Spectroscopic binaries among Hipparcos M giants II. Binary frequency

TL;DR

This study determines the binary frequency among field M giants using spectroscopic data, finding it significantly lower than that of K giants, and explores the relation between spectroscopic parameters and stellar properties.

Contribution

It provides the first detailed comparison of binary frequencies between M and K giants and introduces the use of the Sb parameter to identify fast rotators among M giants.

Findings

Binary detection rate among M giants is 6.3% with low measurements, less than half of K giants.

Higher detection rate of 11.1% confirmed plus 2.7% possible binaries with more measurements.

Sb parameter correlates with stellar radius and identifies fast rotators as binaries.

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.