Harnessing the Potential of Radial Velocity Exoplanet Surveys

open_access
Open Access
Author:
Gupta, Arvind
Graduate Program:
Astronomy and Astrophysics
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
May 12, 2023
Committee Members:
Randy McEntaffer, Major Field Member
Jason Wright, Chair & Dissertation Advisor
Eric Ford, Major Field Member
Randy McEntaffer, Professor in Charge/Director of Graduate Studies
Bradford Foley, Outside Unit & Field Member
Ian Czekala, Major Field Member
Keywords:
exoplanets
radial velocity
spectrograph
fisher information
oscillations
stellar variability
survey
Abstract:
Dedicated exoplanet surveys which combine generous time allocations with the cutting-edge precisions afforded by current extreme precision radial velocity (EPRV) spectrographs have placed a longstanding goal within reach: the first detection of an Earth analog exoplanet. But survey success will depend critically on our capacity to enact optimized observing strategies and mitigate known sources of noise to extract the weak signals induced by the exoplanets we seek. Here, I discuss steps that I have taken to address this challenge through work with NEID, a new spectrograph on the WIYN 3.5m telescope at Kitt Peak National Observatory, and with a novel approach to EPRV exoplanet survey analysis. First, I describe the NEID Earth Twin Survey (NETS), a 5-year RV exoplanet survey being conducted with the NEID spectrograph. In addition to discussing the goals and constraints of the survey and outlining our top level observing strategy, I break down the various sources of external noise that will degrade the achieved precision relative to the instrumental floor. I then formulate a target prioritization metric with which I identify the sample of nearby, Sun-like stars that is most conducive to RV exoplanet detection. Next, I describe the design of the graphical user interfaces (GUIs) with which NEID spectrograph operations are conducted. These GUIs are integrated into the NEID instrument control system and facilitate seamless communication with spectrograph subsystems and enable automatic spectrograph configuration and target ingestion from the nightly observing queue, which improves operational efficiency and consistency across epochs. By interfacing with the NEID exposure meter, the GUIs also allow observers to monitor the progress of individual exposures and trigger the shutter on user-defined SNR thresholds, ensuring the collection of high precision data and minimizing noise contributions from detector charge transfer inefficiency. I then discuss the detection and analysis of p-mode oscillation signals in the subgiant star HD 35833 with both NEID and TESS. These results validate models for the correlated noise impact of oscillations on RV observations and strategies for mitigating this noise contribution. While the signal is detected in both RVs and photometry, the observed RV amplitudes are suppressed significantly relative to expectations. I discuss possible causes of this discrepancy and comment on implications for RV exoplanet observations. I also present an update model for predicting the residual RV oscillation amplitudes for sequences of exposures separated by nonzero readout time. Acknowledging the obstacles presented by oscillations and other correlated noise contributions from stellar variability, I outline a new EPRV survey simulation and analysis framework, in which the Fisher information content of a survey is used to determine exoplanet detection limits. I use this framework to assess the efficacy of common observing strategies and to identify those that will optimize detection of Earth analog exoplanets under various assumptions about stellar variability signals. Finally, I present a detailed analysis of the first two years of NETS observations. I outline various diagnostic measures of the survey performance to date and I identify and discuss several RV signals detected in our data set, including a new exoplanet orbiting the nearby Sun-like star HD 86728. This is the first exoplanet discovered with NETS, and it showcases the state-of-the-art precision NEID is able to deliver. I conclude with a discussion of how the work presented herein will inform the design of future EPRV exoplanet surveys.