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Astron. Astrophys. 332, 93-101 (1998)

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1. Introduction

Globular cluster cores are now known to harbor -if not produce- a variety of exotic objects. Extreme horizontal branch stars (Brown et al. 1997; Dorman et al. 1993), blue straggler stars (Bailyn 1995), low mass X-ray binaries (Parmar 1992), cataclysmic variables (Grindlay 1992; Bailyn et al. 1990), millisecond pulsars (Lyne 1995) or binaries (Hut et al. 1992) are test objects whose presence brings clues to important aspects of stellar evolution. Their nature, number and radial distribution provide also insights on the combined influences of dynamics and star density on the evolution of a coeval, simple stellar population (Fusi Pecci et al. 1993; Djorgovski et al. 1991). Most are variable or suspected variable and have a signature in the ultraviolet. These objects are actively searched in cluster cores and in their outskirts.

The bright galactic globular cluster M 3 (NGC 5272) is a typical benchmark for this type of studies. It has an intermediate metallicity and concentration, and it displays a well populated horizontal-branch (HB), extending from the red to the blue side of the instability strip, but with a very sparse blue extension (Sandage 1953, Buonanno et al. 1994). M3 has a large population of blue straggler stars (BSS) (Ferraro et al. 1993; Bolte et al. 1993; Burgarella et al. 1995; Guhathakurta et al. 1994) with a very unusual bimodal radial distribution (Ferraro et al. 1997a) which suggests either different formation mechanisms or, more specifically, that special destruction/survival and segregation effects have taken place in the core of this cluster. An unsuccessful search for cataclysmic variables down to [FORMULA] was also reported by Shara et al. (1985).

In addition, and most relevant, deep radio synthesis images have provided evidence for a non-pulsating, point radio source in the direction of the very cluster centre, possibly related to a pulsar in a very compact binary system or to an unrelated background object (Kulkarni et al. 1990). To our knowledge, the source has not been identified at optical wavelengths. Two radio sources located in the cluster outskirts [FORMULA], tentatively identified as cluster stars at a modest radio resolution (Klemola 1979), are now thought to be associated with background QSO's (Carney 1976; Harris et al. 1992) as probably is an other nearby radio source M 3A (McLean et al. 1983). The fact that a typical low-redshift QSO would also appear as a blue object emphasizes the need for a search for faint blue objects within the cluster core and for a precise positioning of the radio source at optical wavelengths.

In this paper we report a search for faint blue objects in the core of M 3 carried out using HST/WFPC2 images aimed, in particular, at the detection of any possible candidate counterparts for the known central radio source.

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© European Southern Observatory (ESO) 1998

Online publication: March 10, 1998