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chornedsnorkack
2015-Nov-22, 09:51 AM
Is there any globular cluster for which the peculiar motions - both radial and proper motions - of members are known?
The structure of globular clusters is of course unknown. No globular cluster is anywhere near enough to measure parallax at all - let alone measure parallax with error significantly smaller than the size of the cluster!

StupendousMan
2015-Nov-22, 10:45 PM
Go to ADS:

http://adsabs.harvard.edu/abstract_service.html

Type into the "Abstract words" box the terms "globular cluster proper motion distance". Then click "Send Query". You'll be
given a long list of papers which discuss the motions and distances of globular clusters.

I suggest you read in particular the recent paper by Watkins et al.

http://adsabs.harvard.edu/abs/2015ApJ...812..149W

which discusses in detail how photometry and radial-velocity measurements of a number of
globular clusters allow one to estimate their distances.

Note that if one can find RR Lyrae stars, or Cepheids, or eclipsing binaries, in a globular
cluster, then one can determine the distance to the cluster. For an illustration, consider
Neeley et al. (2015ApJ...808...11N)

http://adsabs.harvard.edu/abs/2015ApJ...808...11N

antoniseb
2015-Nov-23, 12:29 AM
Is there any globular cluster for which the peculiar motions - both radial and proper motions - of members are known?
The structure of globular clusters is of course unknown. No globular cluster is anywhere near enough to measure parallax at all - let alone measure parallax with error significantly smaller than the size of the cluster!
I think you are right that there is no direct way to tell the distances of the individual stars with enough accuracy to get precise internal dynamics, but statistical dynamics is possible, and you can infer distance to the whole cluster in ways that the search StupendousMan has pointed to, and there are studies showing proper motion of the stars, and each star has a peculiar velocity (hard to measure when they are so close together, but they do have them). So what you are asking for is difficult or impossible with current equipment, but possible with some future instruments.

chornedsnorkack
2015-Nov-23, 10:06 AM
Wel, Im more interested about what could be inferred about the internal structure of clusters.
Globular clusters obviously do not consist of stars orbiting in the same plane, because if they did, they should be discs and we should see some of them edge on. Of course we cannot tell apart a face on disc from a sphere (remember, we do not know the depth) but if discs were common some should be seen edge on.

Do the outskirts of globular clusters consist of stars which are near apoapse of eccentric orbit, and return to core of cluster each orbit, or do they consist of stars following low eccentricity orbits, in different planes, that are always in outskirts of the cluster?

antoniseb
2015-Nov-23, 12:51 PM
... Do the outskirts of globular clusters consist of stars which are near apoapse of eccentric orbit, and return to core of cluster each orbit, or do they consist of stars following low eccentricity orbits, in different planes, that are always in outskirts of the cluster?
I don't have a paper to point to, but I have read in years past that the stars in the outskirts are in very elliptical orbits. One of the leading models for GCs is that the form in large star-forming clouds, and start off not looking very organized, but that billions of years of gravitational interactions have resulted in a nearly spherical distribution.