Spectroscopic binaries among Hipparcos M giants

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 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.

Simulating multiple stars in preparation for Gaia

“…Following the results initially given by Famaey et al [27], Mermilliod et al [28] studied the K giants and Frankowski et al [29] the M giants. For the latter, an upper bound for the binarity fraction is 0.111 ((confirmed ) + 0.027 (suspected), apparently corresponding to a 0.43 fraction of the former, which gives an upper bound (32%) for the binarity of the K giant, still quite uncertain as it varies from either 13.7% [27] or 22% [28] in the field, or 30.8% in open clusters.…”

“…Beside, the statistics about giants, although marginally consistent to the upper level of observed properties [29] by chance only, probably indicate the presence of selection TABLE 1. Statistics computed on the generated samples, in order to check whether the simulation results (column 4) can recover the observational statistics (column 2) found in the bibliographical references indicated column 3.…”

Spectroscopic binaries among Hipparcos M giants

“…The intringuing finding by Goldin & Makarov (2007) of a very short period of 141 d in the Hipparcos Intermediate Astrometric Data for the M5III semiregular variable star HIP 34 922 (=HD 56 096 = L 2 Pup) deserves a specific discussion. This 141 d period is clearly related to pulsation, since that periodicity is found in both the light and radial-velocity curves (Cummings et al 1999;Lebzelter et al 2005).…”

“…For the sake of clarity, all panels start at a period of 30 d, although field K giants may be found in binary systems with periods as short as 4 d (see Fig. 6 of Paper II, Frankowski et al 2009). The lines and symbols are as in Fig.…”

Cannibals in the thick disk: the youngα-rich stars as evolved blue stragglers

“…Since this result is based on a small sample with few measurements, it is worth evaluating whether or not this difference of binary frequencies between Y and O stars is statistically significant. To do so, we use the hypergeometric test as in Frankowski et al (2009). We define N y = N o = 13 for the number of Y and O stars, and N t = N y + N o = 26 for the total number of stars.…”