When A Black Hole (Partially) Destroys a Star

Close encounters between stars and supermassive black holes generally don’t end well for the stars. Under the influence of a black hole’s strong gravitational forces, an unsuspecting passing star can be completely shredded, resulting in a spectacular tidal disruption flare. But what happens when the star is only partially destroyed?

Unfortunate Encounters

Tidal disruption event

This simulated TDE shows the looping tidal stream caused when a star is pulled apart by the gravitational forces of a black hole. When this stream intersects with itself, material collides and rains onto the black hole, causing it to light up. [NASA/S. Gezari (JHU)/J. Guillochon (UCSC)]

Stars don’t often wander close to supermassive black holes — which means that tidal disruption events (TDEs) are relatively infrequent. Nonetheless, we’ve spotted around 40 of these destructive encounters so far, and they’re being discovered at an ever-increasing rate.

During a TDE, a passing star is torn apart and stretched into a stream of debris by the tidal forces of the supermassive black hole. Part of this stellar material escapes the black hole’s pull and scatters; the rest collides with itself during looping orbits, eventually raining down on the black hole and accreting. This accretion emits radiation, causing the black hole to briefly flare, producing a characteristic light curve that we might observe.

A Universal Decay?

The shape of the light curve is what makes TDEs distinctive. When we search for tidal flares, we hunt for transient signals that feature a sharp rise in the light curve followed by a long, decaying tail with a shape that’s governed by the physics of the fallback and accretion of the stellar material. For TDEs, that decay asymptotes to a characteristic power-law slope — a slope that was thought to be universal for all such stellar destruction.

But is it truly? A new study by scientists Eric Coughlin (Princeton University; Columbia Astrophysics Laboratory) and Chris Nixon (University of Leicester) explores whether we can expect to see differences when a star is only partially destroyed in its encounter with a supermassive black hole.

Survival Under Stress

fallback rates

Fallback rate onto a million-solar-mass supermassive black hole as a function of time for a disrupted Sun-like star. Different color curves represent different masses of surviving stellar cores. The curve representing a fully destroyed star (µ=0, red) asymptotes to a shallower slope (~t-5/3). The other curves, which all represent partial disruptions of varying degrees, all asymptote to a more steeply decaying slope (~t-9/4). Click to enlarge. [Coughlin & Nixon 2019]

In a partial stellar destruction, material is stripped from the star, but some fraction remains bound together as a stellar core. This core then orbits around the black hole along with the stream of colliding, accreting debris. Coughlin and Nixon show that the gravitational pull of this surviving stellar core affects the rate at which material falls back onto the black hole, causing different behavior than if the star had been fully destroyed.

What does this mean for observations? The authors argue that we should expect to see two types of TDEs: those representing complete stellar disruptions, whose light curves asymptote to a shallower slope, and those representing partial stellar disruptions, whose light curves asymptote to a steeper slope.

Coughlin and Nixon estimate that, for stars that undergo tidal disruptions, just under half of low-mass stars and around 70% of high-mass stars will be only partially disrupted. They therefore expect that a substantial fraction of TDEs detected by future facilities — like the Large Synoptic Survey Telescope (LSST) coming online in 2020 — will represent stars that partially survived their close encounter with a supermassive black hole … though perhaps a little the worse for wear.


“Partial Stellar Disruption by a Supermassive Black Hole: Is the Light Curve Really Proportional to t −9/4?,” Eric R. Coughlin and C. J. Nixon 2019 ApJL 883 L17. doi:10.3847/2041-8213/ab412d