Spectroscopic binaries among Hipparcos M giants. II. Binary frequency

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 S b (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 S b parameter is discussed in relation to global stellar parameters, and the S b 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 S b was found to correlate better with the stellar radius than with either luminosity or effective temperature separately. Two outliers of the S b 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. © 2009 ESO.SCOPUS: ar.j