Binary Stars in Globular Clusters?

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BDanielMayfield
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Binary Stars in Globular Clusters?

Postby BDanielMayfield » Wed Nov 08, 2017 12:51 am

Binary stars are extremely common in general, but are binary stars as common inside globular clusters as they are in the disk population of galaxies like ours?

Bruce
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Ann
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Re: Binary Stars in Globular Clusters?

Postby Ann » Wed Nov 08, 2017 5:42 am

I would think that wide binaries are a lot less common in many globular clusters than in field stars. That is because the stars in many globulars are packed so close together that tidal forces are pulling at them from all directions, and therefore wide binaries should be relatively easily broken apart. But tight binaries should be unaffected by almost all of the tidal forces in globulars.

Another question to take into account is how the globular has evolved over times. It seems certain that all currently existing globulars were more closely packed with stars some time in the past than they are now, or at the very least, all currently existing globulars contained more stars in the past than they do now. Were they also more concentrated? Perhaps they were, some time in the past. But maybe some of the most centrally concentrated globulars, like 47 Tuc and M15, have grown more centrally concentrated over time.

I think it is quite possible that wide binaries are (relatively) easily torn apart inside globulars. But another interesting possibility is that binaries just might form inside globulars, as one star gets caught by the gravity of another when stars are constantly passing close to one another.

Maybe the math whizzes will disagree with me on the latter point.

Ann
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BDanielMayfield
Don't bring me down
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Joined: Thu Aug 02, 2012 11:24 am
AKA: Bruce
Location: East Idaho

Re: Binary Stars in Globular Clusters?

Postby BDanielMayfield » Wed Nov 08, 2017 6:34 am

Ann wrote:I would think that wide binaries are a lot less common in many globular clusters than in field stars. That is because the stars in many globulars are packed so close together that tidal forces are pulling at them from all directions, and therefore wide binaries should be relatively easily broken apart. But tight binaries should be unaffected by almost all of the tidal forces in globulars.

That makes sense Ann.

Another question to take into account is how the globular has evolved over times. It seems certain that all currently existing globulars were more closely packed with stars some time in the past than they are now, or at the very least, all currently existing globulars contained more stars in the past than they do now. Were they also more concentrated? Perhaps they were, some time in the past. But maybe some of the most centrally concentrated globulars, like 47 Tuc and M15, have grown more centrally concentrated over time.

A simple way of picturing complex 3 body interactions that must be common in globular clusters is to remember that overall momentum is conserved but it is also exchanged between the interacting bodies. One of the 3 can be given enough energy to be moved outward while the two other bodies become closer. The trend would be that the outskirts of the cluster grows less dense as stars are slowly lost to the cluster while the core of the cluster becomes denser.

I think it is quite possible that wide binaries are (relatively) easily torn apart inside globulars. But another interesting possibility is that binaries just might form inside globulars, as one star gets caught by the gravity of another when stars are constantly passing close to one another.

Partner swapping might also occur, if binaries are commonly found in globulars.

My main question remains though, are (close) binary stars as common in globulars as they are in ordinary field stars?

Bruce
"Happy are the peaceable ... "


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