Supernova emission in the first days of explosions contains rich information about the physical properties of their progenitors. This motivated me to conduct fast-cadence transient surveys in the intermediate Palomar Transient Factory (iPTF), with the scientific goal of constraining the poorly-understood progenitor systems of hydrogen-free supernovae. In the following, I summarize the major results of this thesis:
1. In these fast-cadence surveys, fast discovery and rapid follow-up ob- servations are equally important for infant supernovae (Chapter 2). I showed that (1) the nightly survey cadences were strictly maintained to ensure the young age of discovered supernovae, (2) our realtime image subtraction pipeline managed to deliver supernova candidates within ten minutes of images being taken, and (3) we successfully performed com- prehensive follow-up observations of interesting transients within hours of discovery through our pre-approved target-of-opportunity programs on a variety of ground-based and space telescopes. In the years 2013 – 2015, we discovered over a hundred supernovae within a few days of their explosions, forty-nine of which we performed spectroscopic follow-up ob- servations to within the same night or next night following discoveries.
2. A strong and declining ultraviolet pulse was detected from a low-velocity (subclass prototype: SN2002es) Type Ia supernova iPTF14atg within four days of its explosion (Chapter 3). The pulse had a peak luminosity ' 3×1041ergs s−1 and decayed in a few days. This pulse can be best interpreted as the diffusive thermal emission produced by collision be- tween the supernova ejecta and a companion star. This result provides evidence that at least some Type Ia supernovae arise from the single degenerate channel in which accretion onto a white dwarf from a nonde- generate companion star triggers the supernova explosion of the white dwarf.
3. Although iPTF14atg and a sibling low-velocity event iPTF14dpk both resemble the prototypical event SN2002es in optical light curve and spectra around and after maximum, they show distinct early-rise be- havior (Chapter 4): the early rising R-band light curve in iPTF14atg before −17days with respect to its maximum is completely missing in iPTF14dpk. Instead, iPTF14dpk abruptly jumped to R = −17mag with a rise rate of > 2mag day−1 at −17days. I showed that the early emission from iPTF14atg can be entirely attributed to the supernova- companion collision. If SN2002es-like events all arise from the single degenerate channel, the missing early emission from iPTF14dpk can be explained as a natural consequence of an unfavored viewing angle along which the supernova-companion interaction is shaded by the optically thick ejecta. I also show that a supernova has a dark phase between its explosion (as estimated by the supernova-explosion collision date) and the first light of its radioactively-powered light curve. The duration of the dark phase depends on the depth of the shallowest ejecta layer in which 56Ni is deposited.
4. A peculiar Type Ia supernova iPTF13asv (Chapter 5) showed strong near-UV emission and absence of iron in the early-phase optical spectra, which implies weak mixing of iron group elements in the fast-moving ejecta and thus a stratified ejecta structure. Compared to the state- of-the-art simulations, I showed that the stratification probably results from a prompt explosion of a white dwarf merger. I further showed that iPTF13asv resembles the super-Chandrasekhar Type Ia supernovae in the large peak luminosity, low expansion velocity, and persistent carbon absorption, and derived a total ejecta mass of about 1.6M. Such a super-Chandrasekhar ejecta mass also supports the double-degenerate origin of iPTF13asv.
5. I reported the first direct detection of the progenitor system of a Type Ib supernova iPTF13bvn in the pre-explosion HST archival images (Chap- ter 6). This progenitor has been confirmed by a post-explosionHST re- visit to the site of the supernova. Separately, comparing the early-phase optical light curve of the supernova to post-shock-breakout cooling mod- els, I constrained the radius of the progenitor being no larger than several solar radii. From the early-phase radio observations of the supernova,
I also derived a mass loss rate of 3×10−5Myr−1 for the progenitor star in its last few years prior to the supernova explosion. The identified progenitor system in the pre-explosion image and the constraints on the radius and mass loss rate of the progenitor form a comprehensive dataset that Type Ib supernova explosion theories need be tested on.
6. Flash spectroscopy of Type II supernovae is outside the scope of this thesis. However, I briefly summarize it here for completeness, as it is another major result from our fast-cadence surveys. If a supernova ex- plodes within a dense circumstellar medium which is established by the mass loss of its progenitor, the high-energy photons released in the su- pernova shock breakout may ionize the circumstellar medium. Before the supernova ejecta sweeps over the circumstellar medium, we anticipate to observe a hot thermal continuum emission from the supernova photo- sphere superposed by recombination lines from the ionized circumstellar medium. In 2013 – 2015, my colleagues and I have observed these re- combination lines in a dozen young Type II supernovae (Gal-Yam et al., 2014; Khazov et al., 2016). The line shapes and their evolution provide new diagnostics to the density profile, the wind velocity and the chemical abundance of the circumstellar medium and thus the properties of the mass loss history of the progenitors.
Moving forward, the next frontier in the young supernova field is to expand these single-object results into sample studies. Specifically, early ultravio- let observations of a large sample of thermonuclear supernovae are warranted to determine the fraction of events of different subtypes with early-phase ul- traviolet pulses. Then we can estimate the fraction of supernovae arising from the single degenerate channel and constrain the binary geometry of these progenitor systems. This study will enhance our understanding towards the population of white dwarf binaries and potentially improve the measurement precision of supernova cosmology.
In the core-collapse supernovae, the early optical light curves of a large num- ber of supernovae will put constraints on the radii of their progenitors. The early-phase flash spectra and radio observations will determine the physical properties of the mass loss from their progenitors at last stages. The last- stage mass loss of massive stars is a key to understanding the core-collapse
supernovae, because it is probably driven by the core-collapse mechanism and because it sets up the initial condition for the core collapse. Compared to the direct pre-explosion imaging, these new methods can be applied to supernovae in a much larger volume and therefore collect a sample within a reasonable timescale.
iPTF will cease operation in February 2017 and will make way for the Zwicky Transient Facility (ZTF; Smithet al.2014) which equipped with a new camera of 47deg2 field of view. ZTF survey speed will be order-of-magnitude faster than that of iPTF and is expected to find one young supernova every night (the exact survey strategy is yet to be determined). The newly commissioned SED Machine (an imager and integrated field unit) on the Palomar 60-inch will automatically undertake immediate follow-up observations of interesting transients discovered in ZTF to provide color information and spectroscopic classification. In parallel to ZTF, a few space missions of large-area fast- cadence ultraviolet surveys, such as ULTRASAT (Sagiv et al., 2014), have also been proposed. These ultraviolet surveys will be more sensitive to high- temperature phases of cosmic explosions, such as the supernova shock breakout and supernova-companion interaction.
In a broad picture, large-scale astronomical surveys including iPTF and ZTF are transforming astronomy from a data-starved to a data-swamped discipline, fundamentally changing traditional methodologies. The data volume that will be collected in the next decade will supersede everything accumulated over the preceding four thousand years of astronomy. Our experience in the iPTF era has demonstrated that applying advanced technologies in computer and data sciences to processing, analyzing and storing the big volume of data will lead to incredible astronomical discoveries in near future.
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