EDP Sciences
Free Access
Issue
A&A
Volume 370, Number 1, April IV 2001
Page(s) 157 - 169
Section Formation, structure and evolution of stars
DOI https://doi.org/10.1051/0004-6361:20010197
Published online 15 April 2001


A&A 370, 157-169 (2001)
DOI: 10.1051/0004-6361:20010197

ROSAT all-sky survey of W Ursae Majoris stars and the problem of supersaturation

K. Stepien1, J. H. M. M. Schmitt2 and W. Voges3

1  Warsaw University Observatory, Al. Ujazdowskie 4, 00-478 Warszawa, Poland
2  Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
3  Max-Planck-Institut für extraterrestrische Physik, Giessenbachstr. 1, 85741 Garching, Germany

(Received 30 November 2000 / Accepted 31 January 2001)

Abstract
From ROSAT all-sky survey (RASS) data we obtained X-ray fluxes for 57 W UMa type contact systems. In our sample we detected three stars which are the shortest period main sequence binaries ever found as X-ray sources. For stars with (B-V)0 < 0.6 the normalized X-ray flux decreases with a decreasing color index but for (B-V)0 > 0.6 a plateau is reached, similar to the saturation level observed for single, rapidly rotating stars. The X-ray flux of W UMa stars is about 4-5 times weaker than that of the fastest rotating single stars. Because early type, low activity variables have longer periods, an apparent period-activity relation is seen among our stars, while cool stars with (B-V)0 > 0.6 and rotation periods between 0.23 and 0.45 days do not show any such relation. The lower X-ray emission of the single, ultra fast rotators (UFRs) and W UMa stars is interpreted as the result of a decreased coronal filling factor. The physical mechanisms responsible for the decreased surface coverage differs for UFRs and W UMa systems. For UFRs we propose strong polar updrafts within a convection zone, driven by nonuniform heating from below. The updrafts should be accompanied by large scale poleward flows near the bottom of the convective layer and equatorward flows in the surface layers. The flows drag dynamo generated fields toward the poles and create a field-free equatorial region with a width depending on the stellar rotation rate. For W UMa stars we propose that a large scale horizontal flow embracing both stars will prevent the magnetic field from producing long-lived structures filled with hot X-ray emitting plasma. The decreased activity of the fastest rotating UFRs increases the angular momentum loss time scale of stars in a supersaturated state. Thus the existence of a period cutoff and a limiting mass of W UMa stars can be naturally explained.


Key words: stars: activity -- (stars:) binaries: eclipsing -- stars: coronae -- stars: late type -- stars: rotation -- X-rays: stars

Offprint request: K. Stepien, kst@astrouw.edu.pl

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© ESO 2001

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