Two types of nuclear transients - tidal disruption events and disruptive collisions

AGENDA Séminaire Lagrange
mardi 26 septembre 2023 - 10:30 mardi 26 septembre 2023 - 11:30

Two types of nuclear transients - tidal disruption events and disruptive collisions
Taeho Ryu (MPA Garching) - hybrid seminar (in-person in Salle NEF + remote via Zoom)
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 disruption (i.e., partial mass loss and surviving remnant) to full disruption 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, causing 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. 

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