Today's main effort was to try and set up a stable rotating disc of gas. It failed miserably but at least it looks nice.
What happens is that the outer parts of the disc expand into infinity, while the inner parts simultaneously collapse. When the density gets too high, numerical errors in the simulation become significant and momentum is not conserved. That's why you see a few dense clumps suddenly moving on straight lines (most noticeably the big bright one in the center at the end). That's not too much of a problem provided I can figure out why it's collapsing in the first place.
My hypothesis is that it's because the thermal motions of the gas (3.5 km/s) are significant compared to the maximum rotation speed (12.2 km/s) so simply by random motions, the gas particles move into regions where their carefully determined initial rotation velocities are no longer stable. If I make the gas much more massive (so the rotation speed higher) or colder it should fix the problem.
However, why so much of the gas is able to escape I do not know. None of the particles should ever reach the escape velocity. Hmmm.
Great stuff! Now, try adding some hydrogen snowflakes for stability:
ReplyDeleteManly Astrophysics claims to have discovered Earth mass to 1% solar mass clumps of gravitationally-bound hydrogen/helium at circa 5 Kelvins and 1 AU radius which they suggest are stabilized by hydrogen snowflakes. And they think they're so stable they coined the term "paleon", meaning old.
From a study of 'interstellar scintillation' they suggest that suggested paleons may out number the stars in the Milky Way by a factor of 1000 !
And I'm keeping score:
Baryonic dark matter: 1
Exotic-particle dark matter: 0
Manly Astrophysics:
http://manlyastrophysics.org/Projects/InterstellarScintillation/index.html
Cool! (And pretty!) Mind if I ask what algorithm you're using for the simulation? Is it SPH?
ReplyDeleteDavid Carlson Aaargh, I'm sorry, I still didn't get around to reading your response properly. I'll try and do that later. But... haven't MACHOs been ruled out by microlensing experiments ?
ReplyDeletehttps://kellyoakes.wordpress.com/2010/09/24/machos-wimps-and-the-mystery-of-the-missing-mass/
Jonah Miller Thanks ! Yes it's an SPH code, an obscure one by the name of gf.
The last time I tried setting up a stable disc it was using the FLASH grid code. It did not go well.
https://www.youtube.com/watch?v=pZsPfzxv13w
Took me ages to realise the boundary conditions were bugged and that through no fault of mine the simulation was gaining more than a factor ten in mass.
This time I think it's partially because of the strong thermal motions and partially because I seem to have resolved the velocity xy components assuming the thing is in solid body rotation, for some strange reason. Oh well, try again...
Can the simulation cope with relativistic effects? Would be interesting to see particles at 0.5c
ReplyDeleteRhys Taylor ah. These big frameworks can be difficult to get to work. The correct settings are often a matter of oral tradition. ;)
ReplyDeleteRhys Taylor
ReplyDeleteMicrolensing: Good question--I don't know.
If memory serves from reading through one of the Manly papers, paleons are indeed suggested to cause microlensing effects in the visible spectrum as well as radio-wave scintillation from plasma in the bow shock and wake, but whether Large Magellanic Cloud (LMC) microlensing studies designed to detect small hard objects like rougue super jupiters, brown dwarfs, neutron stars and black holes would have identified vastly larger and more diffuse objects with proportionately-longer dimming times is questionable.
Indeed, a greater number of microlensing events were discovered than expected in the 2000 study, 13-17 detected vs. 2-4 expected, but the column density of Milky Way DM between us and the LMC may be strongly dependent on its distribution, as in "A Heavy Baryonic Galactic Disc" in spiral galaxies formed by gravitational collapse; however, the LMC is relatively near the Milky Way disk plane compared to many satellite galaxies:
http://www.astronomy.org/StarWatch/May/5-09-milky-way-satellites.gif
I'll scan through the two papers referenced in the MACHOs and WIMPs blog:
http://arxiv.org/abs/astro-ph/0001272
The MACHO Project: Microlensing Results from 5.7 Years of LMC Observations
http://xxx.lanl.gov/abs/astro-ph/0607207
Limits on the Macho Content of the Galactic Halo from the EROS-2 Survey of the Magellanic Clouds
Oliver Hamilton Not to my knowledge, it's designed for galaxy-scale simulations where relativistic effects can be neglected.
ReplyDeleteJonah Miller Indeed. My hypothesis about why it's failing appears to be disproved. After fixing both the way the velocity components are resolved and reducing the thermal motion, I get a nice, stable... ring ! Booo. Hardly any particles escape but everything in the centre moves out. If I make it colder or more massive, this just happens all the faster. Expect more pretty failures later. :)
This is probably going to be a case of, "email someone who knows what they're doing."
Rhys Taylor
ReplyDeleteRe: Microlensing
Apparently I was mistaken in recalling a reference to gravitational/refractive lensing of visible light by suggested 'paleons'. I went through all the 16 Manly Astrophysics papers searching on the word "lens" and they don't appear to have addressed either gravitational or refractive lensing of visible light, only refractive lensing of radio waves through plasma. Come to think of it, an Earth mass dispersed throughout a 1 AU radius wouldn't create much of a gravitational differential, although I can't imagine the refractive effects of cold molecular hydrogen and helium on visible light, which might depend significantly on the density distribution with radius.
I'm having trouble thinking this through: quasars give a vastly-longer baseline than Large Magellanic Cloud stars, but if the lens is in the Milky Way foreground, then the source distance shouldn't have much effect.?
They seem to dismiss gravitational microlensing out of hand, as in the quote from this (early) 2007 paper, as follows:
http://manlyastrophysics.org/MaterialForAstronomers/PublishedPapers/2007Johnston.pdf
Science with the Australian Square Kilometre Array Pathfinder
"Extreme Scattering Events (ESEs) are a type of transient in which the flux variations are not intrinsic to the source but are caused by variations in refraction along the line-of-sight (Fiedler et al. 1987b; Romani et al. 1987). In other words ESEs are a lensing phenomenon; not gravitational
lensing, but refraction of radio waves in ionised gas. It has long been recognised that the lenses which cause ESEs must be Galactic, probably within a few kiloparsecs, but in the 20 years since the phenomenon was discovered no satisfactory physical model has emerged. In part this
is a reflection of the difficulty of explaining the existing data using established ideas about the interstellar medium. Recently, however, Walker (2007) has suggested that the lenses must be associated with neutral, self-gravitating gas clouds and may be the source of the Galaxy’s dark matter."
Rhys Taylor
ReplyDeleteThis is probably going to be a case of, "email someone who knows what they're doing."
The story of scientific computing. :)
Jonah Miller
ReplyDelete(That generally only works if you have a .edu email address.)
David Carlson haha not so! The majority of people on the Einstein Toolkit and yt Project mailing lists, for example, use gmail for their correspondence.
ReplyDeleteyt is especially good about this. They have an open IRC channel on freenode, which anybody is welcome to join.
Of course, using IRC might be a better test of academic affiliation and overall technical competence than a .edu email. :P