Mapelli, M.; Ripamonti, E.; Tolstoy, E.; Sigurdsson, S.; Irwin, M. J.; Battaglia, G.
Bibliographical reference
Monthly Notices of the Royal Astronomical Society, Volume 380, Issue 3, pp. 1127-1140.
Advertised on:
9
2007
Citations
48
Refereed citations
41
Description
Blue straggler star (BSS) candidates have been observed in all old dwarf
spheroidal galaxies (dSphs), however whether or not they are authentic
BSSs or young stars has been a point of debate. To both address this
issue and obtain a better understanding of the formation of BSSs in
different environments, we have analysed a sample of BSS candidates in
two nearby Galactic dSphs, Draco and Ursa Minor. We have determined
their radial and luminosity distributions from wide field multicolour
imaging data extending beyond the tidal radii of both galaxies.
BSS candidates are uniformly distributed through the host galaxy,
whereas a young population is expected to show a more clumpy
distribution. Furthermore, the observed radial distribution of BSSs,
normalized to both red giant branch (RGB) and horizontal branch (HB)
stars, is almost flat, with a slight decrease towards the centre. Such a
distribution is at odds with the predictions for a young stellar
population, which should be more concentrated. Instead, it is consistent
with model predictions for BSS formation by mass transfer in binaries
(MT-BSSs). Such results, although not decisive, suggest that these
candidates are indeed BSSs and that MT-BSSs form in the same way in
Draco and Ursa Minor as in globular clusters. This favours the
conclusion that Draco and Ursa Minor are truly `fossil' galaxies, where
star formation ceased completely more than 8 billion years ago.