[X] My Assistant

[dvandorn]

Neutron stars hold their shape from gravitation which treats them as if they were fluid, even the super-resistant crust. In the case of super fast rotating neutron star, they keep from flying appart from the intense gravitation field, not from the resistance of the crust.

That's what I meant when I said that I didn't think a neutron star would lose its cohesiveness enough to allow significant amounts of matter to be stripped. I wasn't thinking in terms of a solid body that couldn't be broken -- I was thinking of the gravitation holding it so tightly together that the only way anything, including a black hole, could tear it apart would be for the neutron star to barrel into the black hole -- and that the neutron star would be "swallowed" before any significant amount of mass could be stripped.

I say again, I'd want to see what the process of stripping mass off of a neutron star would look like. And, for something as incredibly dense and difficult to break pieces off of as a neutron star, would the Roche limit actually occur inside the Schwartzchild radius of the black hole? My gut feeling is that it would -- the neutron star, I think, ends up getting swallowed whole no matter what you do...

-the other Doug

[Guest_Richard Trigaux_*]

dvandorn, I think only a calculus or simulation would tell us what scenario is possible, the neutron star swallowed at a whole, or it being stripped during the last chaotic orbits around the black hole. My personnal inclination is for the second scenario, but...


if it is stripped, the result may be tremendous. The surface of a neutron star is relatively cold, while the inner fluid are at million degrees or more. So exposing it at the surface may produce burst of gamma rays. There is an example of this, a relatively close neutron star (around 400 light-years) had a small adjustment of shape, allowing liquid to come on the surface. This only produced enough gamma rays and X rays to disturb Earth's ionosphere and affect radio transmissions! So tearing a neutron star may be a really horrible thing...

By the way, neutron stars are cooling, and in the proces they may have plate tectonics, volcanoes, etc. Or they have not, due to the solidity of the outer crust, and they cool only by heat diffusion through this crust.

And the inner fluid? It surely undergoes some convection. And magnetic lines, etc. like in Earth's core.

And superfast neutron stars, what shape do they get? Ellipsoids? peanuts? or more complex? How do they adjust their shape?

[helvick]

And the inner fluid? It surely undergoes some convection. And magnetic lines, etc. like in Earth's core.
And superfast neutron stars, what shape do they get? Ellipsoids? peanuts? or more complex? How do they adjust their shape?

Would heating\convection effects as we know them have any meaning in a superfluid? The models I've seen say that the "neutronium" should be a superfluid. They are supposedly intensely hot (1 million K) so what does that mean for something that is effectively one great big atomic nucleus? Gravitional forces at the surface are 100 billion g, that's a hell of a lot of force keeping it spherical but the centripetal acceleration at the surface is almost exactly the same (assuming it's a 20km sphere rotating at 1000 revs/sec that comes to about 80 billion g). So the surface (at the equator at least) is balanced more or less on a knife edge - bring that close to anything that is massive, let alone a black hole, and the whole thing would probably shed outler layers catastrophically.

I can't see one staying stable as it gets close to a Black hole.

[jamescanvin]

Neutron stars hold their shape from gravitation which treats them as if they were fluid, even the super-resistant crust. In the case of super fast rotating neutron star, they keep from flying appart from the intense gravitation field, not from the resistance of the crust.

Note, I didn't make any ludicrous claims that the structual properties of the crust had anything to do with holding the NS star together as your reply seems to imply.

Would heating\convection effects as we know them have any meaning in a superfluid?

No, and not only that but the intense magnetic fields would also goven the interior motion over any convection effects even if it wasn't superfluid.

Gravitional forces at the surface are 100 billion g, that's a hell of a lot of force keeping it spherical but the centripetal acceleration at the surface is almost exactly the same (assuming it's a 20km sphere rotating at 1000 revs/sec that comes to about 80 billion g). So the surface (at the equator at least) is balanced more or less on a knife edge - bring that close to anything that is massive, let alone a black hole, and the whole thing would probably shed outler layers catastrophically.

1000 Hz is a little fast, the fastest known pulsar is around 1.5ms I beleve (600 Hz) and note millisecond pulsars are extremly uncommon compared to your average common or garden NS (and probably more massive and hence more stable to disruption)

The thing that makes me think that it would not be disrupted by ths method is that even using your very fast 1khz model you stiil have tens of billions of g's to overcome but not only that you've got to accelerate the matter to escape velocity within (less than) half a revolution else it's going to get to a position where the tidal force vanishes and it will fall back to the surface.

I say again, I'd want to see what the process of stripping mass off of a neutron star would look like. And, for something as incredibly dense and difficult to break pieces off of as a neutron star, would the Roche limit actually occur inside the Schwartzchild radius of the black hole? My gut feeling is that it would -- the neutron star, I think, ends up getting swallowed whole no matter what you do...

That's my gut feeling also.

However I beleve the final accretion of material onto a BH is extremely complex and all this speculating is rather pointless without some rather complex simulation to back it up. I've done a bit of work in this field looking into jet production (jets are the day job) and it is very poorly understood (The fact that jets are produced at all tells you something complicated is going on!)


James

[helvick]

Nice feedback James. My estimates were based on some quick googling and it's good to see someone who has done some real work thinking about this bringing some reality into the picture.

Your comment about "reacing escape velocity within 1 half revolution" does kill my argument. Escape velocity will be about 0.5 c while the equatorial surface is "only" moving at 0.4c under my assumptions. It's relatively close but there is no practical way to accelerate to that extra 0.1c via gravitational forces using known physics.

However the physics of what happens at the surface itself is intriquing. If we take it that my 1khz speed is too high but say that the rotational speed is more realistically 600hz then the velocity of the eqatorial surface is still 25% of light speed and the centripetal force is 28 billion g. A couple of g is enough to make normal atomic matter at this sort of scale hold to a spherical shape so is it safe to assume that the ~70billion g surplus at the surface will ensure that neutron stars are effectively perfect spheres despite their rotational velocity?

And what behaviour should we expect during a black hole:neutron star collission? Will it strip off layers as it intersects the event horizon? is there a different "roche" limit at which it will go chaotic? Or are those g levels sufficient to hold it together even as it disappears over the event horizon.

[dvandorn]

Hmmmm... if escape velocity at the surface of the NS is about .5c, and if the attraction of the BH reaches exactly 1c at the event horizon, then it stands to reason that at some point *outside* of the event horizon, the BH will pull at the NS hard enough to cause some particles to reach escape velocity.

The real question is, how far outside of the event horizon does the BH's attraction exceed the escape velocity from the NS? The curvature of space that close to a BH is extremely sharp -- it's possible that the BH doesn't exert enough force to induce the NS's disintegration until just before it touches the event horizon.

Also, as the NS disintegrates, the extremely dense mass of the stream of matter being sucked into the BH would exert its own gravitational force on the remaining mass of the NS.

What bends my brain is that there *must* be effects on the NS mass that has yet to fall within the event horizon based on the gravitational field fluctuations caused by the mass that has already fallen below the EH. But since Einsteinian physics cannot predict or describe the properties of mass or energy that has been accelerated beyond (or at least exactly to) 1c, it's very, very difficult to gain any insight into what's happening to the mass that falls within the EH, and how its gravitation affects the behavior of the remaining mass as it's swallowed.

-the other Doug

[jamescanvin]

Nice feedback James. My estimates were based on some quick googling and it's good to see someone who has done some real work thinking about this bringing some reality into the picture.

Well, I wouldn't say I'd done any 'real' work thinking about this. As I said I work on understanding the radio jets produced from black holes and I once looked into jet production just long enough to realise that it's really hard!

A couple of g is enough to make normal atomic matter at this sort of scale hold to a spherical shape so is it safe to assume that the ~70billion g surplus at the surface will ensure that neutron stars are effectively perfect spheres despite their rotational velocity?

I think so, I seem to remember reading a couple of years back about a theory where once a NS is spun up to such a rate that it starts to be deformed (kHz range) it starts radiating gravitational waves quite strongly, thus limiting its speed and hence is deformation.

And what behaviour should we expect during a black hole:neutron star collission? Will it strip off layers as it intersects the event horizon? is there a different "roche" limit at which it will go chaotic? Or are those g levels sufficient to hold it together even as it disappears over the event horizon.

I really don't know. Generally of course the NS is going to be bigger than the BH so in my mind I imagine the BH being swallowed by the NS. After that I don't know what to imagine, clearly all hell is about to break loose! The part of the NS closest to the BH is obviously going to be consumed but I suspect that it not all going to fall straight in and you may end up in a supernova like scenario where some of the outer layers falling at close to the speed of light bouncing off other layers causing a massive explosion.

These are the type of questions that we may be able to answer better once gravitational wave antennas are sesitive enough. The most powerful gravitational waves are likely to come from such events and will contain a lot of infomation about the coalescence procedure.

James

[Guest_Richard Trigaux_*]

Thank you all for this nice conversation. Just are missing some more ingredients for the "hell breaking lose":

-if the neutron star is submitted to a quickly changing gravitationnal field, it will have waves on its surface. And these waves will in turn produce gravitational waves which effect will not be negligible...

-The same wih the the black hole, which can oscillate in a wave-like pattern.

-Add to this the Lens-Thiring effect of both bodies (NS will have some too) (for non-specialist readers, this effect is the equivalent of the magnetic field into the gravitation domain: when electric charges rotate, they produce a magnetic field, which can in turn make rotate other electric charges, like in a transformer. In the same way a rotating black hole will induce rotation of all the surrounding bodies).

-And also magnetic fields, which can be very strong in neutron stars. It is currently believed (but, as far as I know, not really proven) that it is the magnetic field which produces jets in quasars, accreting stars, etc... We can expect that the result of swallowing a magnetar (neutron star with very strong magnetic field) will be more complex than with a weak field neutron star.


So to know what happens will require a complex simulation accounting with all the parametres: magnetic field, states of matter, gravitationnal waves... some simulations of this kind were done with coalescence of black holes, and they are already very complicated (for instance we can know if a point is within the horizon event only after many steps of simulation).


However I think that such kind of coalescence, in more of being relatively common, may be the most powerful source of gravitationnal waves (a perfectly symmetrical supernova should produce none, in fact, but recent works showed that supernovas are turbulent and thus not symmetrical).

So that simulations of coalescence of neutron stars/black holes are an important domain.

A simple simulation would however, I think, be possible to set the lower limit of mass for a neutron star (the lowest stable mass for a lump of broken neutron star, not the minimum mass at formation, which is known to be 1.3 solar mass). For this some parametres are needed: -the density of neutronic matter -the mimimum pressure to maintain it in this state. Things are complicated by the fact there would be a white-dwarf-like layer of matter around it, more a layer of ordinary gas with variable density.

[jamescanvin]

Well wadayaknow, turns out somebody has thought a bit more about this than us.

Going back to the origonal question about what happens to a NS that goes below it's minimum mass. A quick astro-ph search gives a paper asking that very question in the title!

The fate of a neutron star just below the minimum mass: does it explode?

Very interesting, particualrly the introduction which give a whole load of references to other papers relevent to this disscussion (stripping matter off a NS)

[deglr6328]

So the answer seems to be then, that NSs CAN in fact explode but the paper does not seem to concern itself with a mechanism for the stripping off of that 20% of mass from its surface necessary to do so.....

[tedstryk]

I agree. Which is why science has nothing to say about supernatural phenomena.
Please provide the name of ONE such "fundamentalist scientist".
What domain of science would this be? If you're thinking of string theory, then I'd argue that the scientific status of the theory is questionable. Even so, string theorists don't let the non-scientific status of their discipline stop them from asking questions and finding objectively determinable answers. And they DON'T hire public relations firms to campaign for including their field in the high school science curriculum. (Nor does Pat Robertson call down the wrath of God onto school districts that decide not to teach string theory.)

I think this had better be my last contribution on this topic.

There are plenty, so long as you leave "scientist" in quotes.

[Tom Ames]

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[Guest_Richard Trigaux_*]

Thank you jamescanvin fo the paper about mimimum mass for a neutron star.. If I read well, it is enough to remove a bit of a "mimimum" neutron star to make it explode in a matter of 20s. And, back to the idea of a black hole sripping a neutron star, if the neutron star is engaged into the last "death dance" (the last chaotic relativistic orbits around the black hole, some seconds before being swallowed) it may occasionally pass close enough to have a bit of it into the Schwarzchild radius, while the bulk of it is still on an escape trajectory. So in this case, I think it may be stripped, and thus it will explode, if the black hole does not swallow it before the 20s delay.

Interesting.

And really frightening about what happens after.

[ljk4-1]

Maybe it shouldn't, but it still gets me how it is almost the year 2006 and there are still plenty of people who think that a supernatural being made everything and punishes people as a group with storms and earthquakes.

If someone or something did create this Universe, which is theoretically possible, it will not be the kind of "god" most people think it is.

Today is Voltaire's birthday. This quote I found sums things up pretty well:

"Men will always be mad, and those who think they can cure them are the maddest of all." - Voltaire, 1762

http://en.wikipedia.org/wiki/Voltaire

[Guest_Richard Trigaux_*]

Maybe it shouldn't, but it still gets me how it is almost the year 2006 and there are still plenty of people who think that a supernatural being made everything and punishes people as a group with storms and earthquakes.

If someone or something did create this Universe, which is theoretically possible, it will not be the kind of "god" most people think it is.

Today is Voltaire's birthday. This quote I found sums things up pretty well:

"Men will always be mad, and those who think they can cure them are the maddest of all." - Voltaire, 1762

http://en.wikipedia.org/wiki/Voltaire

I Agree. We must in no case confuse arbitrarian religious beliefs as they were made throughout history (often an evil history of power fights, crusades, dictatures and fundamentalism) and what could be a real spiritual realm.

If someone or something did create this Universe, which is theoretically possible, it will not be the kind of "god" most people think it is.

I like this kind of reasoning. The hypothesis of a God is not completelly intestable. We can still do some reasonings, for instance to eliminate the idea of an arbitrarian fulminating god punishing us with earthquakes and cyclones, because group punishing is very unfair, and such a god would be really an evil and racist thing. Only intellectually limited and heart impaired humans could imagine a creator like this, as they were unable to imagine something better than they were.