1,035 research outputs found
Dynamically triggered star formation in giant molecular clouds
A Lagrangian, particle-based numerical method (tree code gravity plus
smoothed particle hydrodynamics) was used to simulate clump-clump collisions
occurring within GMCs.
The collisions formed shock-compressed layers, out of which condensed
approximately co-planar protostellar discs of 7-60 solar masses and 500-1000AU
radius. Binary and multiple systems were the usual final state. Lower mass
objects were also produced, but commonly underwent disruption or merger. Such
objects occasionally survived by being ejected via a three-body slingshot event
resulting from an encounter with a binary system.
Varying the impact parameter, b, altered the processes by which the
protostellar systems formed. At low b a single central disc formed initially,
and was then spun-up by an accretion flow, causing it to produce secondaries
via rotational instabilities. At mid b the shocked layer w hich formed
initially broke up into fragments, and discs were then formed via fragment
merger. At large b single objects formed within the compressed leading edge of
each clump. These became unbound from each other as b was increased further.
The effect of changing numerical factors was examined by : (i) colliding
clumps which had been re-oriented before the collision (thus altering the
initial particle noise), and (ii) by quadrupling the number of particles in
each clump (thus increasing the resolution of the simulation). Both changes
were found to affect the small-scale details of a collision, but leave the
large scale morphology largely unaltered.
It was concluded that clump-clump collisions provide a natural mechanism by
which multiple protostellar systems may form.Comment: 15 pages, 12 low resolution figures in 50 files, accepted by MNRA
SECRETION RATE AND METABOLIC CLEARANCE RATE OF PROLACTIN IN THE RAT DURING MID-AND LATE LACTATION
SUMMARY The prolactin concentration in the plasma of lactating rats rose less rapidly and attained a significantly lower plateau level in response to suckling on day 20\p=n-\21of lactation than it did on day 13\p=n-\14of lactation. Neither differences in suckling stimulation of the older pups nor a higher metabolic clearance rate (MCR) of prolactin were implicated in the reduced prolactin concentration seen in the late-lactating rats. The MCR was, in fact, slightly reduced in both conscious and late-lactating rats anaesthetized with urethane when compared with those in mid-lactation. The MCR of prolactin was not significantly altered by urethane anaesthesia in rats on either day of lactation. However, the secretion rate of prolactin, computed from the MCR multiplied by the equilibrum concentration of prolactin during suckling, was considerably reduced (665 to 392 ng/min) from mid-to late lactation. We conclude from these data that the reduced plasma concentration of prolactin in response to suckling in late lactation is the result of an impairment within the prolactin secretory mechanism
Numerical simulations of protostellar encounters I. Star-disc encounters
It appears that most stars are born in clusters, and that at birth most stars
have circumstellar discs which are comparable in size to the separations
between the stars. Interactions between neighbouring stars and discs are
therefore likely to play a key role in determining disc lifetimes, stellar
masses, and the separations and eccentricities of binary orbits. Such
interactions may also cause fragmentation of the discs, thereby triggering the
formation of additional stars.
We have carried out a series of simulations of disc-star interactions using
an SPH code which treats self-gravity, hydrodynamic and viscous forces. We find
that interactions between discs and stars provide a mechanism for removing
energy from, or adding energy to, the orbits of the stars, and for truncating
the discs. However, capture during such encounters is unlikely to be an
important binary formation mechanism.
A more significant consequence of such encounters is that they can trigger
fragmentation of the disc, via tidally and compressionally induced
gravitational instabilities, leading to the formation of additional stars. When
the disc-spins and stellar orbits are randomly oriented, encounters lead to the
formation of new companions to the original star in 20% of encounters. If most
encounters are prograde and coplanar, as suggested by simulations of
dynamically-triggered star formation, then new companions are formed in
approximately 50% of encounters.Comment: 17 pages, submitted to MNRAS; low resolution figures onl
Numerical simulations of protostellar encounters III. Non-coplanar disc-disc encounters
It is expected that an average protostar will undergo at least one impulsive
interaction with a neighbouring protostar whilst a large fraction of its mass
is still in a massive, extended disc. If protostars are formed individually
within a cluster before falling together and interacting, there should be no
preferred orientation for such interactions. As star formation within clusters
is believed to be coeval, it is probable that during interactions, both
protostars possess massive, extended discs.
We have used an SPH code to carry out a series of simulations of non-colpanar
disc-disc interactions. We find that non-coplanar interactions trigger
gravitational instabilities in the discs, which may then fragment to form new
companions to the existing stars. (This is different from coplanar
interactions, in which most of the new companion stars form after material in
the discs has been swept up into a shock layer, and this then fragments.) The
original stars may also capture each other, leading to the formation of a
small-N cluster. If every star undergoes a randomly oriented disc-disc
interaction, then the outcome will be the birth of many new stars.
Approximately two-thirds of the stars will end up in multiple systems.Comment: 12 pages, submitted to MNRAS; low resolution figures onl
Jet-Induced Emission-Line Nebulosity and Star Formation in the High-Redshift Radio Galaxy 4C41.17
The high redshift radio galaxy 4C41.17 consists of a powerful radio source in
which previous work has shown that there is strong evidence for jet-induced
star formation along the radio axis. We argue that nuclear photoionization is
not responsible for the excitation of the emission line clouds and we construct
a jet-cloud interaction model to explain the major features revealed by the
data. The interaction of a high-powered jet with a dense cloud in the halo of
4C41.17 produces shock-excited emission-line nebulosity through ~1000 km/s
shocks and induces star formation. The CIII to CIV line ratio and the CIV
luminosity emanating from the shock, imply that the pre-shock density in the
line-emitting cloud is high enough (~1-10 cm^-3) that shock initiated star
formation could proceed on a timescale of order a few x 10^6 yrs, well within
the estimated dynamical age of the radio source. Broad (FWHM ~ 100 - 1400 km/s)
emission lines are attributed to the disturbance of the gas cloud by a partial
bow--shock and narrow emission lines (FWHM ~ 500 - 650 km/s) (in particular
CIV) arise in precursor emission in relatively low metallicity gas. The implied
baryonic mass ~ 8 \times 10^{10} solar masses of the cloud is high and implies
that Milky Way size condensations existed in the environments of forming radio
galaxies at a redshift of 3.8. Our interpretation of the data provides a
physical basis for the alignment of the radio, emission-line and UV continuum
images in some of the highest redshift radio galaxies and the analysis
presented here may form a basis for the calculation of densities and cloud
masses in other high redshift radio galaxies.Comment: 18 pages, 5 figures; uses astrobib.sty and aaspp4.sty. Better
versions of figures available via anonymous from
ftp://mso.anu.edu.au:pub/pub/geoff/4C41.1
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
Biomarkers of Tuberculosis Severity and Treatment Effect: A Directed Screen of 70 Host Markers in a Randomized Clinical Trial.
More efficacious treatment regimens are needed for tuberculosis, however, drug development is impeded by a lack of reliable biomarkers of disease severity and of treatment effect. We conducted a directed screen of host biomarkers in participants enrolled in a tuberculosis clinical trial to address this need. Serum samples from 319 protocol-correct, culture-confirmed pulmonary tuberculosis patients treated under direct observation as part of an international, phase 2 trial were screened for 70 markers of infection, inflammation, and metabolism. Biomarker assays were specifically developed for this study and quantified using a novel, multiplexed electrochemiluminescence assay. We evaluated the association of biomarkers with baseline characteristics, as well as with detailed microbiologic data, using Bonferroni-adjusted, linear regression models. Across numerous analyses, seven proteins, SAA1, PCT, IL-1β, IL-6, CRP, PTX-3 and MMP-8, showed recurring strong associations with markers of baseline disease severity, smear grade and cavitation; were strongly modulated by tuberculosis treatment; and had responses that were greater for patients who culture-converted at 8weeks. With treatment, all proteins decreased, except for osteocalcin, MCP-1 and MCP-4, which significantly increased. Several previously reported putative tuberculosis-associated biomarkers (HOMX1, neopterin, and cathelicidin) were not significantly associated with treatment response. In conclusion, across a geographically diverse and large population of tuberculosis patients enrolled in a clinical trial, several previously reported putative biomarkers were not significantly associated with treatment response, however, seven proteins had recurring strong associations with baseline radiographic and microbiologic measures of disease severity, as well as with early treatment response, deserving additional study
HLA-A, -B, -C, -DRB1, DRB3, DRB4, DRB5 and DQB1 polymorphism detected by PCR-SSP in a semi-urban HIV-positive Ugandan population.
PCR-SSP was used to HLA-type a cohort of Ugandan HIV-positive individuals. The results represent a more comprehensive description of HLA in an African population than previously described and are in concordance with data from a general Black population. Substantial differences exist between this population and Caucasoid populations in which immunological responses to HIV have been investigated; this emphasises that the main HLA-restrictive elements for HIV-specific cytotoxic T lymphocytes will most likely be different for each population
- …