112 Yushan Young Fellow Program-Ting-Wan Chen

Exploring extreme transients. In the last 10 years, the wide-field sky surveys have revolutionised time- domain astronomy. They led to the surprising discovery of transients, such as “superluminous supernovae (SLSNe)”, the brightest stellar explosions known to astronomy, which challenge our understanding of how massive stars explode (Quimby et al. 2011; Gal-Yam 2019). Probing the physics of these transients can reveal the variety in the final stages of stellar evolution and can o↵er the tantalising possibility of a new era in the study of the massive stars and galaxy evolution. Recently, the first binary neutron star (NS) merger was detected by LIGO-Virgo (Abbott et al. 2017a) and subsequently an electromagnetic counterpart of the GW source was found in multi-wavelengths (Abbott et al. 2017b), the so-called “kilonovae (KNe)”. This event opens up a new era of multi-messenger astronomy. Within one day, the KN reached its peak brightness and then rapidly faded 100 times within 10 days, behaving as the fastest evolving event in the transient phase space. The fast decline and redder colour indicate that neutron-rich material was ejected, which then synthesised heavy elements via the r-process. This is the first time that a site of heavy-element nucleosynthesis has been directly identified (e.g. Smartt, Chen et al. 2017). Despite these celebrated triumphs, two key questions remain: “What is the energy source of SLSNe?” and “What roles do KNe play in the production of heavy elements in the Universe?”. With new powerful survey instruments launching we might discover currently unknown new transient objects. The future of transient astronomy is exciting.Understanding the physical underlying mechanisms of these exotic transients is the principal aim of this Yushan Young Fellow Program. The Graduate Institute of Astronomy of National Central University (IANCU) is the host institute to support my project. I propose to combine their rich observational resources such as their own Lulin Observatory and a 2-m telescope in Mexico, and their access to an exciting new development, the Vera C. Rubin Observatory and its Legacy Survey of Space and Time (LSST), with my unique experience in observations and analysis and my membership in several leading international collab- orations. I aim to expand and synthesise several parallel branches of investigation to explore two extreme classes of transients, the brightest and the fastest, which are not well understood. The two major goals of this program are to address both conundrums by I. to investigate the energy source of SLSNe, and II. to verify heavy-element production from KNe.