Title:  Starfall: A heavy rain of stars in ‘turning on’ AGN

Authors:  B. McKernan, K.E.S. Ford, M. Cantiello, et al.

First Author’s Institution: American Museum of Natural History

Status: Open access on ArXiV, submitted to MNRAS

Imagine you are a star trapped within the whirling blue accretion disk of an active galactic nucleus (AGN). Gas far denser than any nebula you have ever traversed now crushes against your face, compressing, heating. Your orbital velocity slows. You plunge. What will be your ultimate fate? Will you escape this inferno unscathed? Or does your eternal damnation lie down the gravitational gullet?

Setting the Stage/Dinner Table

In the centers of galaxies lay nuclear star clusters, great cities of stars where the population density is millions of times that in the provincial outskirts. In the midst of it all lies the hungry heart: a supermassive black hole, surrounded by the thronging mobs of orbiting stars. Hungry though the heart may be, the stars cannot be eaten by it, for they move too fast to be pulled in. Something must cause the stars to lose orbital angular momentum for the black hole to be able to eat them.

The authors consider the possibility that this could occur when gas enters the center of the galaxy and gathers into an accretion disk around the black hole. This would occur when an AGN is first ignited. By pure chance, some of the stars in a nuclear star cluster will have orbits that lie in the plane of the accretion disk. Of these, about half will be orbiting in the opposite direction the accretion disk is rotating–a so-called retrograde orbit, depicted in the first panel of Figure 1. 

These stars are in for a hell of a time.

Into the Inner Circles of Hell

For a retrograde-orbiting star, the disk is akin to a headwind traveling at supersonic speeds. A shock front forms at the star’s surface, radiating away heat. For a sunlike star (temperature ~ 5920 K), the authors calculate the temperature of this shock can far exceed that of an O-type star (temperature ~ 41,000 K)! The luminosity generated will far exceed the star’s own luminosity by several orders of magnitude, and the scouring action of the wind will strip expanded stars like red giants of most of their envelopes.

The raking superheated wind eats away at the stars’ orbital velocity. The stars slowly spiral into the inner accretion disk. This process takes about ten thousand orbits for a solar-type star, five thousand orbits for an A-type star, and three thousand orbits for an O-type star. The most massive stars spiral in the fastest.

In the crowded melee of stars that collect around the supermassive black hole, gravitational interactions between multiple stars will be common. Multi-body interactions are highly chaotic, and can cause some unfortunate stars to be scattered towards the black hole. The case of a binary scattering a single star towards the black hole is depicted in the third panel of Figure 1. In the final plunge of the star through the innermost reaches of the accretion disk, it will generate a weeks-long flare that rises to a peak luminosity of 10³⁵ Watts, comparable to that of an AGN itself. For O-type stars, the peak luminosity can exceed 10³⁷ Watts, approaching the power output of a quasar!

Stars are Best Eaten Filleted

After its long and luminous plunge through the accretion disk, the star now lies at the gates of the supermassive black hole itself. From here on, one of two things can happen. For black holes more massive than 10⁸ solar masses, the star is simply swallowed whole in a single gulp. Black holes smaller than 10⁸ solar masses chew their food first: they generate tidal forces powerful enough to rip the star apart in a tidal disruption event (TDE). Many TDEs around isolated black holes have been observed, and their typical light curve is depicted in the central panel of Figure 2. However, the authors are interested in the case of a TDE in an AGN, and their calculations indicate the light curve will look different.

The precise light curve depends on whether the star is still orbiting retrograde after being scattered towards the black hole, or whether it has switched to orbiting prograde (in the same direction as the accretion disk). Panels 3, 4, and 5 of Figure 1 depict the retrograde case: the remnants of the disemboweled star collide with the gas in the innermost disk and cancel out its angular momentum, causing it to rapidly drain into the black hole. This leaves behind a cavity that temporarily drops the luminosity of the AGN after the initial flare of the star being disrupted (the final panel of Figure 1). 

In a prograde TDE, the stellar viscera are orbiting in the same direction as the gas in the accretion disk, so they mix into it without much drama. The accretion disk expands from the added gas, radiating more profusely than before. Thus, there is an increase in luminosity of the AGN after the star is disrupted (the last panel in the third row of Figure 1). 

Stars fall to their doom rapidly enough that half the tidal disruption events occur within the first 100,000 years of the AGN being active. This means that the distribution of TDEs observed in AGN can give clues to the total AGN lifetime. AGN have tremendous influence on their host galaxies, and understanding precisely how long they stay lit up is key to understanding the magnitude of this influence. Maybe someday, the authors posit, we will know the answer thanks to the screams of numerous damned suns.

Astrobite edited by Kayla Kornoelje, Clarissa Do O, and Pratik Gandhi

Featured image credit: Myself

Disclaimer: I have collaborated with the first two authors in the past, and the final author is my PhD advisor.