Prospective GW sources

MarkF
MarkF
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Ben: First thing thanks for

Ben:
First thing thanks for taking the time to respond. I understand how pressure works to sustain a fluid against gravity. I also understand how to use spherical harmonic functions to model deviations from the maximally symmetric solution.

It seems you are saying two things. First that the bulk of the deviation and hence the gravitation radiation comes from an essentially fluid outer shell of the pulsar. Second such deviations will only dissipate slowly if at all. The first seems completely reasonable; I hope I have it right now. I find the second harder to understand. I can understand how angular momentum and energy conservation could slow the dissipation if the viscosity where high enough. But without that or substantial yield strength I don’t understand how the deviation can avoid collapse.

If I were to somehow remove Earth’s continents and form a globe circling wave 10 meters high the wave wouldn’t just stand there. It would collapse and repeatedly reform in a progressively weaker and more distorted form until it became unidentifiable.

klasm
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Mark: Neutron stars are

Mark:
Neutron stars are expected to have a thin solid crust as well as a fluid interior. Given the density of neutron star matter even a thin crust will hold an enormous mass.
Just like the earth, neutron stars are belive to undergo "earthquakes", now caleld starquakes, when their crust cracks or changes in other ways.
Starquakes in neutron stars with strong magnetic fields, called magnetars, can be some of the most spectacular astronomical events seen.
See eg
recent starquake

or something a bit more technical

starquakes and neutron star evolution

Ben:
Thanks for the suggestion of the post with the survey paper. Nice to read about research outide ones own field every now and then.

/Klas

MarkF
MarkF
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klasm: Thank you for the

klasm:
Thank you for the links. The idea of star quakes has always fascinated me.

Unless I am totally miss-reading Ben's post his point is the crust is not that solid.

Quote:
this stuff is more like Jell-O than anything else


I am familiar with the "standard" model of neutron stars but apparently misunderstand some of the properties. I have gone back my continuum mechanics models to see if I can make sense of this new information.

klasm
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Mark: Just have to be glad e

Mark:
Just have to be glad e don't have any magnetars in our stellar neighbourhood. Starquakes are fascinating, but best viewed from a distance.

I think that "not so solid" correct impression. As you see there will not be vary large deviation in height.

Quote:

In the solid parts of neutron stars, that ratio should be around 10^-3. So the solid parts can hold up mountains, but they'll be relatively short. On Earth the highest mountain is 1/600 of the radius, but on a neutron star the highest mountain is probably no more than 1/10^6, certainly no more than 1/10^4.

However with material as dense as the crust of a neutron star even a tiny hill will hold an enormous amount of mass, and thereby momentum.
One figure I have seen for the density of a neutron star is 2.4 x 10^17 kg/m3 For comparison the earth has an average density 5515 kg/m3. Even given the larger radius of the earth, around 6300km vs around 10km for a neutron star, there is a lot more mass in a small neutron star mountain than in a large earth mountain.

Ben Owen
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Mark, it seems I was

Message 15110 in response to message 15106

Mark, it seems I was unclear.

The main emission mechanism for anything Einstein@Home could see is deformations of the solid parts (probably just the crust) of neutron stars. In a word, mountains.

When I say that the crust is almost a fluid, I mean that (like Jell-O) it can't pile up very well. It tends to crush under its own weight, which leads to small mountain heights. As Klas says, that weight is a lot even for a small mountain. A millimeter mountain on a neutron star generates more signal than all the Earth's continents combined.

But a true fluid would have zero sustainable mountain height. As you said, if you pile up a big blob of ocean and let it go, you get a wave which oscillates until something like viscosity leaches all the energy away. If you pile up a solid too high and let it go, it slumps back to some nonzero height and then stays there.

As on Earth, mountain height and mountain lifetimes are very different subjects. We have rough estimates for the former on neutron stars, but basically none yet for the latter. On Earth the lifetimes of mountains are determined by plate tectonics and erosion mechanisms. On neutron stars we have very rough ideas of mountain building mechanisms (although they're not really plate tectonics), but we have almost no idea of erosion mechanisms, if any.

The globe-circling fluid wave you were talking about is another possible emission mechanism. The trouble is, our best estimates of the viscosity say that those die off extremely quickly unless conditions are just right, including something feeding the wave. There is an instability where the right kind of fluid wave can basically feed itself, but our guess these days is that viscosity kills it in almost all isolated neutron stars. The binaries where it might work tend to be highly visible in x-rays, so they are being targeted specifically (and separately from Einstein@Home).

Hope this helps,
Ben

MarkF
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Ben: Thank you (again) for

Ben:
Thank you (again) for the clarification and sorry for my "density". I thought you meant the crust but then I got confused. I know from other readings about the density of the crust but have never seen an estimate of its shear modulus.
From what I know degenerate matter I assumed it would be much higher. This was based on the assumption that it would be difficult to create any relative movement of the majority of the matter far from the Fermi surface. Is the figure you stated for shear modulus (10^30 erg/cm^3) for the degenerate electron region or for the degenerate neutron region?

SpikeAr
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Mildly off-topic...but these

Mildly off-topic...but these are the only lengthly fictional treatments I know off about n-star makeup, crust and starquakes. Robert L. Forward wrote two sf novels, Dragon's Egg (1980) & its sequel Starquake, about the surface environment on a neutron star.

If he were alive today, what would he have to change in his descriptions, if anything, with the advantage of a quarter-century's increase in scientific knowledge about neutron stars?

Spike R. MacPhee Owner Science Fantasy sf Bookstore Harvard Sq 1977-89 spikerATtiacDOTnet
When retired RedSox pitcher "Spaceman" Bill Lee was asked his plans: "I want to spend the rest of my life fighting gravity."

SpikeAr
SpikeAr
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I've created a new thread for

I've created a new thread for my Forward/starquake question at
http://einsteinathome.org/node/189778
(after learning that the option to edit a post is only good for one hour after posting).

Spike R. MacPhee Owner Science Fantasy sf Bookstore Harvard Sq 1977-89 spikerATtiacDOTnet
When retired RedSox pitcher "Spaceman" Bill Lee was asked his plans: "I want to spend the rest of my life fighting gravity."

Ben Owen
Ben Owen
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Mark, RE: Is the

Message 15114 in response to message 15111

Mark,

Quote:
Is the figure you stated for shear modulus (10^30 erg/cm^3) for the degenerate electron region or for the degenerate neutron region?

That's the highest in a normal star. So it's coming from the bottom of the crust, where you've still got nuclei (although extremely neutron-rich) with neutron and electron fluids between.

Actually, it's got nothing to do with the Fermi surface. You might be thinking of the shear viscosity, which is how the fluid part resists shearing motion. That, and a lot of other things, depend on the Fermi surface because the involve particles trading momentum and in degenerate matter only a few near the Fermi surface are available to do that.

The shear modulus comes from the mutual electrical repulsion of the positively charged nuclei in the crust. The calculation just depends on the charges and how far apart they are. Just like normal matter, and doesn't care much whether they're degenerate. The particles don't have to trade momentum, just sit there.

When I started working on this I found that the original calculation was done by Klaus Fuchs in the 1930s. Small world. I suppose it makes sense - they hired him for a good reason; they needed to calculate properties of materials and he was in the forefront.

Of course they fired him for a good reason too...

Ben

MarkF
MarkF
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Ben, thanks again. I

Ben, thanks again.
I understand the nuclei are not degenerate in the outer portions of neutron star and that degeneracy does not play a major role in the outer most layer. My understanding was that as the electron become more degenerate their electrical charge distribution becomes the major factor in determining the distribution of the nuclei. At least as long as they are the dominate fermions. Once the nuclei start to breakdown and free neutrons become numerous the neutrons of course would be only minimally influenced by the charge distribution. Under these circumstances what controls the distribution of the neutrons if not degeneracy?
I have seen an animation attributed to you showing how a neutron star can oscillate like a ball of Jell-O. Can you point me to any public articles that details the model used?

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