XPost: sci.astro.research
From: helbig@asclothestro.multivax.de
In article , Martin Brown
<'''newspam'''@nezumi.demon.co.uk> writes:
> > The Jeans length is important for star formation, but the stuff which
> > forms (rocky) planets is only a small fraction of a larger cloud which
> > collapsed (as described by Jeans) to form a star. There doesn't seem to
> > be a lower limit on the size of "planets". There is an obvious upper
> > limit for (gaseous) planets---stars. The sizes of planets are
> > determined more by accretion, where gravitation is only one factor.
>
> That suggests an interesting question.
>
> Is it possible to compute either by simulation or from observations what
> percentage of ordinary matter is tightly bound together (either
> gravitationally or electromagnetically) as a function of length scale
> (or mass).
At larger scales, dark matter is important, but we don't know what it
is. In particular, we don't know whether it is self-interacting (other
than via gravity) and even if it isn't, it might not be in the form of
isolated particles (though that is what many people assume); the was a
paper by Bernard Carr and co-authors recently which pointed out that
there is still a mass range where it could be in primordial black holes.
At smaller scales, the last I heard, the IMF (initial mass function) for
stars was not computable from first principles. From observations, we
have a pretty good idea what it is locally, but it was probably
different at high redshift.
With certain assumptions, the Press-Schechter formalism allows one to
calculate a mass function, and, not surprisingly (but a good consistency
test and sanity check), this also comes out of simulations with the same
assumptions.
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