-10 keV) emission, meter radio bursts and Balmer line emission begin to rise, but their maxima occur later on, in the gradual (thermal) phase of the flare.Impulsive stellar flares are often observed as a significant increase in optical continuum, especially in the U-band, of similar duration (1-3 min), and this time interval is, like in the solar case, the rise phase of the soft X-ray emission.Modern observations demonstrate that both the impulsive phase of a flare or an impulsive flare develops in low-lying loops. Earlier only indirect evidence existed in optical and radio data. Recently, however, the heights of the hard X-ray sources in impulsive solar events were determined directly from YOHKOH’s HXT (Kosugi 1994, Masuda 1994) (Fig. 1a). Statistically, the height of the hard X-ray source in the 14-23 keV range is 9700 ± 2000 km above the photosphere, and this height reduces to 6500 km in the 53-93 keV range. Besides two hard X-ray sources in the loop footpoints, a third hard X-ray source exists at the top of the loop at least in some cases. The authors of this experiment suppose that the appearance of this loop-top source is due to reconnection in the impulsive phase. Note that the reconnection begins close to the apex of the loop, when this loop is filled by hot plasma that evaporated from both footpoints.

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Section: Accelerated Particles In Impulsive Solar Flaresmentioning

confidence: 99%

Physical Processes During Impulsive Solar and Stellar Flares

Katsova

Livshits

1995

International Astronomical Union Colloquium

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“…This model was applied to the problem of flares on red dwarf stars in Katsova et al (1981). The authors fulfilled calculations of a one-dimensional radiative shock wave and demonstrated the formation of a region with a small gradient of the plasma parameters: temperature, pressure, and density (see Figures 3 and 4 in Katsova et al (1981)).…”

Section: Introductionmentioning

confidence: 99%

“…The authors fulfilled calculations of a one-dimensional radiative shock wave and demonstrated the formation of a region with a small gradient of the plasma parameters: temperature, pressure, and density (see Figures 3 and 4 in Katsova et al (1981)). This result is confirmed by the calculations of the radiative cooling behind a shock front in the atmospheres of cool stars (see Fadeyev & Gillet (2001) and Belova et al (2014)) with a more detailed account for the elementary processes in the plasma: ionization, recombination, bremsstrahlung, excitation and de-excitation of discrete levels of atoms and ions as well as radiation scattering at the frequencies of spectral lines.…”

Section: Introductionmentioning

confidence: 99%

Continuum and line emission of flares on red dwarf stars

Morchenko

Bychkov²,

Livshits³

2015

Astrophys Space Sci

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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.

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“…The disturbance propagating toward the photosphere ("downward"[5]) "is described in subsequent times by a solution of the type of the second-kind temperature wave [7]." "The latter is characterized by a subsonic-velocity propagation of the thermal wave [temperature jump]" [6], "in front of which a [non-stationary] shock wave ..." develops [5].In these papers [5,6] it was assumed that a region of thickness ∆z ≈ 10 km between the temperature jump and the shock front (referred to below as a chromospheric condensation) 1 The brief version of this article has been published in the Proceedings of the conference in honor of the 100th birthday of Academician V. V. Sobolev (St. Petersburg, September 21 25, 2015), pp. 219 221.…”

mentioning

confidence: 99%

“…is the source of quasi-blackbody radiation at wavelengths around 4500Å (see Fig. 4 in [5] and identical Fig. 8 in [6]).…”

mentioning

confidence: 99%

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

Morchenko

2016

Astrophysics

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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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Origin of the optical continuum of flares of red dwarfs

Cited by 11 publications

References 7 publications

Physical Processes During Impulsive Solar and Stellar Flares

Physical Processes During Impulsive Solar and Stellar Flares

Continuum and line emission of flares on red dwarf stars

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

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