534 research outputs found
Turbulent Formation of Interstellar Structures and the Connection Between Simulations and Observations
I review recent results derived from numerical simulations of the turbulent
interstellar medium (ISM), in particular concerning the nature and formation of
turbulent clouds, methods for comparing the structure in simulations and
observations, and the effects of projection of three-dimensional structures
onto two dimensions. Clouds formed as turbulent density fluctuations are
probably not confined by thermal pressure, but rather their morphology may be
determined by the large-scale velocity field. Also, they may have shorter
lifetimes than normally believed, as the large-scale turbulent modes have
larger associated velocities than the clouds' internal velocity dispersions.
Structural characterization algorithms have started to distinguish the best
fitting simulations to a particular observation, and have opened several new
questions, such as the nature of the observed line width-size relation and of
the relation between the structures seen in channel maps and the true spatial
distribution of the density and velocity fields. The velocity field apparently
dominates the morphology seen in intensity channel maps, at least in cases when
the density field exhibits power spectra steep enough. Furthermore, the
selection of scattered fluid parcels along the line of sight (LOS) by their
LOS-velocity inherent to the construction of spectroscopic data may introduce
spurious small-scale structure in high spectral resolution channel maps.Comment: 15 pages, no figures. To appear in the Proceedings of "The Chaotic
Universe", Roma/Pescara, Italy, 1-5 Feb. 1999, eds. V. Gurzadyan and L.
Bertone. Uses included .cls fil
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
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
Molecular Cloud Evolution VI. Measuring cloud ages
This article has been published in Monthly Notices of the Royal Astronomical Society © 2018 The Author(s). Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.In previous contributions, we have presented an analytical model describing the evolution and star formation rate (SFR) of molecular clouds (MCs) undergoing hierarchical gravitational contraction. The cloud’s evolution is characterized by an initial increase in its mass, density, SFR, and star formation efficiency (SFE), as it contracts, followed by a decrease of these quantities as newly formed massive stars begin to disrupt the cloud. The main parameter of the model is the maximum mass reached by the cloud during its evolution. Thus, specifying the instantaneous mass and some other variable completely determines the cloud’s evolutionary stage. We apply the model to interpret the observed scatter in SFEs of the cloud sample compiled by Lada et al. as an evolutionary effect so that, although clouds such as California and Orion A have similar masses, they are in very different evolutionary stages, causing their very different observed SFRs and SFEs. The model predicts that the California cloud will eventually reach a significantly larger total mass than the Orion A cloud. Next, we apply the model to derive estimated ages of the clouds since the time when approximately 25 per cent of their mass had become molecular. We find ages from ∼1.5 to 27 Myr, with the most inactive clouds being the youngest. Further predictions of the model are that clouds with very low SFEs should have massive atomic envelopes constituting the majority of their gravitational mass, and that low-mass clouds (M ∼ 103–104M⊙) end their lives with a mini-burst of star formation, reaching SFRs ∼300–500M⊙ Myr−1. By this time, they have contracted to become compact (∼1 pc) massive star-forming clumps, in general embedded within larger giant molecular clouds.Peer reviewe
An Evolutionary Model for Collapsing Molecular Clouds and Their Star Formation Activity
We present an idealized, semi-empirical model for the evolution of
gravitationally contracting molecular clouds (MCs) and their star formation
rate (SFR) and efficiency (SFE). The model assumes that the instantaneous SFR
is given by the mass above a certain density threshold divided by its free-fall
time. The instantaneous number of massive stars is computed assuming a Kroupa
IMF. These stars feed back on the cloud through ionizing radiation, eroding it.
The main controlling parameter of the evolution turns out to be the maximum
cloud mass, \Mmax. This allows us to compare various properties of the model
clouds against their observational counterparts. A giant molecular cloud (GMC)
model (\Mmax \sim 10^5 \Msun) adheres very well to the evolutionary scenario
recently inferred by Kawamura et al. (2009) for GMCs in the Large Magellanic
Cloud. A model cloud with \Mmax \approx 2000 \Msun evolves in the
Kennicutt-Schmidt diagram first passing through the locus of typical low-
to-intermediate mass star-forming clouds, and then moving towards the locus of
high-mass star-forming ones over the course of Myr. Also, the stellar
age histograms for this cloud a few Myr before its destruction agree very well
with those observed in the -Oph stellar association, whose parent cloud
has a similar mass, and imply that the SFR of the clouds increases with time.
Our model thus agrees well with various observed properties of star-forming
MCs, suggesting that the scenario of gravitationally collapsing MCs, with their
SFR regulated by stellar feedback, is entirely feasible and in agreement with
key observed properties of molecular clouds.Comment: Version accepted for publication in ApJ. At referee's suggestion,
includes comparison with numerical models in addition to comparison with
observational dat
From the warm magnetized atomic medium to molecular clouds
{It has recently been proposed that giant molecular complexes form at the
sites where streams of diffuse warm atomic gas collide at transonic
velocities.} {We study the global statistics of molecular clouds formed by
large scale colliding flows of warm neutral atomic interstellar gas under ideal
MHD conditions. The flows deliver material as well as kinetic energy and
trigger thermal instability leading eventually to gravitational collapse.} {We
perform adaptive mesh refinement MHD simulations which, for the first time in
this context, treat self-consistently cooling and self-gravity.} {The clouds
formed in the simulations develop a highly inhomogeneous density and
temperature structure, with cold dense filaments and clumps condensing from
converging flows of warm atomic gas. In the clouds, the column density
probability density distribution (PDF) peaks at \sim 2 \times 10^{21} \psc
and decays rapidly at higher values; the magnetic intensity correlates weakly
with density from to 10^4 \pcc, and then varies roughly as
for higher densities.} {The global statistical properties of such
molecular clouds are reasonably consistent with observational determinations.
Our numerical simulations suggest that molecular clouds formed by the
moderately supersonic collision of warm atomic gas streams.}Comment: submitted to A&
Molecular Cloud Evolution III. Accretion vs. stellar feedback
We numerically investigate the effect of feedback from the ionizing radiation
heating from massive stars on the evolution of giant molecular clouds (GMCs)
and their star formation efficiency (SFE). We find that the star-forming
regions within the GMCs are invariably formed by gravitational contraction.
After an initial period of contraction, the collapsing clouds begin forming
stars, whose feedback evaporates part of the clouds' mass, opposing the
continuing accretion from the infalling gas. The competition of accretion
against dense gas consumption by star formation (SF) and evaporation by the
feedback, regulates the clouds' mass and energy balance, as well as their SFE.
We find that, in the presence of feedback, the clouds attain levels of the SFE
that are consistent at all times with observational determinations for regions
of comparable SF rates (SFRs). However, we observe that the dense gas mass is
larger in general in the presence of feedback, while the total (dense gas +
stars) is nearly insensitive to the presence of feedback, suggesting that the
total mass is determined by the accretion, while the feedback inhibits mainly
the conversion of dense gas to stars. The factor by which the SFE is reduced
upon the inclusion of feedback is a decreasing function of the cloud's mass,
for clouds of size ~ 10 pc. This naturally explains the larger observed SFEs of
massive-star forming regions. We also find that the clouds may attain a
pseudo-virialized state, with a value of the virial mass very similar to the
actual cloud mass. However, this state differs from true virialization in that
the clouds are the center of a large-scale collapse, continuously accreting
mass, rather than being equilibrium entities.Comment: Submitted to ApJ (abstract abridged
- …