UV-(2000 ANGSTROM) IMAGING OF GLOBULAR-CLUSTERS .2. THE BLUE STRAGGLER STARS OF M3

UV-(2000 Angstrom) observation of M3 with a 40-cm balloon-borne imagin g telescope is used to investigate the nature of the blue straggler st ars outside the central region. Identifications of the ultraviolet sou rces by their position in a B, V catalogue provide a list of 25 stars which occupy the blue stragglers region of the visible colour-magnitud e diagram. In the colour-colour (B - V, m(2000) - V)-plot, the blue st ragglers are found on or above the horizontal branch, at roughly a con stant ultraviolet colour, extending to a region which is marginally po pulated by the colours of single, normal stars. The luminous blue stra gglers in the visible and some of the bluest have ultraviolet colours compatible with that expected for cluster main-sequence stars. Half th e blue stragglers however, including those at a low-luminosity in the visible, are in a region of the colour-colour diagram which is not acc ounted for by the usual metallicity or gravity effects on the colours of stars. The data suggest a difference in nature among blue straggler s. Simulation of their position in the colour-colour plot by unresolve d binary stars indicates, at a first order, that the blue stragglers w ith a large ultraviolet colour excess can be reproduced by a low-lumin osity hot secondary orbiting a turnoff or a subgiant star; the composi te V-mag and B - V fit in the colour-magnitude diagram and, the requir ed hot secondaries are similar to observed O- and B-type disk subdwarf s in the (M(V), B - V)-plane. At least for some blue stragglers of the outer part of M3, the present ultraviolet colours could support the e xistence of a hot, low-luminosity, unseen secondary star, possible rem nant of binary evolution with mass exchange.