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I Direct collapse to black hole without supernova?

  1. May 29, 2017 #1
    Hi.

    I saw this:

    https://www.forbes.com/sites/starts...a-third-way-to-form-black-holes/#52d7225f216d

    suggesting they saw a star undergo a "direct collapse" to a black hole with no supernova. Now this seems like it could be sensible -- if there's enough gravity then it could just keep on crushing past even any other further sources of explosive energy release. Yet the part I wonder about is why wouldn't there at least be SOME kind of attendant energy pulse with this collapse? In particular, wouldn't as all that gas collapses on itself but before it reaches the black hole size, heat up tremendously due to compression and thus emit an enormous light (EM radiation) pulse just before vanishing into the black hole? Or is it just far too fast for that? Or were they just not looking when the energy pulse was emitted (note the large discrepancy in dates between the two photographs)?
     
  2. jcsd
  3. May 30, 2017 #2

    stefan r

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    The caption below one of the images:
     
  4. Jun 4, 2017 #3

    Chronos

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    Direct collapse is a strong contender to explain the origin of supermassive black holes. In this scenario a primordial gas cloud skips right past the stellar phase and directly forms a 'seed' black hole. The seed serves to anchor a protogalaxy and fattens up off further infalling material. The star to black hole route is a more leisurely process. It is pretty much certain that only a truly obese star can form a black hole - somewhere above 25 solar masses - and are quite rare. Only a handful are known to exist in our galaxy. It is extremely difficult to identify these behemoths in other galaxies. It usually requires measurement of the motion of a companion star to pin down a mass range which is, of course, problematic at the vast distances between galaxies.
     
  5. Jun 5, 2017 #4
    As Chronos pointed out, direct collapse black holes is the best contender for how super massive black holes could have formed in the early universe. Stellar mass black holes have the theoretical range between 3 and 69 solar masses. The smallest observed stellar mass black hole is XTE J1650-500 which is 3.8 ± 0.5 solar masses. The most massive observed stellar mass black hole is 62 ± 4 solar masses detected by its gravitational waves.

    Stars between 100 and 250 solar masses do not form black holes. Stars in the 100 to 130 solar mass range go through phases where they become unstable, shed mass, and find equilibrium again until they drop below 100 solar masses. Stars between 130 and 250 solar masses experience a pair-instability supernova, leaving absolutely nothing behind.

    This is where it gets a little strange. Stars more massive than 250 solar masses experience photodisintegration, spawn what has been termed a "hypernova" and do leave behind a black hole. Although I could not tell you its theoretical size.

    It would take billions of years for even the most massive stellar mass black holes to merge together and form some of these super massive black holes that were have observed. Therefore, there must be another mechanism for how super massive black holes are created. The prevailing theory at the moment is the direct collapse method. However, it has a problem with the Eddington Limit. As a gas cloud collapses eventually it will reach a point where the radiation acting outward will become equal to the gravitational forces acting inward. Thus achieving hydrostatic equilibrium. When a star exceeds its Eddington Luminosity it creates stellar winds that push the rest of the collapsing gas cloud away, thereby theoretically limiting its mass. However, there are stars with super-Eddington Luminosities, and they are now even talking about hyper-Eddington Luminosities. In order for the direct collapse black hole theory to work, it must somehow overcome hydrostatic equilibrium.

    Sources:
    Observation of Gravitational Waves from a Binary Black Hole Merger - Physical Review Letters 116, 061102, Published February 11, 2016 (free issue)
    Hyper-Eddington mass accretion on to a black hole with super-Eddington luminosity - Monthly Notices, Royal Astronomical Society, Volume 461, Issue 4 : 4496-4504, Published July 8, 2016 (free preprint)
     
  6. Jun 6, 2017 #5

    stefan r

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    The solar wind leaves from the outer surface of a star. That does not prevent the compression of the core. Does it need to ever be at "equilibrium"?
     
  7. Jun 6, 2017 #6
    No, it doesn't and in case of collapsing protogalactic clouds there is also a lot of dark matter involved which has no radiation problem at all.
     
  8. Jun 6, 2017 #7
    Black hole formation is not an equilibrium process. The Eddington Limit doesn't quite apply if you never reach equilibrium.

    A few to tens of solar masses moving at relativistic velocities (say few % of the speed of light) is not going to bother paying attention to equilibrium conditions especially if there is a singularity forming in the direction of movement.
     
  9. Jun 7, 2017 #8
    You are right, the solar wind has no effect on the compression of the gas cloud that is occurring. However, the solar winds would have an effect on the remaining gas, limiting the overall mass of the star that can be compressed.

    Hydrostatic equilibrium is not achieved overnight. That is why we have Cepheids and other pulsating stars. Like a pendulum, it keeps overshooting its point of perfect balance but with each pass it gets a little closer to achieving that equilibrium.

    It should also be noted that the early universe was very different from the universe we find ourselves in today. Population III stars were much more massive than stars are able to form today because they were primarily hydrogen, with a little helium, and no metals. There were no metal-rich gas clouds in the early universe, so I am not entirely sure the same thresholds we use to explain today's universe can apply then. Who knows what role dark matter could have played in the formation of super massive black holes in the early universe?
     
    Last edited: Jun 7, 2017
  10. Jun 7, 2017 #9

    stefan r

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    Speculating... Could we model the early universe events as "voids" instead of "stars". Suppose the south side of the expanding void is the first part to collide with another void and heat up. Light from the south will sweep out the interior of the void. The material in the center will be propelled north and begin to catch up with material on the northern boundary. We might expect supermassive blackholes to form when the northern boundary collides with another void. The pressure from photons would have an effect inverse from Eddington. Photons would propel mass toward gravity and away from center of the void.
     
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