Research

Stellar characterization

Most directly imaged planet host stars are poorly characterized in terms of their abundances, primarily due to their very high rotation velocities ( $v \sin i > 30 k m / s$ ), and some even in excess of $100 k m / s$ . Such high velocities can cause significant rotational broadening in these stars and lead to blending of adjacent spectral absorption lines, making spectral characterizationa challenging. However, decoding the formation and evolution pathways of these directly imaged planets requires abundance measurements for both the planets and their host stars. A slew of ongoing projects involving both ground-based and space-based instruments have led to measurements of elemental abundance ratios, primarily the carbon-to-oxygen (C/O) ratio , for the companions. My work aims to measure the abundance of 15 elements (C, O, Na, Mg, Si, S, Ca, Sc, Ti, Cr, Mn, Fe, Ni, Zn, Y), as well as certain abundance ratios (C/O, but also carbon-to-sulfur: C/S and oxygen-to-sulfur: O/S ratios) for directly imaged planet host stars.

The paper corresponding to the above analysis of this initial sample was submitted to AJ and is currently under review. The fastest rotator in the currently submitted paper is the host star 51 Eridani ( $v \sin i \sim 70 k m / s$ ). The next paper in this project will feature hotter directly imaged exoplanet host stars (B and A types), which are among the fastest-rotating stars ( $v s i n i > 100 k m / s$ ).

Exoplanet characterization

The James Webb Space Telescope (JWST) opened up new frontiers in exoplanet characterization with its excellent data quality allowing us to identify previous undetected spectral features in exoplanet atmospheres in the 3-5 micron range. The most significant was the first detection of $S O_{2}$ in the atmosphere of an exoplanet: Alderson et al. (2022) detected $S O_{2}$ in the atmosphere of the hot Jupiter WASP-39b using the NIRSpec instrument onboard JWST. Identifying signatures of molecules like sulfur dioxide, hydrogen sulfide, and ammonia in the atmospheres of exoplanets might allow us access to elemental ratios like C/S, O/S, and N/O, which in addition to C/O, could act as formation tracers, allowing us to decode planet formation.

I am leading the analysis of JWST GO 2778 (PI Perrin) NIRSpec IFU data for the coldest directly imaged exoplanet GJ 504b. I have my own approved JWST program (GO 5485; PI Baburaj) to observe the 28MJup companion HD 206893B using the NIRSpec IFU. Measuring the detailed abundances of this closely separated companion (separation ~ 190mas) will provide a window into the utility of formation diagnostic tools at the extreme upper end of the planet formation process. The observations for this program are scheduled for October-November 2024. The host stars for the above companions (GJ 504 and HD 206893 respectively) have already been analyzed by me as part of my work on the host stars.

Population studies

The composition of the host star encodes the composition of the environment in which the planet forms. Variations in this composition can play a significant role in the occurrence of planets around the host star. Recent work has found relationships between planet occurrence and the abundance of individual elements such as Mg, Si, and Ni for the transiting planets discovered by Kepler. However, no such study has been conducted among the directly imaged planet population. Measuring the detailed chemical makeup of the host star (in addition to carbon and oxygen) could allow us to investigate some of these population trends among this population of companions.

On the companion front, I am interested in performing a population-level analysis of the C/S and O/S ratios as formation tracers and compare them to similar tracers measured for the transiting planet population. It would be interesting if these ratios clearly segregate the two populations of planets like was seen for the C/O ratio.