[HN Gopher] A Sun-like star orbiting a boson star
___________________________________________________________________
A Sun-like star orbiting a boson star
Author : bookofjoe
Score : 61 points
Date : 2023-05-30 19:15 UTC (3 hours ago)
(HTM) web link (arxiv.org)
(TXT) w3m dump (arxiv.org)
| retbull wrote:
| The original paper that is referred to in this is
| https://arxiv.org/abs/2209.06833
| pmontra wrote:
| There is a section about boson stars at
| https://en.wikipedia.org/wiki/Exotic_star
|
| > [...] For this type of star to exist, there must be a stable
| type of boson with self-repulsive interaction; one possible
| candidate particle is the still-hypothetical "axion" (which is
| also a candidate for the not-yet-detected "non-baryonic dark
| matter" particles, which appear to compose roughly 25% of the
| mass of the Universe). It is theorized that unlike normal stars
| (which emit radiation due to gravitational pressure and nuclear
| fusion), boson stars would be transparent and invisible. [...]
| m3kw9 wrote:
| What happens if I fly into a boson star?
| einpoklum wrote:
| You get a ticket from the boson police of course. The thing
| is, they only have a secret police.
| sbierwagen wrote:
| A non-interacting boson star would be transparent, but it
| still has gravity, so it would probably pick up baryonic
| matter. Should look like an ordinary star that has too much
| mass for its diameter and burns more brightly due to the
| additional gravity compression.
|
| Dropping straight through a clean solar-mass boson star,
| though, would be uneventful. A little bit of blueshift on
| starlight, but probably not enough to detect by eye.
|
| Avoid a slingshot orbit. If it has the mass of the Sun and
| the same density, then it would have a "surface" gravity of
| 28g one million kilometers out. With that sharp of a
| gradient, a close orbit will want to pull your spacecraft
| apart as each atom tries to take a different trajectory,
| tidal forces. https://en.wikipedia.org/wiki/Roche_limit (A
| plot point in many a Larry Niven story)
| raattgift wrote:
| Depends very much on the boson and its interaction with the
| (extended) Standard Model that includes it.
|
| Axions, if they exist, take part in nuclear interactions.
| You can expect to have a bad day when encountering them at
| high density (e.g. expect fissions from neutron
| disruptions).
|
| In the preprint at the top, the Action (eqn (1)) and
| Lagrangian densities (eqn (2)) are non-interacting (except
| for self-interaction and gravitation) for tractability
| reasons, so the scalar boson version is only sorta like the
| axion. The paper's setting only considers each type of
| boson in isolation from any other type of matter in the
| universe. However any infalling body which has any nonzero
| distribution of these proposed vector or scalar bosons in
| them would expect the self-interaction term to be
| important, so you could still have a bad day for non-
| gravitational reasons.
|
| I don't think a non-relativistic analysis of a manifestly
| relativistic compact object is particularly enlightening.
| Boson stars can be more compact than neutron stars (see
| Figure 2 on p. 7 of the preprint). Qualitatively though,
| you'd have a bad day if you got too close; you are right
| that tidal stresses are probably what will get you in the
| case of only-self-interacting bosons.
|
| The Roche limit, developed well before even special
| relativity, matters for slowly-orbiting (hyperbolic orbits
| count) self-gravitating bodies (so big asteroids and comets
| count). Your parent commentator's flight would presumably
| be in a spacecraft (or suit) that is held together by
| intermolecular forces instead of its own gravity. Those
| forces are typically much stronger than gravity; gravity
| only overcomes them in relativistic systems.
| akomtu wrote:
| "Boson stars have also been proposed as candidate dark matter
| objects, and it has been hypothesized that the dark matter
| haloes surrounding most galaxies might be viewed as enormous
| [toroidal] boson stars."
|
| That's quite a sci-fi plot.
| metalliqaz wrote:
| absolutely not confirmed to be a boson star. Most likely a black
| hole found its way there under unusual circumstances
| eterevsky wrote:
| Still exciting. Do we have any other examples of stars closely
| orbiting around black holes?
| daniel-thompson wrote:
| [1] S62 comes within 16 AU at closest approach to Sag A* but
| its orbit is highly elliptical.
|
| The distance between objects in this system 1.4 AU, which is
| _incredibly_ close.
|
| https://en.wikipedia.org/wiki/S62_(star)
| dylan604 wrote:
| Do we know the distance to the event horizon for Sgr A*?
| I'm no astrophysicist, but 1.4AU sounds _really_
| _incredibly_ close. In my sci-fi thoughts, something at
| 1.4AU would be attempting to go "in, through, and beyond"
| and not orbiting. So excuse me while I try to re-evaluate
| whatever pre-existing notions I might have
| daniel-thompson wrote:
| I'm no astrophysicist, but the paper and some web data
| show
|
| - 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.
| spuz wrote:
| This paper doesn't seem to agree:
|
| > 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?
| nawgz wrote:
| Is a "boson star" even a confirmed physical construct? I can
| see confirmed neutron stars like RXJ1856 that they think
| might be a strange star, but... Just based on what is a
| confirmed physical construct in the universe, a black hole
| seems far likelier. Obviously they address this in a way I'm
| not knowledgeable enough to argue against.
|
| Regardless, it will be an interesting scenario to watch
| evolve!
| pja wrote:
| > Is a "boson star" even a confirmed physical construct?
|
| No. We don't even have evidence of a candidate boson you
| could use to make one.
| jwuphysics wrote:
| > What makes you think their proposal isn't more likely than
| a black hole?
|
| Because a boson star is purely theoretical and I have a much
| stronger observational prior that the object is a black hole.
| bookofjoe wrote:
| Once upon a time black holes were purely theoretical.
|
| 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_Colonel wrote:
| Survivorship bias. Aether was once also a theory which
| "just" needed an experimental evidence.
| jiggawatts wrote:
| P-hacking.
|
| 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!
| burnte wrote:
| This is one of the hardest things to grasp about the size
| of the universe; it's so damn big pretty much anything is
| possible somewhere.
| metalliqaz wrote:
| that's what I mean by "unusual circumstances". As you point
| out, the "usual evolutionary channels" don't really fit.
| spuz wrote:
| This is why I included this part of the quote:
|
| > 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.
| raattgift wrote:
| The paper rests on the MNRAS published version of
| <https://arxiv.org/abs/2209.06833> (ref. [EBRQ+22] in the
| preprint at the top, cited at the end of the paragraph
| you quote upthread). [EBRQ+22] itself _proposes_ a triple
| as a possibility:
|
| "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>.
| jheriko wrote:
| runaway stars suggest that interactions occur leaving
| binaries behind in these kinds of configurations. i see no
| need for unphysical fine-tuning.
|
| 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...
| raattgift wrote:
| First see footnote 1 of the first page of the preprint linked
| at the top. The authors are jumping the gun. That's fine for
| theorists, but bear in mind that they are literally in the
| dark on a number of telescopic observables, any of which
| would shoot down the idea in the preprint. Some of these are
| achievable by EHT. See e.g. Olivares et al. "How to tell an
| accreting boson star from a black hole" (2020) <https://disco
| very.ucl.ac.uk/id/eprint/10112389/1/staa1878.pd...> [pdf].
|
| 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).
| daniel-thompson wrote:
| More likely != confirmed
___________________________________________________________________
(page generated 2023-05-30 23:00 UTC)