47,104 research outputs found
Tidal stability of giant molecular clouds in the Large Magellanic Cloud
Star formation does not occur until the onset of gravitational collapse
inside giant molecular clouds. However, the conditions that initiate cloud
collapse and regulate the star formation process remain poorly understood.
Local processes such as turbulence and magnetic fields can act to promote or
prevent collapse. On larger scales, the galactic potential can also influence
cloud stability and is traditionally assessed by the tidal and shear effects.
In this paper, we examine the stability of giant molecular clouds (GMCs) in the
Large Magellanic Cloud (LMC) against shear and the galactic tide using CO data
from the Magellanic Mopra Assessment (MAGMA) and rotation curve data from the
literature. We calculate the tidal acceleration experienced by individual GMCs
and determine the minimum cloud mass required for tidal stability. We also
calculate the shear parameter, which is a measure of a clouds susceptibility to
disruption via shearing forces in the galactic disk. We examine whether there
are correlations between the properties and star forming activity of GMCs and
their stability against shear and tidal disruption. We find that the GMCs are
in approximate tidal balance in the LMC, and that shear is unlikely to affect
their further evolution. GMCs with masses close to the minimal stable mass
against tidal disruption are not unusual in terms of their mass, location, or
CO brightness, but we note that GMCs with large velocity dispersion tend to be
more sensitive to tidal instability. We also note that GMCs with smaller radii,
which represent the majority of our sample, tend to more strongly resist tidal
and shear disruption. Our results demonstrate that star formation in the LMC is
not inhibited by to tidal or shear instability.Comment: 18 pages, 10 Figures, Accepted in PAS
Heavy Quarkonium Dissociation Cross Sections in Relativistic Heavy-Ion Collisions
Many of the hadron-hadron cross sections required for the study of the
dynamics of matter produced in relativistic heavy-ion collisions can be
calculated using the quark-interchange model. Here we evaluate the low-energy
dissociation cross sections of , , , , and
in collision with , , and , which are important for
the interpretation of heavy-quarkonium suppression as a signature for the quark
gluon plasma. These comover dissociation processes also contribute to
heavy-quarkonium suppression, and must be understood and incorporated in
simulations of heavy-ion collisions before QGP formation can be established
through this signature.Comment: 38 pages, in LaTe
Constraints on Dark Photon from Neutrino-Electron Scattering Experiments
A possible manifestation of an additional light gauge boson , named
as Dark Photon, associated with a group is studied in neutrino
electron scattering experiments. The exclusion plot on the coupling constant
and the dark photon mass is obtained. It is shown
that contributions of interference term between the dark photon and the
Standard Model are important. The interference effects are studied and compared
with for data sets from TEXONO, GEMMA, BOREXINO, LSND as well as CHARM II
experiments. Our results provide more stringent bounds to some regions of
parameter space.Comment: 22 pages, 6 figures, 2 tables, text improved, fig.6 updated,
references adde
Spectral multigrid methods for elliptic equations 2
A detailed description of spectral multigrid methods is provided. This includes the interpolation and coarse-grid operators for both periodic and Dirichlet problems. The spectral methods for periodic problems use Fourier series and those for Dirichlet problems are based upon Chebyshev polynomials. An improved preconditioning for Dirichlet problems is given. Numerical examples and practical advice are included
Universal Behavior in Large-scale Aggregation of Independent Noisy Observations
Aggregation of noisy observations involves a difficult tradeoff between
observation quality, which can be increased by increasing the number of
observations, and aggregation quality which decreases if the number of
observations is too large. We clarify this behavior for a protypical system in
which arbitrarily large numbers of observations exceeding the system capacity
can be aggregated using lossy data compression. We show the existence of a
scaling relation between the collective error and the system capacity, and show
that large scale lossy aggregation can outperform lossless aggregation above a
critical level of observation noise. Further, we show that universal results
for scaling and critical value of noise which are independent of system
capacity can be obtained by considering asymptotic behavior when the system
capacity increases toward infinity.Comment: 10 pages, 3 figure
Patterns of antihypertensive prescribing, discontinuation and switching among a Hong Kong Chinese population from over one million prescriptions
Hypertension is an alarming public health problem among Chinese. The present study evaluated the prescribing patterns, discontinuation and switching profiles of antihypertensive agents and their associated factors in one Hong Kong Chinese population. Data were retrieved from computerized records for patients prescribed anti-hypertensive agents in government primary care clinics of Hong Kong from January, 2004 to June, 2007. A total of 1,069,836 antihypertensive drug visits, representing 67,028 patients, were analyzed. The most commonly prescribed drugs were Calcium Channel Blockers (CCBs) (49%), b-Blockers (BBs) (46%) and Angiotensin-Converting Enzyme Inhibitors (ACEIs) (19%). Thiazide diuretic prescribing was low (13%) and on the decline (14% in 2004 to 12% in 2007). Prescribing of ACEIs was rising (16% in 2004 to 23% in 2007). Patients’ age, gender, and socio-economic status were independent predictors of class of anti-hypertensive prescribed but explained less than 3.5% of the variation observed. Drug discontinuation was highest for BBs (21%) and lowest for CCBs (12%). The high rates of discontinuation in BBs remained apparent after controlling for confounding variables. Switching was less common than discontinuation and was most likely with thiazide diuretics. To summarize, prescribing of CCBs and BBs were high and that of thiazide diuretics particularly low in this Chinese population when compared with international trends. CCBs may be a particularly favorable antihypertensive treatment in Chinese, given the high discontinuation rates of BBs and international guidelines advising against the use of BBs as first-line therapy. The low use of thiazide diuretics warrants further clinical and cost effectiveness studies among Chinese
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