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Missing Black Holes

Artists concept of a black hole. Image credit: Gallery of Tempolimit Lichtgeschwindigkeit

As astronomers began working out how stars die, they expected that the mass of remnants, whether white dwarfs, neutron stars, or black holes, should be essentially continuous. In other words, there should be a smooth distribution of remnant masses from a fraction of a solar mass, up to nearly 100 times the mass of the sun. Yet observations have shown a distinct lack of objects at the borderline of neutron stars and black holes weighing 2-5 solar masses. So where have they all gone and what might this imply about the explosions that create such objects?

The gap was first noted in 1998 and was originally attributed to a lack of observations of black holes at the time. But in the past 13 years, the gap has held up.

In an attempt to explain this, a new study has been conducted by a team of astronomers led by Krzystof Belczynski at Warsaw University. Following the recent observations, the team assumed the paucity was not caused by a lack of observations or selection effect, but rather, there simply weren’t many objects in this mass range.

Instead, the team looked at the engines of supernovae that would create such objects. Stars less than ~20 solar masses are expected to explode into supernovae, leaving behind neutron stars, while ones greater than 40 solar masses should collapse directly into black holes with little to no fanfare. Stars between these ranges were expected to fill this gap of 2-5 solar mass remnants.

The new study proposes that the gap is created by a fickle switch in the supernova explosion process. In general, supernovae occur when the cores are filled with iron which can no longer create energy through fusion. When this happens, the pressure supporting the star’s mass disappears and the outer layers collapse onto the immensely dense core. This creates a shockwave which is reflected by the core and rushes outwards, slamming into more collapsing material and creates a stalemate, where the outwards pressure balances the infalling material. For the supernova to proceed, that outwards shockwave needs an extra boost.

While astronomers disagree on exactly what might cause this revitalization, some suggest that it is generated as the core, superheated to hundreds of billions of degrees, emits neutrinos. Under normal densities, these particles travel right past most matter, but in the superdense regions inside the supernova, many are captured, reheating the material and driving the shockwave back out to create the event we observe as a supernova.

Regardless of what causes it, the team suggests that this point is critical for the final mass of the object. If it explodes, much of the mass of the progenitor will be lost, pushing it towards a neutron star. If it fails to push outwards, the material collapses and enters the event horizon, piling on mass and driving the final mass upwards. It’s an all or nothing moment.

And moment is a good description of how fast this occurs. At most, astronomers suggest that this interplay between the outwards shock and the inwards collapse takes a single second. Other models place the timescale at a tenth of a second. The new study notes that the more quickly the decision takes place, the more pronounced the gap is in the resulting objects. As such, the fact that the gap exists may be taken as evidence for this being a split second decision.

About 

Jon is a science educator currently living in Missouri. He is a high school teacher and does outreach with the St. Louis Astronomical society as well as presenting talks on science and related topics at regional conventions. He graduated from the University of Kansas with his BS in Astronomy in 2008 and has maintained the Angry Astronomer blog since 2006.
For more of his work, you can find his website here.

Comments on this entry are closed.

  • Anonymous October 14, 2011, 6:51 PM

    This article reminds… there’s usually more than one way to explain remotely observable phenomenon. Of course, given our current interpretation(s) of stellar and galactic evolution, it is ‘logical’ to point at the most energetic known event(s) as part of a possible sequence for creating black holes. BUT, there are other possibilities: “The chicken or the egg” argument continues…

    A small sample (127,000 hits) of the Google question: “Which came first, galaxies or black holes’

    http://news.discovery.com/space/galaxy-black-hole-110920.html
    http://bigthink.com/ideas/24404 (I LIKE Dr. Michio Kaku – he seems to be on to something!)
    http://www.scientificamerican.com/article.cfm?id=baby-black-holes
    http://news.ucsc.edu/2011/09/black-holes.html

    I for one am not entirely convinced that at the core of every galaxy, globular cluster, star, brown dwarf, gas giant or even atoms may lurk a small black hole… or ‘hole’ in the fabric of space/time.

    • Torbjörn Larsson October 14, 2011, 9:59 PM

      Sure, the public likes Kaku; I don’t think anyone in physics do. Too much self promotion, too much speculation, too little physics.

      A black hole isn’t a “hole” in spacetime, but a spacetime region with high energy. An analogy would be a particle (say photon), but we don’t think of them as “holes” in fields (say, EM field). It is no mystery in that, instead black holes are pretty awesome by their own properties.

  • Torbjörn Larsson October 14, 2011, 10:06 PM

    That is one finetuned process then! Impressive.

  • Anonymous October 15, 2011, 1:44 AM

    Strange matter composed of metamaterials, near absolute zero vacuums, and magnetic fields could explain “the GAP”. neutron stars could really be instead quark stars, and black holes have never been observed merely inferred to give existence. We can’t see them at galaxy centers, so we don’t see them at stars 2-5 solar masses. These seem to be facts that best explain why the gap remains today, rather then the “fickle switch” that the scientists invented.

    • Anonymous October 15, 2011, 4:02 PM

      – Strange matter composed of metamaterials, near absolute zero vacuums, and magnetic fields could explain “the GAP”. —

      So you are replacing “dark” matter with “strange” matter and problem solved?

      • metamaterials October 16, 2011, 2:19 AM

        Not replacing, but eliminating dark matter, because strange matter forms quark stars, or densely confined quarks held together by EM forces, the quark gluon plasma. Quark stars are believed formed by exploding neutron stars that rapidly supernova a second time, about 2,000 years later, and believed to be magnetars. Magnetohydrodynamics of magnetized superfluid metamaterials, rules these stars in outer space vacuum w/o gravity. This means the gravity from dark matter is secondary and residual to the primary primordial EM source. Neutron star models are gravitational, but strange quark star models are becoming popular, because they do not require dark matter and black holes.

  • Anonymous October 15, 2011, 3:14 AM

    The mechanism seems tuned for the creation of BH with a mass > 5M_{sol}. I have thought about a form of implosion by a neutron star by a flavor-locking mechanism of a quark-gluon plasma. This would lead to a 2.3 solar mass BH.

    An elementary particle is a bit like a black hole, but turned inside out. The relationship between elementary particles and quantum black holes has been a fairly active topic in physics.

    Michio Kaku is a bit odd in a way. I read his book “Future Physics,” which was interesting in some ways, but had funny aspects to it. Kaku has stepped forwards to try to be the next Carl Sagan.

    LC

    • Torbjörn Larsson October 15, 2011, 1:12 PM

      Maybe, but Sagan was firmly rooted in the science of the day, as I remember descriptions of him. None of that “future physics” stuff, I think.

      I may easily be overstating this, because I haven’t followed Kaku as closely as you have for example. So I’ll abide with what more expert commentators say here.

      • Anonymous October 15, 2011, 3:03 PM

        Sagan imbibed on this sort of stuff. He worked a lot on the idea of ETIs and contacting them. He got into some pretty speculative stuff.

        LC

        • Torbjörn Larsson October 15, 2011, 8:27 PM

          OK. YMMV.

          ETI isn’t fanciful. Rather replies to Fermi’s question is, because it is too loosely constrained. I resisted to answer that for the longest time, before I realized there was a sound reply from a sociological standpoint.

          So I wouldn’t call that “future physics” as much as astrobiology.

          But I see your point.

          • Anonymous October 16, 2011, 2:23 PM

            The hypothesis that if ETI exist they would send radio waves is not entirely out of line. Yet in Sagan’s books he did tend to run with some highly speculative ideas about star faring civilizations and promoted the conjecture about type I through V civilization types and so forth. If I remember right type III civilizations control an entire galaxy, which is a pretty highly speculative idea. The V civilizations as I recall are capable generating new cosmologies and “wormholing” their way into them. At this point the distinction between intelligent life and theological ideas of gods or a God starts to get blurred.

            LC

  • Anonymous October 15, 2011, 4:33 AM

    I think there is room for argument about the first premise that the gap in observation is due to a lack of these objects. The gap is still most likely due to a lack of sensitive enough instruments to detect these small black holes. I think this notion is more plausible than trying to rework the dynamics of how large stars implode/explode during a supernova event. This hypothesis still makes for fun and fanciful reading though.

    • Torbjörn Larsson October 15, 2011, 1:26 PM

      Not that it is important or rare, but now I am confused.

      The paper claims that there is debate between modelers how different instabilities work here. And just the other day I had to take my first layman steps into this area (due to the OPERA results), to find out (it seemed to me) that it is hard to make the supernova explode in the first place.

      So I bought the idea that you could well attach larger models to the current status of the field without having a “rework” of the basic mechanisms. Where did they rework the dynamics?

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