Spectroscopic binaries among Hipparcos M giants I. Data, orbits, and intrinsic variations

TL;DR

This study compiles and analyzes radial velocity data for M giant stars to identify spectroscopic binaries, revealing intrinsic stellar variations and providing new orbital solutions, while highlighting challenges in distinguishing true binaries from pulsation effects.

Contribution

It presents an extensive collection of spectroscopic orbits for M giants, including 12 new solutions, and discusses the impact of intrinsic stellar variability on binary detection.

Findings

Intrinsic radial-velocity variations correlate with spectral-line width and photometric variability.

Large radial-velocity jitter can mimic orbital signals, complicating binary identification.

Most M giants identified as X-ray sources are confirmed binaries.

Abstract

This paper is a follow-up of the vast effort to collect radial velocity data for stars belonging to the Hipparcos survey. We aim at extending the orbital data available for binaries with M giant primaries. The data will be used in the companion papers of this series to (i) derive the binary frequency among M giants and compare it to that of K giants (Paper II), and (ii) analyse the eccentricity-period diagram and the mass-function distribution (Paper III). Keplerian solutions are fitted to radial-velocity data. However, for several stars, no satisfactory solution could be found, despite the fact that the radial-velocity standard deviation is larger than the instrumental error, because M giants suffer from intrinsic radial-velocity variations due to pulsations. We show that these intrinsic radial-velocity variations can be linked with both the average spectral-line width and the photometric variability. We present an extensive collection of spectroscopic orbits for M giants, with 12 new orbits, plus 17 from the literature. Moreover, to illustrate the fact that the large radial-velocity jitter present in Mira and semi-regular variables may easily be confused with orbital variations, we also present examples of pseudo-orbital variations (in S UMa, X Cnc and possibly in HD 115521, a former IAU radial-velocity standard). Because of this difficulty, M giants involving Mira variables were excluded from our monitored sample. We finally show that the majority of M giants detected as X-ray sources are actually binaries.