2 research outputs found
On the interaction of a thin, supersonic shell with a molecular cloud
Molecular clouds (MCs) are stellar nurseries, however, formation of stars
within MCs depends on the ambient physical conditions. MCs, over a free-fall
time are exposed to numerous dynamical phenomena, of which, the interaction
with a thin, dense shell of gas is but one. Below we present results from
self-gravitating, 3-D smoothed particle hydrodynamics ({\small SPH})
simulations of the problem; seven realisations of the problem have been
performed by varying the precollision density within the cloud, the nature of
the post-collision shock, and the spatial resolution in the computational
domain. Irrespective of the type of shock, a complex network of dense
filaments, seeded by numerical noise, readily appears in the shocked cloud.
Segregation of the dense and rarefied gas phases also manifests itself in a
bimodal distribution of gas density. We demonstrate that the power-spectrum for
rarefied gas is Kolomogorov like, while that for the denser gas is considerably
steeper. As a corollary to the main problem, we also look into the possibly
degenerative effect of the {\small SPH} artificial viscosity on the impact of
the incident shell. It is observed that stronger viscosity leads to greater
post-shock dissipation, that strongly decelerates the incident shock-front and
promotes formation of contiguous structure, albeit on a much longer timescale.
We conclude that too much viscosity is likely to enhance the proclivity towards
gravitational boundedness of structure, leading to unphysical fragmentation.On
the other hand, insufficient resolution appears to suppress fragmentation.
Convergence of results is tested at both extremes, first by repeating the test
case with more than a million particles and then with only half the number of
particles in the original test case.Comment: 15 pages, 15 figures, and 1 Table; To appear in Monthly Notices to
the RA
On turbulent fragmentation and the origin of the stellar IMF
Two varieties of the universal stellar initial mass function (IMF) viz., the
Kroupa and the Chabrier IMF, have emerged over the last decade to explain the
observed distribution of stellar masses. The possibility of the universal
nature of the stellar IMF leads us to the interesting prospect of a universal
mode of star-formation. It is well-known that turbulent fragmentation of gas in
the interstellar medium produces a lognormal distribution of density which is
further reflected by the mass-function for clumps at low and intermediate
masses. Stars condense out of unstable clumps through a complex interplay
between a number of dynamic processes which must be accounted for when tracing
the origin of the stellar IMF. In the present work, applying the theory of
gravitational fragmentation we first derive the mass function (MF) for clumps.
Then a core mass function (CMF) is derived by allowing the clumps to fragment,
having subjected each one to a random choice of gas temperature. Finally, the
stellar IMF is derived by applying a random core-to-star conversion efficiency,
, in the range of 5%-15% to each CMF. We obtain a power-law IMF that
has exponents within the error-bars on the Kropua IMF. This derived IMF is
preceded by a similar core mass function which suggests, gravoturbulent
fragmentation plays a key role in assembling necessary conditions that relate
the two mass-functions. In this sense the star-formation process, at least at
low redshifts where gas cooling is efficient, is likely to be universal. We
argue that the observed knee in the CMF and the stellar IMF may alternatively
be interpreted in terms of the characteristic temperature at which gas in
potential star-forming clouds is likely to be found. Our results also show that
turbulence in star-forming clouds is probably driven on large spatial scales
with a power-spectrum steeper than Kolmogorov-type.Comment: 10 pages, 5 figures; To appear in New Astronomy; Figure numbers
corrected in this versio