5,852 research outputs found
On column density thresholds and the star formation rate
We present the results of a numerical study designed to address the question
of whether there is a column density threshold for star formation within
molecular clouds. We have simulated a large number of different clouds, with
volume and column densities spanning a wide range of different values, using a
state-of-the-art model for the coupled chemical, thermal and dynamical
evolution of the gas. We show that star formation is only possible in regions
where the mean (area-averaged) column density exceeds . Within the clouds, we also show that there is a good correlation
between the mass of gas above a K-band extinction and the
star formation rate (SFR), in agreement with recent observational work.
Previously, this relationship has been explained in terms of a correlation
between the SFR and the mass in dense gas. However, we find that this
correlation is weaker and more time-dependent than that between the SFR and the
column density. In support of previous studies, we argue that dust shielding is
the key process: the true correlation is one between the SFR and the mass in
cold, well-shielded gas, and the latter correlates better with the column
density than the volume density.Comment: 21 pages and 12 figures. Accepted for publication in MNRA
Is atomic carbon a good tracer of molecular gas in metal-poor galaxies?
Carbon monoxide (CO) is widely used as a tracer of molecular hydrogen (H2) in
metal-rich galaxies, but is known to become ineffective in low metallicity
dwarf galaxies. Atomic carbon has been suggested as a superior tracer of H2 in
these metal-poor systems, but its suitability remains unproven. To help us to
assess how well atomic carbon traces H2 at low metallicity, we have performed a
series of numerical simulations of turbulent molecular clouds that cover a wide
range of different metallicities. Our simulations demonstrate that in
star-forming clouds, the conversion factor between [CI] emission and H2 mass,
, scales approximately as . We recover a
similar scaling for the CO-to-H2 conversion factor, , but find that
at this point in the evolution of the clouds, is consistently
smaller than , by a factor of a few or more. We have also examined
how and evolve with time. We find that
does not vary strongly with time, demonstrating that atomic carbon remains a
good tracer of H2 in metal-poor systems even at times significantly before the
onset of star formation. On the other hand, varies very strongly
with time in metal-poor clouds, showing that CO does not trace H2 well in
starless clouds at low metallicity.Comment: 16 pages, 9 figures. Updated to match the version accepted by MNRAS.
The main change from the previous version is a new sub-section (3.6)
discussing the possible impact of freeze-out and other processes not included
in our numerical simulation
Does the CO-to-H2 conversion factor depend on the star formation rate?
We present a series of numerical simulations that explore how the `X-factor',
-- the conversion factor between the observed integrated CO emission
and the column density of molecular hydrogen -- varies with the environmental
conditions in which a molecular cloud is placed. Our investigation is centred
around two environmental conditions in particular: the cosmic ray ionisation
rate (CRIR) and the strength of the interstellar radiation field (ISRF). Since
both these properties of the interstellar medium have their origins in massive
stars, we make the assumption in this paper that both the strength of the ISRF
and the CRIR scale linearly with the local star formation rate (SFR). The cloud
modelling in this study first involves running numerical simulations that
capture the cloud dynamics, as well as the time-dependent chemistry, and ISM
heating and cooling. These simulations are then post-processed with a line
radiative transfer code to create synthetic 12CO (1-0) emission maps from which
can be calculated. We find that for 1e4 solar mass virialised clouds
with mean density 100 cm, is only weakly dependent on the local
SFR, varying by a factor of a few over two orders of magnitude in SFR. In
contrast, we find that for similar clouds but with masses of 1e5 solar masses,
the X-factor will vary by an order of magnitude over the same range in SFR,
implying that extra-galactic star formation laws should be viewed with caution.
However, for denser ( cm), super-virial clouds such as those found
at the centre of the Milky Way, the X-factor is once again independent of the
local SFR.Comment: 16 pages, 5 figures. Accepted by MNRA
Star Formation in Transient Molecular Clouds
We present the results of a numerical simulation in which star formation
proceeds from an initially unbound molecular cloud core. The turbulent motions,
which dominate the dynamics, dissipate in shocks leaving a quiescent region
which becomes gravitationally bound and collapses to form a small multiple
system. Meanwhile, the bulk of the cloud escapes due to its initial supersonic
velocities. In this simulation, the process naturally results in a star
formation efficiency of 50%. The mass involved in star formation depends on the
gas fraction that dissipates sufficient kinetic energy in shocks. Thus, clouds
with larger turbulent motions will result in lower star formation efficiencies.
This implies that globally unbound, and therefore transient giant molecular
clouds (GMCs), can account for the low efficiency of star formation observed in
our Galaxy without recourse to magnetic fields or feedback processes.
Observations of the dynamic stability in molecular regions suggest that GMCs
may not be self-gravitating, supporting the ideas presented in this letter.Comment: 5 pages, 3 figures, accepted for MNRAS as a lette
On the effects of rotation during the formation of population III protostars
It has been suggested that turbulent motions are responsible for the
transport of angular momentum during the formation of Population III stars,
however the exact details of this process have never been studied. We report
the results from three dimensional SPH simulations of a rotating
self-gravitating primordial molecular cloud, in which the initial velocity of
solid-body rotation has been changed. We also examine the build-up of the discs
that form in these idealized calculations.Comment: 4 pages, AIP Conference Proceedings, First Stars IV from Hayashi to
the Future (Kyoto, Japan
The First Stellar Cluster
We report results from numerical simulations of star formation in the early
universe that focus on gas at very high densities and very low metallicities.
We argue that the gas in the central regions of protogalactic halos will
fragment as long as it carries sufficient angular momentum. Rotation leads to
the build-up of massive disk-like structures which fragment to form protostars.
At metallicities Z ~ 10^-5 Zsun, dust cooling becomes effective and leads to a
sudden drop of temperature at densities above n = 10^12 cm^-3. This induces
vigorous fragmentation, leading to a very densely-packed cluster of low-mass
stars. This is the first stellar cluster. The mass function of stars peaks
below 1 Msun, similar to what is found in the solar neighborhood, and
comparable to the masses of the very-low metallicity subgiant stars recently
discovered in the halo of our Milky Way. We find that even purely primordial
gas can fragment at densities 10^14 cm^-3 < n < 10^16 cm^-3, although the
resulting mass function contains only a few objects (at least a factor of ten
less than the Z = 10^-5 Zsun mass function), and is biased towards higher
masses. A similar result is found for gas with Z = 10^-6 Zsun. Gas with Z <=
10^-6 Zsun behaves roughly isothermally at these densities (with polytropic
exponent gamma ~ 1.06) and the massive disk-like structures that form due to
angular momentum conservation will be marginally unstable. As fragmentation is
less efficient, we expect stars with Z <= 10^-6 Zsun to be massive, with masses
in excess of several tens of solar masses, consistent with the results from
previous studies.Comment: 9 pages, 6 figures. Accepted by ApJ for publicatio
Interpreting the sub-linear Kennicutt-Schmidt relationship: The case for diffuse molecular gas
Recent statistical analysis of two extragalactic observational surveys
strongly indicate a sublinear Kennicutt-Schmidt (KS) relationship between the
star formation rate (Sigsfr) and molecular gas surface density (Sigmol). Here,
we consider the consequences of these results in the context of common
assumptions, as well as observational support for a linear relationship between
Sigsfr and the surface density of dense gas. If the CO traced gas depletion
time (tau_mol) is constant, and if CO only traces star forming giant molecular
clouds (GMCs), then the physical properties of each GMC must vary, such as the
volume densities or star formation rates. Another possibility is that the
conversion between CO luminosity and Sigmol, the XCO factor, differs from
cloud-to-cloud. A more straightforward explanation is that CO permeates the
hierarchical ISM, including the filaments and lower density regions within
which GMCs are embedded. A number of independent observational results support
this description, with the diffuse gas comprising at least 30% of the total
molecular content. The CO bright diffuse gas can explain the sublinear KS
relationship, and consequently leads to an increasing tau_mol with Sigmol. If
Sigsfr linearly correlates with the dense gas surface density, a sublinear KS
relationship indicates that the fraction of diffuse gas fdiff grows with
Sigmol. In galaxies where Sigmol falls towards the outer disk, this description
suggests that fdiff also decreases radially.Comment: 8 pages, 4 figures, to appear in MNRAS, comments welcom
The star formation efficiency and its relation to variations in the initial mass function
We investigate how the dynamical state of a turbulently supported, 1000 solar
mass, molecular cloud affects the properties of the cluster it forms, focusing
our discussion on the star formation efficiency (SFE) and the initial mass
function (IMF). A variety of initial energy states are examined in this paper,
ranging from clouds with PE = 0.1 KE to clouds with PE = 10 KE, and for both
isothermal and piece-wise polytropic equations of state (similar to that
suggested by Larson). It is found that arbitrary star formation efficiencies
are possible, with strongly unbound clouds yielding very low star formation
efficiencies. We suggest that the low star formation efficiency in the
Maddelena cloud may be a consequence of the relatively unbound state of its
internal structure. It is also found that competitive accretion results in the
observed IMF when the clouds have initial energy states of PE >= KE. We show
that under such conditions the shape of the IMF is independent of time in the
calculations. This demonstrates that the global accretion process can be
terminated at any stage in the cluster's evolution, while still yielding a
distribution of stellar masses that is consistent with the observed IMF. As the
clouds become progressively more unbound, competitive accretion is less
important and the protostellar mass function flattens. These results predict
that molecular clouds should be permeated with a distributed population of
stars that follow a flatter than Salpeter IMF.Comment: 8 pages, 6 figures, accepted by MNRAS for publictaion. Now available
through the 'Online Early' schem
Clump Lifetimes and the Initial Mass Function
Recent studies of dense clumps/cores in a number of regions of low-mass star
formation have shown that the mass distribution of these clumps closely
resembles the initial mass function (IMF) of field stars. One possible
interpretation of these observations is that we are witnessing the
fragmentation of the clouds into the IMF, and the observed clumps are bound
pre-stellar cores. In this paper, we highlight a potential difficulty in this
interpretation, namely that clumps of varying mass are likely to have
systematically varying lifetimes. This timescale problem can effectively
destroy the similarity bewteen the clump and stellar mass functions, such that
a stellar-like clump mass function (CMF) results in a much steeper stellar IMF.
We also discuss some ways in which this problem may be avoided.Comment: 7 pages, 3 figures, accepted to MNRA
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