Kohki Uno
Postdoctoral Research Fellow
Columbia Astrophysics Laboratory, Columbia University
I work on transient science through various approaches — theoretical modeling, observations, and data science — with the Dynamic Universe Initiative (led by Prof. Kishalay De) at Columbia University, New York, USA.
Research Interests
The Universe has a hierarchical structure, with galaxies and the cosmic web at larger scales and stars at smaller scales. Stars are the most fundamental unit of the Universe. They are not only the factories synthesizing elements, but also the engines driving material cycles through stellar explosive phenomena, known as astronomical transients. These transient events, which play a key role in determining the nature of the Universe, have been an important target of astronomy and astrophysics since the 1970s. However, they still remain a long-standing puzzle today.
Astronomical transients are characterized by their luminous and short-timescale properties. Until the 2000s, most known transients had relatively long timescales, such as supernovae and novae. However, powerful time-domain surveys started in the 2010s led to a dramatic increase in the number of transient discoveries. These surveys revealed a greater diversity among transients than previously expected, uncovering rare and new populations, and reshaping the classical understanding of stellar evolution. In the 2020s, the Vera C. Rubin Observatory's Legacy Survey of Space and Time (LSST) is strongly pushing the frontiers of transient astronomy. In this data-rich and dynamic era, it is essential to revisit well-established classical transients and to uncover the hidden characteristics of newly identified populations.
In my research, I aim to deepen our understanding of the diverse nature of transients and their implications for the Universe.
Stellar Evolution & Supernovae
When a star reaches the endpoint of its evolution, it explodes through thermonuclear runaway or gravitational collapse, resulting in a supernova. Supernovae become so bright that we can observe them even outside the Milky Way. Combining observational data with theoretical calculations of the explosion physics, I am approaching the nature of supernovae and the final stages of stellar evolution — including the poorly understood mass-loss episodes that occur just before the explosion.
Related papers
Fast Blue Optical Transients
FBOTs are peculiar transients showing rapid rise and decay with high luminosity. Their timescales are much shorter than those of supernovae, while their peak luminosities can be roughly 100 times brighter. I suspect that FBOTs originate from physical mechanisms different from supernovae — such as super-Eddington accretion and outflows in compact-object systems. These enigmatic phenomena are one of my main research topics.
Related papers
Tidal Disruption Events
When a star approaches too close to a supermassive black hole, it is torn apart by the black hole's strong tidal forces — a phenomenon known as a TDE. In the classical picture, TDEs are bright in X-rays; however, new-generation surveys have discovered TDEs that are bright in the optical and ultraviolet (UV). The radiation mechanisms of optical/UV-bright TDEs are intensively debated but still unknown. Using spectropolarimetry and multi-wavelength observations, I am exploring the origin of optical/UV-bright TDEs.
Related papers
Approaches
To reveal the nature of these transients, I combine three approaches. In theoretical studies, I perform order-of-magnitude estimates, semi-analytic modeling, and numerical simulations. In observational studies, I carry out rapid follow-up observations using facilities such as the Seimei 3.8m telescope, the Subaru 8.2m telescope, Keck, and the Hubble Space Telescope, together with survey data from ZTF, ATLAS, and NEOWISE. In addition, I apply data-science techniques, including machine learning, to large spectroscopic and photometric datasets.