Physics – Plasma Physics
Scientific paper
Oct 1993
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993a%26a...278..109h&link_type=abstract
Astronomy and Astrophysics (ISSN 0004-6361), vol. 278, no. 1, p. 109-128
Physics
Plasma Physics
11
Energy Budgets, Kinetic Energy, Late Stars, Plasma Physics, Stellar Flares, Stellar Magnetic Fields, Stellar Spectra, Absorption Spectra, Emissivity, Optical Thickness, Spectral Signatures
Scientific paper
In two previous publications (Paper I and II in this series), we described the spectral line Doppler shifts and asymmetries observed during a large flare on AD Leo, and interpreted those in terms of mass motions. Here, in order to further constrain the plasma physical properties and estimate the amount of energy released under mass motions, we developed simple methods so as to infer lower limits to the plasma cross-section, volume, column density, mass, average emissivity and optical depth. This allows us to derive the first kinetic energy and momentum budget of a stellar flare spectral signatures at visible wavelengths. The investigation of the line flux variation along the Balmer series indicates that up to 40% or more of the CaII H emission is likely to arise from radiative pumping by the Balmer H-epsilon line. For the preflare motion of the solar like 'dark filament', we estimate that its minimum mass, kinetic energy and radius are respectively about 6.4 1013 kg, 2.7 1032 erg and 5 107 m. The impulsive phase downward motion which attains deep photospheric layers in 'kernels' is believed to carry a mass of only 1.3 1013 kg and energy 2.4 1032 erg. The area/extent values we inferred from the Balmer and the CaII lines using escape probability methods agree well with NLTE-radiation transfer modelling. We obtain a minimum kernel area of approximately 6 1013 sq m and an estimated area of approximately 1014 sq m. A flaring prominence whose spectral signature is oscillatory Doppler shifts has an estimated mass and radius of respectively 1.2 1013 kg and 2 107 m. The minimum kinetic energy associated with its motion is about 2.9 1031 erg. We find that the kinetic energy associated with the detected mass motions strongly depends on the localization of the flare on the stellar disc. Ignoring or taking into account this possible projection factor for the velocities gives a total kinetic energy of respectively approximately 3 1030 erg and approximately 6 1032 erg (without the impulsive phase CME). The latter figure is about a factor of 3 times more than the energy radiated in the U-band and 2.3 times less than the 'missing energy' in the K band. This points to an interesting aspect of stellar flare energetics, that is, for this flare the kinetic energy may be a significant fraction of the total energy budget. Along the same lines, our results also indicate that possibly only a small fraction of the kinetic energy is carried by cool plasmas and that most mass motions may occur in coronal plasmas at a higher temperature regime. This important aspect of flare energetics should be assessed by simultaneous observations in optical and UV/EUV spectroscopy with a higher time resolution.
Doyle Gerry J.
Foing Bernard H.
Houdebine E. R.
Rodono' Marcello
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