244 research outputs found
Hydrodynamical simulations of the decay of high-speed molecular turbulence. I. Dense molecular regions
We present the results from three dimensional hydrodynamical simulations of
decaying high-speed turbulence in dense molecular clouds. We compare our
results, which include a detailed cooling function, molecular hydrogen
chemistry and a limited C and O chemistry, to those previously obtained for
decaying isothermal turbulence.
After an initial phase of shock formation, power-law decay regimes are
uncovered, as in the isothermal case. We find that the turbulence decays faster
than in the isothermal case because the average Mach number remains higher, due
to the radiative cooling. The total thermal energy, initially raised by the
introduction of turbulence, decays only a little slower than the kinetic
energy.
We discover that molecule reformation, as the fast turbulence decays, is
several times faster than that predicted for a non-turbulent medium. This is
caused by moderate speed shocks which sweep through a large fraction of the
volume, compressing the gas and dust. Through reformation, the molecular
density and molecular column appear as complex patterns of filaments, clumps
and some diffuse structure. In contrast, the molecular fraction has a wider
distribution of highly distorted clumps and copious diffuse structure, so that
density and molecular density are almost identically distributed during the
reformation phase. We conclude that molecules form in swept-up clumps but
effectively mix throughout via subsequent expansions and compressions.Comment: 12 pages, 12 figures; For a version of the article with higher
resolution figures, see http://star.arm.ac.uk/preprints/381.p
Dependence of the Star Formation Efficiency on the Parameters of Molecular Cloud Formation Simulations
We investigate the response of the star formation efficiency (SFE) to the
main parameters of simulations of molecular cloud formation by the collision of
warm diffuse medium (WNM) cylindrical streams, neglecting stellar feedback and
magnetic fields. The parameters we vary are the Mach number of the inflow
velocity of the streams, Msinf, the rms Mach number of the initial background
turbulence in the WNM, and the total mass contained in the colliding gas
streams, Minf. Because the SFE is a function of time, we define two estimators
for it, the "absolute" SFE, measured at t = 25 Myr into the simulation's
evolution (sfeabs), and the "relative" SFE, measured 5 Myr after the onset of
star formation in each simulation (sferel). The latter is close to the "star
formation rate per free-fall time" for gas at n = 100 cm^-3. We find that both
estimators decrease with increasing Minf, although by no more than a factor of
2 as Msinf increases from 1.25 to 3.5. Increasing levels of background
turbulence similarly reduce the SFE, because the turbulence disrupts the
coherence of the colliding streams, fragmenting the cloud, and producing
small-scale clumps scattered through the numerical box, which have low SFEs.
Finally, the SFE is very sensitive to the mass of the inflows, with sferel
decreasing from ~0.4 to ~0.04 as the the virial parameter in the colliding
streams increases from ~0.15 to ~1.5. This trend is in partial agreement with
the prediction by Krumholz & McKee (2005), since the latter lies within the
same range as the observed efficiencies, but with a significantly shallower
slope. We conclude that the observed variability of the SFE is a highly
sensitive function of the parameters of the cloud formation process, and may be
the cause of significant scatter in observational determinations.Comment: 19 pages, submitted to MNRA
Sub-Alfvenic Non-Ideal MHD Turbulence Simulations with Ambipolar Diffusion: II. Comparison with Observation, Clump Properties, and Scaling to Physical Units
Ambipolar diffusion is important in redistributing magnetic flux and in
damping Alfven waves in molecular clouds. The importance of ambipolar diffusion
on a length scale is governed by the ambipolar diffusion Reynolds
number, \rad=\ell/\lad, where \lad is the characteristic length scale for
ambipolar diffusion. The logarithmic mean of the AD Reynolds number in a sample
of 15 molecular clumps with measured magnetic fields (Crutcher 1999) is 17,
comparable to the theoretically expected value. We identify several regimes of
ambipolar diffusion in a turbulent medium, depending on the ratio of the flow
time to collision times between ions and neutrals; the clumps observed by
Crutcher (1999) are all in the standard regime of ambipolar diffusion, in which
the neutrals and ions are coupled over a flow time. We have carried out
two-fluid simulations of ambipolar diffusion in isothermal, turbulent boxes for
a range of values of \rad. The mean Mach numbers were fixed at \calm=3 and
\ma=0.67; self-gravity was not included. We study the properties of
overdensities--i.e., clumps--in the simulation and show that the slope of the
higher-mass portion of the clump mass spectrum increases as \rad decreases,
which is qualitatively consistent with Padoan et al. (2007)'s finding that the
mass spectrum in hydrodynamic turbulence is significantly steeper than in ideal
MHD turbulence. For a value of \rad similar to the observed value, we find a
slope that is consistent with that of the high-mass end of the Initial Mass
Function for stars. However, the value we find for the spectral index in our
ideal MHD simulation differs from theirs, presumably because our simulations
have different initial conditions. This suggests that the mass spectrum of the
clumps in the Padoan et al. (2007) turbulent fragmentation model for the IMF
depends on the environment, which would conflict with evidence ...Comment: 33 pages, 7 figure
Clump morphology and evolution in MHD simulations of molecular cloud formation
Abridged: We study the properties of clumps formed in three-dimensional
weakly magnetized magneto-hydrodynamic simulations of converging flows in the
thermally bistable, warm neutral medium (WNM). We find that: (1) Similarly to
the situation in the classical two-phase medium, cold, dense clumps form
through dynamically-triggered thermal instability in the compressed layer
between the convergent flows, and are often characterised by a sharp density
jump at their boundaries though not always. (2) However, the clumps are bounded
by phase-transition fronts rather than by contact discontinuities, and thus
they grow in size and mass mainly by accretion of WNM material through their
boundaries. (3) The clump boundaries generally consist of thin layers of
thermally unstable gas, but these layers are often widened by the turbulence,
and penetrate deep into the clumps. (4) The clumps are approximately in both
ram and thermal pressure balance with their surroundings, a condition which
causes their internal Mach numbers to be comparable to the bulk Mach number of
the colliding WNM flows. (5) The clumps typically have mean temperatures 20 < T
< 50 K, corresponding to the wide range of densities they contain (20 < n <
5000 pcc) under a nearly-isothermal equation of state. (6) The turbulent ram
pressure fluctuations of the WNM induce density fluctuations that then serve as
seeds for local gravitational collapse within the clumps. (7) The velocity and
magnetic fields tend to be aligned with each other within the clumps, although
both are significantly fluctuating, suggesting that the velocity tends to
stretch and align the magnetic field with it. (8) The typical mean field
strength in the clumps is a few times larger than that in the WNM. (9) The
magnetic field strength has a mean value of B ~ 6 mu G ...Comment: substantially revised version, accepted by MNRAS, 13 pages, 14
figures, high resolution version:
http://www.ita.uni-heidelberg.de/~banerjee/publications/MC_Formation_Paper2.pd
A method for reconstructing the variance of a 3D physical field from 2D observations: Application to turbulence in the ISM
We introduce and test an expression for calculating the variance of a
physical field in three dimensions using only information contained in the
two-dimensional projection of the field. The method is general but assumes
statistical isotropy. To test the method we apply it to numerical simulations
of hydrodynamic and magnetohydrodynamic turbulence in molecular clouds, and
demonstrate that it can recover the 3D normalised density variance with ~10%
accuracy if the assumption of isotropy is valid. We show that the assumption of
isotropy breaks down at low sonic Mach number if the turbulence is
sub-Alfvenic. Theoretical predictions suggest that the 3D density variance
should increase proportionally to the square of the Mach number of the
turbulence. Application of our method will allow this prediction to be tested
observationally and therefore constrain a large body of analytic models of star
formation that rely on it.Comment: 8 pages, 9 figures, accepted for publication in MNRA
Comparing the statistics of interstellar turbulence in simulations and observations: Solenoidal versus compressive turbulence forcing
We study two limiting cases of turbulence forcing in numerical experiments:
solenoidal (divergence-free) forcing, and compressive (curl-free) forcing, and
compare our results to observations reported in the literature. We solve the
equations of hydrodynamics on grids with up to 1024^3 cells for purely
solenoidal and purely compressive forcing. Eleven lower-resolution models with
mixtures of both forcings are also analysed. We find velocity dispersion--size
relations consistent with observations and independent numerical simulations,
irrespective of the type of forcing. However, compressive forcing yields
stronger turbulent compression at the same RMS Mach number than solenoidal
forcing, resulting in a three times larger standard deviation of volumetric and
column density probability distributions (PDFs). We conclude that the strong
dependence of the density PDF on the type of forcing must be taken into account
in any theory using the PDF to predict properties of star formation. We supply
a quantitative description of this dependence. We find that different observed
regions show evidence of different mixtures of compressive and solenoidal
forcing, with more compressive forcing occurring primarily in swept-up shells.Comment: 28 pages, 20 figures, published as Highlight Paper in A&A, 512, A81
(2010); simulation movies available at
http://www.ita.uni-heidelberg.de/~chfeder/videos.shtml?lang=e
The Density Variance Mach Number Relation in the Taurus Molecular Cloud
Supersonic turbulence in molecular clouds is a key agent in generating
density enhancements that may subsequently go on to form stars. The stronger
the turbulence - the higher the Mach number - the more extreme the density
fluctuations are expected to be. Numerical models predict an increase in
density variance with rms Mach number of the form: sigma^{2}_{rho/rho_{0}} =
b^{2}M^{2}, where b is a numerically-estimated parameter, and this prediction
forms the basis of a large number of analytic models of star formation. We
provide an estimate of the parameter b from 13CO J=1-0 spectral line imaging
observations and extinction mapping of the Taurus molecular cloud, using a
recently developed technique that needs information contained solely in the
projected column density field to calculate sigma^{2}_{rho/rho_{0}}. We find b
~ 0.48, which is consistent with typical numerical estimates, and is
characteristic of turbulent driving that includes a mixture of solenoidal and
compressive modes. More conservatively, we constrain b to lie in the range
0.3-0.8, depending on the influence of sub-resolution structure and the role of
diffuse atomic material in the column density budget. We also report a break in
the Taurus column density power spectrum at a scale of ~1pc, and find that the
break is associated with anisotropy in the power spectrum. The break is
observed in both 13CO and dust extinction power spectra, which, remarkably, are
effectively identical despite detailed spatial differences between the 13CO and
dust extinction maps. [ abridged ]Comment: 8 pages, 9 figures. Accepted for publication in A&
Gravity or turbulence? II. Evolving column density PDFs in molecular clouds
It has been recently shown that molecular clouds do not exhibit a unique
shape for the column density probability distribution function (Npdf). Instead,
clouds without star formation seem to possess a lognormal distribution, while
clouds with active star formation develope a power-law tail at high column
densities. The lognormal behavior of the Npdf has been interpreted in terms of
turbulent motions dominating the dynamics of the clouds, while the power-law
behavior occurs when the cloud is dominated by gravity. In the present
contribution we use thermally bi-stable numerical simulations of cloud
formation and evolution to show that, indeed, these two regimes can be
understood in terms of the formation and evolution of molecular clouds: a very
narrow lognormal regime appears when the cloud is being assembled. However, as
the global gravitational contraction occurs, the initial density fluctuations
are enhanced, resulting, first, in a wider lognormal Npdf, and later, in a
power-law Npdf. We thus suggest that the observed Npdf of molecular clouds are
a manifestation of their global gravitationally contracting state. We also show
that, contrary to recent suggestions, the exact value of the power-law slope is
not unique, as it depends on the projection in which the cloud is being
observed.Comment: 9 pages, 4 figures. MNRAS, accepte
Probing the evolution of molecular cloud structure: From quiescence to birth
Aims: We derive the probability density functions (PDFs) of column density
for a complete sample of prominent molecular cloud complexes closer than 200
pc. Methods: We derive near-infrared dust extinction maps for 23 molecular
cloud complexes, using the "nicest" colour excess mapping technique and data
from the 2MASS archive. The extinction maps are then used to examine the column
density PDFs in the clouds. Results: The column density PDFs of most molecular
clouds are well-fitted by log-normal functions at low column densities (0.5 mag
< A_v < 3-5 mag). However, at higher column densities prominent, power-law-like
wings are common. In particular, we identify a trend among the PDFs: active
star-forming clouds always have prominent non-log-normal wings. In contrast,
clouds without active star formation resemble log-normals over the whole
observed column density range, or show only low excess of higher column
densities. This trend is also reflected in the cumulative PDFs, showing that
the fraction of high column density material is significantly larger in
star-forming clouds. These observations are in agreement with an evolutionary
trend where turbulent motions are the main cloud-shaping mechanism for
quiescent clouds, but the density enhancements induced by them quickly become
dominated by gravity (and other mechanisms) which is strongly reflected by the
shape of the column density PDFs. The dominant role of the turbulence is
restricted to the very early stages of molecular cloud evolution, comparable to
the onset of active star formation in the clouds.Comment: 7 pages, 11 figures, accepted to A&A Letter
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