Issue |
A&A
Volume 370, Number 1, April IV 2001
|
|
---|---|---|
Page(s) | 157 - 169 | |
Section | Interstellar and circumstellar matter | |
DOI | https://doi.org/10.1051/0004-6361:20010197 | |
Published online | 15 April 2001 |
ROSAT all-sky survey of W Ursae Majoris stars and the problem of supersaturation
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
Corresponding author: K. Stępień, kst@astrouw.edu.pl
Received:
30
November
2000
Accepted:
31
January
2001
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 the
normalized X-ray flux decreases with a decreasing color index but for
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
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
© ESO, 2001
Current usage metrics show cumulative count of Article Views (full-text article views including HTML views, PDF and ePub downloads, according to the available data) and Abstracts Views on Vision4Press platform.
Data correspond to usage on the plateform after 2015. The current usage metrics is available 48-96 hours after online publication and is updated daily on week days.
Initial download of the metrics may take a while.