Ursa Minor dwarf galaxy: still an old stellar system

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DOIResolve DOI: http://doi.org/10.1086/113925
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Journal titleAstronomical Journal
Pages22212238; # of pages: 18
Subjectabundance; dwarf galaxies; elliptical galaxies; galactic evolution; star clusters; stellar evolution; stellar systems; binary stars; charge coupled devices; color-magnitude diagram; irregular galaxies; main sequence stars; Milky Way galaxy; star distribution
AbstractWe have constructed a color-magnitude diagram of the Ursa Minor dwarf spheroidal to m/sub V/ = 24.8 mag from charged-coupled device (CCD) observations with the Kitt Peak 4 m telescope. The main-sequence turnoff is easily visible. Fits to evolutionary isochrones and the globular M92 indicate that Ursa Minor has an age and metal abundance very similar to that of the latter cluster. No evidence for stars younger than about 16 billion years is seen, with the possible exception of approximately 20 stars believed to be blue stragglers. Ursa Minor is therefore an extreme-age galaxy, unlike superficially similar objects such as the Carina dwarf. Indeed, Ursa Minor may be the only outer-halo spheroidal whose stellar content lives up to the classical ideals of a Population II system. A distance modulus of (m-M)0 = 19.0 mag is derived from a sliding fit to the M92 ridge lines. However, this modulus is uncertain by approx.0.1 mag, for the horizontal branch in our color-magnitude diagram is poorly populated. The ratio of blue stragglers to anomalous Cepheids in Ursa Minor is estimated to be approx.100, a number that may provide an important constraint on binary models for the origin of these stars. A surprising result of our study is the discovery of clumpiness in the distribution of stars. This finding may give more weight to the idea that dwarf spheroidal galaxies were previously dwarf irregular galaxies, although clearly, if so, Ursa Minor must have lost its gaseous content very soon after formation.
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AffiliationNational Research Council Canada
Peer reviewedYes
NRC number24838
NPARC number21273619
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Record identifierfa32ecb5-5621-4f81-a3c9-dddafd7352a4
Record created2015-01-16
Record modified2016-05-09
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