absolutely not confirmed to be a boson star. Most likely a black hole found its way there under unusual circumstances
> the scenario of a central black hole requires unreasonable amount of fine-tuning within the usual evolutionary channels. In particular, if the system is expected to have formed as a binary in isolation, a common envelope formation scenario is rather unlikely, given the system’s arrangement. This requires an extreme, and possibly unphysical, tuning of the relevant parameters of the evolutionary channel under consideration. Moreover, formation within a globular cluster is also improbable given the geometrical characteristics of the observed orbit. Other evolutionary channels such as formation without a common envelope or via a hierarchical triple also seem unlikely for similar reasons.
What makes you think their proposal isn't more likely than a black hole?
Regardless, it will be an interesting scenario to watch evolve!
No. We don’t even have evidence of a candidate boson you could use to make one.
Because a boson star is purely theoretical and I have a much stronger observational prior that the object is a black hole.
See, for example:
>The Country Parson Who Conceived of Black Holes [in 1783]
https://www.amnh.org/learn-teach/curriculum-collections/cosm....
>A Brief History of Black Holes
https://www.amnh.org/learn-teach/curriculum-collections/cosm....
>At a conference in New York in 1967, Dr. Wheeler, seizing on a suggestion shouted from the audience, hit on the name “black hole” to dramatize this dire possibility for a star and for physics.
https://www.nytimes.com/2008/04/14/science/14wheeler.html#:~....
The universe is big and has lots of stars. Even “very unlikely” things ought to turn up.
E.g.: one in a billion chance ought to occur hundreds of times… in our galaxy alone!
> Other evolutionary channels such as formation without a common envelope or via a hierarchical triple also seem unlikely for similar reasons.
Maybe it's hard to say without getting into specifics but it seems that the paper has considered the "unusual circumstances" in your statement.
"The system's evolution may be better-explained in models in which the G star was initially a wide tertiary companion to a close binary containing two massive stars. In this case, interactions between the two stars could have prevented either one from expanding to become a red supergiant, such that the G star could have formed in an orbit similar to its current orbit (but somewhat tighter) and remained there ever since. High-precision RV follow-up offers the tantalizing possibility of testing this scenario."
RV there is "radial velocity measurements".
Moreover, the part you quote and in particular "Moreover, formation within a globular cluster is also improbable" does not touch on their reference's "... dynamical formation in an open cluster that has since dissolved is more plausible."
Disruptions of open clusters are well known. <https://en.wikipedia.org/wiki/Open_cluster#Eventual_fate>.
The exotic bosons available in a 3+1 spacetime have never been detected at energy levels low enough that one might expect them to be reasonably bound by the weak (yes, weak) gravitation required by the boson star's internal repulsive forces. If the internal repulsive forces are too weak (or their self-gravitation isn't weak enough), the bosons cannot support the star against ultimate gravitational collapse (into a black hole). If the internal repulsive forces are too strong, the bosons all escape rather than stick around in the neighbourhood of the star. The latter is especially acute since the boson masses considered by the paper may be smaller than that of neutrinos.
Until a suitable slightly self-repelling exotic boson is discovered, achieving and maintaining equilibrium between it and gravitation is pretty fatal to the boson star idea.
Cold massive bosons are an idea for particle dark matter, incidentally. An example dark matter candidate is the axion. Axions have not been convincingly observed, and might not exist. A compact object made of essentially only axions could plausibly meet the criteria in the preprint linked at the top (cf their axion-motivated scalar field equation at eqn (9)), depending on details of heating during gravitational compaction. Also see e.g. Mohapatra et al., "Dense Axion Stars" (2016) <https://arxiv.org/abs/1512.00108> which is the preprint version of the PRL letter <https://doi.org/10.1103/PhysRevLett.117.121801>. There is as far as I know no clear mechanism to have anything like the axion density required for self-gravitation except in the very early universe, a long time before the first galaxies. So then why would small primordial axion stars be in galaxies, as the binary in the article at the top appears to be? How do you keep them small, rather than coalescing into supermassive compact objects?
The preprint at the top discusses other Beyond The Standard Model (of particle physics; BTSM) possibilities, but they are less convincing than the axion (for starters, such BTSM extensions need to have some non-physical degrees of freedom strongly suppressed, since such BTSM theories are almost always "haunted" by <https://en.wikipedia.org/wiki/Ghost_(physics)>s.). Additionally, the axion is motivated by the strong CP problem in the Standard Model, and if there is a lot of them they become gravitationally relevant. The non-axion options in the preprint at the top are afaik only motivated by a lack of knowledge about the microscopic details of gravitational physics. (See Koberlein's 2021 blog entry on Proca Stars <https://briankoberlein.com/blog/proca-stars/> for a brief intro to (light) massive vector boson stars).
looking at the paper they cite as reference for this claim, those authors thought exactly the same...
https://academic.oup.com/mnras/article-abstract/518/1/1057/6...
The distance between objects in this system 1.4 AU, which is _incredibly_ close.
- Sag A* has a mass of ~4,000,000 times the sun, and an event horizon of approximately 12,000,000 km (0.08 AU). This is just the event horizon; Sag A*'s accretion disk has a diameter of about 150,000,000,000 km (1000 AU). So your intuition would certainly be right about Sag A* - there is a huge disk of gas and other junk you'd be flying through.
- This object has a mass of at most ~12 times the sun, and (if it's a black hole) an event horizon of approximately 35 km. The paper doesn't say anything about an accretion disk given that it's exploring the idea that the object is not a black hole. Regardless, a star orbiting at 1.4 AU would probably clear out everything in the immediate neighborhood.