54 research outputs found
Modeling Collapse and Accretion in Turbulent Gas Clouds: Implementation and Comparison of Sink Particles in AMR and SPH
We implemented sink particles in the adaptive mesh refinement (AMR)
hydrodynamics code FLASH. Sink particles are created in regions of local
gravitational collapse, and their trajectories and accretion can be followed
over many dynamical times. We perform a series of tests including the time
integration of circular and elliptical orbits, the collapse of a Bonnor-Ebert
sphere and a rotating, fragmenting cloud core. We compare the collapse of a
highly unstable singular isothermal sphere to the theory by Shu (1977), and
show that the sink particle accretion rate is in excellent agreement with the
theoretical prediction.
To model eccentric orbits and close encounters of sink particles accurately,
we show that a very small timestep is often required, for which we implemented
subcycling of the N-body system. We emphasize that a sole density threshold for
sink particle creation is insufficient in supersonic flows, if the density
threshold is below the opacity limit. In that case, the density can exceed the
threshold in strong shocks that do not necessarily lead to local collapse.
Additional checks for bound state, gravitational potential minimum, Jeans
instability and converging flows are absolutely necessary for a meaningful
creation of sink particles.
We apply our new sink particle module for FLASH to the formation of a stellar
cluster, and compare to a smoothed particle hydrodynamics (SPH) code with sink
particles. Our comparison shows encouraging agreement of gas properties,
indicated by column density distributions and radial profiles, and of sink
particle formation times and positions. We find excellent agreement in the
number of sink particles formed, and in their accretion and mass distributions.Comment: 30 pages, 17 figures, ApJ accepted, simulation movies available at
http://www.ita.uni-heidelberg.de/~chfeder/videos.shtml?lang=e
Natural Gas Pyrolysis in a Liquid Metal Bubble Column Reaction System. Part I: Experimental Setup and Methods
Limiting Accretion onto Massive Stars by Fragmentation-Induced Starvation
Massive stars influence their surroundings through radiation, winds, and
supernova explosions far out of proportion to their small numbers. However, the
physical processes that initiate and govern the birth of massive stars remain
poorly understood. Two widely discussed models are monolithic collapse of
molecular cloud cores and competitive accretion. To learn more about massive
star formation, we perform simulations of the collapse of rotating, massive,
cloud cores including radiative heating by both non-ionizing and ionizing
radiation using the FLASH adaptive mesh refinement code. These simulations show
fragmentation from gravitational instability in the enormously dense accretion
flows required to build up massive stars. Secondary stars form rapidly in these
flows and accrete mass that would have otherwise been consumed by the massive
star in the center, in a process that we term fragmentation-induced starvation.
This explains why massive stars are usually found as members of high-order
stellar systems that themselves belong to large clusters containing stars of
all masses. The radiative heating does not prevent fragmentation, but does lead
to a higher Jeans mass, resulting in fewer and more massive stars than would
form without the heating. This mechanism reproduces the observed relation
between the total stellar mass in the cluster and the mass of the largest star.
It predicts strong clumping and filamentary structure in the center of
collapsing cores, as has recently been observed. We speculate that a similar
mechanism will act during primordial star formation.Comment: extended version, ApJ in pres
High- and Low-Mass Star Forming Regions from Hierarchical Gravitational Fragmentation. High local Star Formation Rates with Low Global Efficiencies
We investigate the properties of "star forming regions" in a previously
published numerical simulation of molecular cloud formation out of compressive
motions in the warm neutral atomic interstellar medium, neglecting magnetic
fields and stellar feedback. In this simulation, the velocity dispersions at
all scales are caused primarily by infall motions rather than by random
turbulence. We study the properties (density, total gas+stars mass, stellar
mass, velocity dispersion, and star formation rate) of the cloud hosting the
first local, isolated "star formation" event in the simulation and compare them
with those of the cloud formed by a later central, global collapse event. We
suggest that the small-scale, isolated collapse may be representative of low-
to intermediate-mass star-forming regions, while the large-scale, massive one
may be representative of massive star forming regions. We also find that the
statistical distributions of physical properties of the dense cores in the
region of massive collapse compare very well with those from a recent survey of
the massive star forming region in the Cygnus X molecular cloud. The star
formation efficiency per free-fall time (SFE_ff) of the high-mass SF clump is
low, ~0.04. This occurs because the clump is accreting mass at a high rate, not
because its specific SFR (SSFR) is low. This implies that a low value of the
SFE_ff does not necessarily imply a low SSFR, but may rather indicate a large
gas accretion rate. We suggest that a globally low SSFR at the GMC level can be
attained even if local star forming sites have much larger values of the SSFR
if star formation is a spatially intermittent process, so that most of the mass
in a GMC is not participating of the SF process at any given time.Comment: Accepted by ApJ. Revised version, according to exchanges with
referee. Original results unchanged. Extensive new discussion on the low
global efficiency vs. high local efficiency of star formation. Abstract
abridge
The linewidth-size relationship in the dense ISM of the Central Molecular Zone
The linewidth (sigma) - size (R) relationship has been extensively measured
and analysed, in both the local ISM and in nearby normal galaxies. Generally, a
power-law describes the relationship well with an index ranging from 0.2-0.6,
now referred to as one of "Larson's Relationships." The nature of turbulence
and star formation is considered to be intimately related to these
relationships, so evaluating the sigma-R correlations in various environments
is important for developing a comprehensive understanding of the ISM. We
measure the sigma-R relationship in the Central Molecular Zone (CMZ) of the
Galactic Centre using spectral line observations of the high density tracers
N2H+, HCN, H13CN, and HCO+. We use dendrograms, which map the hierarchical
nature of the position-position-velocity (PPV) data, to compute sigma and R of
contiguous structures. The dispersions range from ~2-30 km/s in structures
spanning sizes 2-40 pc, respectively. By performing Bayesian inference, we show
that a power-law with exponent 0.3-1.1 can reasonably describe the sigma-R
trend. We demonstrate that the derived sigma-R relationship is independent of
the locations in the PPV dataset where sigma and R are measured. The uniformity
in the sigma-R relationship suggests turbulence in the CMZ is driven on the
large scales beyond >30 pc. We compare the CMZ sigma-R relationship to that
measured in the Galactic molecular cloud Perseus. The exponents between the two
systems are similar, suggestive of a connection between the turbulent
properties within a cloud to its ambient medium. Yet, the velocity dispersion
in the CMZ is systematically higher, resulting in a coefficient that is nearly
five times larger. The systematic enhancement of turbulent velocities may be
due to the combined effects of increased star formation activity, larger
densities, and higher pressures relative to the local ISM.Comment: 11 pages, 8 figures, Accepted for publication in MNRA
Importance of the Initial Conditions for Star Formation - I. Cloud Evolution and Morphology
We present a detailed parameter study of collapsing turbulent cloud cores,
varying the initial density profile and the initial turbulent velocity field.
We systematically investigate the influence of different initial conditions on
the star formation process, mainly focusing on the fragmentation, the number of
formed stars, and the resulting mass distributions. Our study compares four
different density profiles (uniform, Bonnor-Ebert type, ,
and ), combined with six different supersonic turbulent
velocity fields (compressive, mixed, and solenoidal, initialised with two
different random seeds each) in three-dimensional simulations using the
adaptive-mesh refinement, hydrodynamics code FLASH. The simulations show that
density profiles with flat cores produce hundreds of low-mass stars, either
distributed throughout the entire cloud or found in subclusters, depending on
the initial turbulence. Concentrated density profiles always lead to the
formation of one high-mass star in the centre of the cloud and, if at all,
low-mass stars surrounding the central one. In uniform and Bonnor-Ebert type
density distributions, compressive initial turbulence leads to local collapse
about 25% earlier than solenoidal turbulence. However, central collapse in the
steep power-law profiles is too fast for the turbulence to have any significant
influence. We conclude that (I) the initial density profile and turbulence
mainly determine the cloud evolution and the formation of clusters, (II) the
initial mass function (IMF) is not universal for all setups, and (III) that
massive stars are much less likely to form in flat density distributions. The
IMFs obtained in the uniform and Bonnor-Ebert type density profiles are more
consistent with the observed IMF, but shifted to lower masses.Comment: 20 pages, MNRAS in pres
The Small-Scale Dynamo and Non-Ideal MHD in Primordial Star Formation
We study the amplification of magnetic fields during the formation of
primordial halos. The turbulence generated by gravitational infall motions
during the formation of the first stars and galaxies can amplify magnetic
fields very efficiently and on short timescales up to dynamically significant
values. Using the Kazantsev theory, which describes the so-called small-scale
dynamo - a magnetohydrodynamical process converting kinetic energy from
turbulence into magnetic energy - we can then calculate the growth rate of the
small-scale magnetic field. Our calculations are based on a detailed chemical
network and we include non-ideal magnetohydrodynamical effects such as
ambipolar diffusion and Ohmic dissipation. We follow the evolution of the
magnetic field up to larger scales until saturation occurs on the Jeans scale.
Assuming a weak magnetic seed field generated by the Biermann battery process,
both Burgers and Kolmogorov turbulence lead to saturation within a rather small
density range. Such fields are likely to become relevant after the formation of
a protostellar disk and, thus, could influence the formation of the first stars
and galaxies in the Universe.Comment: 10 pages, 8 figures, ApJ accepte
Modeling H2 formation in the turbulent ISM: Solenoidal versus compressive turbulent forcing
We present results from high-resolution three-dimensional simulations of the
turbulent interstellar medium that study the influence of the nature of the
turbulence on the formation of molecular hydrogen. We have examined both
solenoidal (divergence-free) and compressive (curl-free) turbulent driving, and
show that compressive driving leads to faster H2 formation, owing to the higher
peak densities produced in the gas. The difference in the H2 formation rate can
be as much as an order of magnitude at early times, but declines at later times
as the highest density regions become fully molecular and stop contributing to
the total H2 formation rate. We have also used our results to test a simple
prescription suggested by Gnedin et al. (2009) for modeling the influence of
unresolved density fluctuations on the H2 formation rate in large-scale
simulations of the ISM. We find that this approach works well when the H2
fraction is small, but breaks down once the highest density gas becomes fully
molecular.Comment: 13 pages, 8 figures, accepted for publication in MNRA
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