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 parameterSb (measuring the average line width) vs. radial-velocity sta ndard deviation for our samples were used to define appropriate bin arity criteria. These then served to extract the binarity fr action 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 rotator s. Results. We find that the spectroscopic binary detection rate among fie ld 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 dif ference originates in the greater difficulty of finding binaries amon g 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% poss ible binaries. The CORAVEL spectroscopic parameterSb was found to correlate better with the stellar radius than wi th 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 bot h objects turn out to be spectroscopic binaries.