M. Laget et al., UV-(2000 ANGSTROM) IMAGING OF GLOBULAR-CLUSTERS .2. THE BLUE STRAGGLER STARS OF M3, Astronomy and astrophysics, 282(1), 1994, pp. 37-44
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.