My research spans several subfields of Galactic archaeology, interstellar dust, and the cosmic distance scale, among them:
- The origin and properties of second generations in Galactic globular clusters.
- Interstellar extinction curve and variations thereof, particularly toward the inner Galaxy
- The Milky Way’s bulge, morphology, ages, nature
- Red giant stellar evolution the red giant branch bump, the red clump
- The WFIRST microlensing planet survey, systematics and survey design
- The cosmic distance scale, RR Lyrae, Mira variables
My primary research interests are inferences of helium abundance variations in the stellar field, improving understanding of and from probes of Galactic archaeology such as extremely-metal-poor stars and RR Lyrae stars, and the wavelength-dependence of the interstellar extinction curve and variations thereof. I have worked extensively with data from the APOGEE, OGLE, VVV, and EMBLA surveys.
Optical/Near-Infrared Extinction curve variations toward the inner Milky Way
At left, the colour-magnitude diagrams for two sightlines (orange and blue) in two different colours, (V-I) on the left and (J-Ks) on the right. Though the two sightlines show a similar (V-I) colour distribution, they show a distinct (J-Ks) colour distribution, with the ratio differing by 40%. This is indicative of the extinction curve toward the inner Galaxy, which is non-standard in the mean, variable, and has at least two components to its shape.
The age and helium abundance of the Galactic bulge
Shown at left, the disparate star formation histories of the Galactic bulge. They are fully inconsistent, with mean ages ranging from 3 to 13 billion years. I have speculated that this inconsistency may be due to elevated helium enrichment, due to the fact that it specifically solves the low number counts and high brightness of the red giant branch bump, and the the fact that it can exactly resolve the turnoff age discrepancy. Efforts to further constrain the helium abundance with detached red giant eclipsing binary twins are ongoing.
The distance scale with Mira variable stars
Mira variable stars, with an typical brightness MKs ~ -7, are observable to extragalactic distances, even in sparse (less blended) haloes as they trace both young and old stellar populations. I recently supervised Johns Hopkins undergraduate student Wenzer Qin, who investigated the systematics in the Mira-based distance to the Galactic centre as a case study. Once geometry, extinction, and photometric completeness were accounted for, a final distance estimate of 7.9 ± 0.3 kpc was achieved, consistent with other literature values. At left, we show the observed dereddened period-colour relationships as the black points, along with some theoretical predictions shown as the coloured lines.