|
I will discuss recent results linking the topology of the velocity and
magnetic fields in molecular clouds (MCs), and the star formation
efficiency (SFE). Both observational and numerical results suggest that
the velocity field in MCs may have a significant component of inward
motions, that may either trigger or be a consequence of gravitational
contraction. This result implies that a significant fraction of the
kinetic energy in MCs is not available for support against gravity, but
rather either promotes collapse or is a consequence of it. In numerical
simulations of driven, isothermal, non-magnetic turbulence with
different box sizes and Mach numbers but keeping the virial parameter
fixed, we observe that there is an anti-correlation between the mean
density of clumps and the mean divergence of the velocity field,
indicating the convergent nature of the motions within the clumps. We
measure the SFE in these simulations and plot it against the rms Mach
number of the turbulence, finding marginal agreement (within the
uncertainties) with the predictions of the model by Krumholz & McKee
(2005), although the most probable trend appears to be steeper.
In multi-temperature ideal MHD simulations of cloud formation out of
compressions in the warm neutral medium with self-gravity, we find that
the presence of magnetic field fluctuations allows the formation of
locally supercritical regions that may collapse even in cases in which
the mean field would imply that the whole box is magnetically
subcritical. For future simulations, we discuss the importance of
including the stellar feedback in calculations of this sort, in order to
assess its effects on regulating the SFE.
|
