The Stellar Mass-Weighted Orbit and Metallicity Distribution of the Milky Way

Coming soon ...

For more information, you can refer to my AAS247 iPoster.

Constraining Mass Loss on the Red Giant Branch Using Wide Binaries and Asteroseismology

Mass loss on the red giant branch (RGB) is poorly constrained, despite being a key ingredient in stellar models. In this work, we leverage wide binaries, whose diversity in parameter space and coeval nature are advantageous to understanding mass loss. From the El-Badry+2021 catalog of wide binaries based on Gaia EDR3 astrometry, we identify a sample of 9 wide binaries with a primary red clump (RC) star and a secondary upper main sequence/subgiant star. We then estimate the mass lost on the RGB for the RC star by jointly fitting the two stars to a grid of stellar models. These stellar models are generated from MESA and GYRE, where mass loss is encoded using the Reimers prescription with various mass-loss efficiencies. For our fittings, we use classical observable quantities for both the primary and secondary star, individual asteroseismic frequency modes of oscillation for the primary star, and apply binary constraints that account for the coevality of the binary system. From our fits, we derive an average integrated mass loss and an average Reimer's mass-loss efficiency. This integrated mass loss value is more consistent with the smaller values predicted by seismic studies of open clusters, rather than the larger values predicted by globular clusters. We do not observe a dependence on stellar metallicity, both internally within our sample and when extrapolating mass loss-metallicity trends determined from studies of metal-poor globular clusters.

For more information, you can watch my REU Final presentation or refer to my AAS245 iPoster, for which I won the Chambliss Astronomy Student Achievement Award.

Willman 1 Revisited: The Kinematics, Chemistry, and Orbital Properties of a Potentially Disrupting Dwarf Galaxy

The ultra-faint Milky Way satellite Willman 1 (W1) was the first stellar overdensity found via resolved stars in the Sloan Digital Sky Survey, yet its classification as either a dwarf galaxy or star cluster remains ambiguous. Using new Keck/DEIMOS spectroscopy, Hubble Space Telescope photometry, and orbital modeling, we re-examine the nature of W1. From our updated sample of 56 member stars, we find that past analyses included four binaries and seven nonmembers, identified here using Gaia proper motions and updated velocities. We continue to find a velocity dispersion consistent with previous analyses. If W1 is in equilibrium, this suggests a dynamical mass of ~10^6 solar masses and a mass-to-light ratio indicative of a galaxy. Based on Ca II triplet measurements, we estimate an iron abundance of [Fe/H]=-2.45 and a non-negligible metallicity dispersion. We confirm that W1 does not exhibit mass segregation inside one half-light radius. Our best-fit orbital model predicts that W1 is at apocenter, implying that W1 was closer to the Milky Way in the recent past, reaching a pericentric distance <25 kpc from the Galactic center ~0.3 Gyr ago. Given its internal kinematics, metallicity spread, and lack of mass segregation, we conclude that W1 is a galaxy. However, given its orbit and structural properties, which suggest that W1 might be tidally disrupted, and the difficulty of identifying a pure member sample, we caution that the measured internal velocity dispersion may not accurately reflect the dynamical mass of this system.

More information can be found in the paper, Chiu et al. (2026) (or arXiv version), and by accessing the corresponding code. You can learn more about and access the dataset here. An accessible doodle of the paper is available here, or you can read more about Willman 1 on its Wikipedia page.