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Trapezium: "Multiple Star" or "Cluster"?

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#1 jrbarnett


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Posted 23 October 2012 - 05:08 PM

Along the lines of something bruce (CNer drollere) posted a while back, when does a multiple star system, particularly one where we have no orbital data for the components and assume relationship solely based on shared proper motion, belong in the "cluster" bucket?

I suppose a better question might be, "does it matter?"


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#2 drollere


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Posted 23 October 2012 - 08:04 PM

i'm not sure there is definitive answer, and it is bound to be arbitrary. the simplest answer is that a double (multiple) star can be analyzed as a closed system in terms of stable (or slowly changing) orbital elements, whereas a cluster is a stochastic averaging of mutually interacting orbits around a common center of mass, for example, as having a tidal radius or half mass radius. multiple stars also have a strongly hierarchical organization, with very close binaries orbiting at far distances around other binaries or single stars. in one recent study, the average difference in orbital separations between levels in a multiple star hierarchy was on the order of 1:1000. in clusters the stars would have more similar orbits in relation to a common center of mass.

since you've taken solved orbits off the table, you have to derive the system potential from the separate stellar masses, trajectories and distances. i think the current outer bound for "fragile" binaries that are just waiting to be broken up is (depending on which source you look at) a comoving pair separated by 10,000 to 30,000 AU; i use 25,000 AU only because i have seen it cited more than once. that is 0.12 parsec, or about 0.4 light year. since a cluster radius is usually multiples of light years, the fundamental difference is perhaps simply dimensional.

you could also set an arbitrary number of components as the cluster cutoff. WDS includes a few systems with over a dozen components, and a multiple star such as BU 442 -- several very faint companions around a delicate triple star hidden in the cygnus milky way -- looks to me like cluster remnant, or perhaps a minicluster.

there is a similar vagueness about the difference between a cluster and an association. trumpler 10 and collinder 121 were both originally classified as clusters, but are now classified as OB associations. they are comoving, but so large and sparse that whether they are gravitationally bound or not is difficult to determine: the member motions are not orbitally around a center, but moving at equal speed away from a distant point.

#3 azure1961p


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Posted 27 October 2012 - 07:03 AM

Mr. Barnett - thats a nice question raised and Bruce thats a terrific reply particularly in the first paragraph.


#4 The Ardent

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Posted 27 October 2012 - 05:34 PM

What about NGC 2017 ???

#5 GlennLeDrew


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Posted 29 October 2012 - 09:33 AM

An interesting thing about the Trapezium is that the current best evidence suggests that it ejected the stars iota Ori (immediately S of M42), AE Aur and Mu Col, in possibly the same event. Iota Ori is a massive spectroscopic binary, and so the velocity it acquired was small, whereas the other two stars (singletons, as far as is known) were hurtled off at 100 or so km/s.

Imagine the Trapezium if today it still contained these three stars.

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