Not all stars with black hole companions die gruesome deaths, scientists say

Death by black hole isn't always inevitable for stars that exist in binary systems with one of these cosmic titans.

There may be a way for small stars in such systems to dodge their expected fates: violent supernovas that end in the creation of another black hole. This expected mechanism is also believed to turn the small stars into snacks for their black hole partners.

The revelation comes courtesy of two strange black hole binary systems discovered by the high-precision star-tracking Gaia space telescope. The systems actually contain two of the closest black holes to Earth.

The black holes are designated BH1 and BH2 and are located just 1,560 and 3,800 light-years from Earth in the direction of the constellation Ophiuchus. The systems in which they dwell contain stars in wide orbits that aren't being fed upon by their black hole companions and are still at the same stage of evolution as the sun.

This is extraordinary, as companion stars aren't supposed to survive their massive partner star's transformation into a black hole. That is because the transformation to a black hole is violent and turbulent, seeing massive stars swell up and engulf companions before exploding and lashing smaller companions with stellar material and enough energy to destroy them or kick them out of orbit. That means small companion stars to black holes are generally either destroyed, devoured, or ejected.

This fate is generally sealed by interactions like mass transfers between the small companion stars and the massive stars that birth black holes, or "progenitor stars," before their deaths.

"We investigated a way to avoid an interaction of the black hole progenitor with its companion," research lead author Matthias Kruckow told Space.com.

These results will blow you away

With masses around 10 times that of the sun, the stars that died to birth black holes BH1 and BH2 should have been so large that they interacted with their smaller companion stars during their deaths.

However, if this had been the case, then the companion stars of BH1 and BH2 should have been forced out of their main sequence lifetime; the stage accounts for 90% of a star's life, during which it is an ordinary star like the sun that burns hydrogen to forge helium in its core. However, it seems that the stars accompanying BH1 and BH2 may still be in the main sequence phase, though this still isn't clear.

Additionally, the transfer of mass between the companion stars and the stars that died to create these black holes should have tightened the orbits of these systems.

In these instances, systems develop with black holes in binaries with companion stars. However, in those cases, the companion stars in these tight orbits would be so close to the black hole that this cosmic titan would begin stripping away stellar material. This material would form a disk of material around the black hole that gradually feeds it.

The immense gravity of the black hole causes tremendous tidal forces to arise in the accretion disks, causing fiction and heating that leads to the emission of electromagnetic radiation, including X-rays.

Thus, black hole/star binaries are usually identified by their strong X-ray emissions. However, the BH1 and BH2 systems lack these X-ray signatures.

If the stars had been close enough to their black hole partners, they could have even been squashed and squeezed into a strand of stellar pasta by the immense gravity and tidal forces generated within them.

This death by "spaghettification" and the devouring of stellar material is known as a Tidal Disruption Event or a "TDE." They are associated with bright emissions of radiation. And, again, it is something the stars around BH1 and BH2 have avoided.

But how?

The answer is blowing in the wind

Following the evolution of stars with masses in excess of 80 times that of the sun, Krucow and team hit upon a solution and a potential survival mechanism for small stars around black holes.

Kruckow explained that the key to survival is the strong winds that blow from extremely massive stars with high concentrations of elements heavier than hydrogen and helium, which astronomers call "metals."

These winds not only help prevent the mass transfer from occurring between the stars before the black hole forms, but also cause the massive progenitor star to lose mass and shrink. The strong winds from the massive star also push the companion star into a wider orbit.

These elements help protect the companion star from being engulfed as their massive companions go supernova. The same factors also create a wider orbit that prevents the star from experiencing a gruesome death via TDE or from being fed on by its black hole companion.

The fact that both BH1 and BH2 have similar masses indicated to the team that both systems followed this same channel of evolution.

"It was enlightening that the winds at different phases of the stellar evolution are responsible for different characteristics of these kinds of binary systems," Kruckow said.

Gaia has also uncovered a third black hole binary system, which Kruckow revealed seems to be a different kind of system.

"Gaia BH3 is very different in many aspects, not only its different mass, but the companion has a different metallicity, which is much lower than for Gaia BH1 and BH2," Kruckow said. "Additionally, Gaia BH3 is associated with a stream of stars, which is believed to be the remnant of a star cluster."

While these winds and the wide binaries the winds help to create are helpful for the survival of small stars around black holes, they are somewhat of a hindrance to astronomers hunting such systems.

Kruckow explained that this wider orbit initially made it difficult to confirm BH1 and BH2 as binaries because astronomers could only observe the sun-like star elements while their black hole partners remained invisible.

"Additionally, the long periods required to reobserve the star over a long time to cover the full orbit. This is needed to differentiate binaries from random flybys," Kruckow added.

The researcher explained that Gaia is designed to make images with very high resolution, which means a comparison of images allows astronomers to see the tiny movement those distant stars have.

"At the moment, this is the only way to detect these wide black hole/ star binary systems," Kruckow said.

"Unfortunately, it is very uncertain how common systems like those of BH1 and BH2 may be, but we expect to have hundreds to thousands of them in our galaxy," Kruckow said. "The uncertainty comes from a lack of knowledge of extremely massive stars that form black holes. Especially when those stars are in binaries."

Despite this uncertainty, the team expects that Gaia should be able to discover several percent of these binaries in the Milky Way.

"Thus, we can hope for tens to a hundred to be discovered with the upcoming data releases from Gaia," Kruckow concluded.

The team's research is available as a preprint on the repository site arXiv.