Astron. Astrophys. Suppl. Ser. 134, 241-254

Rotation in molecular clouds

J.P. Phillips

Send offprint request: J.P. Phillips

Instituto de Astronomía y Meteorología, Universidad de Guadalajara, Av. Vallarta 2602, CP. 44100 Sector Juárez, Guadalajara, Jalisco, Mexico
e-mail: jpp@udgserv.cencar.udg.mx

Received February 23; accepted July 31, 1998

Abstract:

We have investigated a broad range of evidence concerning rotation in molecular clouds. As a consequence, we show that trends in specific angular momentum J/M and angular velocity $\Omega$ are inconsistent with certain models of isothermal, non-magnetic cloud rotation. Similarly, models of rotation which invoke turbulent vorticity may have only limited applicability to clumps and condensations. There is evidence to favour an important rôle for rotation in maintaining the stability of disks, larger cloud structures, and perhaps a large fraction of intermediate sized clouds, whilst rotation may also be implicated in maintaining observed departures from cloud sphericity. Although it is conceivable that magnetic braking is responsible for the radial decrement in specific angular momentum, it appears that observed gradients dln(J/M)/dln(R) are significantly shallower than is normally anticipated through this mechanism.

The variation of angular momentum with cloud mass M (viz. $J\propto M^{1.7}$) appears to be highly correlated, and is consistent with models of clump merging in isothermal rotating clouds. Similarly, the orientations of the angular velocity vectors for clumps and condensations appear broadly random, suggesting a turbulent origin for observed components of $\Omega$ or, alternatively, a process of randomisation through magnetic and/or dynamic clump interactions. By contrast, isolated clouds (and perhaps also disks) are shown to have angular velocity vectors oriented predominantly towards the north and south galactic poles; a distribution which would be anticipated were components of J to arise from galactic shear.

We show, finally, that most of the cloud subgroups appear to follow similar functional trends in J, J/M, and $\Omega$. Disks and rings, on the other hand, appear to depart from these variations to a significant degree; a difference which presumably derives from their distinct spatio-kinematic structures.

Key words: ISM: clouds -- ISM: kinematics and dynamics -- ISM: molecules -- ISM: structure