I work in the intermediate Palomar Transient Factory. One of its primary goals is to explore and understand transients within a few days of their explosion. In order to achieve this scientific goal, we use modern technology to make transient discovery fast. My PhD thesis is to characterize young type I supernovae and is divided into the technical and scientic parts. Besides my PhD thesis, I am also involved in a few other projects about M31 novae and R Coronae Borealis stars, and an infrared transient survey with Spitzer.

Developing iPTF Fast-Cadence Transient Survey

The iPTF Fast-cadence transient survey is the base of my PhD researches. In order to rapidly and reliably find young transients from massive survey data, supervised by Peter Nugent, I developed and am maintaining the real-time image subtraction pipeline at NERSC. This pipeline uses existing software such as WCSTools, astrometry.net, AstrOmatic software and HOTPANTS, and also incorporates machine-learning classifiers which are developed at Berkeley, JPL and LANL to boost our discovery efficiency. The current pipeline generates transient candidates within ten minutes after a survey image is taken. Each night, the pipeline processes more than 90 GB survey data and generates half a million transient candidates which are dumped into a PostgreSQL database. In addition, it stacks images in real time to increase our detection sensitivity. Recently I am helping the Extreme-Scale Scientific Workflow Analysis and Prediction (X-SWAP) project at NERSC which aims to develop an integrated analytic performance modeling framework for distributed extreme-scale science workflows.

In order for collaborators to visually inspect transient candidates, I learned some common-gateway interface to develop and maintain a dynamic web portal for people to easily access the data.

I am also in charge of field selection in the fast-cadence experiments. In the meanwhile, I am responsible to trigger target-of-opportunity observations to young supernova candidates with the 200-inch Hale telescope at Palomar and the Keck telescopes at Mauna kea, as well as the space Swift mission.

Young Transients

Supernova-Companion Interaction

Many supernovae are believed to originate from binary systems, including both thermonuclear (Type Ia) and core-collapse types. In such a supernova explosion, when the supernova ejected material slams into a companion star, the collision will produce strong emissions primarily in the ultraviolet superposed on the supernova light. The collision is only visible in a young supernova for a short period of time along particular directions, because the fast-moving ejected material quickly runs over the companion and blocks the emission of the collision. In order to search for this interaction signature, we trigger the Swift space mission to observe young supernovae in the ultraviolet. After many attempts, eventually in a low-velocity thermonuclear event iPTF14atg, for the first time, we observed a strong and declining emission in the ultraviolet within four days of its explosion. This is consistent with the theoretical expectation of supernova-companion interaction. Our observation strongly supports the single-degenerate origin of Type Ia supernovae. Our result, together with other recent researches, strongly suggest that Type Ia supernovae have multiple progenitor channels (Cao et al. 2015, Nature, 521, 328). Now we are searching for more supernovae with the companion interaction signature in order to determine the fraction of Type Ia supernovae from the single-degenerate channel.

Progenitors of Stripped Envelope Supernovae

None of Type Ib/c supernova progenitors has been identified in the pre-supernova images. It is in debate whether stripped envelope supernovae are from massive Wolf-Rayet stars or less massive helium stars in binaries. In 2013, we discovered a Type Ib supernova within a day of its explosion and reported a possible progenitor candidate in the pre-supernova HST image. Our comprehensive follow-up observation of this young supernova also constrains the size of its progenitor to be as compact as several solar radii and its mass loss rate as large as 10-5 solar mass per year (Cao et al. 2013, ApJL, 775, 7, Fremling et al. 2014). HST will revisit this field again in late 2015 to verify the progenitor candidate.

Flash Spectroscopy

Massive stars may loss a significant amount of its mass which forms dense circumstellar media. At the onset of supernova explosion, these media can be ionized by the flash emission of supernova shock breakout. The recombination of these lines provides chemical and dynamic information about the circumstellar media. But these recombination lines are short-lived. They disappear when the supernova ejected material sweeps out these media. In 2013, we find a circumstellar medium around a type IIb supernova iPTF13ast very similar to those surrounding Wolf-Rayet stars. This strongly suggests that the progenitor of this type IIb supernova is a Wolf-Rayet star (Gal-Yam et al. 2014). Now we are searching for these flash recombination lines in all types of young core-collapse supernovae.

M31 Classical Novae and R Coronae Borealis Stars

Classical nova systems share many similarities to hypothetical progenitors of Type Ia supernovae. For this reason, they are among the most promising Type Ia supernova progenitor candidates. Though several to a few ten classical novae are found in the Milky Way and nearby galaxies, their diverse light curves and event rates are still poorly understood. PTF/iPTF provides a unique data set to explore the diversity of the light curves of novae in M31 and constrain their rates. As my first-year project, I published lightcurves of M31 novae in 2009-2012 (Cao et al. 2012, ApJ, 752, 133). We also found a recurrent nova with the shortest recurrent period of one year which indicates a white dwarf mass close to the Chandrasekhar mass limit (Tang et al. 2014). Moving forward, the PTF/iPTF monthly Hα survey of M31 provides a unique data set to build a complete list of M31 novae. This will strongly constrain the nova rate problem.

R Coronae Borealis (RCB) stars are hydrogen-deficit but carbon-rich stars. Observationally they show irregular decline of several magnitudes over timescales of weeks to months. They may be produced by mergers of two low-mass white wharfs and therefore serve as a low mass counterpart of type Ia supernovae. In M31, we confirmed two RCB and found two other RCB candidates. They are the first discovered beyond the Milky Way and Magellanic clouds (Tang et al. 2013).

Infrared Transients

I am also involved in the Spitzer InfraRed Intensive Transient Survey (SPIRITS; PI Kasliwal). SPIRITS uses the Spitzer space telescope to characterize transients and variables in galaxies within 20 Mpc in the 3.6 and 4.5 microns on timescales ranging from one week to one year. I help with scheduling the Spitzer observations and organizing ground-based follow-up. We are now finding a zoo of infrared-only transients and investigating their natures.