My expertise is in time-domain astronomy, particularly the use of ground-based optical surveys for large-scale supernova discovery and classification (e.g. ZTF), and in developing data-driven tools for modeling spectra using massively multiplexed spectrographs (e.g. LAMOST). My primary scientific interests are engine-driven explosions (gamma-ray burst afterglows, AT2018cow) and pre-supernova mass-loss.
With the commissioning of wide-field high-cadence optical surveys such as the Zwicky Transient Facility (ZTF), large fractions of the sky are being monitored on nightly or faster cadences, enabling the first systematic exploration of the fast and luminous corner of transient phase-space. For my thesis, I am using ZTF to discover fast-luminous transients in real-time, and watch them unfold with telescopes across the electromagnetic spectrum. My filters enabled the discovery of ZTF18abukavn (SN2018gep), discovered as a rapidly rising transient (1.4 mag/hr) only 20 minutes after shock breakout and spectroscopically classified as a Ic-BL SN. The early discovery enabled an intensive spectroscopic campaign, including the earliest spectral sequence of a stripped-envelope supernova to-date. A retrospective search revealed emission in the weeks prior to explosion. Taken together, we concluded that the data are best explained by shock breakout in a massive shell of dense circumstellar material that was ejected in eruptive pre-explosion mass-loss episodes. Our results suggest that late-stage eruptive mass-loss may be present in a greater variety of massive stars than had been previously thought. See the paper for more details.
The explosion AT2018cow (“The Cow”) was undoubtedly the biggest surprise in time-domain astronomy in 2018. I led a study using millimeter-wavelength and X-ray telescopes that showed that The Cow was likely powered by a central engine, a newborn black hole or neutron star. I established that this new class of explosions are prime targets for millimeter-wavelength observatories such as the Submillimeter Array (SMA), traditionally not used to study cosmic explosions. For more details, see my paper, the review I wrote in the SMA Newsletter, my presentation at the AAS press conference, and articles in Nature News, the Washington Post, Science News, and WIRED.
For my thesis, I am using the Zwicky Transient Facility (ZTF) to conduct the first systematic search for relativistic explosions outside the gamma-ray band, to unearth phenomena invisible to GRB detectors. This is challenging because dirty fireballs and orphan afterglows are expected to produce afterglow emission that appears and disappears on the timescale of hours, with a formidable foreground of false positives. Using archival data from the Palomar Transient Factory, I developed strategies for identifying these contaminants. I found that M-dwarf flares fade more rapidly than afterglows, and that they can be easily mitigated via the color of the host. See the paper for more details.
Today, the quality of stellar spectra from large-scale surveys such as APOGEE has surpassed the precision of the physical models used to fit them. The Cannon is a data-driven approach to spectral modeling that transfers information from high-quality, high-SNR data to lower quality, low-SNR data. This approach can be used to bring qualitatively different surveys onto the same scale. By using The Cannon to reanalyze the LAMOST spectra of 450,000 giants, my work resulted in the largest catalog to-date of stellar masses, ages, and individual abundances (alpha enhancement, carbon, and nitrogen). I also wrote a version of The Cannon for use by the community, along with detailed documentation and tutorials. I also taught a workshop on The Cannon at Gemini Observatory in Chile.
The dense millisecond pulsar (MSP) populations in globular clusters (GCs) provide a unique opportunity to measure and map the galactic magnetic field on small (pc) scales, using the Faraday rotation technique. The globular cluster Terzan 5 has the highest-known population of MSPs: radio flux measurements predict a much higher total than the current population of 35. As a summer student at the NRAO (a program funded by the NSF) I used nearly five full days of 1.5 GHz and 2 GHz Green Bank Telescope data to calculate precise rotation measures (RMs) for 24 of the 35 millisecond pulsars in Terzan 5. Dividing these values by the corresponding DMs enabled me to measure the mean line-of-sight galactic magnetic field strength to each pulsar. I found a 15-20% gradient in line-of-sight magnetic field strength across the cluster with fluctuations on the order of 0.1 microGauss and measured a structure function power law index of alpha = 1.13 +/- 0.15.