Understanding the Impacts of Stellar Companions on Planet Formation and Evolution: A Survey of Stellar and Planetary Companions within 25 pc

Abstract

We explore the impact of outer stellar companions on the occurrence rate of giant planets detected with radial velocities. We searched for stellar and planetary companions to a volume-limited sample of solar-type stars within 25 pc. Using adaptive optics imaging observations from the Lick 3 m and Palomar 200″ Telescopes, we characterized the multiplicity of our sample stars, down to the bottom of the main sequence. With these data, we confirm field star multiplicity statistics from previous surveys. We additionally combined three decades of radial velocity (RV) data from the California Planet Search with newly collected RV data from Keck/HIRES and the Automated Planet Finder/Levy Spectrometer to search for planetary companions in these same systems. Using an updated catalog of both stellar and planetary companions, as well as detailed injection/recovery tests to determine our sensitivity and completeness, we measured the occurrence rate of planets among the single-and multiple-star systems. We found that planets with masses in the range of 0.1-10 M J and with semimajor axes of 0.1-10 au have an occurrence rate of -+ 0.18 0.03 0.04 planets per star when they orbit single stars and an occurrence rate of 0.12 ± 0.04 planets per star when they orbit a star in a binary system. Breaking the sample down by the binary separation, we found that only one planet-hosting binary system had a binary separation <100 au, and none had a separation <50 au. These numbers yielded planet occurrence rates of -+ 0.20 0.06 0.07 planets per star for binaries with separation a B > 100 au and -+ 0.04 0.02 0.04 planets per star for binaries with separation a B < 100 au. The similarity in the planet occurrence rate around single stars and wide primaries implies that wide binary systems should actually host more planets than single-star systems, since they have more potential host stars. We estimated a system-wide planet occurrence rate of 0.3 planets per wide binary system for binaries with separations a B > 100 au. Finally, we found evidence that giant planets in binary systems have a different semimajor-axis distribution than their counterparts in single-star systems. The planets in the single-star sample had a significantly higher occurrence rate outside of 1 au than inside 1 au by nearly 4σ, in line with expectations that giant planets are most common near the snow line. However, the planets in the wide binary systems did not follow this distribution, but rather had equivalent occurrence rates interior and exterior to 1 au. This may point to binary-mediated planet migration acting on our sample, even in binaries wider than 100 au.

Architecture of Hierarchical Stellar Systems and Their Formation

“…The proportions of single, binary, triple, quadruple (and higher-order) systems in this sample are 54:29:12:5, so a 0.17 fraction of the total population are hierarchies. Hirsch et al [14] confirm this fraction for the small but well-studied 25-pc sample. Complex hierarchies containing more than four stars can be viewed as combinations of elementary binaries, triples, etc.…”

“…This increase was due to the discovery of additional subsystems in known binaries. The latest update on the 25-pc sample by Hirsch et al [14] gives the 17% fraction of hierarchies, larger than in the earlier studies (new subsystems were discovered mostly by high-resolution imaging). According to these authors, only 2/3 of non-single stars are pure binaries, while the remaining 1/3 of the systems host three or more stars.…”

California Legacy Survey. II. Occurrence of Giant Planets beyond the Ice Line

“…We continued to observe those stars and an additional 134 stars at Keck Observatory through 2020. CLS also includes observations of a subset of these stars made with the Hamilton spectrometer at Lick Observatory during 1988-2011, high-cadence Keck-HIRES observations of 235 magnetically inactive stars as part of the Eta-Earth Survey (Howard et al 2010), and high-cadence Lick-APF observations of 135 of those stars (Fulton et al 2016;Hirsch et al 2021). The average star has been observed for 22 years and has 71 RVs with a precision of ∼2 m s −1 .…”

The California Legacy Survey. I. A Catalog of 178 Planets from Precision Radial Velocity Monitoring of 719 Nearby Stars over Three Decades

“…This selection process left us with 719 stars. Figure 1 shows the entire CLS samples as a Venn diagram, illustrating the overlap of the Cumming et al (2008) sample with the Eta-Earth (Howard et al 2010b) and 25 pc northern hemisphere volumelimited (Hirsch et al 2021) samples. The 25 pc sample includes 255 G and early K dwarfs with apparent V magnitudes ranging from V ≈ 3 to V ≈ 9.…”