60 research outputs found
3D MHD Modeling of the Gaseous Structure of the Galaxy: Synthetic Observations
We generated synthetic observations from the four-arm model presented in
Gomez & Cox (2004) for the Galactic ISM in the presence of a spiral
gravitational perturbation. We found that velocity crowding and diffusion have
a strong effect in the l-v diagram. The v-b diagram presents structures at the
expected spiral arm velocities, that can be explained by the off-the-plane
structure of the arms presented in previous papers of this series. Such
structures are observed in the Leiden/Dwingeloo HI survey. The rotation curve,
as measured from the inside of the modeled galaxy, shows similarities with the
observed one for the Milky Way Galaxy, although it has large deviations from
the smooth circular rotation corresponding to the background potential. The
magnetic field inferred from a synthetic synchrotron map shows a largely
circular structure, but with interesting deviations in the midplane due to
distortion of the field from circularity in the interarm regions.Comment: Accepted for publication in ApJ. Better quality figures in
http://www.astro.umd.edu/~gomez/publica/3d_galaxy-3.pd
The effect of the Coriolis force on Kelvin-Helmholtz-driven mixing in protoplanetary disks
We study the stability of proto-planetary disks with vertical velocity
gradients in their equilibrium rotation rates; such gradients are expected to
develop when dust settles into the midplane. Using a linear stability analysis
of a simple three-layer model, we show that the onset of instability occurs at
a larger value of the Richardson number, and therefore for a thicker layer,
when the effects of Coriolis forces are included. This analysis also shows that
even-symmetry (midplane-crossing) modes develop faster than odd-symmetry ones.
These conclusions are corroborated by a large number of nonlinear numerical
simulations with two different parameterized prescriptions for the initial
(continuous) dust distributions. Based on these numerical experiments, the
Richardson number required for marginal stability is more than an order of
magnitude larger than the traditional 1/4 value. The dominant modes that grow
have horizontal wavelengths of several initial dust scale heights, and in
nonlinear stages mix solids fairly homogeneously over a comparable vertical
range. We conclude that gravitational instability may be more difficult to
achieve than previously thought, and that the vertical distribution of matter
within the dust layer is likely globally, rather than locally, determined.Comment: Accepted for publication in Ap
3D MHD Modeling of the Gaseous Structure of the Galaxy: Setup and Initial Results
We show the initial results of our 3D MHD simulations of the flow of the
Galactic atmosphere as it responds to a spiral perturbation in the potential.
In our standard case, as the gas approaches the arm, there is a downward
converging flow that terminates in a complex of shocks just ahead of the
midplane density peak. The density maximum slants forward at high z, preceeded
by a similarly leaning shock. The latter diverts the flow upward and over the
arm, as in a hydraulic jump. Behind the gaseous arm, the flow falls again,
generating further secondary shocks as it approaches the lower z material.
Structures similar to the high z part of the gaseous arms are found in the
interarm region of our two-armed case, while broken arms and low column density
bridges are present in the four-armed case.
We present three examples of what can be learned from these models.Comment: 33 pages, 17 figures. Accepted for publication in Apj. Better quality
images in
http://www.journals.uchicago.edu/ApJ/journal/preprints/ApJ55782.preprint.pd
Formation and Collapse of Quiescent Cloud Cores Induced by Dynamic Compressions
(Abridged) We present numerical hydrodynamical simulations of the formation,
evolution and gravitational collapse of isothermal molecular cloud cores. A
compressive wave is set up in a constant sub-Jeans density distribution of
radius r = 1 pc. As the wave travels through the simulation grid, a
shock-bounded spherical shell is formed. The inner shock of this shell reaches
and bounces off the center, leaving behind a central core with an initially
almost uniform density distribution, surrounded by an envelope consisting of
the material in the shock-bounded shell, with a power-law density profile that
at late times approaches a logarithmic slope of -2 even in non-collapsing
cases. The resulting density structure resembles a quiescent core of radius <
0.1 pc, with a Bonnor-Ebert-like (BE-like) profile, although it has significant
dynamical differences: it is initially non-self-gravitating and confined by the
ram pressure of the infalling material, and consequently, growing continuously
in mass and size. With the appropriate parameters, the core mass eventually
reaches an effective Jeans mass, at which time the core begins to collapse.
Thus, there is necessarily a time delay between the appearance of the core and
the onset of its collapse, but this is not due to the dissipation of its
internal turbulence as it is often believed. These results suggest that
pre-stellar cores may approximate Bonnor-Ebert structures which are however of
variable mass and may or may not experience gravitational collapse, in
qualitative agreement with the large observed frequency of cores with BE-like
profiles.Comment: Accepted for publication in ApJ. Associated mpeg files can be found
in http://www.astrosmo.unam.mx/~g.gomez/publica.htm
Direct observational evidence of the multi-scale, dynamical mass accretion toward a high-mass star forming hub-filament system
There is growing evidence that high-mass star formation and hub-filament
systems (HFS) are intricately linked. The gas kinematics along the filaments
and the forming high-mass star(s) in the central hub are in excellent agreement
with the new generation of global hierarchical high-mass star formation models.
In this paper, we present an observational investigation of a typical HFS
cloud, G310.142+0.758 (G310 hereafter) which reveals unambiguous evidence of
mass inflow from the cloud scale via the filaments onto the forming
protostar(s) at the hub conforming with the model predictions. Continuum and
molecular line data from the ATOMS and MALT90 surveys are used that cover
different spatial scales. Three filaments (with total mass ) are identified converging toward the central hub region where
several signposts of high-mass star formation have been observed. The hub
region contains a massive clump () harbouring a central
massive core. Additionally, five outflow lobes are associated with the central
massive core implying a forming cluster. The observed large-scale, smooth and
coherent velocity gradients from the cloud down to the core scale, and the
signatures of infall motion seen in the central massive clump and core, clearly
unveil a nearly-continuous, multi-scale mass accretion/transfer process at a
similar mass infall rate of over all scales,
feeding the central forming high-mass protostar(s) in the G310 HFS cloud.Comment: Accepted to publish in ApJ. 10 pages with 6 figures and 2 table
ATOMS : ALMA Three-millimeter Observations of Massive Star-forming regions - XI. From inflow to infall in hub-filament systems
We investigate the presence of hub-filament systems in a large sample of 146 active proto-clusters, using (HCO+)-C-13 J = 1-0 molecular line data obtained from the ATOMS survey. We find that filaments are ubiquitous in proto-clusters, and hub-filament systems are very common from dense core scales (similar to 0.1 pc) to clump/cloud scales (similar to 1-10 pc). The proportion of proto-clusters containing hub-filament systems decreases with increasing dust temperature (T-d) and luminosity-to-mass ratios (L/M) of clumps, indicating that stellar feedback from H ii regions gradually destroys the hub-filament systems as proto-clusters evolve. Clear velocity gradients are seen along the longest filaments with a mean velocity gradient of 8.71 km s(-1) pc(-1) and a median velocity gradient of 5.54 km s(-1) pc(-1). We find that velocity gradients are small for filament lengths larger than similar to 1 pc, probably hinting at the existence of inertial inflows, although we cannot determine whether the latter are driven by large-scale turbulence or large-scale gravitational contraction. In contrast, velocity gradients below similar to 1 pc dramatically increase as filament lengths decrease, indicating that the gravity of the hubs or cores starts to dominate gas infall at small scales. We suggest that self-similar hub-filament systems and filamentary accretion at all scales may play a key role in high-mass star formation.Peer reviewe
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