Cut to the Core

NASA/JPL-Caltech

We all know that black holes can devour stars. Rip them apart and consume their remnants. But that only happens if a star passes too close to a black hole. What if a star gets close enough to a star to experience strong tidal effects, but not close enough to be immediately devoured? This scenario is considered in a recent paper on the arXiv.¹

The study considered a dying, 2 solar-mass star known as a subgiant. These stars are reaching the end of their lives. Most of the hydrogen in their cores has been transformed to helium, but they haven’t quite swelled to a red giant. Our sun will become a subgiant star before ending its life as a red giant and eventually a white dwarf.

In the model they consider, the star is initially part of a binary system at the center of our galaxy. The binary system passes close enough to the supermassive black hole, Sag A*, so that the subgiant is captured in close orbit while its companion escapes. Over time, the orbit of the subgiant decays and the star starts to enter the danger zone of Sag A*. This is where things get interesting.

Because the outer layers of the subgiant are somewhat swollen, they are the first to be captured by the black hole. Essentially, the black hole can rip off the outer layers of the star, leaving a dense helium core. This bare core star continues to orbit ever closer to the black hole until finally being consumed.

Olejak, et al

The authors consider this model because it poses an interesting observational challenge. Unlike when a black hole rips apart a star in a tidal disruption event, the stripping of a star wouldn’t produce a bright flare we can detect. Certainly not for Sag A*, which is shrouded in gas and dust. But since the dense helium core remains close to the black hole, its dying orbit could be detected through gravitational waves. The waves are too small to be detected by current gravitational observatories such as LIGO and Virgo, but a space-based gravitational observatory such as LISA could detect them.

The authors run the numbers to see what is possible. Could LISA actually detect an inspiraling helium core, and how likely would we detect it? They found that these stars could be well within the observing range of LISA, and gravitationally bright enough we might be able to detect them orbiting supermassive black holes beyond our galaxy. Even as far as a billion light years away. They estimate that during its initial 4-year mission, LISA should detect the gravitational waves of at least a few such stars, and that there is about a 1% chance of finding one within our own galaxy. If the star is particularly dynamic and experiencing helium flashes, the X-rays they produce might be observable, leading to multi-messenger observations.