A study of two high-velocity red horizontal branch stars⋆,⋆⋆,⋆⋆⋆
1 Observatório Nacional, Rua José Cristino 77, São Cristóvão, 20921-400 Rio de Janeiro-RJ, Brazil
e-mail: claudio@on.br; jilinski@on.br; drake@on.br; vladimir@on.br; froig@on.br
2 Instituto de Física, Universidade do Estado do Rio de Janeiro, Rua São Francisco Xavier 524, Maracanã, 20550-900 Rio de Janeiro-RJ, Brazil
3 Pulkovo Observatory, Russian Academy of Sciences, 65, Pulkovo, 196140 Saint Petersburg, Russia
4 Sobolev Astronomical Institute, Saint Petersburg State University, Universitetski pr. 28, 198504 Saint Petersburg, Russia
Received: 27 December 2012
Accepted: 28 July 2013
Context. High-velocity halo stars provide important information about the properties of the extreme Galactic halo. The study of Population II unbound and bound stars enables us better estimate the mass of the halo.
Aims. We carried out a detailed spectroscopic and kinematic study of two red horizontal branch stars, CD-41°15048 and HD 214362.
Methods. The atmospheric parameters, chemical abundances, and kinematical properties were determined using high-resolution optical spectroscopy and employing the local thermodynamic equilibrium model atmospheres of Kurucz and the spectral analysis code moog.
Results. We found that CD-41°15048 and HD 214362 are metal-poor red horizontal branch stars. Their abundance patterns are similar to other metal-poor halo stars already investigated. Our kinematic analysis of the 3D space motions shows that HD 214362 has a highly eccentric (e = 0.95) orbit and passes at 2 kpc from the Galactic center. CD-41°15048, for an adopted distance of 1.3 kpc, has an extreme retrograde motion and travels with very high velocity relative to the Galactocentric reference frame (VGRF = 583 km s-1).
Conclusions. CD-41°15048 is a bound or an unbound star, depending on the adopted Galactic potential. We also show that the red horizontal branch star BD+09°3223 is another example of a hypervelocity star. Whether it is bound or unbound to the Galaxy depends on the assumed mass and adopted Galactic potential. Possible origins of these two high-velocity stars are briefly discussed. CD-41°15048 and BD+09°3223 are further examples of evolved stars to join the restricted group of hypervelocity stars. Finally, our results seem to contradict the idea that a passage of a star very close to the Galactic center is the only possible origin of hypervelocity stars.
Key words: stars: horizontal-branch / Galaxy: kinematics and dynamics / stars: evolution
Based on observations made with the 2.2 m telescope at the European Southern Observatory (La Silla, Chile) under the agreement between ESO and Observatório Nacional (2007−2010).
Figures 3−5, 8, 10−12, Tables 1, 3, and 5 are available in electronic form at http://www.aanda.org
Reduced spectra for both stars (FITS files) are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/559/A12
© ESO, 2013