Search

Multi-wavelength observations of supernovae (SN) not only probe the explosion mechanism, but also carry information about the configuration of the star at the moment of collapse and the mass-loss history of the progenitor system in the years immediately preceding its death. The study of supernovae therefore offers a rare observational view of the final stages of stellar evolution. As a result, the recent advent of wide-field SN searches---which are discovering new classes of astronomical transients at an ever-increasing rate---has both expanded the types of stellar systems that we can directly probe and challenged theoretical models of the late-stages of stellar evolution. This thesis explores several new regimes of transient space that have been opened by these modern, wide-field, time domain surveys. We present a series of observational studies which constrain the explosion properties, progenitor systems, and intrinsic rates for several classes of peculiar astronomical transients. First, we investigate the properties of transients that reach SN luminosities but evolve on rapid timescales. We present a detailed study of the rapidly-declining hydrogen-poor SN 2005ek, showing that it could be produced by the core-collapse of a stripped massive star with a very small ejecta mass. We then describe results from the first systematic search for rapidly-evolving and luminous transients in a wide-field survey with a rapid cadence. Using data from the PanSTARRS1 Medium-Deep Survey (PS1-MDS), we both identified a new class of rapidly-evolving transients and calculated their intrinsic rates---demonstrating that these explosions are not intrinsically rare. Second, we utilize very early UV/spectroscopic observations and detailed multi-wavelength follow-up of the stripped-envelope core-collapse SN 2013ge to place constraints on the final configuration of the progenitor system. Finally, we investigate the progenitor stars and mass-loss mechanism that operates in a class of luminous SN that explode within a dense circumstellar medium (Type IIn SN). This is accomplished through a joint analysis of the explosion properties and host galaxy environments for a large number of events, including the full sample of Type IIn SN discovered by the PS1-MDS. We find that luminous Type IIn SN explode in a wide range of galaxies which are are robustly distinguished from the host galaxies of hydrogen-poor super-luminous SN in being more luminous, massive, and metal-rich. Furthermore, we show that a significant fraction of luminous Type IIn SN explode in the bright central regions of their host galaxies, indicating that the physical conditions for star formation in these environments are conducive for the formation of luminous Type IIn progenitors., Astronomy

Despite the prevalence of M dwarfs, the smallest and most common type of main sequence star, their sizes, compositions, and ages are not well-constrained. Empirical determination of these properties is important for gaining insight into their stellar structure, magnetic field generation, and angular momentum evolution. I obtained low-resolution (R = 2000) near-infrared spectra of 447 nearby mid-to-late M dwarfs. I measured their absolute radial velocities with an accuracy of 4.4 km/s by exploiting telluric lines to establish an absolute wavelength calibration. I estimated their metallicities from the equivalent width of the sodium absorption feature at 2.2 μm to a precision of 0.12 dex, and from 2MASS colors to a precision of 0.15 dex. Using stars with radii measured from interferometry, I showed that the equivalent widths of aluminum and magnesium absorption features can be used to infer K and M dwarf temperatures to a precision of 69 K, and radii to 0.027 R⊙. I applied these relations to planet-hosting stars from Kepler, showing that the typical planet is 15% larger than inferred when adopting stellar parameters from other recent catalogs. Using photometry from the MEarth-North Observatory, I measured rotation periods from 0.1 to 140 days for 387 M dwarfs. I found a prevalence of stable spot patterns, and no correlation between period and amplitude for fully-convective stars. Using galactic kinematics as a proxy for age, I found that rapid rotators (P < 10 days) are < 2 Gyr, and that the slowest are on average 5 ± 3 Gyr old. I then showed that for early M dwarfs the typical stellar rotation period at 5 Gyr coincides with the orbital period at which habitable planets are found, and I suggest that mid-to-late M dwarfs are optimal targets around which to search for habitable-zone planets. I obtained optical spectra of 247 nearby M dwarfs, and measured the strength of the chromospheric Hα emission line. I identified a well-defined boundary in the mass–period plane that separates active and inactive M dwarfs. Hα activity is therefore a simple, accessible diagnostic for stellar rotation period, and I present a mass–period relation for inactive M dwarfs. I found a significant (p value < 10e−4) positive correlation between Hα emission strength and photometric variability amplitude, which implies that stars with stronger magnetic fields have both higher levels of chromospheric activity and larger or more abundant spots. I suggest that fully convective stars maintain rapid rotation rates and saturated magnetic activity for about 2 Gyr. They then undergo rapid angular momentum evolution upon reaching some critical threshold. Only upon reaching long rotation periods (around 70 days for a 0.2 M⊙ star) do their magnetic activity levels drop below what is required for Hα to be seen in emission. The stars I have observed in pursuit of this work are the nearest low-mass stars. As such they are the best targets around which to search for habitable, rocky worlds, and my work provides the means to constrain the sizes, temperatures, and ages of those planets., Astronomy

Superluminous supernovae (SLSNe) are a rare class of stellar explosions discovered by wide-field optical transient surveys in the past decade. They are characterized by peak luminosities 10-100 times that of ordinary core-collapse and Type Ia SNe, and radiated energies of order 10^51 erg, comparable to the entire kinetic energy of a canonical supernova explosion. Proposed sources of these tremendous energies include interaction between the supernova ejecta and dense circumstellar material (CSM), energy injection from the spin-down of a rapidly rotating and highly magnetized neutron star, or the pair-instability explosion of a very massive star producing several solar masses of radioactive nickel. In this thesis, I present results from the Pan-STARRS1 Medium Deep Survey (PS1/MDS), which discovered 15 hydrogen-poor SLSNe out to redshift 1.6 over the four years of its operation. I address the nature of SLSNe from two different angles. First, I characterize the SNe themselves, and compare their observed properties to model predictions. The PS1/MDS SLSN sample exhibits a diversity of light curve properties, and a wider range of peak luminosities than previously reported, particularly when accounting for the flux-limited nature of the survey. The light curves can generally be fit with magnetar spin-down models, though our sample also contains one very slowly evolving event that could plausibly be powered by radioactive decay. Second, I present the first comprehensive study of SLSN host galaxy environments and the sub-galactic environments, demonstrating that H-poor SLSNe preferentially occur in low-luminosity, low-mass, low-metallicity galaxies with high specific star formation rates. Their host galaxies are statistically distinct from the hosts of core-collapse SNe, but share many similarities with the galaxies that host long gamma-ray bursts (LGRBs). This suggests that the environmental factors leading to a massive star forming either a SLSN or a LGRB are similar, with a possible common ingredient being a preference for low-metallicity environments through the need of a progenitor with high core angular momentum. In terms of their local environments, resolved Hubble Space Telescope imaging reveals that SLSN locations are correlated with the UV light, though not as strongly as LGRBs are. Although a larger sample size is needed to distinguish them statistically, this trend is also consistent with the interpretation that SLSN progenitors are lower-mass than those of LGRBs, collapsing to form a rapidly spinning neutron star rather than a black hole launching a relativistic jet., Astronomy

Gamma-ray bursts (GRBs) are the most energetic explosions in the Universe, thus providing a unique laboratory for the study of extreme astrophysical processes. In parallel, their large luminosity makes GRBs a premier probe of the early Universe. My thesis has explored and exploited both aspects of GRB science by addressing the following fundamental open questions: 1) what is the nature of the GRB ejecta?, 2) how does the GRB progenitor population evolve with redshift, and 3) how can GRBs be used to probe the high-redshift Universe? To answer these questions, I present the first multi-wavelength detection and modeling of a GRB reverse shock, a comprehensive analysis of the plateau phase of GRB light curves, studies of the evolution of the progenitor population to redshifts, z~9, and demonstrate the use of GRBs as probes of galaxy formation and evolution through the first galaxy mass-metallicity relation at z~3-5. I find support for baryonic ejecta in GRB~130427A, evidence that GRB jets contain a large amount of energy in slow-moving ejecta, and proof that the GRB progenitor population does not evolve to the highest redshifts at which it has yet been observed. Building on the decade of observations by the Swift GRB mission, future observations and modeling of GRBs and their host galaxies will provide clues to these and other open questions in GRB science, allowing for the first statistical studies of their progenitors and host environments to the epoch of reionization and beyond., Astronomy

Gamma-ray bursts (GRBs) are relativistic explosions which originate at cosmological distances, and are among the most luminous transients in the universe. Following the prompt gamma-ray emission, a fading synchrotron "afterglow" is detectable at lower energies. While long-duration GRBs (duration > 2 sec) are linked to the deaths of massive stars, the progenitors of short-duration GRBs (duration < 2 sec) have remained elusive. Theoretical predictions formulated over the past two decades have suggested that they are the mergers of two compact objects, involving either two neutron stars (NS-NS) or a neutron star and a black hole (NS-BH). Such merging systems are also important to understand because they are premier candidates for gravitational wave detections with upcoming facilities and are considered likely sites of heavy element nucleosynthesis. The launch of the Swift satellite in 2004, with its rapid multi-wavelength monitoring and localization capabilities, led to the first discoveries of short GRB afterglows and therefore robust associations to host galaxies. At a Swift detection rate of ~8 events per year, the growing number of well-localized short GRBs enables comprehensive population studies of their afterglows and environments for the first time. In this thesis, I undertake a multi-wavelength observational campaign to address testable predictions for the progenitors of short GRBs. From their local environments, I show that short GRBs explode in diffuse regions of their host galaxies and are weakly correlated with the distribution of stellar mass and star formation in their host galaxies. I study the host galaxy demographics for the entire population and find that ~20-40% of short GRBs originate from elliptical galaxies, implying an older stellar progenitor. From their afterglows, I present evidence that some short GRBs are collimated in narrow jets of ~5-10 degrees, directly affecting the true energy scale and event rate. Finally, taking advantage of a decade of broad-band afterglow observations at radio through X-ray wavelengths, I find that short GRBs have median isotropic-equivalent energies of ~10^51 erg and that their local environments have low densities, ~10^-3-10^-2 cm^-3. Taken together, this thesis comprises several lines of independent evidence to demonstrate that short GRBs originate from the mergers of two compact objects, and also provides the first constraints on the explosion properties for a large sample of events. With the direct detection of gravitational waves from compact object mergers on the horizon, these studies provide necessary inputs to inform the next decade of joint electromagnetic-gravitational wave search strategies., Astronomy