Stellar Multiplicity Meets Stellar Evolution and Metallicity: The APOGEE View

Abstract

We use the multi-epoch radial velocities acquired by the Apache Point Observatory Galactic Evolution Experiment (APOGEE) survey to perform a large-scale statistical study of stellar multiplicity for field stars in the Milky Way, spanning the evolutionary phases between the main sequence (MS) and the red clump. We show that the distribution of maximum radial velocity shifts (Delta RVmax) for APOGEE targets is a strong function of log g, with MS stars showing Delta RVmax as high as similar to 300 km s(-1), and steadily dropping down to similar to 30 km s(-1) for log g similar to 0, as stars climb up the red giant branch (RGB). Red clump stars show a distribution of Delta RVmax values comparable to that of stars at the tip of the RGB, implying they have similar multiplicity characteristics. The observed attrition of high Delta RVmax systems in the RGB is consistent with a lognormal period distribution in the MS and a multiplicity fraction of 0.35, which is truncated at an increasing period as stars become physically larger and undergo mass transfer after Roche Lobe overflow during H-shell burning. The Delta RVmax distributions also show that the multiplicity characteristics of field stars are metallicity-dependent, with metal-poor ([Fe/H] less than or similar to -0.5) stars having a multiplicity fraction a factor of 2-3 higher than metal-rich ([Fe/H] less than or similar to 0.0) stars. This has profound implications for the formation rates of interacting binaries observed by astronomical transient surveys and gravitational wave detectors, as well as the habitability of circumbinary planets.