Starfall: A heavy rain of stars in 'turning on' AGN

Abstract

As active galactic nuclei (AGN) `turn on', some stars end up embedded in accretion disks around supermassive black holes (SMBHs) on retrograde orbits. Such stars experience strong headwinds, aerodynamic drag, ablation and orbital evolution on short timescales. Loss of orbital angular momentum in the first $\sim 0.1$~Myr of an AGN leads to a heavy rain of stars (`starfall') into the inner disk and onto the SMBH. A large AGN loss cone (${\theta }_{\mathrm{AGN},\mathrm{lc}}$) can result from binary scatterings in the inner disk and yield tidal disruption events (TDEs). Signatures of starfall include optical/UV flares that rise in luminosity over time, particularly in the inner disk. If the SMBH mass is $M_{\mathrm{SMBH}}\ge 10^8{M}_{\odot }$, flares truncate abruptly and the star is swallowed. If $M_{\mathrm{SMBH}}<10^8{M}_{\odot }$, and if the infalling orbit lies within ${\theta }_{\mathrm{AGN},\mathrm{lc}}$, the flare is followed by a TDE which can be prograde or retrograde relative to the AGN inner disk. Retrograde AGN TDEs are over-luminous and short-lived as in-plane ejecta collide with the inner disk and a lower AGN state follows. Prograde AGN TDEs add angular momentum to inner disk gas and so start off looking like regular TDEs but are followed by an AGN high state. Searches for such flare signatures test models of AGN `turn on', SMBH mass, as well as disk properties and the embedded population.

10 pages, 1 figure, MNRAS submitted

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