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Astron. Astrophys. 357, 139-144 (2000)


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A ROSAT HRI study of the open cluster NGC 3532

E. Franciosini 1, S. Randich 2 and R. Pallavicini 1

1 Osservatorio Astronomico di Palermo "G.S. Vaiana", Piazza del Parlamento 1, 90134 Palermo, Italy (francio@oapa.astropa.unipa.it; pallavic@oapa.astropa.unipa.it)
2 Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy (randich@arcetri.astro.it)

Received 15 December 1999 / Accepted 1 March 2000

Abstract

NGC 3532 is a very rich southern open cluster of age [FORMULA] Myr; it is therefore a good candidate to investigate the X-ray activity-age-rotation relationship at ages intermediate between the Pleiades and the Hyades, where, to our knowledge, X-ray studies exist for only one cluster (NGC 6475). We have performed an X-ray study of NGC 3532 using HRI observations retrieved from the ROSAT archive. The observations have a limiting sensitivity [FORMULA] erg sec-1 in the center of the field. We detected [FORMULA] X-ray sources above a [FORMULA] threshold, half of which have a known optical counterpart within 10 arcsec; 15 of the X-ray sources have at least one cluster member as optical counterpart.

A comparison of NGC 3532 with the nearly coeval cluster NGC 6475 indicates that the former cluster is considerably X-ray underluminous with respect to NGC 6475. However, because of the existence of possible selection effects, additional X-ray and optical observations are needed before definitively concluding that the X-ray properties of NGC 3532 and NGC 6475 are significantly different.

Key words: Galaxy: open clusters and associations: individual: NGC 3532 – stars: coronae – X-rays: stars

Send offprint requests to: E. Franciosini

© European Southern Observatory (ESO) 2000

Online publication: May 3, 2000

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