Companions to APOGEE Stars: A Stellar Populations View of the Milky Way's Stellar and Substellar Companion Hosts

Over the past few decades, the power of statistical population studies of multiplicity has been unleashed, allowing us to derive general properties such as period, eccentricity, and mass ratio distributions for binary stars. With the explosion of candidate and confirmed planetary systems recently made available, due in large part to NASA's Kepler mission, these types of studies are now becoming possible for exoplanet systems as well. The Sloan Digital Sky Survey's (SDSS-III/IV) Apache Point Observatory Galactic Evolution Experiment (APOGEE) is an ongoing stellar chemodynamical survey of the Milky Way and its closest galactic neighbors, but given its typical radial velocity (RV) precision of ~100 m/s, APOGEE can also detect companions with masses as low as a few Jupiter masses around solar-mass stars. This gives APOGEE a unique place among the aforementioned population studies as the first Galaxy-wide survey for stellar and substellar companions, capable of sampling a diverse set of stars and environments.

In three years of operation (2011-2014), the APOGEE-1 survey observed >14,000 stars with enough epochs over a sufficient temporal baseline for the fitting of Keplerian orbits to RV variations induced by companions. We present the custom orbit-fitting pipeline used to create this catalog, which includes novel quality metrics that account for the phase and velocity coverage of a fitted Keplerian orbit. Here we present initial results from a catalog of 382 of the most compelling stellar and substellar companion candidates detected by APOGEE-1, which orbit a variety of host stars in diverse Galactic environments. The most surprising result from this catalog is a large number of close-in BD companion candidates. We propose a few potential explanations of this finding, some which invoke this catalog's many small separation companion candidates found orbiting evolved stars, which suggests that the canonical BD desert may only be a special case for Sun-like stars.

Several improvements to the APOGEE pipelines were implemented for the start of APOGEE-2 in SDSS-IV, including treatment for stellar rotational velocities in dwarfs and new methods for RV determination for low S/N spectra which we present here. These upgrades improve APOGEE's ability to derive orbits for stars in exciting new Galactic environments and for stellar populations never before probed with long-term RV monitoring. We exploit these improvements and the extended baseline of APOGEE-2 to build a new catalog of ~700 companion candidates that includes the first two years of APOGEE-2 observations. This new catalog supports the primary result from our previous sample of a large number of BD companion candidates from 0.1 AU < a < 3 AU. We combine the orbital parameters in this catalog with the uniformly-derived stellar parameters and chemical abundances available with the first APOGEE-2 data release to investigate the plausible formation mechanisms of the brown dwarf companions in our catalog.

Finally, we present some of our ongoing follow-up efforts to further vet our candidate systems, and our dedicated APOGEE-2 goal science program to intentionally gain a large number of additional epochs of APOGEE-1 targets. We also introduce the APOGEE Time-Domain Legacy Survey (ATLaS), a major After Sloan 4 (AS4) program that will expand upon the work presented here and extend it well into the next decade.