The structure of radiative shock waves

Fadeyev, Yu. A.; Gillet, Denis

doi:10.1051/0004-6361:20010064

Cited by 17 publications

(32 citation statements)

References 22 publications

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“…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: Introductionsupporting

confidence: 57%

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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Section: Introductionsupporting

confidence: 57%

Continuum and line emission of flares on red dwarf stars

Morchenko

Bychkov²,

Livshits³

2015

Astrophys Space Sci

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“…They are followed by recombinations and de-excitations that produce the observed emission lines. In this radiative region of the shock wake, the gas density is typically 50 to 100 times that of the unperturbed gas in front of the shock (Fadeyev & Gillet 2001). If the shock Mach number is high enough, the temperature and the density in the de-excitation region of the wake could be sufficient to produce He I and He II emission lines.…”

Section: Helium Emissions In Rr Lyrae Starsmentioning

confidence: 99%

Emission lines and shock waves in RR Lyrae stars

Gillet

Фокин

2014

A&A

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Context. Emission lines observed in radially pulsating stars are thought to be produced by atoms de-exciting after being excited by a shock wave that is traveling into and then compressing, heating, and accelerating the atmospheric gas. Aims. With the help of recent observations, we examine the origin of all the different types of emission lines of hydrogen and helium that appear during a pulsation cycle. Methods. To analyze the physical origin of emission lines, we used the different models of atmospheric dynamics of RR Lyrae stars that have been calculated so far. Results. In contrast to a recent explanation, we propose that the redshifted emission component of Hα, which occurs near the pulsation phase 0.3, is produced by the main shock. In this case, the emission is the natural consequence of the large extension of the expanding atmosphere. Therefore, this (weak) emission should only be observed in RR Lyrae stars for which the main shock will propagate far enough from the photosphere. It appears as a P-Cygni type profile. We estimate the shock front velocity during the shock propagation in the atmosphere and show that it decreases by 40% when the Hα emitting-shock passes from the photospheric level to the upper atmosphere. The Hα P-Cygni profile observed in long-period Cepheids also seems to be caused by the main shock wave. Although to date He II has only been detected in some Blazhko stars, a comprehensive survey of RR Lyrae stars is necessary to confirm this trend, so we can say that the most intense shocks will only be observed in Blazhko stars. Conclusions. The development of a model of atmospheric pulsation that takes the effects of 2D and 3D convection into account, seems to be a necessary step to fully quantify the effects of shock waves on the atmospheric dynamics of radially pulsating stars.

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“…One of the most important aspects characterizing the radiative shocks is that the largest part of their energy is lost as radiation. Indeed, comparing the preshock and post-shock outer boundaries, Fadeyev & Gillet (2001) have shown that in a pure hydrogen gas, most of the kinetic energy of the gas flow is irreversibly lost. When the shock velocity becomes higher than 60 km s −1 , radiative losses increase very rapidly with increasing upstream velocity.…”

Section: The Second Required Process: the Decrease Of The Average Effmentioning

confidence: 99%

Atmospheric dynamics in RR Lyrae stars

Gillet

2013

A&A

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Context. Discovered in 1907, the Blazhko effect is a modulation of the light variations of about half of the RR Lyr stars. It has remained unexplained for over 100 years, despite more than a dozen proposed explanations. Today it represents an ongoing challenge in variable-star research. Aims. We propose a new explanation. It is based on the observation that Blazhko stars seem to be located in the region of the instability strip where fundamental and first overtone modes are excited at the same time.Methods. An analysis of nonlinear and nonadiabatic pulsation models of RR Lyrae stars shows that a specific shock (called first overtone shock) may be generated by the perturbation of the fundamental mode by the transient first overtone. Results. The first overtone shock induces a sharp slowdown of the atmospheric layers during their infalling motion. This slowdown in turn affects the compression rate on the deep photospheric layers and the intensity of the κ-mechanism. After an amplification phase, the intensity of the main shock wave before the Blazhko maximum becomes high enough to provoke large radiative losses. These can be at least equal to 70% of the total energy flux of the shock, which induces a small decrease of the effective temperature at each pulsation cycle. In these conditions, when the intensity of the main shock reaches its highest critical value at the Blazhko maximum, it completely desynchronizes the motion of the phostospheric layers. At this point, the atmosphere relaxes and reaches a new synchronous state that occurs at the Blazhko minimum. Conclusions. The combined effects of these two shocks on the atmosphere cause the Blazhko effect. This effect can only exist if the first overtone mode is excited together with the fundamental mode. Because the involved physical mechanisms are essentially nonlinear (shocks, atmospheric dynamics, radiative losses, mode excitations), the Blazhko process is expected to be unstable and irregular. Consequently, the Blazhko process has a specific random nature that is in contrast with the pulsation of non-Blazhko stars.

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The structure of radiative shock waves

Cited by 17 publications

References 22 publications

Continuum and line emission of flares on red dwarf stars

Continuum and line emission of flares on red dwarf stars

Emission lines and shock waves in RR Lyrae stars

Atmospheric dynamics in RR Lyrae stars

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