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Thread: 2-body inspirals that circularize

  1. #1
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    2-body inspirals that circularize

    Other than compact binaries, are there 2-body systems in which inspirals circularize? 3-body?
    Thx

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    Three Bodies? Maybe.

    Quote Originally Posted by Inquirer View Post
    Other than compact binaries, are there 2-body systems in which inspirals circularize? 3-body?
    Thx
    The Wiki article on degenerate orbitss is here:

    https://en.wikipedia.org/wiki/Orbital_decay

    Worth noting is that all binary systems degenerate over astronomical time; compact binaries do it fast.

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    Quote Originally Posted by John Mendenhall View Post
    Worth noting is that all binary systems degenerate over astronomical time; compact binaries do it fast.
    Yes, I would think there’s nothing particularly special about the bodies being a compact binary, but the same would apply (to lesser extent) to any bodies of approximately equal mass orbiting each other.

    Pluto-Charon springs to mind, although Charon is only a bit more than a tenth the mass of Pluto, that’s enough for both to orbit the common barycentre outside Pluto’s body. Of course, Charon’s orbit is corresponding that much greater radius than Pluto’s, (so Charon making a large circle around Pluto making a small one) but very very close to circular.

    A three-body system in an intricate looping dance would not be stable, so the alternative is a two close bodies at the centre with a third orbiting a long way out, an example being Alpha-Centauri.


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    Quote Originally Posted by John Mendenhall View Post
    The Wiki article on degenerate orbitss is here:

    https://en.wikipedia.org/wiki/Orbital_decay

    Worth noting is that all binary systems degenerate over astronomical time; compact binaries do it fast.
    Right re astronomical time — to clarify, my primary interest was to confirm that the only known cause where a binary inspiral naturally circularizes — before decaying over the longer term — is from gravitational waves.

    A better link for that progression would have been this: “When binary black holes form in the field, it is expected that their orbits typically circularize before coalescence.”

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    Quote Originally Posted by Ufonaut99 View Post
    Yes, I would think there’s nothing particularly special about the bodies being a compact binary, but the same would apply (to lesser extent) to any bodies of approximately equal mass orbiting each other.

    Pluto-Charon springs to mind, although Charon is only a bit more than a tenth the mass of Pluto, that’s enough for both to orbit the common barycentre outside Pluto’s body. Of course, Charon’s orbit is corresponding that much greater radius than Pluto’s, (so Charon making a large circle around Pluto making a small one) but very very close to circular.

    A three-body system in an intricate looping dance would not be stable, so the alternative is a two close bodies at the centre with a third orbiting a long way out, an example being Alpha-Centauri.


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    Yes, gravitational waves would work toward circularizing inspirals for objects of any size. The question becomes — for other than compact binaries — are there conditions where that effect would be enough to offset (for some non-insignificant time) inspiraling from other causes of orbital decay as noted in John’s link above.

    Thx re Charon — I see from here that “Tidal interaction inevitably leads to tidal drag on the primary object if the satellite is above synchronous orbit or tidal acceleration of the primary object if the satellite is in sub-synchronous orbit and zero tidal interaction if the two bodies are tidally interlocked. If there is a tidal interaction then satellite orbits will be an evolving orbit like an expanding spiral or contracting spiral (Burns 1978; Lambeck 1979; Szeto 1983; Sharma, Ishwar & Rangesh 2009). Satellites like our Moon and Deimos which are in extra-synchronous orbit are in expanding spiral orbit whereas Phobos which is in sub-synchronous orbit is in contracting spiral orbit whereas as Charon (moon of Pluto) which is exactly in mutually interlocked tidally synchronous orbit is in stationary circular orbit.”

    I haven’t yet found a source that states Charon’s orbit was an inspiral (though it arguably was from some source of decay) rather than outspiral or ellipse before circularizing from gravitational tides.

    And thx for the prompt re Alpha-Centauri. The question there becomes (similar to my first para above) whether grav wave interaction of A and B would be enough to circularize (for some non-insignificant time) in view of continued interaction with C. Hard to imagine an inspiral–to–circularization scenario with more than 2 bodies from any combination of causes.
    Last edited by Inquirer; 2019-Jun-12 at 11:04 AM. Reason: Added link

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    Quote Originally Posted by Inquirer View Post
    Other than compact binaries, are there 2-body systems in which inspirals circularize? 3-body? Thx
    The theories of planetary evolution I've read nowadays include planetary migration, in which outer worlds gradually spiral inward until they settle into near-circular orbits. The TRAPPIST-1 system is thought to be an extreme example of this, with its worlds packed in now like eggs in a carton, though they must have once been farther out and moved inward slowly.
    There is something fascinating about science. One gets such wholesale returns of conjecture out of such a trifling investment of fact.
    — Mark Twain, Life on the Mississippi (1883)

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    Quote Originally Posted by Roger E. Moore View Post
    The theories of planetary evolution I've read nowadays include planetary migration, in which outer worlds gradually spiral inward until they settle into near-circular orbits. The TRAPPIST-1 system is thought to be an extreme example of this, with its worlds packed in now like eggs in a carton, though they must have once been farther out and moved inward slowly.
    @Roger E. Moore — great example — that might be the archetypical isolated n-body case (where the masses are constant except for grav radiation), where surviving inspirals are circularized for quite a while prior to eventual merge. This setup wouldn’t apply where the bodies, e.g., stars in planetary systems, lose mass (and where that eventually leads to outspiral).
    Last edited by Inquirer; 2019-Jun-13 at 12:41 AM. Reason: all —> surviving; clarifying re mass

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