K. Bui

Hydrogen-poor superluminous stellar explosions

Supernovae are stellar explosions driven by gravitational or thermonuclear energy that is observed as electromagnetic radiation emitted over weeks or more. In all known supernovae, this radiation comes from internal energy deposited in the outflowing ejecta by one or more of the following processes: radioactive decay of freshly synthesized elements (typically 56Ni), the explosion shock in the envelope of a supergiant star, and interaction between the debris and slowly moving, hydrogen-rich circumstellar material. Here we report observations of a class of luminous supernovae whose properties cannot be explained by any of these processes. The class includes four new supernovae that we have discovered and two previously unexplained events (SN 2005ap and SCP 06F6) that we can now identify as members of the same class. These supernovae are all about ten times brighter than most type Ia supernova, do not show any trace of hydrogen, emit significant ultraviolet flux for extended periods of time and have late-time decay rates that are inconsistent with radioactivity. Our data require that the observed radiation be emitted by hydrogen-free material distributed over a large radius (∼1015 centimetres) and expanding at high speeds (>104 kilometres per second). These long-lived, ultraviolet-luminous events can be observed out to redshifts z > 4.

Supernova PTF 09uj: A possible shock breakout from a dense circumstellar wind

Type-IIn supernovae (SNe IIn), which are characterized by strong interaction of their ejecta with the surrounding circumstellar matter (CSM), provide a unique opportunity to study the mass-loss history of massive stars shortly before their explosive death. We present the discovery and follow-up observations of an SN IIn, PTF 09uj, detected by the Palomar Transient Factory (PTF). Serendipitous observations by Galaxy Evolution Explorer (GALEX) at ultraviolet (UV) wavelengths detected the rise of the SN light curve prior to the PTF discovery. The UV light curve of the SN rose fast, with a timescale of a few days, to a UV absolute AB magnitude of about –19.5. Modeling our observations, we suggest that the fast rise of the UV light curve is due to the breakout of the SN shock through the dense CSM (n ≈ 10^(10) cm^(–3)). Furthermore, we find that prior to the explosion the progenitor went through a phase of high mass-loss rate (~0.1 M_⊙ yr^(–1)) that lasted for a few years. The decay rate of this SN was fast relative to that of other SNe IIn.

RAPIDLY DECAYING SUPERNOVA 2010X: A CANDIDATE “.Ia” EXPLOSION

We present the discovery, photometric, and spectroscopic follow-up observations of SN 2010X (PTF 10bhp). This supernova decays exponentially with τ_d = 5 days and rivals the current recordholder in speed, SN 2002bj. SN 2010X peaks at M_r = −17 mag and has mean velocities of 10,000 km s^(−1). Our light curve modeling suggests a radioactivity-powered event and an ejecta mass of 0.16M_⊙. If powered by Nickel, we show that the Nickel mass must be very small (≈0.02 M_⊙) and that the supernova quickly becomes optically thin to γ -rays. Our spectral modeling suggests that SN 2010X and SN 2002bj have similar chemical compositions and that one of aluminum or helium is present. If aluminum is present, we speculate that this may be an accretion-induced collapse of an O-Ne-Mg white dwarf. If helium is present, all observables of SN 2010X are consistent with being a thermonuclear helium shell detonation on a white dwarf, a “.Ia” explosion. With the 1 day dynamic-cadence experiment on the Palomar Transient Factory, we expect to annually discover a few such events.

The Palomar Transient Factory Survey Camera: first year performance and results

The Palomar Transient Factory (PTF) is a new fully-automated, wide-field survey conducting a systematic exploration of the optical transient sky. The transient survey is performed using a new 8.1 square degree, 101 megapixel camera installed on the 48-inch Samuel Oschin Telescope at Palomar Observatory. The PTF Camera achieved first light at the end of 2008, completed commissioning in July 2009, and is now in routine science operations. The camera is based on the CFH12K camera, and was extensively modified for use on the 48-inch telescope. A field-flattening curved window was installed, the cooling system was re-engineered and upgraded to closed-cycle, custom shutter and filter exchanger mechanisms were added, new custom control software was written, and many other modifications were made. We here describe the performance of these new systems during the first year of Palomar Transient Factory operations, including a detailed and long term on-sky performance characterization. We also describe lessons learned during the construction and commissioning of the upgraded camera, the photometric and astrometric precision currently achieved with the PTF camera, and briefly summarize the first supernova results from the PTF survey.

Robo-AO: autonomous and replicable laser-adaptive-optics and science system

We have created a new autonomous laser-guide-star adaptive-optics (AO) instrument on the 60-inch (1.5-m) telescope at Palomar Observatory called Robo-AO. The instrument enables diffraction-limited resolution observing in the visible and near-infrared with the ability to observe well over one-hundred targets per night due to its fully robotic operation. Robo-AO is being used for AO surveys of targets numbering in the thousands, rapid AO imaging of transient events and long-term AO monitoring not feasible on large diameter telescope systems. We have taken advantage of cost-effective advances in deformable mirror and laser technology while engineering Robo-AO with the intention of cloning the system for other few-meter class telescopes around the world.