High-contrast imaging has revealed a population of substellar companions, generally classified as giant planets (~2-13 MJup) or brown dwarfs (~13-75 MJup), orbiting at large separations (~3-1000 au) from their host stars. Past studies have mostly relied on low-resolution spectroscopy (R~20-100) to study their atmospheres, but encountered hurdles in measuring reliable atmospheric abundances. In my thesis, I work to overcome these challenges by studying these objects using high-resolution spectroscopy from Keck/KPIC, a unique single-mode fiber feed into NIRSPEC that provides R~35,000 spectra in the near-infrared. Besides studying substellar atmospheres with KPIC, I contributed significantly to its data reduction pipeline and calibration procedure.
\r\n\r\nWith KPIC, I used atmospheric retrievals to characterize a large sample of planetary-mass companions and brown dwarfs to shed light on their formation history. First, I measured the carbon and oxygen abundances of high-mass brown dwarfs and low-mass M dwarfs (m\u224860-90 MJup) and showed they are chemically homogeneous to their host stars (Chapters 2 and 3). I also made one of the first estimates of the vertical mixing rate in a L/T transition brown dwarf companion from its relative H\u2082O, CO, and CH\u2084 abundances (Chapter 2). Next, I carried out a survey of eight planetary-mass companions with estimated masses between 10-30 MJup (Chapter 4). I found that these companions also have C and O abundances clustered around the solar value, similar to abundances of stars in the same star-forming regions. In these studies, I made several isotopologue ratio measurements including \u00b9\u00b2CO/\u00b9\u00b3CO and showed that a late-M dwarf companion has the same \u00b9\u00b2C/1\u00b9\u00b3C and \u00b9\u2076O/\u00b9\u2078O as its K6V host star. Overall, my KPIC studies show that companions with m\u227310 MJup likely form as the tail-end of star formation, consistent with the conclusions from demographic and orbital architecture studies of substellar companions.
\r\n\r\nNext, I worked on addressing the over-massive brown dwarf problem, an emerging phenomenon where several brown dwarf companions have dynamical masses higher than predictions from evolutionary models given their luminosities. This problem can be solved if these objects are not single entities. Using VLTI/GRAVITY and VLT/CRIRES+, I resolved the first brown dwarf companion, Gliese 229B, into two nearly-equal mass brown dwarfs, Gliese 229 Ba and Bb, on a 12 day orbit (Chapter 5). Gliese 229Bab is the tightest substellar binary orbiting a star, and indicates that other over-massive brown dwarfs might also be unresolved, tight binaries. As a follow-up study, I analyzed JWST/MIRI spectrum (5-14 \u00b5m) of Gliese 229 Bab to show that both brown dwarfs have similar C/O and metallicities as their host star, as expected for a star-like formation scenario (Chapter 6).
", "doi": "10.7907/2w2m-qw02", "publication_date": "2025", "thesis_type": "phd", "thesis_year": "2025" }, { "id": "thesis:16076", "collection": "thesis", "collection_id": "16076", "cite_using_url": "https://resolver.caltech.edu/CaltechTHESIS:06032023-031445650", "primary_object_url": { "basename": "PhD_Thesis (44).pdf", "content": "final", "filesize": 26896529, "license": "other", "mime_type": "application/pdf", "url": "/16076/2/PhD_Thesis (44).pdf", "version": "v5.0.0" }, "type": "thesis", "title": "The Orbits of Young Extrasolar Planets as Formation Probes", "author": [ { "family_name": "Blunt", "given_name": "Sarah", "orcid": "0000-0002-3199-2888", "clpid": "Blunt-Sartah" } ], "thesis_advisor": [ { "family_name": "Howard", "given_name": "Andrew W.", "orcid": "0000-0001-8638-0320", "clpid": "Howard-Andrew-W" } ], "thesis_committee": [ { "family_name": "Knutson", "given_name": "Heather A.", "orcid": "0000-0002-5375-4725", "clpid": "Knutson-H-A" }, { "family_name": "Stevenson", "given_name": "David John", "orcid": "0000-0001-9432-7159", "clpid": "Stevenson-D-J" }, { "family_name": "Ravi", "given_name": "Vikram", "orcid": "0000-0002-7252-5485", "clpid": "Ravi-Vikram" }, { "family_name": "Mawet", "given_name": "Dimitri", "orcid": "0000-0002-8895-4735", "clpid": "Mawet-D" }, { "family_name": "Howard", "given_name": "Andrew W.", "orcid": "0000-0001-8638-0320", "clpid": "Howard-Andrew-W" }, { "family_name": "Wang", "given_name": "Jason", "orcid": "0000-0003-0774-6502", "clpid": "Wang-Jason" } ], "local_group": [ { "literal": "div_pma" } ], "abstract": "In this thesis, I have translated a few flavors of exoplanet timeseries measurements into constraints on exoplanet orbital parameters, and used these to make inferences about planet formation. I begin by introducing the two main observational techniques I used to perform these analyses: optical interferometry and stellar radial velocity monitoring. I then discuss some of the big open questions of exoplanet formation, particularly the mechanism for forming giant planets close to and far from their stars, where core accretion is thought to be too inefficient to form giant planet cores in time for them to accrete atmospheres.
\r\n\r\nHigh cadence radial velocity monitoring enables advances in our understanding of stellar activity, the fundamental stumbling block in the path to discovering and characterizing planets like the Earth. In my second thesis chapter, I present an argument that previously published RV-derived activity models of the PMS star V1298~Tau suffer from overfitting, casting doubt on published mass estimates of the young planets in the system which necessitated rapid contraction after formation, in tension with formation theory. I walk through several potential explanations for this overfitting, pointing out that the star has a strong differential rotation signal which is not included in published model fits, and encourage broader use of cross validation techniques in stellar activity model evaluation.
\r\n\r\nOptical interferometry, particularly using the VLTI/GRAVITY instrument, enables astrometry measurements that are orders of magnitude more precise than contemporary coronographic instruments, which translates to precise orbital parameters. In my third thesis chapter, I present and analyze two new VLTI/GRAVITY astrometric measurements of a young, widely separated planet and use them to make a preliminary argument that the planet's eccentricity is low or moderate. This sets an upper limit on the time (relative to disk dispersal) that the planet attained its current wide separation, and downweights the possibility of scattering after disk dispersal.
\r\n\r\nIn the next two chapters, I showcase my contributions to two widely used open-source orbit-fitting software toolkits: orbitize! and radvel. I highlight two major new features of orbitize! that are available in the main code base as of the release of version 2: jointly fitting radial velocity measurements and jointly fitting absolute astrometry measurements. In the radvel chapter, I motivate and describe an updated Gaussian Process regression model for stellar activity modeling that reduces the potential for overfitting.
", "doi": "10.7907/2j2b-f649", "publication_date": "2023", "thesis_type": "phd", "thesis_year": "2023" } ]