Volume 382, Number 3, FebruaryII 2002
|Page(s)||888 - 898|
|Section||Galactic structure, stellar clusters, and populations|
|Published online||15 February 2002|
Photometric variability in the open cluster M 67
I. Cluster members detected in X-rays
Astronomical Institute, Utrecht University, PO Box 80000, 3508 TA Utrecht, The Netherlands
2 Department of Astronomy, University of Wisconsin-Madison, 475 N Charter St, Madison, WI 53706, USA
Corresponding author: M. van den Berg, email@example.com
Accepted: 23 October 2001
We study photometric variability among the optical counterparts of X-ray sources in the old open cluster M 67. The two puzzling binaries below the giant branch are both variables: for S 1113 the photometric period is compatible with the orbital period, S 1063 either varies on a period longer than the orbital period, or does not vary periodically. For the spectroscopic binaries S 999, S 1070 and S 1077 the photometric and orbital periods are similar. Another new periodic variable is the main-sequence star S 1112, not known to be a binary. An increase of the photometric period in the W UMa system S 1282 (AH Cnc) is in agreement with a previously reported trend. Six of the eight variables we detected are binaries with orbital periods of 10 days or less and equal photometric and orbital periods. This confirms the interpretation that their X-ray emission arises in the coronae of tidally locked magnetically active stars. No variability was found for the binaries with orbital periods longer than 40 days; their X-ray emission remains to be explained.
Key words: stars: activity / binaries: general / stars: variables: general / open clusters and associations: individual: M 67 / X-rays: stars
© ESO, 2002
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