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Astron. Astrophys. 326, 608-613

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Soft X-ray emission from intermediate-age open clusters:
NGC 6940

T. Belloni1 and G. Tagliaferri2

1Astronomical Institute ''Anton Pannekoek'', University of Amsterdam and Center for High-Energy Astrophysics, Kruislaan 403, 1098 SJ Amsterdam, The Netherlands
2Osservatorio Astronomico di Brera, Via Bianchi 46, I-22055 Merate, Italy

Received 20 March 1997 / Accepted 22 May 1997


We present the results of soft X-ray observations of the intermediate-age open cluster NGC 6940 performed with the ROSAT PSPC. We detected 18 sources, four of which are identified with member stars of the cluster and five with active field stars. Another source is identified with an active giant star that could belong to the cluster, although it is not classified as a cluster member. The remaining eight, most likely of extragalactic nature, need to be identified through optical follow-up. As in the case of old clusters such as M 67 and NGC 752, a high fraction of the detected members are binaries: in NGC 6940 three out of four of the identified members are among the only six binaries known in the cluster. All four sources have X-ray luminosities typical of RS CVn binaries. However, only two of the three detected binaries have system characteristics typical of active X-ray binaries. The high X-ray luminosity of the other two sources needs to be explained. All four identified members are red giants, in agreement with the presence of a saturation level. The four field stars identified as counterparts of X-ray sources are all very active, with X-ray luminosities in the range FORMULA erg s-1, and deserve further optical investigation. We confirm the results found in other old open clusters, i.e. that binaries are still very active coronal sources at an age greater than 1 Gyr, and that good spatial resolution images in the X-ray band are a powerful tool to investigate their evolution in clusters of different ages.

Key words: stars: activity - open clusters and associations: individual: NGC 6940 - X-rays: stars

Send offprint requests to: T. Belloni

© European Southern Observatory (ESO) 1997

Online publication: September 18, 1997
Last change: October 15, 1997