About me

I am a fourth-year PhD student and NASA FINESST Fellow at Caltech.

My primary research interest lies in exoplanet atmospheres. Specifically, I am working on direct imaging and high-resolution spectroscopy to measure the compositions, spins, and radial velocities of giant planets and brown dwarf companions.

Before starting my PhD, I pursued a MPhil at the University of Cambridge (2020) as a Downing Scholar. There, I worked with Mark Wyatt on calculating the 3-D orbits and dynamical masses of giant planets discovered by the radial velocity technique.

I received a BA in Physics from Pomona College (2019), where I spent 4 enriching years learning about the universe and widening my worldview through a philosophical reading group I organized.

View my CV for more information.

Research Projects

High-resolution spectroscopy with KPIC

Advised by Dimitri Mawet, Heather Knutson, Jason Wang, and Jean-Baptiste Ruffio

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 the Paper I on benchmark brown dwarf companion HD 4747 B, I demonstrated 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 am currently working on a sample of eight young, planetary-mass companions (~10-30 MJup) to uniformly measure their abundances and constrain their formation pathway.

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 undergrad) 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.

JWST 1-14 micron spectroscopy of the first T dwarf companion

I am leading a JWST Cycle 2 GO program to acquire an extensive spectrum for Gl 229 B, the first T dwarf discovered. We will be observing Gl 229 B from 1-5 micron with NIRSpec (R~2700), and 5-14 micron with MIRI LRS (R~50-140). The MIRI spectrum has arrived, and retrievals are in progress!

JWST/NIRSpec R~3000 spectroscopy of AF Lep b, the lowest-mass directly imaged planet

Co-PIs: Jean-Baptiste Ruffio, Yapeng Zhang

I'm very excited about our 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 we have imaged, and we will collect 3-5 micron spectra at R~3000 to characterize the planet's atmosphere in detail.

Direct measurements of orbital mutual inclinations in exoplanet systems

Advised by Mark Wyatt

I'm also interested in combining different methods (imaging, radial velocity, astrometry) to constrain 3D orbital architectures and 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.

Characterizing the Keck/NIRC2 vortex coronagraph for high-contrast imaging

Advised by Dimitri Mawet, Garreth Ruane, and Henry Ngo

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.

Selected Publications
Xuan, W.J., Wang, J., Finnerty, L., et al. (2023) Validation of elemental and isotopic abundances in late-M spectral types with the benchmark HIP 55507 AB system. ApJ, 962, 10

Xuan, W.J., Wang, J., Ruffio, J.-B., et al. (2022) A Clear View of a Cloudy Brown Dwarf Companion from High-resolution Spectroscopy. ApJ, 937, 54

Xuan, W.J. & Wyatt, M.C. (2020) Evidence for a high mutual inclination between the cold Jupiter and transiting super Earth orbiting π Men. MNRAS, 497, 2096

Xuan, W.J., Kennedy, G.M., Wyatt, M.C., Yelverton, B. (2020) Mutual inclinations between giant planets and their debris discs in HD 113337 and HD 38529. MNRAS, 499, 5059

Xuan, W.J., Bryan, M.L., Knutson., et al (2020) A Rotation Rate for the Planetary-mass Companion DH Tau b. AJ, 159, 979

Xuan, W.J., Mawet, D., Ngo, H., Ruane., et al. (2018) Characterizing the Performance of the NIRC2 Vortex Coronagraph at W. M. Keck Observatory. AJ, 156, 156

Finnerty, L., Xuan, W.J., Xin, Y., et al. (2023) Atmospheric metallicity and C/O of HD 189733 b from high-resolution spectroscopy. AJ, 167, 43

Xin Y., Xuan, W.J., Mawet, D., et al. (2023) On-sky speckle nulling through a single-mode fiber with the Keck Planet Imager and Characterizer. JATIS 9, 035001

Echeverri, D., Xuan, W.J., Jovanovic, N., et al. (2023) Vortex Fiber Nulling for Exoplanet Observations: Implementation and First Light. JATIS, 9, 035002

Mawet, D., Ruane, G., Xuan, W., et al. (2017) Observing Exoplanets with High-dispersion Coronagraphy. II. Demonstration of an Active Single-mode Fiber Injection Unit. ApJ, 838, 92

Click below for a full list of my publications.

Outreach

In the summer of 2019, I worked for 6 weeks as Teaching and Residential Assistant for the Summer Science Program, where I am alumnus (2014). In the Astrophysics camp, students participate in an intensive research project on asteroid orbital determination, organizing their own observations and reducing real telescope data. As TA, I helped with late-night observing runs, homework sessions, and answering questions. I also lived together with the students, organized fun activites including sports, board games, boba and hiking trips. It was an incredibly rewarding experience to see the participants grow both as scientists and human beings.

During college, I worked as TA for several physics and astronomy classes, including Observational Astrophysics (twice), Intro to Astronomy, and Advanced Intro Physics.

At Caltech, I volunteered for several astronomy outreach events. For example, in the partial solar eclipse of Oct 2023, I made pinhole cameras with many enthusiastic kids.