Star formation efficiency in turbulent clouds

Here we present a simple, but nevertheless, instructive model for the star formation efficiency in turbulent molecular clouds. The model is based on the assumption of log-normal density distribution which reflects the turbulent nature of the interstellar medium (ISM). Together with the number count of cloud cores, which follows a Salpeter-like core mass function (CMF), and the minimum mass for the collapse of individual cloud cores, given by the local Jeans mass, we are able to derive the SFE for clouds as a function of their Jeans masses. We find a very generic power-law, SFE \propto (M_cloud/M_J)^{-0.26} and a maximum SFE_max \sim 1/3 for the Salpeter case. This result is independent of the turbulent Mach number but fairly sensitive to variations of the CMF.Comment: submitted to A&A, comments are welcom

Formation of star-forming clouds from the magnetised, diffuse interstellar medium

Molecular clouds, the birthplaces of stars in galaxies, form dynamically from the diffuse atomic gas of the interstellar medium (ISM). The ISM is also threaded by magnetic fields which have a large impact on its dynamics. In particular, star forming regions must be magnetically supercrit- ical in order to accomodate gas clumps which can collapse under their own weight. Based on a parameter study of three dimensional magneto-hydrodyamical (MHD) simulations, we show that the long-standing problem of how such supercritical regions are generated is still an open issue.Comment: Invited contribution to the NIC proceedings 2016 for the John von Neumann-Institut f\"ur Computing (NIC) Symposium 201

Magnetic heating across the cosmological recombination era: Results from 3D MHD simulations

The origin of cosmic magnetic fields is an unsolved problem and magnetogenesis could have occurred in the early Universe. We study the evolution of such primordial magnetic fields across the cosmological recombination epoch via 3D magnetohydrodynamic numerical simulations. We compute the effective or net heating rate of baryons due to decaying magnetic fields and its dependence on the magnetic field strength and spectral index. In the drag-dominated regime ($z \gtrsim 1500$), prior to recombination, we find no real heating is produced. Our simulations allow us to smoothly trace a new transition regime ($600 \lesssim z \lesssim 1500$), where magnetic energy decays, at first, into the kinetic energy of baryons. A turbulent velocity field is built up until it saturates, as the net heating rate rises from a low value at recombination to its peak towards the end of the transition regime. This is followed by a turbulent decay regime ($z \lesssim 600$) where magnetic energy dissipates via turbulent decay of both magnetic and velocity fields while net heating remains appreciable and declines slowly. Both the peak of the net heating rate and the onset of turbulent decay are delayed significantly beyond recombination, by up to 0.5 Myr (until $z\simeq 600-700$), for scale-invariant magnetic fields. We provide analytic approximations and present numerical results for a range of field strengths and spectral indices, illustrating the redshift-dependence of dissipation and net heating rates. These can be used to study cosmic microwave background constraints on primordial magnetic fields.Comment: Submitted to MNRAS, comments are welcome; 22 pages, 26 figures, 2 table