My graduate studies in the supernova group at Las Cumbres Observatory have allowed me to take part in an amazing variety of projects, both single-event case studies and large sample analyses, of supernovae of all classes, tidal disruption events, and gravitational wave follow-up. The three projects on which I've taken the lead have centered around the use of supernovae as probes of circumstellar environments. I discuss each of these in detail below, followed by other significant projects in which I've participated.
Type Ia supernovae are the thermonuclear explosions of white dwarf stars in a binary system. However, the nature of the binary companion has been debated for several decades: it may be a nondegenerate main sequence or giant star, or it may be another white dwarf. Understanding the physics of Type Ia progenitors has the potential to improve their application as standard candles for cosmology, as well as shed light on some of the uncertainties in binary stellar evolution.
SN 2017cbv was discovered very young, within about a day of explosion, by the DLT40 nearby galaxy survey. During the first five days of observations, we observed extra U-band (350 nm) emission compared to the smooth rise of a normal Type Ia supernova. We fit models of interaction between the material ejected by the supernova and a nondegenerate binary companion. Overall, these models reproduced the time dependence of the excess emission quite well, but they greatly overpredicted the observed UV (200–300 nm) flux. This discrepancy may indicate significant deviation from a blackbody spectrum due to line absorption in the UV.
From rates, a lack of a surviving companion star, the lack of hydrogen, and the rarity of these blue bumps, the consensus is that many if not most Type Ia supernovae do not have a nondegenerate companion star.Yet there are a handful of cases where a nondegenerate companion is inferred, and our letter on SN 2017cbv presents the strongest evidence yet. This has many implications, including the fact that there are at least two progenitor paths to Type Ia supernovae, and that accretion of hydrogen or helium is a viable path to grow a white dwarf.
Core-collapse supernovae that interact strongly with their circumstellar material are excellent probes of mass loss in massive stars (more than 8 times the mass of the sun). Most interacting supernovae show narrow hydrogen emission lines in their spectra (Type IIn supernovae), which are produced as photons and/or ejected material excite slowly-moving (less than 1000 km/s) material present around the star since before it exploded. Such supernovae can have a wide variety of light curves, lasting up to several years, since the circumstellar hydrogen can occupy a relatively large volume and may even have a non-uniform density profile.
Type Ibn supernovae, which show narrow helium lines in their spectra (and no hydrogen), are a rare and poorly understood class of interacting supernovae. Our paper combined photometry and spectroscopy on six new events with all other events from the literature to form the largest-yet sample of Type Ibn supernovae. We found that, unlike Type IIn supernovae, Type Ibn supernovae evolve quickly, rising in a few days and declining at a rate of 0.1 mag/day. Their light curves are homogeneous (with a few exceptions) but their maximum-light spectra can either show P Cygni lines on a blue continuum or more complicated emission features. This may be evidence for diversity in the progenitor state at the time of explosion: either two discrete states or a continuum of properties.
In the course of working on this paper, I learned about PS1-12sk, a Type Ibn supernova that exploded in a giant elliptical galaxy. Elliptical host galaxies are exceedingly rare (less than 1%) for a core-collapse supernova, since massive stars have relatively short lifetimes. The fact that one of only 22 Type Ibn supernovae exploded in an elliptical galaxy could be an indication that these supernovae may not come from massive stars. I proposed for and was allocated time on the Hubble Space Telescope to search for star formation near the explosion location of PS1-12sk in order to investigate this possibility. I expect observations to be taken sometime in the next year.
Traditionally, stellar evolution theory has predicted the lives of massive stars to be relatively uneventful until they explode as a core-collapse supernova. However recent observations of some Type II (hydrogen-rich) supernovae indicate the presence of a confined shell of material around the star, ejected by an unknown mechanism during the last months to years of the star's life. The shell manifests itself as narrow high-ionization lines in the supernova spectra present for only a few days after explosion. These lines have been termed "flash ionization" lines, because they are excited by a burst high-energy photons produced as the shock wave breaks out of the stellar surface. The lines disappear as material ejected in the explosion sweeps up the circumstellar shell.
Previous work has suggested that flash ionization lines only appear in the most luminous Type II supernovae, perhaps because their progenitors are hotter and more massive. However, we classified SN 2016bkv, which lies in the low-luminosity, low-velocity tail of Type II supernovae, very early, probably within a few days of explosion. Spectra during the first two days of follow-up show a hot thermal continuum with narrow flash ionization features, which quickly give way to low-velocity Type II spectra. The fact that circumstellar material was present even in this low-energy explosion suggests that pre-supernova mass loss is ubiquitous in progenitors of Type II supernovae, or even core-collapse supernovae in general, which may help to unify Type IIP and IIL supernovae.
This year, the LIGO/Virgo Collaboration announced the first detection of gravitational waves from a binary neutron star merger (GW170817). Our group at Las Cumbres Observatory was one of six to independently discover an optical counterpart of the merger, called a kilonova. The kilonova peaked less than one day after the merger and faded beyond detection in the optical in just five days, making follow-up very difficult. However, using our global network of telescopes, we were able to observe the kilonova every eight hours to produce one of the best-sampled light curves of this first-of-its-kind event.
I have been heavily involved in many aspects of our gravitational wave follow-up project since its inception in mid-2015. Because of the small fields of view of our telescopes and the poor localization of the gravitational wave source, we adopted a galaxy-targeted follow-up strategy. After receiving an alert from LIGO/Virgo, we choose targets from a galaxy catalog, prioritized by probability in the 3D sky map, galaxy mass, and inverse distance. The kilonova host galaxy was fifth on our list for GW170817. Preparatory work involved designing and testing software to receive gravitational-wave alerts and produce the galaxy list, upgrading our image subtraction software to aid in transient detection, and testing the full system on earlier gravitational-wave alerts (some of which were spurious detections). I also contributed to the analysis of the kilonova data once it was obtained.
My current group (4–6 people) leads the Global Supernova Project, which will produce light curves and spectral series of hundreds of supernovae over the course of three years using Las Cumbres Observatory's unique capabilities. Much of my time is spent developing the infrastructure to observe dozens of targets at a time, store and reduce the data semi-automatically, and display it to users in a useful way. I have taken the lead in maintaining and improving our photometric and spectroscopic reduction pipelines (originally written by Stefano Valenti) and have participated in all other aspects of our data management. Our database currently contains over 300,000 photometry points and 4,500 spectra. This is a science in itself—one that will become increasingly relevant as we approach the era of the Large Synoptic Survey Telescope, when we will discover millions of transients each night! In order to formalize my astronomical data science training, I have been accepted into the LSSTC Data Science Fellowship Program, consisting of six week-long schools over a two-year period.