galactic nuclei are extreme environments where stars are densely packed around a supermassive black hole (SMBH). Occasionally, dynamical interactions in the galactic center lead the stars to interact violently at short distances with each other or with the SMBH, resulting in the formation of nuclear transients. In this talk, I will discuss two types of nuclear transients, tidal disruption events and high-velocity collisions between stars, based on the results of detailed hydrodynamics simulations. Tidal disruption events are one of the most dramatic nuclear transients in which a star is tidally disrupted by the SMBH in a few hours. The conventional picture has been that a star is fully disrupted at the first pericenter passage and the debris circularizes rapidly. However, these events are in fact more diverse and they can be categorized into several groups with different observational signatures depending on stellar pericenter distance, from partial disruptions (i.e., partial mass loss and surviving remnant) to full disruptions which is further sub-categorized depending on relativistic effects. On the other hand, disruptive collisions are the events where two stars collide at a very high relative velocity near the central SMBH. The collision product, a homologously expanding gas cloud, can generate a flare as bright as tidal disruption events. Subsequently, the expanding gas cloud would interact with the nearby SMBH, generating a second, possibly even brighter accretion-driven flare. Because these can happen near BHs at any mass scale, if the accretion is efficient, these disruptive collisions could contribute to the growth of black holes.
A GPU (Graphic Processing Unit) spectrometer for the ALMA (Atacama Large Millimeter/submillimeter Array) Total Power Array has been developed by the KASI and the NAOJ (National Astronomy Observatory of Japan). It is a first in-kind contribution of the KASI to the ALMA international collaboration. The development project went through a series of milestones, from a concept design review in 2016 to commission and science verification in 2023. During the ALMA maintenance period of February 2022, the GPU spectrometer was installed in a technical building at the Array Operation Site by its developers from the KASI and the NAOJ. Right after the installation, SiO (J=2-1; v=1) 86 GHz maser spectral lines toward the Orion KL region were successfully detected using the spectrometer. The spectrometer will be used from the ALMA Cycle 10 (October 2023) for science observations.