1,247 research outputs found
Constraints on the Formation and Evolution of Circumstellar Disks in Rotating Magnetized Cloud Cores
We use magnetic collapse models to place some constraints on the formation
and angular momentum evolution of circumstellar disks which are embedded in
magnetized cloud cores. Previous models have shown that the early evolution of
a magnetized cloud core is governed by ambipolar diffusion and magnetic
braking, and that the core takes the form of a nonequilibrium flattened
envelope which ultimately collapses dynamically to form a protostar. In this
paper, we focus on the inner centrifugally-supported disk, which is formed only
after a central protostar exists, and grows by dynamical accretion from the
flattened envelope. We estimate a centrifugal radius for the collapse of mass
shells within a rotating, magnetized cloud core. The centrifugal radius of the
inner disk is related to its mass through the two important parameters
characterizing the background medium: the background rotation rate \Omb and
the background magnetic field strength \Bref. We also revisit the issue of
how rapidly mass is deposited onto the disk (the mass accretion rate) and use
several recent models to comment upon the likely outcome in magnetized cores.
Our model predicts that a significant centrifugal disk (much larger than a
stellar radius) will be present in the very early (Class 0) stage of
protostellar evolution. Additionally, we derive an upper limit for the disk
radius as it evolves due to internal torques, under the assumption that the
star-disk system conserves its mass and angular momentum even while most of the
mass is transferred to a central star.Comment: 23 pages, 1 figure, aastex, to appear in the Astrophysical Journal
(10 Dec 1998
Molecular Evolution in Collapsing Prestellar Cores
We have investigated the evolution and distribution of molecules in
collapsing prestellar cores via numerical chemical models, adopting the
Larson-Penston solution and its delayed analogues to study collapse. Molecular
abundances and distributions in a collapsing core are determined by the balance
among the dynamical, chemical and adsorption time scales. When the central
density n_H of a prestellar core with the Larson-Penston flow rises to 3 10^6
cm^{-3}, the CCS and CO column densities are calculated to show central holes
of radius 7000 AU and 4000 AU, respectively, while the column density of N2H+
is centrally peaked. These predictions are consistent with observations of
L1544. If the dynamical time scale of the core is larger than that of the
Larson-Penston solution owing to magnetic fields, rotation, or turbulence, the
column densities of CO and CCS are smaller, and their holes are larger than in
the Larson-Penston core with the same central gas density. On the other hand,
N2H+ and NH3 are more abundant in the more slowly collapsing core. Therefore,
molecular distributions can probe the collapse time scale of prestellar cores.
Deuterium fractionation has also been studied via numerical calculations. The
deuterium fraction in molecules increases as a core evolves and molecular
depletion onto grains proceeds. When the central density of the core is n_H=3
10^6 cm^{-3}, the ratio DCO+/HCO+ at the center is in the range 0.06-0.27,
depending on the collapse time scale and adsorption energy; this range is in
reasonable agreement with the observed value in L1544.Comment: 21 pages, 17 figure
Global Nonradial Instabilities of Dynamically Collapsing Gas Spheres
Self-similar solutions provide good descriptions for the gravitational
collapse of spherical clouds or stars when the gas obeys a polytropic equation
of state, (with ). We study the behaviors of
nonradial perturbations in the similarity solutions of Larson, Penston and
Yahil, which describe the evolution of the collapsing cloud prior to core
formation. Our global stability analysis reveals the existence of unstable
bar-modes () when . In particular, for the collapse of
isothermal spheres, which applies to the early stages of star formation, the
density perturbation relative to the background, , increases as ,
where denotes the epoch of core formation, and is the cloud
central density. Thus, the isothermal cloud tends to evolve into an ellipsoidal
shape (prolate bar or oblate disk, depending on initial conditions) as the
collapse proceeds. In the context of Type II supernovae, core collapse is
described by the equation of state, and our analysis
indicates that there is no growing mode (with density perturbation) in the
collapsing core before the proto-neutron star forms, although nonradial
perturbations can grow during the subsequent accretion of the outer core and
envelope onto the neutron star. We also carry out a global stability analysis
for the self-similar expansion-wave solution found by Shu, which describes the
post-collapse accretion (``inside-out'' collapse) of isothermal gas onto a
protostar. We show that this solution is unstable to perturbations of all
's, although the growth rates are unknown.Comment: 28 pages including 7 ps figures; Minor changes in the discussion; To
be published in ApJ (V.540, Sept.10, 2000 issue
IR Dust Bubbles: Probing the Detailed Structure and Young Massive Stellar Populations of Galactic HII Regions
We present an analysis of wind-blown, parsec-sized, mid-infrared bubbles and
associated star-formation using GLIMPSE/IRAC, MIPSGAL/MIPS and MAGPIS/VLA
surveys. Three bubbles from the Churchwell et al. (2006) catalog were selected.
The relative distribution of the ionized gas (based on 20 cm emission), PAH
emission (based on 8 um, 5.8 um and lack of 4.5 um emission) and hot dust (24
um emission) are compared. At the center of each bubble there is a region
containing ionized gas and hot dust, surrounded by PAHs. We identify the likely
source(s) of the stellar wind and ionizing flux producing each bubble based
upon SED fitting to numerical hot stellar photosphere models. Candidate YSOs
are also identified using SED fitting, including several sites of possible
triggered star formation.Comment: 37 pages, 17 figure
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PDV Probe Alignment Technique
This alignment technique was developed while performing heterodyne velocimetry measurements at LLNL. There are a few minor items needed, such as a white card with aperture in center, visible alignment laser, IR back reflection meter, and a microscope to view the bridge surface. The work was performed on KCP flyers that were 6 and 8 mils wide. The probes used were Oz Optics manufactured with focal distances of 42mm and 26mm. Both probes provide a spot size of approximately 80?m at 1550nm. The 42mm probes were specified to provide an internal back reflection of -35 to -40dB, and the probe back reflections were measured to be -37dB and -33dB. The 26mm probes were specified as -30dB and both measured -30.5dB. The probe is initially aligned normal to the flyer/bridge surface. This provides a very high return signal, up to -2dB, due to the bridge reflectivity. A white card with a hole in the center as an aperture can be used to check the reflected beam position relative to the probe and launch beam, and the alignment laser spot centered on the bridge, see Figure 1 and Figure 2. The IR back reflection meter is used to measure the dB return from the probe and surface, and a white card or similar object is inserted between the probe and surface to block surface reflection. It may take several iterations between the visible alignment laser and the IR back reflection meter to complete this alignment procedure. Once aligned normal to the surface, the probe should be tilted to position the visible alignment beam as shown in Figure 3, and the flyer should be translated in the X and Y axis to reposition the alignment beam onto the flyer as shown in Figure 4. This tilting of the probe minimizes the amount of light from the bridge reflection into the fiber within the probe while maintaining the alignment as near normal to the flyer surface as possible. When the back reflection is measured after the tilt adjustment, the level should be about -3dB to -6dB higher than the probes specified back reflection. This 3 to 6dB increase in back reflection from the surface relative to the probes specified back reflection is the optimal level for acquiring data from the flyer. Data obtained with the LLNL system is shown in Figure 5
Early antenatal prediction of gestational diabetes in obese women: development of prediction tools for targeted intervention
All obese women are categorised as being of equally high risk of gestational diabetes (GDM) whereas the majority do not develop the disorder. Lifestyle and pharmacological interventions in unselected obese pregnant women have been unsuccessful in preventing GDM. Our aim was to develop a prediction tool for early identification of obese women at high risk of GDM to facilitate targeted interventions in those most likely to benefit. Clinical and anthropometric data and non-fasting blood samples were obtained at 15+0â18+6 weeksâ gestation in 1303 obese pregnant women from UPBEAT, a randomised controlled trial of a behavioural intervention. Twenty one candidate biomarkers associated with insulin resistance, and a targeted nuclear magnetic resonance (NMR) metabolome were measured. Prediction models were constructed using stepwise logistic regression. Twenty six percent of women (n = 337) developed GDM (International Association of Diabetes and Pregnancy Study Groups criteria). A model based on clinical and anthropometric variables (age, previous GDM, family history of type 2 diabetes, systolic blood pressure, sum of skinfold thicknesses, waist:height and neck:thigh ratios) provided an area under the curve of 0.71 (95%CI 0.68â0.74). This increased to 0.77 (95%CI 0.73â0.80) with addition of candidate biomarkers (random glucose, haemoglobin A1c (HbA1c), fructosamine, adiponectin, sex hormone binding globulin, triglycerides), but was not improved by addition of NMR metabolites (0.77; 95%CI 0.74â0.81). Clinically translatable models for GDM prediction including readily measurable variables e.g. mid-arm circumference, age, systolic blood pressure, HbA1c and adiponectin are described. Using a âĽ35% risk threshold, all models identified a group of high risk obese women of whom approximately 50% (positive predictive value) later developed GDM, with a negative predictive value of 80%. Tools for early pregnancy identification of obese women at risk of GDM are described which could enable targeted interventions for GDM prevention in women who will benefit the most
Interactions between brown-dwarf binaries and Sun-like stars
Several mechanisms have been proposed for the formation of brown dwarfs, but
there is as yet no consensus as to which -- if any -- are operative in nature.
Any theory of brown dwarf formation must explain the observed statistics of
brown dwarfs. These statistics are limited by selection effects, but they are
becoming increasingly discriminating. In particular, it appears (a) that brown
dwarfs that are secondaries to Sun-like stars tend to be on wide orbits, a\ga
100\,{\rm AU} (the Brown Dwarf Desert), and (b) that these brown dwarfs have a
significantly higher chance of being in a close (a\la 10\,{\rm AU}) binary
system with another brown dwarf than do brown dwarfs in the field. This then
raises the issue of whether these brown dwarfs have formed {\it in situ}, i.e.
by fragmentation of a circumstellar disc; or have formed elsewhere and
subsequently been captured. We present numerical simulations of the purely
gravitational interaction between a close brown-dwarf binary and a Sun-like
star. These simulations demonstrate that such interactions have a negligible
chance () of leading to the close brown-dwarf binary being captured by
the Sun-like star. Making the interactions dissipative by invoking the
hydrodynamic effects of attendant discs might alter this conclusion. However,
in order to explain the above statistics, this dissipation would have to favour
the capture of brown-dwarf binaries over single brown-dwarfs, and we present
arguments why this is unlikely. The simplest inference is that most brown-dwarf
binaries -- and therefore possibly also most single brown dwarfs -- form by
fragmentation of circumstellar discs around Sun-like protostars, with some of
them subsequently being ejected into the field.Comment: 10 pages, 8 figures, Accepted for publication in Astrophysics and
Space Scienc
Collapse of Rotating Magnetized Molecular Cloud Cores and Mass Outflows
Collapse of the rotating magnetized molecular cloud core is studied with the
axisymmetric magnetohydrodynamical (MHD) simulations. Due to the change of the
equation of state of the interstellar gas, the molecular cloud cores experience
several different phases as collapse proce eds. In the isothermal run-away
collapse (), a pseudo-disk is formed and
it continues to contract till the opaque core is fo rmed at the center. In this
disk, a number of MHD fast and slow shock pairs appear running parallelly to
the disk. After the equation of state becomes hard, an adiabatic core is
formed, which is separated from the isothermal contracting pseudo-disk by the
accretion shock front facing radially outwards. By the effect of the magnetic
tension, the angular momentum is transferred from the disk mid-plane to the
surface. The gas with excess angular momentum near the surface is finally
ejected, which explains the molecular bipolar outflow. Two types of outflows
are observed. When the poloidal magnetic field is strong (magnetic energy is
comparable to the thermal one), a U-shaped outflow is formed in which fast
moving gas is confined to the wall whose shape looks like a capit al letter U.
The other is the turbulent outflow in which magnetic field lines and velocity
fi elds are randomly oriented. In this case, turbulent gas moves out almost
perpendicularly from the disk. The continuous mass accretion leads to the
quasistatic contraction of the first core. A second collapse due to
dissociation of H in the first core follows. Finally another quasistatic
core is again formed by atomic hydrogen (the second core). It is found that
another outflow is ejected around the second atomic core, which seems to
correspond to the optical jets or the fast neutral winds.Comment: submitted to Ap
The genetic organisation of prokaryotic two-component system signalling pathways
<p>Abstract</p> <p>Background</p> <p>Two-component systems (TCSs) are modular and diverse signalling pathways, involving a stimulus-responsive transfer of phosphoryl groups from transmitter to partner receiver domains. TCS gene and domain organisation are both potentially informative regarding biological function, interaction partnerships and molecular mechanisms. However, there is currently little understanding of the relationships between domain architecture, gene organisation and TCS pathway structure.</p> <p>Results</p> <p>Here we classify the gene and domain organisation of TCS gene loci from 1405 prokaryotic replicons (>40,000 TCS proteins). We find that 200 bp is the most appropriate distance cut-off for defining whether two TCS genes are functionally linked. More than 90% of all TCS gene loci encode just one or two transmitter and/or receiver domains, however numerous other geometries exist, often with large numbers of encoded TCS domains. Such information provides insights into the distribution of TCS domains between genes, and within genes. As expected, the organisation of TCS genes and domains is affected by phylogeny, and plasmid-encoded TCS exhibit differences in organisation from their chromosomally-encoded counterparts.</p> <p>Conclusions</p> <p>We provide here an overview of the genomic and genetic organisation of TCS domains, as a resource for further research. We also propose novel metrics that build upon TCS gene/domain organisation data and allow comparisons between genomic complements of TCSs. In particular, '<it>percentage orphaned TCS genes</it>' (or 'Dissemination') and '<it>percentage of complex loci</it>' (or 'Sophistication') appear to be useful discriminators, and to reflect mechanistic aspects of TCS organisation not captured by existing metrics.</p
Site amplification in the Kathmandu Valley during the 2015 M7.6 Gorkha, Nepal earthquake
The 25th April 2015 M7.6 Gorkha earthquake caused significant damage to
buildings and infrastructure in both Kathmandu and surrounding areas as well as triggering
numerous, large landslides. This resulted in the loss of approximately 8600 lives. In order
to learn how the impact of such events can be reduced on communities both in Nepal and
elsewhere, the Earthquake Engineering Field Investigation Team (EEFIT) reconnaissance
mission was undertaken, aiming to look at damage patterns within the country. Passive,
microtremor recordings in severely damaged areas of the Kathmandu Valley, as well as at
the main seismic recording station in Kathmandu (USGS station KATNP) are used to
determined preliminary shear wave velocity (Vs) profiles for each site. These profiles are
converted into spectral acceleration using the input motion of the Gorkha earthquake. The
results are limited, but show clear site amplification within the Siddhitol Region. The
resulting ground motions exceed the design levels from the Nepalese Building Codes,
indicating the need for site-specific hazard analysis and for revision of the building code to
address the effect of site amplificatio
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