Pushing the (Convective) Envelope: Mitigating Radial Velocity P-mode Oscillations in Subgiants to Reveal Low-amplitude Companions in Evolved Systems

Evolved stars are an important population for understanding demographics of planets orbiting stars more massive than the Sun, as well as the interplay between stellar and planetary evolution. Legacy radial velocity (RV) surveys like the “Retired” A Star Survey revealed that evolved stars typically host giant planets on long-period orbits, markedly different from what we observe in systems around Sun-like stars. However, such RV surveys of subgiants are limited by a 5-10 m/s astrophysical floor from stellar p-mode oscillations, which hinders the detection of planets smaller than Saturn mass. Particularly, the long p-mode timescales for subgiants preclude the application of mitigation strategies typically used on dwarf stars. Here, we describe a novel approach to mitigating p-mode oscillations for subgiant stars using a tailored observing strategy, which we have begun employing with the Automated Planet Finder (APF). We share initial results from the first year of this ongoing program, the “APF A-star Retirement Program” (AARP), that demonstrate APF’s ability to observe individual stars at specific cadences within a night, and subsequently push through the stellar p-mode noise floor. Specifically, we show that despite the lower instrumental precision of APF, it can outperform Keck-HIRES performance when using our AARP observing strategy. We use these observations to confirm several planet candidates, adding to the small population of planets known to orbit evolved stars. Continued observations with this program will extend the baseline to improve sensitivity to long-period planets and become sensitive to possible sub-Neptunes/super-Earths in these systems.