Vedant Chandra


Center for Astrophysics, Harvard & Smithsonian

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White Dwarf Stars

White dwarfs are the evolutionary end-stages of nearly all stars in the Universe, and exhibit fascinating properties due to their density and composition. There are numerous unsolved problems in white dwarf physics, partly because they have been very difficult to observe in large numbers until recently. Nadia Zakamska gave a talk at the IAS in November 2021 that summarizes our group’s progress studying white dwarfs.

The main challenge with studying white dwarfs is that most current theoretical white dwarf atmospheric models are proprietary and kept under lock and key. Along with Nadia Zakamska and Hsiang-Chih Hwang, I developed a Python package (wdtools) that uses various computational techniques to reverse-engineer white dwarf atmospheric models and enable astronomers to predict white dwarf stellar parameters from spectra (Chandra et al. 2020a).

Additionally, we recently wrote a paper (Chandra et al. 2020b) with Sihao Cheng in which we statistically measured the white dwarf mass-radius relation using the gravitational redshift effect. We combined observations of over three thousand white dwarfs from the Sloan Digital Sky Survey (SDSS) and the Gaia space observatory to average gravitational redshifts as a function of stellar radius, and consequently derived this empirical relation. Our work was featured by the Johns Hopkins HUB, as well as the ScienceNews magazine. I wrote a brief article for astrobites describing our methods and results.

chandra2020b photometric mass-radius plot
The main result from Chandra et al. (2020b), an observationally-derived white dwarf mass-radius relation across a wide range of stellar masses. Our measurements are in excellent agreement with current theoretical models (overlaid in red and blue).

I’ve also been leading a search for double white dwarf (WD+WD) binaries in the fifth-generation Sloan Digital Sky Survey. Our first discovery was a 99-minute orbital period binary presented in Chandra et al. 2021b. This is a nearby WD+WD binary in which both stars are visible on the spectrum, allowing us to precisely measure their masses. It turns out to be one of the strongest known emittors of gravitational waves in the millihertz regime, and could be used as a verification source by future space-based gravitational wave observatories.

chandra2021b spectrogram
Continuum-normalized H-alpha spectra of SDSS J1337+3952 from Gemini GMOS across three runs, separated by black lines, along with the double-lined binary model. The residual significance is the difference between the data and model divided by the data uncertainty. Contributions from both stars are clearly visible, and follow a sinusoidal pattern as expected for a circular orbit.

Resolved Stellar Populations

The vast majority of the galaxies we observe are too faint or distant for us to characterize their individual stars. However, using world-class space telescopes like the Hubble Space Telescope, we can resolve the stellar populations of nearby galaxies and use their color-magnitude diagrams to study their initial mass distribution, composition, and more. It’s an open problem in astrophysics whether all galaxies form in a similar fashion to our own Milky Way. The next generation of telescopes like JWST and RST will probe these galaxies deeper than ever before, perhaps finally answering this problem.

There is a strong need for speedy and accurate techniques to analyse resolved stellar populations to infer parameters that described their formation. I worked with Dr Mario Gennaro at the Space Telescope Science Institute (STScI) to develop a computational framework to fit star formation histories and initial mass functions to resolved stellar populations. We are building an open-source package (starwave) to enable the broader astronomical community to perform this kind of analysis. I worked as a long-term research intern at STScI, gratefully funded by the Director’s office.

The First Stars in the Universe

The very first stars in the Universe were formed only a hundred million years after the Big Bang in an environment completely devoid of metals - indeed, these primordial stars themselves formed the first metals. It is theorized that some low-mass primordial stars could survive in the local Universe to this day. However, to date they have eluded all efforts to observe them. Instead, astronomers have observed extremely metal-poor stars, which are fascinating in their own right due to their peculiar chemical composition, old age, and insights into Galactic star formation. One significant challenge when hunting for metal-poor or metal-free stars is that when observed with spectroscopy, they are nearly indistinguishable from cool white dwarfs. I have recently been working with Professor Kevin Schlaufman on developing an automated technique to differentiate metal-poor main-sequence stars from white dwarfs in large spectroscopic datasets. When applied to future large-scale surveys, this method could help find thousands more extremely metal-poor stars, and provide stronger constraints on the existence of surviving primordial stars in the local Universe. Our work is summarized in Chandra & Schlaufman (2021).

Human Physiology during Long-Duration Spaceflight

The next few decades will likely see a revolution in manned spaceflight, due to new launch platforms like SpaceX and renewed public interest in sending people to other worlds like the Moon and Mars. The effects of such long-duration spaceflight on human physiology are not yet well understood. Various factors like radiation, lack of gravity, and even loneliness are being actively researched these days. I am particularly interested in the effect of spaceflight on the vestibular system or inner ear, which governs balance and orientation. I worked at the Johns Hopkins Human Spaceflight Lab with Professor Mark Shelhamer to investigate this question using controlled human trials. Along with a large team of student collaborators, we developed software to analyze physiological signals (hsltools) and applied it to data we collected on 50 subjects in a spaceflight analog experiment.