SAMUEL CABOT'S RESEARCH SITE

EXOPLANET DETECTION WITH EPRVS

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An orbital stability analysis of the HD 3651 system that I contributed to the first flagship EXPRES study. The known, eccentric, Saturn-mass planet clears a significant region of parameter space, thus making the system an excellent benchmark for extreme-precision RVs.

The radial velocity technique is a well-known and frequently employed approach for discovering non-transiting exoplanets. It is based on the principle of measuring perturbations to a star's velocity owing to a companion planet's gravity. Widespread efforts have focused on improving the minimum observable perturbation in order to detect the smallest and most widely separated exoplanets. I am a member of Debra Fischer's research group which manages the Extreme Precision Spectrometer (EXPRES), a latest-generation radial velocity instrument installed at the 4.3m Lowell-Discovery Telescope (LDT). As EXPRES continues its multi-year campaign for low-mass planets, I have worked on special set of high-activity stars which serve as a testbed for new activity mitigation methods in Extreme-Precision Radial Velocity (EPRV) datasets.

I contributed a stability analysis to the first EXPRES science paper on HD 3651b (Brewer et al. 2020). I also led the development of our in-house Gaussian Process (GP) framework (Cabot et al. 2020) which we applied to HD 101501. I investigated the effect of observing cadence on our capacity to detect low-mass, short-period exoplanets. Our findings indicated that a high-cadence observing schedule is most conducive for GP modeling of quasi-periodic activity signals over timescales of tens of days. We also introduced the concept of studying contemporaneous surface maps of stars toward constraining the activity contribution.

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Testing the efficacy of GP-based RV methods with an injection-recovery scheme. Extreme-precision radial velocities are synthetically generated for a variety of observing cadences. A nested-sampling routine attempts to recover Keplerian signals while using GPs to model correlated noise. High-cadence observing strategies help the GP learn the correlated structure and recover the underlying planets.

The next flagship EXPRES paper presented a new stellar activity radial velocity model (Roettenbacher et al. 2020), in which I contributed a new framework for translating stellar surface maps into radial velocities over a full rotation. Surface maps can be derived from lightcurve inversion or interferometry. In this case, we observed Epsilon Eridani contemporaneously with TESS Sector 31. Our new activity model successfully reduced the RMS scatter by 2 m/s, and we are currently refining the methodology and identifying new targets for its application.

A new radial velocity activity model (Roettenbacher et al. 2022). Lightcurve inversion reveals the actual spot distribution on the stellar surface (left). Our framework converts the surface map into a 2D projection and pixelated grid, and simulates an integrated spectrum, from which we extract an RV curve. Our model closely matches the EXPRES data (right)