Did Scientists Find A 70 Solar Mass Black Hole?
Note: This is a layperson summary but links are provided to the scientific literature for further details.
A few weeks ago, a team of astronomers announced their discovery [1] of a 70 solar mass black hole in a binary system. Today, 3 teams of astronomers have released work showing that this is probably not the case.
In this piece I’ll focus on the work of our team called “Weighing in on black hole binaries with BPASS: LB-1 does not contain a 70M⊙ black hole” [2], but the other two papers are certainly worth a read if you dable in science literature:
-“No signature of the orbital motion of a putative 70 solar mass black hole in LB-1” [3]
-“Not so fast: LB-1 is unlikely to contain a 70 M⊙ black hole” [4]
In any case, what the hell happened?
The Surprising Discovery
The 70 solar mass black hole was a noteworthy finding because black holes of this size are not expected in the Milky Way. Why is that you ask?
Well let’s look at how most black holes are made:
Have a massive star - like, really massive, at least 20 or more solar masses.
Wait….
Star is old and eventually its core collapses. If the core is big enough (roughly 3 times greater than the mass of the sun) it will become a black hole!
So why would a 70 solar mass black hole be hard to make? Don’t you just need a bigger star?
The problem is that extremely massive stars do crazy things. Their unbelievably strong stellar winds strip their outer envelope and they lose a lot of mass. These winds are stronger the more enriched they are with elements created in previous generations of stars (a.k.a greater metallicity). Since this system was observed relatively nearby we know it has a lot of these elements and the stellar winds would have been tremendously strong.
So the star preceding the black hole should have lost so much material it should be impossible for it to leave a 70 solar mass black hole behind.
The original team knows that of course, here is a quote form the first author: “We thought that very massive stars with a chemical composition typical of our galaxy must shed most of their gas in powerful stellar winds, as they approach the end of their life. Therefore, they should not leave behind such a massive remnant” [5]
So there are 2 possibilities, either we don’t understand stellar evolution, or this black hole isn’t quite as big as we thought.
The Rebuttal
The question is this: do astronomers need to entirely rethink the way they understand how massive stars shed their fluffy envelopes into the cosmos?
What if the system observed by Liu et al. 2019 was not made of a 70 solar mass black hole and an 8 solar mass companion star?
Our team had some thoughts about this.
Using the observed parameters of the system reported in the original paper we searched through our own theoretical codes for matching stellar systems… and we found them!
But the black holes did not weigh 70 solar masses, only 7! And the companion stars were closer to 1 solar mass rather than 8.
How did such a major difference arise? There are two main factors:
First, the original team used rapid stellar evolution codes which, although powerful, do not take into account all of the details of stellar evolution. If we don’t model the stars in detail then it is difficult to predict how the stars will react to losing mass — this is a crucial detail in this particular case.
The BPASS code written by JJ. Eldridge and E. Stanway is a very complex model that simulates a whole Universe of stars with detail evolutionary models (scaled down to 1 million times the mass of the sun) and observes how it evolves over millions and billions of years.
As it turns out, this Universe in a box naturally recreates the observational properties that Liu et al. 2019 saw… only with a system that is roughly 1/10 of the mass.
The second big question concerning the paper published a few weeks ago is the claim that distance deduced by the Gaia telescope were not reliable. Put simply, the way Gaia finds the distance a star is by measuring its wobble over time - the more it wobbles the closer it is. The original team said that the presence of a big black hole could mess with the wobble and the inferred distance.
But in the end our work demonstrates that the Gaia distances can be trusted — in fact they even prefer a solution with a lower mass black hole!
In short, although astronomers did find a star orbiting a black hole, it is most likely not 70 times the mass of the Sun.
What does that mean for science?
Although a more moderate size black hole may not be quite as impressive, in the end our understanding of stellar evolution / mass loss is probably safe and that’s good news.
Also, even if the black hole is 10 times less massive than originally thought, it remains a very rare type of system and it is so exciting we get to study it. This is a feat of technology and opens up a new window to a better understanding of how our Universe works.
It’s important to remember that astronomers disagreeing on the conclusion of an observation is perfectly normal, that’s how science works!
We just hope you’ll enjoy this roller coaster of discovery as much as we do :)
This is a brief summary of the publication below. It has been submitted to the Monthly Noticed of the Royal Astronomical Society and is pending review.
“Weighing in on black hole binaries with BPASS: LB-1 does not contain a 70M⊙ black hole” [link]
J.J. Eldridge, E.R. Stanway, K. Breivik, A.R. Casey, D.T.H. Steeghs, H. F. Stevance