Re: Rotation of Protostars and Stellar Disks

[Stephen Murray <murray8@LLNL.GOV>]

Don Polvani wrote:

> The July-August issue of American Scientist has an interesting article on
> protostars.  Unfortunately it fails to answer a question I have had for some
time...

As you say, the collapse of a rotating cloud cannot be perfectly
spherically symmetric.  Gas can only contract towards the axis of
rotation, conserving angular momentum, until its rotational speed is
Keplerian.  It can collapse parallel to the axis of rotation until it
becomes pressure supported.  The result is the formation of a disk.
The aspect ratio of the disk (thickness-to-radius ratio) is
determined by the ratio of the sound speed of the gas to the
Keplerian speed.  The gas from which protostars form is extremely
cold (something like 10 K), giving a very low sound speed, resulting
in very flattened disks (there is shock heating during the collapse,
but the dense gas is very efficient at radiating away the internal
energy gained during a shock passage).

The initial rotation could result from multiple causes.  Molecular
clouds in galaxies are large enough to experience some differential
rotation as they orbit.  Tidal interactions between clumps within the
clouds can impart some small rotations.  Probably more importantly,
molecular clouds are observed to have significant turbulent motions,
which will give angular momentum to individual clumps within the
clouds.  The latter mechanisms will tend to impart a fairly random
distribution of rotational orientations to the clumps within a
molecular cloud.

In the early universe, it is believed that tidal interactions between
overdense regions that were in the process of evolving into galaxies
led to the observed rotation.  Numerical simulations have found that
the specific angular momentum gained from such encounters is similar
to that observed in spiral galaxies.  I believe that small elliptical
galaxies also tend to show significant rotation.  Larger elliptical
galaxies, however, seem to be shaped by anisotropic velocity
dispersions among their stars.  The current thinking is that large
ellipticals formed via the merger of disk (spiral) systems, which is
why ellipticals dominate the populations of rich clusters of galaxies.


================================
Stephen D. Murray
Physicist, A Division
Lawrence Livermore National Laboratory
Email:  sdmurray@llnl.gov
Phone: (925) 423-9382
FAX:    (925) 423-0925
================================