Summary: THE ASTROPHYSICAL JOURNAL, 541 : 396È409, 2000 September 20
2000. The American Astronomical Society. All rights reserved. Printed in U.S.A.
(
EXTREME­ULTRAVIOLET FLARE ACTIVITY IN LATE­TYPE STARS
MARC AUDARD,1,2 MANUEL GU# DEL,1,2 JEREMY J. DRAKE,3 AND VINAY L. KASHYAP3
Received 2000 March 7 ; accepted 2000 May 2
ABSTRACT
Extreme Ultraviolet Explorer Deep Survey observations of cool stars (spectral type F to M) have been
used to investigate the distribution of coronal Ñare rates in energy and its relation to activity indicators
and rotation parameters. Cumulative and di+erential Ñare rate distributions were constructed and Ðtted
with di+erent methods. Power laws are found to approximately describe the distributions. A trend
toward Ñatter distributions for later type stars is suggested in our sample. Assuming that the power laws
continue below the detection limit, we have estimated that the superposition of Ñares with radiated ener­
gies of about 1029È1031 ergs could explain the observed radiative power loss of these coronae, while the
detected Ñares are contributing only B10%. Although the power­law index is not correlated with rota­
tion parameters (rotation period, projected rotational velocity, Rossby number) and only marginally with
the X­ray luminosity, the Ñare occurrence rate is correlated with all of them. The occurrence rate of
Ñares with energies larger than 1032 ergs is found to be proportional to the average total stellar X­ray
luminosity. Thus, energetic Ñares occur more often in X­ray bright stars than in X­ray faint stars. The
normalized occurrence rate of Ñares with energies larger than 1032 ergs increases with increasing L X /L bol

Source: Audard, Marc - INTEGRAL Science Data Centre & Observatory of Geneva, Université de Genève
Guedel, Manuel - Institut für Astronomie, Universität Wien

Collections: Physics