EDP Sciences
Free access
Volume 482, Number 2, May I 2008
Page(s) 639 - 651
Section Stellar atmospheres
DOI http://dx.doi.org/10.1051/0004-6361:20078848
Published online 20 February 2008

A&A 482, 639-651 (2008)
DOI: 10.1051/0004-6361:20078848

Abundance variations and first ionization potential trends during large stellar flares

R. Nordon1 and E. Behar2, 1

1  Department of Physics, Technion, Haifa 32000, Israel
    e-mail: nordon@physics.technion.ac.il
2  Senior NPP Fellow, Code 662, NASA/Goddard Space Flight Center, Greenbelt, MD 20771, USA
    e-mail: behar@physics.technion.ac.il

(Received 14 October 2007 / Accepted 22 January 2008)

Context. In the solar corona, elements of low first ionization potential (FIP $\la$ 10 eV) are enriched relative to their abundances in the photosphere, while high-FIP abundances remain unchanged. This was labeled as the Solar FIP effect. High-resolution X-ray spectroscopy has revealed that active stellar coronae show an opposite effect, which was labeled the inverse-FIP (IFIP) effect. The correlation found between coronal activity and the FIP/IFIP bias suggests that flaring activity is involved in switching from FIP to IFIP.
Aims. This work aims at a more systematic understanding of the FIP trends during stellar flares and complements an earlier study based on Chandra alone.
Methods. The eight brightest X-ray flares observed with XMM-Newton are analyzed and compared with their respective quiescence states. Together with six previous flares observed with Chandra, this establishes the best currently available sample of flares. We look for abundance variations during the flare and their correlation with FIP. For that purpose, we define a new FIP bias measure.
Results. A trend is found where coronae that are IFIP-biased in quiescence, show a FIP bias during flares relative to their quiescence composition. This effect is reversed for coronae that are FIP-biased in quiescence. The observed trend is thus consistent with chromospheric evaporation rather than with a FIP mechanism operating during flares. It also suggests that the quiescent IFIP bias is real and that the large flares are not the direct cause of the IFIP effect in stellar coronae.

Key words: stars: activity -- stars: coronae -- stars: flare -- stars: abundances -- X-rays: stars

© ESO 2008