14,588 research outputs found
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
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
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
An estimate of the stratospheric contribution to springtime tropospheric ozone maxima using TOPSE measurements and beryllium-7 simulations
Measurements of tropospheric ozone (O3) between 30°N and 70°N show springtime maxima at remote locations. The contribution of seasonal changes in stratosphere–troposphere exchange (STE) to these maxima was investigated using measurements from the Tropospheric Ozone Production about the Spring Equinox Experiment (TOPSE) campaign and the beryllium-7 (7Be) distribution from a calculation driven by fields from the Goddard Earth Observing System Data Assimilation System (GEOS DAS). Comparison with TOPSE measurements revealed that upper tropospheric model-calculated 7Be mixing ratios were reasonable (a change from previous calculations) but that lower tropospheric mixing ratios were too low most likely due to an overestimation of scavenging. Temporal fluctuations were well captured although their amplitudes were often underestimated. Analysis of O3measurements indicated that O3 mixing ratios increased by 5–10% month−1 for θ \u3c 300 K (the underworld) and by 10–15% month−1 for θ \u3e 300 K (the tropospheric middleworld). 7Be mixing ratios decreased with time for θ \u3c 290 K and increased with time for θ \u3e 300 K. Model-calculated middleworld increases of 7Be were a factor of 2 less than measured increases. 7Be with a stratospheric source (strat-7Be) increased by 4.6–8.8% month−1 along TOPSE flight paths within the tropospheric middleworld. Increases in strat-7Be were not seen along TOPSE flight paths in the underworld. Assuming changes in tropospheric O3 with a stratospheric source are the same as changes in strat-7Be and that 50% of O3 in the region of interest is produced in the stratosphere, changes in STE explain 20–60% of O3 increases in the tropospheric middleworld and less than 33% of O3 increases in the underworld
The fragmentation of expanding shells III: Oligarchic accretion and the mass spectrum of fragments
We use SPH simulations to investigate the gravitational fragmentation of
expanding shells through the linear and non--linear regimes. The results are
analysed using spherical harmonic decomposition to capture the initiation of
structure during the linear regime; the potential-based method of Smith et al.
(2009) to follow the development of clumps in the mildly non-linear regime; and
sink particles to capture the properties of the final bound objects during the
highly non-linear regime. In the early, mildly non--linear phase of
fragmentation, we find that the clump mass function still agrees quite well
with the mass function predicted by the analytic model. However, the sink mass
function is quite different, in the sense of being skewed towards high-mass
objects. This is because, once the growth of a condensation becomes non-linear,
it tends to be growing non-competitively from its own essentially separate
reservoir; we call this Oligarchic Accretion.Comment: 14 pages, accepted for publication in MNRA
Experimental radio frequency link for Ka-band communications applications
An experimental radio frequency link has been demonstrated to provide two-way communication between a remote user ground terminal and a ground-based Ka-band transponder. Bit-error-rate performance and radio frequency characteristics of the communication link were investigated
\u3cem\u3eRhizobium japonicum\u3c/em\u3e Mutants Defective in Symbiotic Nitrogen Fixation
Rhizobium japonicum strains 3I1b110 and 61A76 were mutagenized to obtain 25 independently derived mutants that produced soybean nodules defective in nitrogen fixation, as assayed by acetylene reduction. The proteins of both the bacterial and the plant portions of the nodules were analyzed by two-dimensional polyacrylamide gel electrophoresis. All of the mutants had lower-than-normal levels of the nitrogenase components, and all but four contained a prominent bacteroid protein not observed in wild-type bacteroids. Experiments with bacteria grown ex planta suggested that this protein was derepressed by the absence of ammonia. Nitrogenase component II of one mutant was altered in isoelectric point. The soluble plant fraction of the nodules of seven mutants had very low levels of heme, yet the nodules of five of these seven mutants contained the polypeptide of leghemoglobin. Thus, the synthesis of the globin may not be coupled to the content of available heme in soybean nodules. The nodules of the other two of these seven mutants lacked not only leghemoglobin but most of the other normal plant and bacteroid proteins. Ultrastructural examination of nodules formed by these two mutants indicated normal ramification of infection threads but suggested a problem in subsequent survival of the bacteria and their release from the infection threads
Feedback from massive stars at low metallicities : MUSE observations of N44 and N180 in the Large Magellanic Cloud
Accepted for publication in MNRAS, 27 pages, 21 figuresWe present MUSE integral field data of two HII region complexes in the Large Magellanic Cloud (LMC), N44 and N180. Both regions consist of a main superbubble and a number of smaller, more compact HII regions that formed on the edge of the superbubble. For a total of 11 HII regions, we systematically analyse the radiative and mechanical feedback from the massive O-type stars on the surrounding gas. We exploit the integral field property of the data and the coverage of the HeII5412 line to identify and classify the feedback-driving massive stars, and from the estimated spectral types and luminosity classes we determine the stellar radiative output in terms of the ionising photon flux . We characterise the HII regions in terms of their sizes, morphologies, ionisation structure, luminosity and kinematics, and derive oxygen abundances via emission line ratios. We analyse the role of different stellar feedback mechanisms for each region by measuring the direct radiation pressure, the pressure of the ionised gas, and the pressure of the shock-heated winds. We find that stellar winds and ionised gas are the main drivers of HII region expansion in our sample, while the direct radiation pressure is up to three orders of magnitude lower than the other terms. We relate the total pressure to the star formation rate per unit area, , for each region and find that stellar feedback has a negative effect on star formation, and sets an upper limit to as a function of increasing pressure.Peer reviewe
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