An optical study of X-ray sources in the old open clusters NGC 752 and NGC 6940*
Astronomical Institute, Utrecht University, PO Box 80000, 3508 TA Utrecht, The Netherlands
Corresponding author: M. van den Berg, firstname.lastname@example.org
Accepted: 22 May 2001
We observed the optical counterparts of X-ray sources in the old open clusters NGC 752 and NGC 6940 to search for the origin of the X-rays. The photometric variability reported earlier for the blue straggler H 209 is not confirmed by our light curves, nor is an indication for variability seen in the spectra; thus its X-rays remain unexplained. The X-rays of VR 111 and VR 114 are likely not a result of magnetic activity as these stars lack strong Ca II H& K emission, while in VR 108 the level of activity could be enhanced. The short-period binary H 313 is a photometric variable; this supports the interpretation that it is a magnetically active binary. From the detection of the Li I 6707.8 Å line, we classify the giant in VR 84 as a first-ascent giant; this leaves its circular orbit unexplained. As a side-result we report the detection of Li I 6707.8 Å in the spectrum of the giant H 3 and the absence of this line in the spectrum of the giant H 11; this classifies H 3 as a first-ascent giant and H 11 as a core-helium-burning clump star, and confirms the faint extension of the red-giant clump in NGC 752.
Key words: stars: activity / stars: blue stragglers / stars: variables: general / open clusters and associations: individual: NGC 752, NGC 6940 / X-rays: stars
© ESO, 2001