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Title:
Blue straggler stars in dwarf spheroidal galaxies
Authors:
Mapelli, M.; Ripamonti, E.; Tolstoy, E.; Sigurdsson, S.; Irwin, M. J.; Battaglia, G.
Affiliation:
AA(Institute for Theoretical Physics, University of Zürich, Winterthurerstrasse 190, CH-8057 Zürich, Switzerland), AB(Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700 AV Groningen, the Netherlands), AC(Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700 AV Groningen, the Netherlands), AD(Department of Astronomy and Astrophysics, The Pennsylvania State University, 525 Davey Lab, University Park, PA 16802, USA), AE(Royal Greenwich Observatory, Madingley Road, Cambridge CB3 0EZ), AF(Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700 AV Groningen, the Netherlands)
Publication:
Monthly Notices of the Royal Astronomical Society, Volume 380, Issue 3, pp. 1127-1140. (MNRAS Homepage)
Publication Date:
09/2007
Origin:
MNRAS
MNRAS Keywords:
stellar dynamics, blue stragglers, galaxies: dwarf, galaxies: individual: Draco, galaxies: individual: Ursa Minor
DOI:
10.1111/j.1365-2966.2007.12148.x
Bibliographic Code:
2007MNRAS.380.1127M

Abstract

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.

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