Structural Parameters for the Eight Galactic Dwarf Spheroidals
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view Abstract Citations (38) References (64) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The stellar content of dwarf spheroidal galaxies. Hirshfeld, A. W. Abstract The stellar content of dwarf spheroidal galaxies has been investigated with an emphasis on determining the nature of the anomalous Cepheid variables. Models of extremely metal-poor horizontal branch stars in the mass range 0.70-1.60 solar masses have been constructed. It is found that the models with masses of 1.30-1.60 solar masses spend considerable time within the instability strip and successfully predict the observational characteristics of the anomalous Cepheids. The results support binary mass transfer as the origin of the anomalous Cepheids. If so, Draco is probably younger than the majority of Galactic globular clusters. Age is held to be the second parameter affecting the horizontal branch distribution in the color-magnitude diagrams of dwarf spheroidal galaxies. Publication: The Astrophysical Journal Pub Date: October 1980 DOI: 10.1086/158322 Bibcode: 1980ApJ...241..111H Keywords: Cepheid Variables; Galactic Structure; Star Distribution; Stellar Models; Galactic Evolution; Mass Transfer; Stellar Evolution; Stellar Mass; Astrophysics; C-M Diagrams:Dwarf Galaxies; Cepheids:Dwarf Galaxies; Dwarf Galaxies:Helium; Dwarf Galaxies:Metal Abundances; Dwarf Galaxies:Stellar Content full text sources ADS | data products SIMBAD (10) NED (1)
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The M81 group consists of a few large galaxies and about 40 dwarf galaxies. It is a highly interacting group which makes it an important nearby laboratory to study the role of the environment and interactions in the dwarf galaxy formation and evolution. We use the resolved stellar populations in a sample of early-type dwarf galaxies in this group to derive the photometric metallicity distribution functions and study the potential presence of population gradients in this dwarf galaxy sample.
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We use a large set of radial velocities in the Ursa Minor and Draco dwarf spheroidal galaxies to search for binary stars and to infer the binary frequency. Of the 118 stars in our sample with multiple observations, six are velocity variables with $χ^2$ probabilities below 0.001. We use Monte Carlo simulations that mimic our observations to determine the efficiency with which our observations find binary stars. Our best, though significantly uncertain, estimate of the binary frequency for stars near the turnoff in Draco and UMi is 0.2--0.3 per decade of period in the vicinity of periods of one year, which is 3--5$\times$ that found for the solar neighborhood. This frequency is high enough that binary stars might significantly affect the measured velocity dispersions of some dwarf spheroidal galaxies according to some previous numerical experiments. However, in the course of performing our own experiments, we discovered that this previous work had inadvertently overestimated binary orbital velocities. Our first set of simulations of the effects of binaries is based on the observed scatter in the individual velocity measurements for the multiply-observed Draco and Ursa Minor stars. This scatter is small compared to measured velocity dispersions and, so, the effect of binaries on the dispersions is slight. This result is supported by our second set of experiments, which are based on a model binary population normalized by the observed binary frequency in Draco and Ursa Minor. We conclude that binary stars have had no significant effect on the measured velocity dispersion and inferred mass-to-light ratio of any dwarf spheroidal galaxy.
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