Research (astro)

Interests

I have worked on:

the origins of the heaviest elements in the Universe,
formation of the earliest galaxies, and
formation of globular clusters, and
Helium spectral modeling of neutron-star merger or "kilonova".

Neutron-cature elements in UFD

The smallest galaxies are relics of the earliest galaxies. Using galaxy formation simulations, we have modeled the enrichment of ultra-faint dwarf galaxies (UFDs) by neutron-capture processes.

r-process

We showed that one NSM in the center of the UFD progenitor explains the high Eu abundance of the highly r-enhanced UFD (Ret II). Also, the moderately r-enriched UFD (Tuc III, Gru II) could be explained by NSM around the virial radius of the galaxy. We have shown that >150 Myr is required to mix r-process elements well and reproduce the observation.

https://ui.adsabs.harvard.edu/abs/2020MNRAS.494..120T/abstract

s-process

We have pointed out that AGB stars cannot synthesize a sufficient amount of s-process elements, particularly strontium (Sr) and barium(Ba), within the short star formation duration of UFDs. Rotating massive stars are possible candidates for the origin of Ba. For Sr, electron-capture supernovae (ECSNe) are promising candidates, and we have constrained the mass range of ECSNe by the stellar masses of Sr-poor UFDs.

https://ui.adsabs.harvard.edu/abs/2021MNRAS.505.3755T/abstract

r-process in MW

Time delay between star formation and the synthesis of r-process elements, ``delay-time'', imprints gradient on [r-process/Mg] against [Fe/H] trend. With a Monte-Carlo simulation, we have shown that the increasing trend of [Ba/Mg] requires a delay time of the source. The delay is naturally expected for neutron-star mergers, therefore supporting the scenario that binary neutron-stars are the dominant origins of r-process elements in the Universe.

https://ui.adsabs.harvard.edu/abs/2021ApJ...913L..30T/abstract

r-process abundance spread in M15

M15, a massive and old globular cluster is known to have a significant r-process element abundance spread within the system. We propose that an r-process event (like wandering binary neutron stars) enriches the molecular cloud that forms globular cluster M15. A lack of correlation between Na and Eu supports the scenario that r-process enrichment occurs at different moments between Na (proton-burning) and Eu (r-process). From the elemental abundances of Na and Eu, we argue that globular cluster stars form in a single short burst.

https://ui.adsabs.harvard.edu/abs/2021ApJ...921L..11T/abstract

Formation of an UFD with an extended stellar halo

A UFD Tucana II has a spatially extended stellar halo. We propose that the extended structure is a consequence of galaxy mergers. Based on this scenario, we simulate the merger of two building-block galaxies. We find that an ultra-faint dwarf galaxy that has experienced a merger will have an extended stellar profile. This feature is quite different from compact UFDs that have not experienced mergers. We propose that the stellar density profile will be useful to infer the merger history of the galaxy.

https://ui.adsabs.harvard.edu/abs/2021ApJ...914L..10T/abstract

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