106 research outputs found
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 (), prior to recombination, we find
no real heating is produced. Our simulations allow us to smoothly trace a new
transition regime (), 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 () 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 ), 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
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