1,355 research outputs found
An investigation of spherical blast waves and detonation waves in a rocket combustion chamber
Spherical blast waves and detonation waves in rocket combustion chambe
The efficiency of star formation in clustered and distributed regions
We investigate the formation of both clustered and distributed populations of
young stars in a single molecular cloud. We present a numerical simulation of a
10,000 solar mass elongated, turbulent, molecular cloud and the formation of
over 2500 stars. The stars form both in stellar clusters and in a distributed
mode which is determined by the local gravitational binding of the cloud. A
density gradient along the major axis of the cloud produces bound regions that
form stellar clusters and unbound regions that form a more distributed
population. The initial mass function also depends on the local gravitational
binding of the cloud with bound regions forming full IMFs whereas in the
unbound, distributed regions the stellar masses cluster around the local Jeans
mass and lack both the high-mass and the low-mass stars. The overall efficiency
of star formation is ~ 15 % in the cloud when the calculation is terminated,
but varies from less than 1 % in the the regions of distributed star formation
to ~ 40 % in regions containing large stellar clusters. Considering that large
scale surveys are likely to catch clouds at all evolutionary stages, estimates
of the (time-averaged) star formation efficiency for the giant molecular cloud
reported here is only ~ 4 %. This would lead to the erroneous conclusion of
'slow' star formation when in fact it is occurring on a dynamical timescale.Comment: 9 pages, 8 figures, MNRAS in pres
Ionizing feedback from massive stars in massive clusters III: Disruption of partially unbound clouds
We extend our previous SPH parameter study of the effects of photoionization
from O-stars on star-forming clouds to include initially unbound clouds. We
generate a set of model clouds in the mass range M
with initial virial ratios =2.3, allow them to form
stars, and study the impact of the photoionizing radiation produced by the
massive stars. We find that, on the 3Myr timescale before supernovae are
expected to begin detonating, the fractions of mass expelled by ionizing
feedback is a very strong function of the cloud escape velocities. High-mass
clouds are largely unaffected dynamically, while lower-mass clouds have large
fractions of their gas reserves expelled on this timescale. However, the
fractions of stellar mass unbound are modest and significant portions of the
unbound stars are so only because the clouds themselves are initially partially
unbound. We find that ionization is much more able to create well-cleared
bubbles in the unbound clouds, owing to their intrinsic expansion, but that the
presence of such bubbles does not necessarily indicate that a given cloud has
been strongly influenced by feedback. We also find, in common with the bound
clouds from our earlier work, that many of the systems simulated here are
highly porous to photons and supernova ejecta, and that most of them will
likely survive their first supernova explosions.Comment: 14 pages, 13 figures (some degraded and greyscaled), accepted by
MNRA
Ionization--induced star formation V: Triggering in partially unbound clusters
We present the fourth in a series of papers detailing our SPH study of the
effects of ionizing feedback from O--type stars on turbulent star forming
clouds. Here, we study the effects of photoionization on a series of initially
partially unbound clouds with masses ranging from --M
and initial sizes from 2.5-45pc. We find that ionizing feedback profoundly
affects the structure of the gas in most of our model clouds, creating large
and often well-cleared bubble structures and pillars. However, changes in the
structures of the embedded clusters produced are much weaker and not well
correlated to the evolution of the gas. We find that in all cases, star
formation efficiencies and rates are reduced by feedback and numbers of objects
increased, relative to control simulations. We find that local triggered star
formation does occur and that there is a good correlation between triggered
objects and pillars or bubble walls, but that triggered objects are often
spatially-mixed with those formed spontaneously. Some triggered objects acquire
large enough masses to become ionizing sources themselves, lending support to
the concept of propagating star formation. We find scant evidence for spatial
age gradients in most simulations, and where we do see them, they are not a
good indicator of triggering, as they apply equally to spontaneously-formed
objects as triggered ones. Overall, we conclude that inferring the global or
local effects of feedback on stellar populations from observing a system at a
single epoch is very problematic.Comment: 17 pages, 11 figures (mostly degraded to get under the submission
size limit), accepted by MNRA
Test for Time Dilation of Intervals Between Pulse Structures in GRBs
If -ray bursts are at cosmological distances, then not only their
constituent pulses but also the intervals between pulses should be
time-dilated. Unlike time-dilation measures of pulse emission, intervals would
appear to require negligible correction for redshift of narrower temporal
structure from higher energy into the band of observation. However, stretching
of pulse intervals is inherently difficult to measure without incurring a
timescale-dependent bias since, as time profiles are stretched, more structure
can appear near the limit of resolution. This problem is compounded in dimmer
bursts because identification of significant structures becomes more
problematic. We attempt to minimize brightness bias by equalizing
signal-to-noise (s/n) level of all bursts. We analyze wavelet-denoised burst
profiles binned to several resolutions, identifying significant fluctuations
between pulse structures and interjacent valleys. When bursts are ranked by
peak flux, an interval time-dilation signature is evident, but its magnitude
and significance are dependent upon temporal resolution and s/n level.Comment: 5 pages in LATeX, REVTEX style, 2 embedded figures. To appear in
Third Huntsville GRB Workshop Proceeding
Calibration of Tests for Time Dilation in GRB Pulse Structures
Two tests for cosmological time dilation in -ray bursts -- the peak
alignment and auto-correlation statistics -- involve averaging information near
the times of peak intensity. Both tests require width corrections, assuming
cosmological origin for bursts, since narrower temporal structure from higher
energy would be redshifted into the band of observation, and since intervals
between pulse structures are included in the averaging procedures. We analyze
long ( 2 s) BATSE bursts and estimate total width corrections for trial
time-dilation factors (TDF = [1+]/[1+]) by
time-dilating and redshifting bright bursts. Both tests reveal significant
trends of increasing TDF with decreasing peak flux, but neither provides
sufficient discriminatory power to distinguish between actual TDFs in the range
2--3.Comment: 5 pages in LATeX, REVTEX style, 2 embedded figures. To appear in
Third Huntsville GRB Workshop Proceeding
Ionisation-induced star formation II: External irradiation of a turbulent molecular cloud
In this paper, we examine numerically the difference between triggered and
revealed star formation. We present Smoothed Particle Hydrodynamics (SPH)
simulations of the impact on a turbulent 10^4 solar-mass molecular cloud of
irradiation by an external source of ionising photons. In particular, using a
control model, we investigate the triggering of star formation within the
cloud. We find that, although feedback has a dramatic effect on the morphology
of our model cloud, its impact on star formation is relatively minor. We show
that external irradiation has both positive and negative effects, accelerating
the formation of some objects, delaying the formation of others, and inducing
the formation of some that would not otherwise have formed. Overall, the
calculation in which feedback is included forms nearly twice as many objects
over a period of \sim0.5 freefall times (\sim2.4 Myr), resulting in a
star--formation efficiency approximately one third higher (\sim4% as opposed to
\sim3% at this epoch) as in the control run in which feedback is absent.
Unfortunately, there appear to be no observable characteristics which could be
used to differentiate objects whose formation was triggered from those which
were forming anyway and which were simply revealed by the effects of radiation,
although this could be an effect of poor statistics.Comment: 12 pages, 9 figures, accepted by MNRA
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