New spectroscopic binary companions of giant stars and updated metallicity distribution for binary systems^⋆

¹ Departamento de Astronomía, Pontificia Universidad Católica, Av.Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile
e-mail: pvbluhm@uc.cl
² Center of Astro-Engineering UC, Pontificia Universidad Católica, 7820436 Macul, Santiago, Chile
³ Department of Electrical Engineering, Pontificia Universidad Católica, 7820436 Macul, Santiago, Chile
⁴ Departamento de Astronomía, Universidad de Chile, Camino El Observatorio 1515, Las Condes, Santiago, Chile
⁵ Instituto de Física y Astronomía, Universidad de Vaparaíso, Cassila 5030, Valparaíso, Chile
⁶ School of Physics and Australian Centre for Astrobiology, University of New South Wales, 2052 Sydney, Australia
⁷ Computational Engineering and Science Research Centre, University of Southern Queensland, 4350 Toowoomba, Australia
⁸ Departamento de Ciencias Fisicas, Universidad Andres Bello, Avda. Republica 252, Santiago, Chile
⁹ Millennium Institute of Astrophysics, Santiago, Chile
¹⁰ Departamento de Astronomía, Universidad de Concepción, Casilla 160- C Concepción, Chile
¹¹ European Southern Observatory, Casilla 19001, Santiago, Chile

Received: 8 March 2016
Accepted: 28 June 2016

Abstract

We report the discovery of 24 spectroscopic binary companions to giant stars. We fully constrain the orbital solution for 6 of these systems. We cannot unambiguously derive the orbital elements for the remaining stars because the phase coverage is incomplete. Of these stars, 6 present radial velocity trends that are compatible with long-period brown dwarf companions. The orbital solutions of the 24 binary systems indicate that these giant binary systems have a wide range in orbital periods, eccentricities, and companion masses. For the binaries with restricted orbital solutions, we find a range of orbital periods of between ~97–1600 days and eccentricities of between ~0.1–0.4. In addition, we studied the metallicity distribution of single and binary giant stars. We computed the metallicity of a total of 395 evolved stars, 59 of wich are in binary systems. We find a flat distribution for these binary stars and therefore conclude that stellar binary systems, and potentially brown dwarfs, have a different formation mechanism than planets. This result is confirmed by recent works showing that extrasolar planets orbiting giants are more frequent around metal-rich stars. Finally, we investigate the eccentricity as a function of the orbital period. We analyzed a total of 130 spectroscopic binaries, including those presented here and systems from the literature. We find that most of the binary stars with periods ≲30 days have circular orbits, while at longer orbital periods we observe a wide spread in their eccentricities.