205 research outputs found
On the turbulence driving mode of expanding HII regions
We investigate the turbulence driving mode of ionizing radiation from massive
stars on the surrounding interstellar medium (ISM). We run hydrodynamical
simulations of a turbulent cloud impinged by a plane-parallel ionization front.
We find that the ionizing radiation forms pillars of neutral gas reminiscent of
those seen in observations. We quantify the driving mode of the turbulence in
the neutral gas by calculating the driving parameter , which is
characterised by the relation between
the variance of the logarithmic density contrast (where with the gas density and its average ), and
the turbulent Mach number . Previous works have shown that
indicates solenoidal (divergence-free) driving and
indicates compressive (curl-free) driving, with producing up to ten
times higher star formation rates than . The time variation of in
our study allows us to infer that ionizing radiation is inherently a
compressive turbulence driving source, with a time-averaged . We also investigate the value of of the pillars, where star
formation is expected to occur, and find that the pillars are characterised by
a natural mixture of both solenoidal and compressive turbulent modes
() when they form, and later evolve into a more compressive turbulent
state with --. A virial parameter analysis of the pillar regions
supports this conclusion. This indicates that ionizing radiation from massive
stars may be able to trigger star formation by producing predominately
compressive turbulent gas in the pillars.Comment: 10 pages, 6 figures. Accepted for publication in MNRA
Outflows Driven by Direct and Reprocessed Radiation Pressure in Massive Star Clusters
We use three-dimensional radiation hydrodynamic (RHD) simulations to study
the formation of massive star clusters under the combined effects of direct
ultraviolet (UV) and dust-reprocessed infrared (IR) radiation pressure. We
explore a broad range of mass surface density -, spanning values typical of weakly
star-forming galaxies to extreme systems such as clouds forming super-star
clusters, where radiation pressure is expected to be the dominant feedback
mechanism. We find that star formation can only be regulated by radiation
pressure for ,
but that clouds with become super-Eddington once high star formation efficiencies
() are reached, and therefore launch the remaining gas in a steady
outflow. These outflows achieve mass-weighted radial velocities of -
, which is - times the
cloud escape speed. This suggests that radiation pressure is a strong candidate
to explain recently observed molecular outflows found in young super-star
clusters in nearby starburst galaxies. We quantify the relative importance of
UV and IR radiation pressure in different regimes, and deduce that both are
equally important for , whereas clouds with higher (lower) density are increasingly
dominated by the IR (UV) component. Comparison with control runs without either
the UV or IR bands suggests that the outflows are primarily driven by the
impulse provided by the UV component, while IR radiation has the effect of
rendering a larger fraction of gas super-Eddington, and thereby increasing the
outflow mass flux by a factor of .Comment: 15 pages, 11 figures. MNRAS accepted. v2: Minor changes in text made
to address referee comment
Infrared Radiation Feedback Does Not Regulate Star Cluster Formation
We present 3D radiation-hydrodynamical (RHD) simulations of star cluster
formation and evolution in massive, self-gravitating clouds, whose dust columns
are optically thick to infrared (IR) photons. We use \texttt{VETTAM} -- a
recently developed, novel RHD algorithm, which uses the Variable Eddington
Tensor (VET) closure -- to model the IR radiation transport through the cloud.
We also use realistic temperature () dependent IR opacities () in
our simulations, improving upon earlier works in this area, which used either
constant IR opacities or simplified power laws (). We
investigate the impact of the radiation pressure of these IR photons on the
star formation efficiency (SFE) of the cloud, and its potential to drive dusty
winds. We find that IR radiation pressure is unable to regulate star formation
or prevent accretion onto the star clusters, even for very high gas surface
densities (), contrary to recent
semi-analytic predictions and simulation results using simplified treatments of
the dust opacity. We find that the commonly adopted simplifications of or constant for the IR dust opacities leads to this
discrepancy, as those approximations overestimate the radiation force. By
contrast, with realistic opacities that take into account the micro-physics of
the dust, we find that the impact of IR radiation pressure on star formation is
very mild, even at significantly high dust-to-gas ratios ( times
solar), suggesting that it is unlikely to be an important feedback mechanism in
controlling star formation in the ISM.Comment: 28 pages, 19 figures. MNRAS accepted. v2: Minor changes made to
address referee comment
Probing the evolution of molecular cloud structure II: From chaos to confinement
We present an analysis of the large-scale molecular cloud structure and of
the stability of clumpy structures in nearby molecular clouds. In our recent
work, we identified a structural transition in molecular clouds by studying the
probability distributions of gas column densities in them. In this paper, we
further examine the nature of this transition. The transition takes place at
the visual extinction of A_V^tail = 2-4 mag, or equivalently, at \Sigma^tail =
40-80 Ms pc^{-2}. The clumps identified above this limit have wide ranges of
masses and sizes, but a remarkably constant mean volume density of n = 10^3
cm^{-3}. This is 5-10 times larger than the density of the medium surrounding
the clumps. By examining the stability of the clumps, we show that they are
gravitationally unbound entities, and that the external pressure from the
parental molecular cloud is a significant source of confining pressure for
them. Then, the structural transition at A_V^tail may be linked to a transition
between this population and the surrounding medium. The star formation rates in
the clouds correlate strongly with the total mass in the clumps, i.e, with the
mass above A_V^tail, dropping abruptly below that threshold. These results
imply that the formation of pressure confined clumps introduces a prerequisite
for star formation. Furthermore, they give a physically motivated explanation
for the recently reported relation between the star formation rates and the
amount of dense material in molecular clouds. Likewise, they give rise to a
natural threshold for star formation at A_V^tail.Comment: 11 pages, 12 figures, accepted for publication in Astronomy and
Astrophysic
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
A semi-analytic model of the turbulent multi-phase interstellar medium
We present a semi-analytic model for the interstellar medium that considers
local processes and structures of turbulent star-forming gas. A volume element
of the interstellar medium is described as a multiphase system, comprising a
cold and a warm gas phase in effective (thermal plus turbulent) pressure
equilibrium, and a stellar component. Since turbulence has a critical impact on
the shape of the gaseous phases, on the production of molecular hydrogen and on
the formation of stars, the consistent treatment of turbulence energy -- the
kinetic energy of unresolved motions -- is an important new feature of our
model. Besides turbulence production by supernovae and by the cooling
instability, we also take into account the forcing by large scale motions.
We formulate a set of ordinary differential equations, which statistically
describes star formation and the exchange between the different budgets of mass
and energy in a region of the interstellar medium with given mean density,
size, metallicity and external turbulence forcing. By exploring the behaviour
of the solutions, we find equilibrium states, in which the star formation
efficiencies are consistent with observations. Kennicutt-Schmidt-like relations
naturally arise from the equilibrium solutions, while conventional star
formation models in numerical simulations impose such relations with observed
efficiency parameters as phenomenological calibrations.
Beyond the semi-analytic approach, a potential application is a complete
subgrid scale model of the unresolved multi-phase structure, star formation and
turbulence in simulations of galaxies or in cosmological simulations. The
formulation presented in this article combines various models focusing on
particular processes and yet can be adopted to specific applications, depending
on the range of resolved length scales.Comment: 25 pages, 22 figures, 1 table, accepted for publication by MNRA
Do Lognormal Column-Density Distributions in Molecular Clouds Imply Supersonic Turbulence?
Recent observations of column densities in molecular clouds find lognormal
distributions with power-law high-density tails. These results are often
interpreted as indications that supersonic turbulence dominates the dynamics of
the observed clouds. We calculate and present the column-density distributions
of three clouds, modeled with very different techniques, none of which is
dominated by supersonic turbulence. The first star-forming cloud is simulated
using smoothed particle hydrodynamics (SPH); in this case gravity, opposed only
by thermal-pressure forces, drives the evolution. The second cloud is
magnetically subcritical with subsonic turbulence, simulated using nonideal
MHD; in this case the evolution is due to gravitationally-driven ambipolar
diffusion. The third cloud is isothermal, self-gravitating, and has a smooth
density distribution analytically approximated with a uniform inner region and
an r^-2 profile at larger radii. We show that in all three cases the
column-density distributions are lognormal. Power-law tails develop only at
late times (or, in the case of the smooth analytic profile, for strongly
centrally concentrated configurations), when gravity dominates all opposing
forces. It therefore follows that lognormal column-density distributions are
generic features of diverse model clouds, and should not be interpreted as
being a consequence of supersonic turbulence.Comment: 6 pages, 6 figures, accepted for publication in MNRA
The <i>Herschel</i> view of the massive star-forming region NGC 6334
Aims: Fundamental to any theory of high-mass star formation are gravity and turbulence. Their relative importance, which probably changes during cloud evolution, is not known. By investigating the spatial and density structure of the high-mass star-forming complex NGC 6334 we aim to disentangle the contributions of turbulence and gravity.
Methods: We used Herschel PACS and SPIRE imaging observations from the HOBYS key programme at wavelengths of 160, 250, 350, and 500 μm to construct dust temperature and column density maps. Using probability distribution functions (PDFs) of the column density determined for the whole complex and for four distinct sub-regions (distinguished on the basis of differences in the column density, temperature, and radiation field), we characterize the density structure of the complex. We investigate the spatial structure using the Δ-variance, which probes the relative amount of structure on different size scales and traces possible energy injection mechanisms into the molecular cloud.
Results: The Δ-variance analysis suggests that the significant scales of a few parsec that were found are caused by energy injection due to expanding HII regions, which are numerous, and by the lengths of filaments seen everywhere in the complex. The column density PDFs have a lognormal shape at low densities and a clearly defined power law at high densities for all sub-regions whose slope is linked to the exponent α of an equivalent spherical density distribution. In particular with α = 2.37, the central sub-region is largly dominated by gravity, caused by individual collapsing dense cores and global collapse of a larger region. The collapse is faster than free-fall (which would lead only to α = 2) and thus requires a more dynamic scenario (external compression, flows). The column density PDFs suggest that the different sub-regions are at different evolutionary stages, especially the central sub-region, which seems to be in a more evolved stage
Turbulent Mixing in the Interstellar Medium -- an application for Lagrangian Tracer Particles
We use 3-dimensional numerical simulations of self-gravitating compressible
turbulent gas in combination with Lagrangian tracer particles to investigate
the mixing process of molecular hydrogen (H2) in interstellar clouds. Tracer
particles are used to represent shock-compressed dense gas, which is associated
with H2. We deposit tracer particles in regions of density contrast in excess
of ten times the mean density. Following their trajectories and using
probability distribution functions, we find an upper limit for the mixing
timescale of H2, which is of order 0.3 Myr. This is significantly smaller than
the lifetime of molecular clouds, which demonstrates the importance of the
turbulent mixing of H2 as a preliminary stage to star formation.Comment: 10 pages, 5 figures, conference proceedings "Turbulent Mixing and
Beyond 2007
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