View Single Post
  #9  
Old March 24th 17, 12:47 PM posted to sci.astro.research
Martin Brown[_3_]
external usenet poster
 
Posts: 189
Default NET Angular Momentum of Globular cluster of stars

On 23/03/2017 22:23, Eric Flesch wrote:
On Thu, 16 Mar 2017 12:46:58 EDT, wrote:
Is the NET angular momentum of a globular cluster of stars, zero?
... Is the NET angular momentum of an elliptical galaxy and or the
central bulge of a spiral, zero or close to zero


I think the current status of thinking is long on theory and short on
observations. I don't think there's been a single comprehensive
survey of the actual stellar movements in any globular cluster. I'm
not aware of any done for an elliptical galaxy either.


I'm no expert in this apart from having known a few of the early
practitioners of computational stellar dynamics but I think there are
now some precision observations of stars in local globular clusters like
Omega Centuri which show that things are never simple.

http://www.aanda.org/articles/aa/pdf.../aa3061-05.pdf

and

https://www.astro.umd.edu/~richard/A...ergy_equip.pdf

There are plenty of velocity dispersion and luminosity measurements
along lines of sight through various of the brighter Messier globular
clusters dating from way back. eg M15

http://articles.adsabs.harvard.edu//...00251.000.html

Not long ago there was an idea of "tri-axial" rotation for these which
implied that individual star orbits could be decomposed into 3 axial
components which would somehow serve to keep the stars from plunging
into the core. That doesn't seem to be de rigueur anymore.

Also there was speculation some decades ago that globular cluster
stars simply fell through the cluster cores in a sort of mass yo-yo
action. Also not popular of late, I don't think.


I'm sure I have seen somewhere a claim that stars out on the fringes of
clusters tend to be in very elongated orbits rather than circular ones.
Certainly the eye tends to pick out spiky rays in for example M13 around
the edges of the core.

I had a notion which I publicized on these forums some years ago, that
there's a gravitational scalar, as yet unmodelled, which allowed stars
to freely float in such environments. I plead guilty for hand-waving
on this, but it seems an elegant concept. Ugly facts begone!


At a handwaving level a single test particle in an evolved spherically
symmetric globular cluster moves in a gravitational potential that far
out is roughly 1/r^2 but closer in becomes weaker and in the dense core
becomes almost linear. This implies that a test particle will no longer
follow an ellipse around the centre of mass but will instead trace out a
somewhat more circular petal shape staying further away from the core.

Self consistent models in phase space seem to be one interesting way of
approaching it using maximum entropy or some other computational
heuristic to create the most non-committal distribution of matter in
consistent with a given mass M, energy E and angular moment L.

Going through the core must increase the possibility of a three body
close encounter from time to time resulting in ejection of stars and the
condensation of the rest.

I mention all this because it looks to me that other answers on this
thread are muddying up the distinction between theory and observation.
If there've been cogent observations made recently on actual stellar
orbits in globular clusters or ellipticals, I'd be glad to be directed
to them.


On the simulation early evolution of star clusters this was published
fairly recently, but I don't know enough to comment on its veracity.

http://rsta.royalsocietypublishing.o...t/368/1913/829

--
Regards,
Martin Brown