1,224 research outputs found
Occlusion Effects and the Distribution of Interstellar Cloud Sizes and Masses
The frequency distributions of sizes of ``clouds" and ``clumps" within clouds
are significantly flatter for extinction surveys than for CO spectral line
surveys, even for comparable size ranges. A possible explanation is the
blocking of extinction clouds by larger foreground clouds (occlusion), which
should not affect spectral line surveys much because clouds are resolved in
velocity space along a given line of sight. We present a simple derivation of
the relation between the true and occluded size distributions, assuming clouds
are uniformly distributed in space or the distance to a cloud comples is much
greater than the size of the complex. Because the occlusion is dominated by the
largest clouds, we find that occlusion does not affect the measured size
distribution except for sizes comparable to the largest size, implying that
occlusion is not responsible for the discrepancy if the range in sizes of the
samples is large. However, we find that the range in sizes for many of the
published observed samples is actually quite small, which suggests that
occlusion does affect the extinction sample and/or that the discrepancy could
arise from the different operational definitions and selection effects involved
in the two samples. Size and mass spectra from an IRAS survey (Wood \etal\
1994) suggest that selection effects play a major role in all the surveys. We
conclude that a reliable determination of the ``true" size and mass spectra of
clouds will require spectral line surveys with very high signal-to-noise and
sufficient resolution and sampling to cover a larger range of linear sizes, as
well as careful attention to selection effects.Comment: 13 pages, LATEX, aas style, Submitted to Ap
Turbulence-Induced Relative Velocity of Dust Particles II: The Bidisperse Case
We extend our earlier work on turbulence-induced relative velocity between
equal-size particles (Pan and Padoan, Paper I) to particles of arbitrarily
different sizes. The Pan and Padoan (PP10) model shows that the relative
velocity between different particles has two contributions, named the
generalized shear and acceleration terms, respectively. The generalized shear
term represents the particles' memory of the spatial flow velocity difference
across the particle distance in the past, while the acceleration term is
associated with the temporal flow velocity difference on individual particle
trajectories. Using the simulation of Paper I, we compute the root-mean-square
relative velocity, ^1/2, as a function of the friction times, tau_p1 and
tau_p2, of the two particles, and show that the PP10 prediction is in
satisfactory agreement with the data, confirming its physical picture. For a
given tau_p1 below the Lagrangian correlation time of the flow, T_L, ^1/2
as a function of tau_p2 shows a dip at tau_p2~tau_p1, indicating tighter
velocity correlation between similar particles. Defining a ratio
f=tau_pl/tau_ph, with tau_pl and tau_ph the friction times of the smaller and
larger particles, we find that ^1/2 increases with decreasing f due to the
generalized acceleration contribution, which dominates at f<1/4. At a fixed f,
our model predicts that ^1/2 scales as tau_ph^1/2 for tau_p,h in the
inertial range of the flow, stays roughly constant for T_L <tau_ph < T_L/f, and
finally decreases as tau_ph^-1/2 for tau_ph>>T_L/f. The acceleration term is
independent of the particle distance, r, and thus reduces the r-dependence of
^1/2 in the bidisperse case.Comment: 23 pages, 12 figures, Accepted to Ap
Wind-Driven Gas Networks and Star Formation in Galaxies: Reaction-Advection Hydrodynamic Simulations
The effects of wind-driven star formation feedback on the spatio-temporal
organization of stars and gas in galaxies is studied using two-dimensional
intermediate-representational quasi-hydrodynamical simulations. The model
retains only a reduced subset of the physics, including mass and momentum
conservation, fully nonlinear fluid advection, inelastic macroscopic
interactions, threshold star formation, and momentum forcing by winds from
young star clusters on the surrounding gas. Expanding shells of swept-up gas
evolve through the action of fluid advection to form a ``turbulent'' network of
interacting shell fragments whose overall appearance is a web of filaments (in
two dimensions). A new star cluster is formed whenever the column density
through a filament exceeds a critical threshold based on the gravitational
instability criterion for an expanding shell, which then generates a new
expanding shell after some time delay. A filament- finding algorithm is
developed to locate the potential sites of new star formation. The major result
is the dominance of multiple interactions between advectively-distorted shells
in controlling the gas and star morphology, gas velocity distribution and mass
spectrum of high mass density peaks, and the global star formation history. The
gas morphology observations of gas in the LMC and in local molecular clouds.
The frequency distribution of present-to-past average global star formation
rate, the distribution of gas velocities in filaments (found to be
exponential), and the cloud mass spectra (estimated using a structure tree
method), are discussed in detail.Comment: 40 pp, 15 eps figs, mnras style, accepted for publication in MNRAS,
abstract abridged, revisions in response to referee's comment
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