SPECTROSCOPY OF CARBON STARS IN THE DRACO AND URSA MINOR DWARF SPHEROIDAL GALAXIES

Abstract

With the ISIS spectrograph on the William Herschel Telescope, we obtained intermediate-resolution optical spectra in three and five carbon stars belonging to the dwarf spheroidal (dSph) galaxies Draco and Ursa Minor, respectively. The metallicity, carbon isotopic ratios, and high-mass s-element abundances were determined by spectral synthesis in LTE using appropriate spherically symmetric, carbon-rich atmosphere models. The infrared colors and derived luminosities suggest that these stars are equivalent to the classical CH-type stars found in the halo of the Milky Way, although the evidence of luminosity variations in the stars Draco 461 and Draco 20733 may be compatible with these being carbon-rich low-mass asymptotic giant branch stars. The derived overall metallicity in the stellar sample ([M/H] ~–2.0) agrees with the average metallicity of the main stellar component in these dSphs obtained by previous studies. The C/O and 12C/13C ratios, and the average large heavy-element (Ba, La, Nd, Sm) enhancements derived ([hs/M] ≥1) are also similar to the values found in galactic CH-type stars at the same stellar metallicity. Although this average excess in heavy elements can be explained by standard s-process nucleosynthesis models, in two stars of Ursa Minor there is a suggestion that their heavy-element abundance pattern bears a closer resemblance to the scaled solar system r-process than the s-process abundance curve. If this is confirmed, these stars would represent an extragalactic example of the s + r carbon-rich (binary) stars found in the galactic halo. This r-process like abundance pattern has been found previously in other red giant stars belonging to Ursa Minor, suggesting a peculiar chemical evolution history in this dSph galaxy.