Volume 570, October 2014
|Number of page(s)||15|
|Section||Galactic structure, stellar clusters and populations|
|Published online||15 October 2014|
Gemini spectroscopy of the outer disk star cluster BH176⋆
1 Special Astrophysical Observatory, Russian Academy of Sciences, N. Arkhyz, KChR, 369167, Russia
2 Instituto de Astronomía Teórica y Experimental IATE, CONICET − Observatorio Astronómico, Universidad Nacional de Córdoba, Laprida 854, X5000BGR Córdoba, Argentina
3 IRAP, Université de Toulouse, CNRS, 14 avenue E. Belin, 31400 Toulouse, France
4 Kazan Federal University, Kremlevskaya 18, 420008 Kazan, Russia
5 Geneva Observatory, University of Geneva, 51 Ch. des Maillettes, 1290 Versoix, Switzerland
Received: 12 February 2014
Accepted: 25 July 2014
Context. BH176 is an old metal-rich star cluster. It is spatially and kinematically consistent with belonging to the Monoceros Ring. It is larger in size and more distant from the Galactic plane than typical open clusters, and it does not belong to the Galactic bulge.
Aims. Our aim is to determine the origin of this unique object by accurately determining its distance, metallicity, and age. The best way to reach this goal is to combine spectroscopic and photometric methods.
Methods. We present medium-resolution observations of red clump and red giant branch stars in BH176 obtained with the Gemini South Multi-Object Spectrograph. We derive radial velocities, metallicities, effective temperatures, and surface gravities of the observed stars and use these parameters to distinguish member stars from field objects.
Results. We determine the following parameters for BH176: Vh = 0 ± 15 km s-1, [Fe/H] = −0.1 ± 0.1, age 7 ± 0.5 Gyr, E(V − I) = 0.79 ± 0.03, distance 15.2 ± 0.2 kpc, α-element abundance [α/Fe] ~ 0.25 dex (the mean of [Mg/Fe], and [Ca/Fe]).
Conclusions. BH176 is a member of old Galactic open clusters that presumably belong to the thick disk. It may have originated as a massive star cluster after the encounter of the forming thin disk with a high-velocity gas cloud or as a satellite dwarf galaxy.
Key words: globular clusters: general / globular clusters: individual: BH176 / open clusters and associations: general / Galaxy: structure / Galaxy: formation
Appendix A is available in electronic form at http://www.aanda.org
Based on observations obtained at the Gemini Observatory, which is operated by the Association of Universities for Research in Astronomy, Inc., under a cooperative agreement with the NSF on behalf of the Gemini partnership: the National Science Foundation (United States), the Science and Technology Facilities Council (United Kingdom), the National Research Council (Canada), CONICYT (Chile), the Australian Research Council (Australia), Ministério da Ciência, Tecnologia e Inovação (Brazil), and the Ministerio de Ciencia, Tecnología e Innovación Productiva (Argentina).
© ESO, 2014
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