I led a study to spatially and spectrally resolve the first brown dwarf companion, Gliese 229 B, into two nearly equal mass brown dwarfs, Gliese 229Ba and Bb. The two brown dwarfs orbit each other every 12 days, as they orbit a M1V star every 250 years. The discovery resolves the apparent conflict between the dynamical mass of Gliese 229 B and substellar evolutionary models. Here's the press release story, and the paper published in Nature. I also wrote a Research Briefing with Dr. Rebecca Oppenheimer.

Since 2025, I'm leading a 25-hour CRIRES+ program to search for more tight brown dwarf binaries that orbit stellar primaries. The program will observe eight young substellar companions, three times each.

I analyze KPIC high-resolution spectra (R~35,000) of directly imaged giant planets and brown dwarfs companions to measure their atmospheric compositions, rotation rates, and radial velocities. I use atmospheric retrievals for this work, specifically with the radiative transfer code petitRADTRANS (Mollière+2019). In Paper I, I studied the benchmark brown dwarf companion HD 4747 B anddemonstrated that high-resolution spectra yielded consistent results independent of assumptions on clouds, while low-resolution data fail to robustly constrain the brown dwarf's abundances.

In Paper II on late-M dwarf companion HIP 55507 B, I detect ¹³CO and H₂¹⁸O isotopologues and validated the companion's ¹²C/¹³C and ¹⁶O/¹⁸O ratios are consistent with those of its K6 primary star, which I also measure using KPIC data.

Paper III: I uniformly measured the C and O abundances, spins, and RVs for eight young, planetary-mass companions (~10-30 MJup, ~50-360 AU). I find that these companions have compositions consistent with solar and their host star compositions, which implies that they most likely form via gravitational instabiliy.

In addition to the science, I am also heavily involved in the instrument commissioning and data pipeline for KPIC. In fact, my first ever research project (as a first year undergraduate) was to build a prototype of the fiber injection system for KPIC (Mawet, Ruane, Xuan et al. 2017). The success of this work laid the foundation for KPIC.

After starting my PhD, I worked with Yinzi Xin on demonstrating speckle nulling through single-mode fibers in KPIC. I'm also involved in the science planning and data analysis for the VFN mode on KPIC, led by Dan Echeverri. VFN is targeting young planets and brown dwarfs orbiting between 1-10 AU (~1 λ/D) from their host stars. Papers on these topics have been published in JATIS. In addition, I worked on streamlining the daytime calibration procedure for KPIC and various other engineering tasks as a Keck Visiting Scholar in 2022.

Xuan et al. 2022: High and low-resolution spectroscopy of HD 4747 B
Xuan et al. 2024a: Carbon and Oxygen Isotopologue Ratios in HIP 55507 AB
Xuan et al. 2024b: Are these planets or brown dwarfs?
Speckle Nulling
Vortex Fiber Nuller: Instrument
Vortex Fiber Nuller: Science
Instrument Paper
Fiber Injection Unit

I am leading the atmospheric retrieval efforts for the 3-5 micron JWST/NIRSpec data (R~2700) of HR 8799 c, d, e (Ruffio & Xuan et al. accepted) and HR 8799 b (Xuan & Ruffio et al. in prep). These iconic planets orbit their host star between 16-71 AU, and have posed challenges to planet formation theories. From the JWST data, we obtain new detections of multiple molecular species in the planet atmospheres, and tightly constrain the C/O and metallicity. These results demonstrate the unprecedented sensitivity of JWST/NIRSpec IFU in studying exoplanet atmospheres, which is feasible for a vast majority of known directly imaged planets.

I am leading a JWST Cycle 2 GO program to acquire an extensive spectrum for Gl 229 Bab, now known to be a tight binary brown dwarf. We observed Gl 229 Bab from 1-5 micron with NIRSpec (R~2700), and 5-14 micron with MIRI LRS (R~50-140). The MIRI spectrum has been published in Xuan et al. 2024d. I found that the two brown dwarfs share the same chemical composition as their host star, as expected for formation via gravitational collapse.

• Xuan et al. 2024d
• Cycle 2 GO

I am leading a JWST Cycle 3 GO program to observe AF Lep b, a 3 MJup giant planet orbiting at 8 AU from its host star. AF Lep b is the closest Jupiter analog that has ever been imaged, and we will collect 3-5 micron spectra at R~3000 to characterize the planet's atmosphere in detail. Situated at the peak of the RV giant planet occurrence rate (Fulton et al. 2021), AF Lep b will serve as a bridge to link the hot Jupiter population with widely separated imaged planets (~10-100 AU).

• Cycle 3 GO

I am leading a JWST Cycle 4 GO program to observe eps Indi Ab, the coldest directly imaged planet with Teff~275 K (Matthews et al. 2024). We will obtain a comprehensive SED from 4-25 microns, as well as brown dwarf-quality spectra to measure multiple elemental and isotopic abundance ratios (e.g. D/H, 15N/14N).

• Cycle 4 GO

I'm also interested in combining different methods (imaging, radial velocity, astrometry) to measure 3D orbital architectures and dynamical masses of exoplanets.

I found that the inner and outer planets in π Men and HAT-P-11 are highly misaligned by ~50 deg in Xuan & Wyatt 2020. I also measured planet-debris disk mutual inclinations in two other planetary systems in Xuan et al. 2020b.

• Xuan & Wyatt 2020
• Xuan+2020b

In the summer of 2017, I wrote software to automate the data processing workflow for the Keck/NIRC2 vortex coronagraph, and helped set up a Mongo database and website to display the data products. These tools are still in use by the group.

With the pipeline, I re-reduced archival data of more than 300 stars, and wrote a paper characterizing the instrument's performance. This paper is the standard citation for work involving Keck/NIRC2 high-contrast science.

• Xuan et al. 2018