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Issue A&A
Volume 374, Number 3, August II 2001
Page(s) 1072 - 1084
Section Stellar atmospheres
DOI http://dx.doi.org/10.1051/0004-6361:20010518



A&A 374, 1072-1084 (2001)
DOI: 10.1051/0004-6361:20010518

Microwave plasma emission of a flare on AD Leo

A. V. Stepanov1, B. Kliem2, V. V. Zaitsev3, E. Fürst4, A. Jessner4, A. Krüger2, J. Hildebrandt2 and J. H. M. M. Schmitt5

1  Pulkovo Observatory, 196140 St. Petersburg, Russia
    e-mail: stepanov@gao.spb.ru
2  Astrophysikalisches Institut Potsdam, 14482 Potsdam, Germany
3  Institute of Applied Physics, 603600 Nizhny Novgorod, Russia
    e-mail: za130@appl.sci-nnov.ru
4  Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
    e-mail: efuerst@mpifr-bonn.mpg.de
5  Universitätssternwarte Hamburg, 21029 Hamburg, Germany
    e-mail: jschmitt@hs.uni-hamburg.de

(Received 29 September 2000 / Accepted 6 April 2001 )

Abstract
An intense radio flare on the dMe star AD Leo, observed with the Effelsberg radio telescope and spectrally resolved in a band of 480 MHz centred at 4.85 GHz is analysed. A lower limit of the brightness temperature of the totally right handed polarized emission is estimated as $T_{\rm b}\sim5\times10^{10}$ K (with values $T_{\rm b}\ga3\times10^{13}$ K considered to be more probable), which requires a coherent radio emission process. In the interpretation we favour fundamental plasma radiation by mildly relativistic electrons trapped in a hot and dense coronal loop above electron cyclotron maser emission. This leads to densities and magnetic field strengths in the radio source of $n\sim2\times10^{11}$ cm-3 and $B\sim800$ G. Quasi-periodic pulsations during the decay phase of the event suggest a loop radius of $r\sim7\times10^8$ cm. A filamentary corona is implied in which the dense radio source is embedded in hot thin plasma with temperature $T\ge2\times10^7$ K and density $n_{\rm ext}\le10^{-2}n$. Runaway acceleration by sub-Dreicer electric fields in a magnetic loop is found to supply a sufficient number of energetic electrons.


Key words: stars: activity -- stars: flare -- stars: coronae -- radio continuum: stars -- radiation mechanisms: non-thermal -- acceleration of particles

Offprint request: B. Kliem, bkliem@aip.de

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© ESO 2001


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