Probing the Origins of Directly Imaged Planets and Brown Dwarfs: From Atmospheric Compositions to Binarity

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. With 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≈60-90 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 H₂O, CO, and CH₄ 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 ¹²CO/¹³CO and showed that a late-M dwarf companion has the same ¹²C/1¹³C and ¹⁶O/¹⁸O as its K6V host star. Overall, my KPIC studies show that companions with m≳10 MJup likely form as the tail-end of star formation, consistent with the conclusions from demographic and orbital architecture studies of substellar companions. Next, 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 µm) 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).