43 research outputs found
Evolution of Angular Momentum Distribution during Star Formation
If the angular momentum of the molecular cloud core were conserved during the
star formation process, a new-born star would rotate much faster than its
fission speed. This constitutes the angular momentum problem of new-born stars.
In this paper, the angular momentum transfer in the contraction of a rotating
magnetized cloud is studied with axisymmetric MHD simulations. Owing to the
large dynamic range covered by the nested-grid method, the structure of the
cloud in the range from 10 AU to 0.1 pc is explored. First, the cloud
experiences a run-away collapse, and a disk forms perpendicularly to the
magnetic field, in which the central density increases greatly in a finite
time-scale. In this phase, the specific angular momentum j of the disk
decreases to of the initial cloud. After the central density of
the disk exceeds , the infall on to the central
object develops. In this accretion stage, the rotation motion and thus the
toroidal magnetic field drive the outflow. The angular momentum of the central
object is transferred efficiently by the outflow as well as the effect of the
magnetic stress. In 7000 yr from the core formation, the specific angular
momentum of the central decreases a factor of 10^{-4} from the
initial value (i.e. from to ).Comment: 15 pages, 2 figures, Astrophysical Journal Letters in pres
Shapes of Molecular Cloud Cores and the Filamentary Mode of Star Formation
Using recent dust continuum data, we generate the intrinsic ellipticity
distribution of dense, starless molecular cloud cores. Under the hypothesis
that the cores are all either oblate or prolate randomly-oriented spheroids, we
show that a satisfactory fit to observations can be obtained with a gaussian
prolate distribution having a mean intrinsic axis ratio of 0.54. Further, we
show that correlations exist between the apparent axis ratio and both the peak
intensity and total flux density of emission from the cores, the sign of which
again favours the prolate hypothesis. The latter result shows that the mass of
a given core depends on its intrinsic ellipticity. Monte Carlo simulations are
performed to find the best-fit power law of this dependence. Finally, we show
how these results are consistent with an evolutionary scenario leading from
filamentary parent clouds to increasingly massive, condensed, and roughly
spherical embedded cores.Comment: 16 pages, incl. 11 Postscript figures. Accepted by Ap
Monte-Carlo Simulations of Globular Cluster Evolution - I. Method and Test Calculations
We present a new parallel supercomputer implementation of the Monte-Carlo
method for simulating the dynamical evolution of globular star clusters. Our
method is based on a modified version of Henon's Monte-Carlo algorithm for
solving the Fokker-Planck equation. Our code allows us to follow the evolution
of a cluster containing up to 5x10^5 stars to core collapse in < 40 hours of
computing time. In this paper we present the results of test calculations for
clusters with equal-mass stars, starting from both Plummer and King model
initial conditions. We consider isolated as well as tidally truncated clusters.
Our results are compared to those obtained from approximate, self-similar
analytic solutions, from direct numerical integrations of the Fokker-Planck
equation, and from direct N-body integrations performed on a GRAPE-4
special-purpose computer with N=16384. In all cases we find excellent agreement
with other methods, establishing our new code as a robust tool for the
numerical study of globular cluster dynamics using a realistic number of stars.Comment: 35 pages, including 8 figures, submitted to ApJ. Revised versio
The "Nessie" Nebula: Cluster Formation in a Filamentary Infrared Dark Cloud
The "Nessie" Nebula is a filamentary infrared dark cloud (IRDC) with a large
aspect ratio of over 150:1 (1.5 degrees x 0.01 degrees, or 80 pc x 0.5 pc at a
kinematic distance of 3.1 kpc). Maps of HNC (1-0) emission, a tracer of dense
molecular gas, made with the Australia Telescope National Facility Mopra
telescope, show an excellent morphological match to the mid-IR extinction.
Moreover, because the molecular line emission from the entire nebula has the
same radial velocity to within +/- 3.4 km/s, the nebula is a single, coherent
cloud and not the chance alignment of multiple unrelated clouds along the line
of sight.
The Nessie Nebula contains a number of compact, dense molecular cores which
have a characteristic projected spacing of ~ 4.5 pc along the filament. The
theory of gravitationally bound gaseous cylinders predicts the existence of
such cores, which, due to the "sausage" or "varicose" fluid instability,
fragment from the cylinder at a characteristic length scale. If turbulent
pressure dominates over thermal pressure in Nessie, then the observed core
spacing matches theoretical predictions. We speculate that the formation of
high-mass stars and massive star clusters arises from the fragmentation of
filamentary IRDCs caused by the "sausage" fluid instability that leads to the
formation of massive, dense molecular cores. The filamentary molecular gas
clouds often found near high-mass star-forming regions (e.g., Orion, NGC 6334,
etc.) may represent a later stage of IRDC evolution.Comment: 5 pages, 2 figures, accepted for publication in The Astrophysical
Journal Letter
Dynamical Mass Estimates of Large-Scale Filaments in Redshift Surveys
We propose a new method to measure the mass of large-scale filaments in
galaxy redshift surveys. The method is based on the fact that the mass per unit
length of isothermal filaments depends only on their transverse velocity
dispersion. Filaments that lie perpendicular to the line of sight may therefore
have their mass per unit length measured from their thickness in redshift
space. We present preliminary tests of the method and find that it predicts the
mass per unit length of filaments in an N-body simulation to an accuracy of
~35%. Applying the method to a select region of the Perseus-Pisces supercluster
yields a mass-to-light ratio M/L_B around 460h in solar units to within a
factor of two. The method measures the mass-to-light ratio on length scales of
up to 50h^(-1) Mpc and could thereby yield new information on the behavior of
the dark matter on mass scales well beyond that of clusters of galaxies.Comment: 21 pages, LaTeX with 6 figures included. Submitted to Ap
Gravitational Collapse of Filamentary Magnetized Molecular Clouds
We develop models for the self-similar collapse of magnetized isothermal
cylinders. We find solutions for the case of a fluid with a constant toroidal
flux-to-mass ratio (Gamma_phi=constant) and the case of a fluid with a constant
gas to magnetic pressure ratio (beta=constant). In both cases, we find that a
low magnetization results in density profiles that behave as rho ~ r^{-4} at
large radii, and at high magnetization we find density profiles that behave as
rho ~ r^{-2}. This density behaviour is the same as for hydrostatic filamentary
structures, suggesting that density measurements alone cannot distinguish
between hydrostatic and collapsing filaments--velocity measurements are
required. Our solutions show that the self-similar radial velocity behaves as
v_r ~ r during the collapse phase, and that unlike collapsing self-similar
spheres, there is no subsequent accretion (i.e. expansion-wave) phase. We also
examine the fragmentation properties of these cylinders, and find that in both
cases, the presence of a toroidal field acts to strengthen the cylinder against
fragmentation. Finally, the collapse time scales in our models are shorter than
the fragmentation time scales. Thus, we anticipate that highly collapsed
filaments can form before they are broken into pieces by gravitational
fragmentation.Comment: 20 pages, 4 figures, accepted to Ap
A Genetic Algorithm-Based Exploration of Three Filament Models: A Case for the Magnetic Support of the G11.11-0.12 Infrared-Dark Cloud
The G11.11-0.12 infrared-dark cloud has a filamentary appearance, both in
extinction against the diffuse infrared emission of the Galactic plane and in
emission at 850 microns. We use a novel computational technique based on an
advanced genetic algorithm to explore thoroughly 3 different models of
self-gravitating, pressure truncated filaments and to constrain their
parameters. Specifically, the models tested are the non-magnetic Ostriker
(1964) model, a generalized version of the magnetic Stodolkiewicz (1963) model,
and the magnetic Fiege & Pudritz (2000) model. Previous results showed that
G11.11-0.12 has a much steeper r^{-4} radial density profile than other
filaments, where the density varies approximately as r^{-2}, and that this
steep density profile is consistent with the Ostriker (1964) model. We present
a more complete analysis that shows that the radial structure of G11.11-0.12 is
consistent with regimes of each of these models. All of the magnetic models
that agree with the data are threaded by a dominant poloidal magnetic field,
and most have dynamically significant fields. Thus, G11.11-0.12 is an excellent
candidate for radial support by a magnetic field that is predominantly
poloidal. We predict the polarization patterns expected for both magnetic
models and show that the two magnetic models produce different polarization
patterns that should be distingished by observations.Comment: To appear in Ap.J. Dec. 1 edition, volume 616. 40 pages and 42
figures. Figures are severely reduced to satisfy astro-ph size limits. A
version with higher quality figures is available by contacting the first
autho
A Parallel Monte Carlo Code for Simulating Collisional N-body Systems
We present a new parallel code for computing the dynamical evolution of
collisional N-body systems with up to N~10^7 particles. Our code is based on
the the Henon Monte Carlo method for solving the Fokker-Planck equation, and
makes assumptions of spherical symmetry and dynamical equilibrium. The
principal algorithmic developments involve optimizing data structures, and the
introduction of a parallel random number generation scheme, as well as a
parallel sorting algorithm, required to find nearest neighbors for interactions
and to compute the gravitational potential. The new algorithms we introduce
along with our choice of decomposition scheme minimize communication costs and
ensure optimal distribution of data and workload among the processing units.
The implementation uses the Message Passing Interface (MPI) library for
communication, which makes it portable to many different supercomputing
architectures. We validate the code by calculating the evolution of clusters
with initial Plummer distribution functions up to core collapse with the number
of stars, N, spanning three orders of magnitude, from 10^5 to 10^7. We find
that our results are in good agreement with self-similar core-collapse
solutions, and the core collapse times generally agree with expectations from
the literature. Also, we observe good total energy conservation, within less
than 0.04% throughout all simulations. We analyze the performance of the code,
and demonstrate near-linear scaling of the runtime with the number of
processors up to 64 processors for N=10^5, 128 for N=10^6 and 256 for N=10^7.
The runtime reaches a saturation with the addition of more processors beyond
these limits which is a characteristic of the parallel sorting algorithm. The
resulting maximum speedups we achieve are approximately 60x, 100x, and 220x,
respectively.Comment: 53 pages, 13 figures, accepted for publication in ApJ Supplement
Monte Carlo Simulations of Globular Cluster Evolution. IV. Direct Integration of Strong Interactions
We study the dynamical evolution of globular clusters containing populations
of primordial binaries, using our newly updated Monte Carlo cluster evolution
code with the inclusion of direct integration of binary scattering
interactions. We describe the modifications we have made to the code, as well
as improvements we have made to the core Monte Carlo method. We present several
test calculations to verify the validity of the new code, and perform many
comparisons with previous analytical and numerical work in the literature. We
simulate the evolution of a large grid of models, with a wide range of initial
cluster profiles, and with binary fractions ranging from 0 to 1, and compare
with observations of Galactic globular clusters. We find that our code yields
very good agreement with direct N-body simulations of clusters with primordial
binaries, but yields some results that differ significantly from other
approximate methods. Notably, the direct integration of binary interactions
reduces their energy generation rate relative to the simple recipes used in
Paper III, and yields smaller core radii. Our results for the structural
parameters of clusters during the binary-burning phase are now in the tail of
the range of parameters for observed clusters, implying that either clusters
are born significantly more or less centrally concentrated than has been
previously considered, or that there are additional physical processes beyond
two-body relaxation and binary interactions that affect the structural
characteristics of clusters.Comment: Accepted for publication in ApJ; 17 pages, 19 figures; changes to
reflect accepted versio
Collapse of Rotating Magnetized Molecular Cloud Cores and Mass Outflows
Collapse of the rotating magnetized molecular cloud core is studied with the
axisymmetric magnetohydrodynamical (MHD) simulations. Due to the change of the
equation of state of the interstellar gas, the molecular cloud cores experience
several different phases as collapse proce eds. In the isothermal run-away
collapse (), a pseudo-disk is formed and
it continues to contract till the opaque core is fo rmed at the center. In this
disk, a number of MHD fast and slow shock pairs appear running parallelly to
the disk. After the equation of state becomes hard, an adiabatic core is
formed, which is separated from the isothermal contracting pseudo-disk by the
accretion shock front facing radially outwards. By the effect of the magnetic
tension, the angular momentum is transferred from the disk mid-plane to the
surface. The gas with excess angular momentum near the surface is finally
ejected, which explains the molecular bipolar outflow. Two types of outflows
are observed. When the poloidal magnetic field is strong (magnetic energy is
comparable to the thermal one), a U-shaped outflow is formed in which fast
moving gas is confined to the wall whose shape looks like a capit al letter U.
The other is the turbulent outflow in which magnetic field lines and velocity
fi elds are randomly oriented. In this case, turbulent gas moves out almost
perpendicularly from the disk. The continuous mass accretion leads to the
quasistatic contraction of the first core. A second collapse due to
dissociation of H in the first core follows. Finally another quasistatic
core is again formed by atomic hydrogen (the second core). It is found that
another outflow is ejected around the second atomic core, which seems to
correspond to the optical jets or the fast neutral winds.Comment: submitted to Ap