2,237 research outputs found
SCUBA observations of the Horsehead Nebula - what did the horse swallow?
We present observations taken with SCUBA on the JCMT of the Horsehead Nebula
in Orion (B33), at wavelengths of 450 and 850 \mum. We see bright emission from
that part of the cloud associated with the photon-dominated region (PDR) at the
`top' of the horse's head, which we label B33-SMM1. We characterise the
physical parameters of the extended dust responsible for this emission, and
find that B33-SMM1 contains a more dense core than was previously suspected. We
compare the SCUBA data with data from the Infrared Space Observatory (ISO) and
find that the emission at 6.75-\mum is offset towards the west, indicating that
the mid-infrared emission is tracing the PDR while the submillimetre emission
comes from the molecular cloud core behind the PDR. We calculate the virial
balance of this core and find that it is not gravitationally bound but is being
confined by the external pressure from the HII region IC434, and that it will
either be destroyed by the ionising radiation, or else may undergo triggered
star formation. Furthermore we find evidence for a lozenge-shaped clump in the
`throat' of the horse, which is not seen in emission at shorter wavelengths. We
label this source B33-SMM2 and find that it is brighter at submillimetre
wavelengths than B33-SMM1. SMM2 is seen in absorption in the 6.75-\mum ISO
data, from which we obtain an independent estimate of the column density in
excellent agreement with that calculated from the submillimetre emission. We
calculate the stability of this core against collapse and find that it is in
approximate gravitational virial equilibrium. This is consistent with it being
a pre-existing core in B33, possibly pre-stellar in nature, but that it may
also eventually undergo collapse under the effects of the HII region.Comment: 11 pages, 6 figures, accepted by MNRA
Simulating star formation in molecular cloud cores IV. The role of turbulence and thermodynamics
We perform SPH simulations of the collapse and fragmentation of low-mass
cores having different initial levels of turbulence
(alpha_turb=0.05,0.10,0.25). We use a new treatment of the energy equation
which captures the transport of cooling radiation against opacity due to both
dust and gas (including the effects of dust sublimation, molecules, and H^-
ions). We also perform comparison simulations using a standard barotropic
equation of state. We find that -- when compared with the barotropic equation
of state -- our more realistic treatment of the energy equation results in more
protostellar objects being formed, and a higher proportion of brown dwarfs; the
multiplicity frequency is essentially unchanged, but the multiple systems tend
to have shorter periods (by a factor ~3), higher eccentricities, and higher
mass ratios. The reason for this is that small fragments are able to cool more
effectively with the new treatment, as compared with the barotropic equation of
state. We find that the process of fragmentation is often bimodal. The first
protostar to form is usually, at the end, the most massive, i.e. the primary.
However, frequently a disc-like structure subsequently forms round this
primary, and then, once it has accumulated sufficient mass, quickly fragments
to produce several secondaries. We believe that this delayed fragmentation of a
disc-like structure is likely to be an important source of very low-mass
hydrogen-burning stars and brown dwarfs.Comment: 14 pages, 8 figures. Accepted for publication by A&
Structural analysis of the GH43 enzyme Xsa43E from Butyrivibrio proteoclasticus
The rumen of dairy cattle can be thought of as a large, stable fermentation vat and as such it houses a large and diverse community of microorganisms. The bacterium Butyrivibrio proteoclasticus is a representative of a significant component of this microbial community. It is a xylan-degrading organism whose genome encodes a large number of open reading frames annotated as fibre-degrading enzymes. This suite of enzymes is essential for the organism to utilize the plant material within the rumen as a fuel source, facilitating its survival in this competitive environment. Xsa43E, a GH43 enzyme from B. proteoclasticus, has been structurally and functionally characterized. Here, the structure of selenomethionine-derived Xsa43E determined to 1.3 Å resolution using single-wavelength anomalous diffraction is reported. Xsa43E possesses the characteristic five-bladed β-propeller domain seen in all GH43 enzymes. GH43 enzymes can have a range of functions, and the functional characterization of Xsa43E shows it to be an arabinofuranosidase capable of cleaving arabinose side chains from short segments of xylan. Full functional and structural characterization of xylan-degrading enzymes will aid in creating an enzyme cocktail that can be used to completely degrade plant material into simple sugars. These molecules have a range of applications as starting materials for many industrial processes, including renewable alternatives to fossil fuels
A Retrospective Analysis of Mortality Distribution in Juba Teaching Hospital, Southern Sudan
No Abstrac
Development of an electron density probe Final report, 28 May 1965 - 28 Feb. 1966
Electron density probe and its application in separated flow region of reentry Apollo spacecraft for typical trajector
The environmental stewardship program: lessons on creating long-term agri-environment schemes
The conservation of biodiversity on private land is both a high priority and a considerable challenge. An effective response to this challenge requires a combination of legislative and incentive mechanisms, coupled with preparedness by government to review and revise administrative arrangements. Preliminary results from the Environmental Stewardship Program, established by the Australian Government, highlight that there is a role for market-based approaches. However, implementation of this program through a Commonwealth bureaucracy was not without its challenges. Here we provide an overview of the program’s implementation from 2007 to 2012, followed by discussion of some key lessons learned
Combining algorithms to predict bacterial protein sub-cellular location: Parallel versus concurrent implementations
We describe a novel and potentially important tool for candidate subunit vaccine selection through in silico reverse-vaccinology. A set of Bayesian networks able to make individual predictions for specific subcellular locations
is implemented in three pipelines with different architectures: a parallel implementation with a confidence level-based decision engine and two serial implementations with a hierarchical decision structure, one initially rooted by
prediction between membrane types and another rooted by soluble versus membrane prediction. The parallel pipeline outperformed the serial pipeline, but took twice as long to execute. The soluble-rooted serial pipeline outperformed
the membrane-rooted predictor. Assessment using genomic test sets was more equivocal, as many more predictions are made by the parallel pipeline, yet the serial pipeline identifies 22 more of the 74 proteins of known location
- …