Astronomy & Astrophysics

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Issue		A&A Volume 411, Number 3, December I 2003
Page(s)		587 - 593
Section		Formation and evolution of planetary systems
DOI		10.1051/0004-6361:20031398

A&A 411, 587-593 (2003)
DOI: 10.1051/0004-6361:20031398

AD Leonis : Flares observed by XMM-Newton and Chandra

E. J. M. van den Besselaar^{1, 2}, A. J. J. Raassen^{1, 3}, R. Mewe¹, R. L. J. van der Meer¹, M. Güdel⁴ and M. Audard⁵

¹ SRON National Institute for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands
    e-mail: A.J.J.Raassen@sron.nl;R.Mewe@sron.nl;R.L.J.van.der.Meer@sron.nl
² Department of Astrophysics, University of Nijmegen, PO Box 9010, 6500 GL Nijmegen, The Netherlands
³ Astronomical Institute "Anton Pannekoek", Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
⁴ Paul Scherrer Institut, Würenlingen & Villigen, 5232 Villigen PSI, Switzerland
    e-mail: guedel@astro.phys.ethz.ch
⁵ Columbia Astrophysics Laboratory, Columbia University, 550 West 120th Street, New York, NY 10027, USA
    e-mail: audard@astro.columbia.edu

(Received 3 February 2003 / Accepted 2 September 2003 )

Abstract
The M-dwarf AD Leonis has been observed with the Reflection Grating Spectrometers and the European Photon Imaging Camera aboard XMM-Newton and also with the Low Energy Transmission Grating Spectrometer aboard the Chandra X-ray Observatory. In the observation taken with XMM-Newton five large flares produced by AD Leo were identified and only one in the observation taken with Chandra. A quiescent level to the lightcurves is difficult to define, since several smaller flares mutually overlap each other. However, we defined a quasi-steady state outside of obvious flares or flare decays. The spectra from the flare state and the quasi-steady state are analysed separately. From these spectra the temperature structure was derived with a multi-temperature model and with a differential emission measure model. The multi-temperature model was also used to determine the relative abundances of $\element{C}$ , $\element{N}$ , $\element{O}$ , $\element$ , $\element$ , $\element$ , $\element{S}$ , and $\element$ . $\element$ -like ions, such as $\ion{O}{vii}$ and $\ion{ix}$ , produce line triplets which are used to determine or constrain the electron temperature and electron density of the corresponding ion. During the flare state a higher emission measure at the hottest temperature is found for both XMM-Newton and Chandra observations. The derived abundances suggest the presence of an inverse First Ionization Potential effect in the corona of AD Leo .

Key words: stars: individual: AD Leonis -- stars: coronae -- stars: flare -- X-rays: stars -- missions: XMM-Newton, Chandra

Offprint request: E. J. M. van den Besselaar, besselaar@astro.kun.nl

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