The Astrophysical Origins of Stellar Radial Velocity Variability and its Impact on Planet Detection
Open Access
- Author:
- Luhn, Jacob
- Graduate Program:
- Astronomy and Astrophysics
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- June 07, 2021
- Committee Members:
- Rebekah Dawson, Major Field Member
Michael Eracleous, Major Field Member
Jason Wright, Chair & Dissertation Advisor
Eric Ford, Major Field Member
James Kasting, Outside Unit & Field Member
Randall Lee Mc Entaffer, Program Head/Chair - Keywords:
- exoplanets
radial velocity
stellar variability
jitter
planet detection
stellar astrophysics - Abstract:
- With current and upcoming radial velocity (RV) spectrographs capable of extreme precision (30 cm/s and below), the next hurdle for measuring precise masses of Earth-mass exoplanets is intrinsic stellar variability, or "jitter," which can imprint RV signals at the m/s level, an order of magnitude larger than the Earth-analog signals we seek to uncover (~10 cm/s). It is crucial that we better understand the effects of stellar RV jitter in order to select and prioritize targets best suited for RV observations. I present the results of an in-depth analysis of stellar RV jitter for more than 600 California Planet search stars with as many as 20 years of observations. The careful approach to calculating RV jitter requires removing the center-of-mass motions due to planets. I describe the discovery of 15 new planet and planet candidate companions to subgiant host stars---a relatively understudied population---and representing an increase in the number of known planets around such stars by 15%. These planets provide a glimpse of planet formation and evolution around more massive stars and build on established trends that these stars are more likely to host Jupiter-mass planets on long (1-2 au) orbits. These detections allow for the robust measurements of RV jitter as a function of evolution. Based on this large sample of RV jitter measurements, I provide an astrophysical framework to describe the evolution of RV jitter and the resulting "jitter minimum"---the period during which a star is most amenable to RV observations. I show that RV jitter depends strongly on a star's mass, surface gravity, and level of activity. Further, I highlight the existence RV-stable F stars---typically avoided in RV surveys---and I provide several metrics for selecting such stars, opening a new domain for planet hunting. Studies of these stars will bridge the gap between solar-type stars of typical surveys and the evolved sample of more massive stars described above. Using this framework for the astrophysical evolution of RV jitter, I construct several models to predict the RV jitter of stars a priori based on their stellar properties. In particular, I use a hierarchical Bayesian model to fit individual astrophysical sources of RV jitter: activity, granulation, and oscillations, making it capable of estimating not only the overall amplitude of stellar RV jitter for a given star, but also each astrophysical component, which will allow for informed observing strategies for RV follow up in order to mitigate individual sources of stellar RV variability. Finally, I investigate the effect of correlated noise due to intrinsic stellar variability on proposed future RV surveys by employing Gaussian process kernels for activity, granulation, and oscillations. I show that these sources of correlated noise can significantly reduce the precision with which we can measure the masses of Earth-analogs.