Coronal properties of the EQ Pegasi binary system

¹ Hamburger Sternwarte, Universät Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany e-mail: cliefke@hs.uni-hamburg.de
² Arizona State University, School of Earth and Space Exploration, ASU, PO Box 871404, Tempe, AZ 85287, USA
³ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy

Received: 25 April 2008
Accepted: 20 September 2008

Abstract

Context. The activity indicators of M dwarfs are distinctly different for early and late types. The coronae of early M dwarfs display high X-ray luminosities and temperatures, a pronounced inverse FIP effect, and frequent flaring to the extent that no quiescent level can be defined in many cases. For late M dwarfs, fewer but more violent flares have been observed, and the quiescent X-ray luminosity is much lower.

Aims. To probe the relationship between coronal properties with spectral type of active M dwarfs, we analyze the M3.5 and M4.5 components of the EQ Peg binary system in comparison with other active M dwarfs of spectral types M0.5 to M5.5.

Methods. We investigate the timing behavior of both components of the EQ Peg system, reconstruct their differential emission measure, and investigate the coronal abundance ratios based on emission-measure independent line ratios from their Chandra HETGS spectra. Finally we test for density variations in different states of activity.

Results. The X-ray luminosity of EQ Peg A (M3.5) is by a factor of 6–10 brighter than that of EQ Peg B (M4.5). Like most other active M dwarfs, the EQ Peg system shows an inverse FIP effect. The abundances of both components are consistent within the errors; however, there seems to be a tendency toward the inverse FIP effect being less pronounced in the less active EQ Peg B when comparing the quiescent state of the two stars. This trend is supported by our comparison with other M dwarfs.

Conclusions. As the X-ray luminosity decreases with later spectral type, so do coronal temperatures and flare rate. The amplitude of the observed abundance anomalies, i.e. the inverse FIP effect, declines; however, clear deviations from solar abundances remain.