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Thread: Origin of angular momentum in accretion disk models?

  1. #1 Origin of angular momentum in accretion disk models? 
    Your Mama! GiantEvil's Avatar
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    What is the origin of angular momentum in accretion disk models?


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  3. #2  
    Universal Mind John Galt's Avatar
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    It is inherited from the motion of the collapsing GMC element.


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  4. #3  
    Your Mama! GiantEvil's Avatar
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    Gravitational M? C?
    I tried to Wiki GMC and got a list of trucks, no really! I suspect a geologist would know about accretion disks, but if this is a joke it better be funny.
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    Quote Originally Posted by GiantEvil
    Gravitational M? C?
    I tried to Wiki GMC and got a list of trucks, no really! I suspect a geologist would know about accretion disks, but if this is a joke it better be funny.
    More something for an astronomer than a geologist. He like me might pick it up on one of the hybrid topics, sort of like the "planetary atmospheres" course I once took which had meteorologist and astronomy students interested in hypothesis about how atmospheres form under different conditions.

    GMC stands for giant molecular cloud. The models presumes they start with some angular momentum and I think several surveys substantiate that assumption.
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    GMC as in Giant Molecular Cloud. Molecular clouds are turbulent. These movements are inherited to fragmenting clumps and cloud cores that can collapse to form stars and then planets within the surrounding discs.
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    I mostly appreciate Ophie's sense of humor so wouldn't have minded a joke. I think my first exposure to accretion disk models was from Jr or Sr high earth sciences. Time could be tricking me on that one though. Giant Molecular Cloud, turbulence, that makes sense. Thank's guy's. Hey Lynx, what part of "Wa" are you from?
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    Administrator KALSTER's Avatar
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    I just always think of water running out of a tub. Some arbitrary direction of spin is just dominant at the beginning and pulls everything along with it AFAIK.
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

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    Sorry, Giant Evil, I wasn't trying to be deliberately obscure. Your knowledge in general and the perceptiveness of the question specifically, led me to mistakenly believe you were familiar with GMCs.

    These GMCs really are vast, with masses running to the order of several thousands stars the size of the sun. Most of them are cold, only a few degrees above abolsute zero, but when a portion of them starts to collapse they heat up. Eventually you get to a point where liquid water is stable. And you have a rich variety of organic compounds present . So - to derail the thread somewhat - a wast volume of water, organic compounds and mineral templates: ideal setting for abiogenesis. Which is why I am more inclined to pan spermia than most.
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  10. #9  
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    pan spermia
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    Quote Originally Posted by GiantEvil
    pan spermia
    A town where a lot of importance is placed on Chimpanzee semen. :-D

    But seriously, what is funny?
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

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    Forum Isotope Bunbury's Avatar
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    The models presumes they start with some angular momentum
    This doesn't really answer the original question - it just kicks it up a level. What is the cause of the GMC's rotation?
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    Universal Mind John Galt's Avatar
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    There is random movement within the cloud. A collapse of a portion of the cloud is triggered by gravitational instability or pressure waves from a supernova. The collapsing cloud will have some net momentum. End of story (or from an antrhopocentric viewpoint, the beginning.)
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  14. #13  
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    OK, not angels then.
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    Quote Originally Posted by KALSTER
    Quote Originally Posted by GiantEvil
    pan spermia
    A town where a lot of importance is placed on Chimpanzee semen. :-D

    But seriously, what is funny?
    Yes, I wondered about that. :?

    (Even a cave dweller would have got your chimp reference. Neat.)
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  16. #15  
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    Quote Originally Posted by Bunbury
    The models presumes they start with some angular momentum
    This doesn't really answer the original question - it just kicks it up a level. What is the cause of the GMC's rotation?
    Angular momentum is conserved, so the origin is in the big bang itself. I would not be surprised if the net angular momentum about each point were zero (but I can't prove it). Be aware that problems involving rotation are notoriously difficult to handlr in general relativity.

    You are in danger of chasing a chicken-and-egg question around forever.
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  17. #16  
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    If matter's density were even a bit heterogeneous there would be velocity differential densities driven by gravity and angular momentum across those differentials.
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  18. #17  
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    Quote Originally Posted by Lynx_Fox
    If matter's density were even a bit heterogeneous there would be velocity differential densities driven by gravity and angular momentum across those differentials.
    This is
    1) irrelevant
    2) trivial
    3) a hell of a convoluted way to say that gravity is attractive and proportional to mass

    It also fails to reflect that the fact that angular momentum is only meaningful when referred to a point.
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  19. #18  
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    Quote Originally Posted by DrRocket
    Quote Originally Posted by Lynx_Fox
    If matter's density were even a bit heterogeneous there would be velocity differential densities driven by gravity and angular momentum across those differentials.
    This is
    1) irrelevant
    2) trivial
    3) a hell of a convoluted way to say that gravity is attractive and proportional to mass

    It also fails to reflect that the fact that angular momentum is only meaningful when referred to a point.
    I don't see it's either irrelevant or trivial. Any contraction around a mass within that broader velocity gradient could be considered angular momentum around that point. If those higher density points start as being randomly distributed I don't how you can anything other than angular momentum around those higher density points. This would be true even is the net angular momentum of the larger cloud were zero. You'd end up pockets of spinning matter, some of which are rotating in opposite directions within that larger cloud. I'll look for some simple models that others have run that illustrate the point.
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  20. #19  
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    Quote Originally Posted by Lynx_Fox
    I don't see it's either irrelevant or trivial..
    I don't doubt that. But it is.
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  21. #20  
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    But seriously, what is funny?
    What's funny is I didn't know what "Panspermia"(one word in the wiki) was. I just thought it was a word Ophie made up to be funny. Fascinating concept though.
    Apparently there is no solid scientific consensus on origin of angular momentum in accretion disk models. Is the accretion disk model also applied to galaxies or is there a different theory concerning their formation?
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    Administrator KALSTER's Avatar
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    Not sure, but I think dark matter halos play a larger role in galaxy formation.
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  23. #22  
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    Quote Originally Posted by KALSTER
    Not sure, but I think dark matter halos play a larger role in galaxy formation.
    Larger than what ?
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  24. #23  
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    Quote Originally Posted by GiantEvil
    But seriously, what is funny?
    What's funny is I didn't know what "Panspermia"(one word in the wiki) was. I just thought it was a word Ophie made up to be funny. Fascinating concept though.
    Apparently there is no solid scientific consensus on origin of angular momentum in accretion disk models. Is the accretion disk model also applied to galaxies or is there a different theory concerning their formation?
    An acretion disk is a bunch of stuff in (a possibly decaying) orbit around something. If there is no angular momentum around the central point, there is no orbit. http://en.wikipedia.org/wiki/Accretion_disc

    Angular momentum is conserved. In fact Kepler's second law is basically conservation of angular momentum.
    http://en.wikipedia.org/wiki/Kepler's_laws_of_planetary_motion

    What do you even mean by "origin of angular momentum" ? Do you have some hypothetical initial condition in mind ? If so, what is it ?

    When something approaches a central body and goes into orbit, it must either already have the necessary angular momentum or interact gravitationally with bodies already in orbit to go into orbit itself. But in any case the angular momentum of the whole system about the central point does not change and the gravitational force from the central body exerts no torque about the central point.
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  25. #24  
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    It doesn't become a disk without some angular momentum.
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  26. #25  
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    This is the process I was thinking about; http://en.wikipedia.org/wiki/Solar_n...ion_of_planets.
    I got Kepler and conservation, it's analogous to a stream of water from a faucet, thick at the top and thinner as the water falls faster.
    I was wondering why a gravitationally collapsing molecular cloud should spin. Why should the particles be falling tangential to the MC's center? If we sum all the random motions in the MC it should come to about zero. Like flipping a coin billion's and billion's of times lead's closer and closer to a fifty/fifty split.
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  27. #26  
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    Quote Originally Posted by DrRocket
    Quote Originally Posted by KALSTER
    Not sure, but I think dark matter halos play a larger role in galaxy formation.
    Larger than what ?
    Than in planetary accretion discs.
    Disclaimer: I do not declare myself to be an expert on ANY subject. If I state something as fact that is obviously wrong, please don't hesitate to correct me. I welcome such corrections in an attempt to be as truthful and accurate as possible.

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  28. #27  
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    Quote Originally Posted by GiantEvil View Post
    Apparently there is no solid scientific consensus on origin of angular momentum in accretion disk models.
    As I recall, the lack of consensus is not about how such disks get angular momentum, but how this angular momentum is transferred outwards to allow matter to migrate inwards. Stuff should "orbit", not "fall into"... unless angular momentum can be transferred. For astrophysicists, this remains one of the really interesting puzzles about how to evolve supermassive black holes from little ones.

    Which touches that other mystery: where are the intermediate sized (20 - 100,000 solar mass) black holes?
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