Astron. Astrophys. 357, 460-470 (2000)

ROSAT HRI survey of the open cluster Stock 2

S. Sciortino ¹, G. Micela ¹, F. Favata ², A. Spagna ³ and M.G. Lattanzi <sup>3

1</sup> Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
² Astrophysics Division - Space Science Department of ESA, ESTEC, Postbus 299, 2200 AG Noordwijk, The Netherlands
³ Osservatorio Astronomico di Torino, Strada Osservatorio 20, 10025 Pino Torinese, Italy

Received 14 October 1999 / Accepted 2 February 2000

Abstract

We have obtained two deep HRI observations toward the open cluster Stock 2 in order to assess the X-ray luminosity level of the 112 high-probability cluster members falling in the area we have surveyed. Members have been selected on the basis of the result of a recent proper motion study based on the same photographic material and the same reduction scheme as the GSC-II project. We have reached a limiting sensitivity of   3 10²⁹ erg/sec for Stock 2 members and have detected 19 X-ray sources. The attained limiting sensitivity has prevented a complete sampling of the coronal emission level of Stock 2, and we have detected only three solar-type and one B9V members. We have derived the X-ray luminosity function for the Stock 2 stars in the 0.43 0.8 color range and have compared it with the X-ray luminosity functions of the dG members of Praesepe, Hyades and Pleiades open clusters. While the limiting sensitivity of our observations and the present uncertainties in Stock 2 and other cluster distances do not allow to derive firm conclusion, yet, our data indicate that the coronal emission level of Stock 2 solar-type stars is more similar to that of the Hyades (or the Praesepe) than to the Pleiades one.

This in turn suggests that either Stock 2 is older than previously reported or that the current age-rotation-activity paradigm need to be revised. In this latter case Stock 2 would likely belong to the small group of open clusters (Praesepe, NGC6333 and IC 4675) for which recent X-ray surveys indicate a more complex age-rotation-activity relation than currently thought.

Key words: Galaxy: open clusters and associations: individual: Stock 2 stars: activity stars: late-type X-rays: stars

Send offprint requests to: S. Sciortino, (sciorti@oapa.astropa.unipa.it)

© European Southern Observatory (ESO) 2000

Online publication: June 5, 2000

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