Spectroscopic binaries among Hipparcos M giants II. Binary frequency

📝 Abstract

This paper is the second one in a series devoted to the study of properties of binaries involving M giants. The binary frequency of field M giants is derived and compared with the binary fraction of K giants. Diagrams of the CORAVEL spectroscopic parameter Sb (measuring the average line-width) vs. radial-velocity standard deviation for our samples are used to define appropriate binarity criteria. These then serve to extract the binarity fraction among the M giants. Comparison is made to earlier data on K giants binarity frequency. The Sb parameter is discussed in relation to global stellar parameters and the Sb vs. stellar radius relation is used to identify fast rotators. We find that the spectroscopic binary detection rate among field M giants, in a sample with a low number of velocity measurements (~2), unbiased toward earlier known binaries, is 6.3%. This is less than half of the analogous rate for field K giants, likely resulting from a real difference. This difference originates in the greater difficulty of finding binaries among M giants because of their smaller orbital velocity amplitudes and larger intrinsic jitter and in the different distributions of K and M giants in the eccentricity-period diagram. A larger detection rate was obtained in a smaller M giant sample with more radial velocity measurements per object: 11.1% confirmed plus 2.7% possible binaries. The CORAVEL spectroscopic parameter Sb was found to correlate better with the stellar radius than with either luminosity or effective temperature separately. Two outliers of the Sb vs. stellar radius relation, HD 190658 and HD 219654, have been recognized as fast rotators. The rotation is companion-induced, as both objects turn out to be spectroscopic binaries.

💡 Analysis

This paper is the second one in a series devoted to the study of properties of binaries involving M giants. The binary frequency of field M giants is derived and compared with the binary fraction of K giants. Diagrams of the CORAVEL spectroscopic parameter Sb (measuring the average line-width) vs. radial-velocity standard deviation for our samples are used to define appropriate binarity criteria. These then serve to extract the binarity fraction among the M giants. Comparison is made to earlier data on K giants binarity frequency. The Sb parameter is discussed in relation to global stellar parameters and the Sb vs. stellar radius relation is used to identify fast rotators. We find that the spectroscopic binary detection rate among field M giants, in a sample with a low number of velocity measurements (~2), unbiased toward earlier known binaries, is 6.3%. This is less than half of the analogous rate for field K giants, likely resulting from a real difference. This difference originates in the greater difficulty of finding binaries among M giants because of their smaller orbital velocity amplitudes and larger intrinsic jitter and in the different distributions of K and M giants in the eccentricity-period diagram. A larger detection rate was obtained in a smaller M giant sample with more radial velocity measurements per object: 11.1% confirmed plus 2.7% possible binaries. The CORAVEL spectroscopic parameter Sb was found to correlate better with the stellar radius than with either luminosity or effective temperature separately. Two outliers of the Sb vs. stellar radius relation, HD 190658 and HD 219654, have been recognized as fast rotators. The rotation is companion-induced, as both objects turn out to be spectroscopic binaries.

📄 Content

arXiv:0901.0937v2 [astro-ph.SR] 9 Feb 2009 Astronomy & Astrophysics manuscript no. binairesM˙frequency.20081213 c⃝ESO 2018 October 31, 2018 Spectroscopic binaries among Hipparcos M giants⋆ II. Binary frequency A. Frankowski⋆⋆,1, B. Famaey⋆⋆⋆,1, S. Van Eck†,1, M. Mayor2, S. Udry2, and A. Jorissen1 1 Institut d’Astronomie et d’Astrophysique, Universit´e libre de Bruxelles, Facult´e des Sciences, CP. 226, Boulevard du Triomphe, B-1050 Bruxelles, Belgium 2 Observatoire de Geneve, Universit´e de Geneve, CH-1290 Sauverny, Switzerland Received date; accepted date ABSTRACT Context. This paper is the second in a series devoted to studying the properties of binaries with M giant primaries. Aims. The binary frequency of field M giants is derived and compared with the binary fraction of K giants. Methods. Diagrams of the CORAVEL spectroscopic parameter Sb (measuring the average line width) vs. radial-velocity standard deviation for our samples were used to define appropriate binarity criteria. These then served to extract the binarity fraction among the M giants. Comparison is made to earlier data on K giant binarity frequency. The Sb parameter is discussed in relation to global stellar parameters, and the Sb vs. stellar radius relation is used to identify fast rotators. Results. We find that the spectroscopic binary detection rate among field M giants, in a sample with few velocity measurements (∼2), unbiased toward earlier known binaries, is 6.3%. This is less than half of the analogous rate for field K giants. This difference originates in the greater difficulty of finding binaries among M giants because of their smaller orbital velocity amplitudes and larger intrinsic jitter and in the different distributions of K and M giants in the eccentricity – period diagram. A higher detection rate was obtained in a smaller M giant sample with more radial velocity measurements per object: 11.1% confirmed plus 2.7% possible binaries. The CORAVEL spectroscopic parameter Sb was found to correlate better with the stellar radius than with either luminosity or effective temperature separately. Two outliers of the Sb vs. stellar radius relation, HD 190658 and HD 219654, have been recognised as fast rotators. The rotation is companion-induced, as both objects turn out to be spectroscopic binaries. Key words. binaries: spectroscopic - stars: late-type

1. Introduction This paper is the second in our series discussing the spectroscopic-binary content of a sample of M giants drawn from the Hipparcos Catalogue (ESA 1997), for which CORAVEL radial velocities have been obtained in a system- atic way (Udry et al. 1997; Famaey et al. 2005). The main driver behind such an extensive monitoring effort lies of course with the kinematics; indeed, the kinematical properties of the present sample of M giants have been fully analysed by Famaey et al. (2005). But this large amout of material may also be used to search for binaries, which is discussed in a series of three papers. In Paper I (Famaey et al. 2009), we present the radial-velocity data and orbital elements of newly-discovered spectroscopic bi- naries, and also discuss the intrinsic variations sometimes caus- ing pseudo-orbital variations. As a follow-up, the present paper presents the first attempt to derive the frequency of spectroscopic binaries in such an extensive sample of M giants. A side topic of this paper is the identification of fast rotators among M giants, using the CORAVEL line-width parameter Sb. This way of find- ⋆Based on observations carried out at the Swiss telescope installed at the Observatoire de Haute Provence (OHP, France), and at the 1.93-m OHP telescope ⋆⋆Postdoctoral Researcher, F.N.R.S., Belgium. Currently at Department of Physics, Technion-Israel Institute of Technology, Haifa 32000, Israel ⋆⋆⋆Postdoctoral Researcher, F.N.R.S., Belgium † Research Associate, F.N.R.S., Belgium ing binaries is independent of radial velocities. Finally, we inves- tigate the relation between the CORAVEL line-width parameter Sb and stellar parameters and conclude that Sb is best correlated with the stellar radius. In Paper III (Jorissen et al. 2009), more evolutionary considerations are presented, based on the analysis of the eccentricity – period diagram of our sample of M giants.
2. Binary frequency among M giants The sample used for the present study and its three subsam- ples are extensively described in Paper I. Sample I consists of 771 stars from the Hipparcos survey, for which at least 2 radial- velocity measurements have been obtained (Udry et al. 1997). Sample II consists of about one-third sample I stars (254 stars), for which at least 4 measurements have been obtained. Sample III consists of 35 sample II stars with σ(Vr) > 1 km s−1, and sample IV of 138 sample II stars with σ(Vr) < 1 km s−1. 2.1. Spectroscopic binaries (sample I) As explained in Paper I, the detection of spectroscopic binaries (SBs) among M giants cannot rely solely on a χ2 test compar- ing the radial-velocity stan

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