3,781 research outputs found
Non-invasive assessment of pulmonary vascular resistance in pulmonary hypertension: Current knowledge and future direction
Pulmonary Hypertension (PHT) is relatively common, dangerous and under-recognised. Pulmonary hypertension is not a diagnosis in itself; it is caused by a number of differing diseases each with different treatments and prognoses. Therefore, timely and accurate recognition of the underlying cause for PHT is essential for appropriate management. This is especially true for patients with Pulmonary Arterial Hypertension (PAH) in the current era of disease-specific drug therapy.
Measurement of Pulmonary Vascular Resistance (PVR) helps separate pre-capillary from post-capillary PHT, and is measured with right heart catheterisation (RHC). Echocardiography has been used to derive a number of non-invasive surrogates for PVR, with varying accuracy. Ultimately, the goal of non-invasive assessment of PVR is to separate PHT due to left heart disease from PHT due to increased PVR, to help streamline investigation and subsequent treatment.
In this review, we summarise the physiology and pathophysiology of pulmonary blood flow, the various causes of pulmonary hypertension, and non-invasive surrogates for PVR
The Evolution of Blue Stragglers Formed Via Stellar Collisions
We have used the results of recent smoothed particle hydrodynamic simulations
of colliding stars to create models appropriate for input into a stellar
evolution code. In evolving these models, we find that little or no surface
convection occurs, precluding angular momentum loss via a magnetically-driven
stellar wind as a viable mechanism for slowing rapidly rotating blue stragglers
which have been formed by collisions. Angular momentum transfer to either a
circumstellar disk (possibly collisional ejecta) or a nearby companion are
plausible mechanisms for explaining the observed low rotation velocities of
blue stragglers. Under the assumption that the blue stragglers seen in NGC 6397
and 47 Tuc have been created solely by collisions, we find that the majority of
these blue stragglers cannot have been highly mixed by convection or meridional
circulation currents at anytime during their evolution. Also, on the basis of
the agreement between the predictions of our non-rotating models and the
observed blue straggler distribution, the evolution of blue stragglers is
apparently not dominated by the effects of rotation.Comment: 36 pages, including 1 table and 7 postscript figures (LaTeX2e). Also
avaliable at http://astrowww.phys.uvic.ca/~ouellet/ . Accepted for
publication in A
Particle size of tobacco mosaic virus
Publication authorized June 22, 1938.Includes bibliographical references (pages 17-18)
Adaptive Lévy processes and area-restricted search in human foraging
A considerable amount of research has claimed that animals’ foraging behaviors display movement lengths with power-law distributed tails, characteristic of Lévy flights and Lévy walks. Though these claims have recently come into question, the proposal that many animals forage using Lévy processes nonetheless remains. A Lévy process does not consider when or where resources are encountered, and samples movement lengths independently of past experience. However, Lévy processes too have come into question based on the observation that in patchy resource environments resource-sensitive foraging strategies, like area-restricted search, perform better than Lévy flights yet can still generate heavy-tailed distributions of movement lengths. To investigate these questions further, we tracked humans as they searched for hidden resources in an open-field virtual environment, with either patchy or dispersed resource distributions. Supporting previous research, for both conditions logarithmic binning methods were consistent with Lévy flights and rank-frequency methods–comparing alternative distributions using maximum likelihood methods–showed the strongest support for bounded power-law distributions (truncated Lévy flights). However, goodness-of-fit tests found that even bounded power-law distributions only accurately characterized movement behavior for 4 (out of 32) participants. Moreover, paths in the patchy environment (but not the dispersed environment) showed a transition to intensive search following resource encounters, characteristic of area-restricted search. Transferring paths between environments revealed that paths generated in the patchy environment were adapted to that environment. Our results suggest that though power-law distributions do not accurately reflect human search, Lévy processes may still describe movement in dispersed environments, but not in patchy environments–where search was area-restricted. Furthermore, our results indicate that search strategies cannot be inferred without knowing how organisms respond to resources–as both patched and dispersed conditions led to similar Lévy-like movement distributions
Another Non-segregated Blue Straggler Population in a Globular Cluster: the Case of NGC 2419
We have used a combination of ACS-HST high-resolution and wide-field SUBARU
data in order to study the Blue Straggler Star (BSS) population over the entire
extension of the remote Galactic globular cluster NGC 2419. The BSS population
presented here is among the largest ever observed in any stellar system, with
more than 230 BSS in the brightest portion of the sequence. The radial
distribution of the selected BSS is essentially the same as that of the other
cluster stars. In this sense the BSS radial distribution is similar to that of
omega Centauri and unlike that of all Galactic globular clusters studied to
date, which have highly centrally segregated distributions and, in most cases,
a pronounced upturn in the external regions. As in the case of omega Centauri,
this evidence indicates that NGC 2419 is not yet relaxed even in the central
regions. This observational fact is in agreement with estimated half-mass
relaxation time, which is of the order of the cluster age.Comment: in press in the Ap
The surprising external upturn of the Blue Straggler radial distribution in M55
By combining high-resolution HST and wide-field ground based observations, in
ultraviolet and optical bands, we study the Blue Straggler Star (BSS)
population of the low density galactic globular cluster M55 (NGC 6809) over its
entire radial extent. The BSS projected radial distribution is found to be
bimodal, with a central peak, a broad minimum at intermediate radii, and an
upturn at large radii. Similar bimodal distributions have been found in other
globular clusters (M3, 47 Tucanae, NGC 6752, M5), but the external upturn in
M55 is the largest found to date. This might indicate a large fraction of
primordial binaries in the outer regions of M55, which seems somehow in
contrast with the relatively low (\sim 10%) binary fraction recently measured
in the core of this cluster.Comment: in press on Ap
Far-Ultraviolet Surveys of Globular Clusters: Hunting for the Products of Stellar Collisions and Near Misses
Globular clusters are gravitationally bound stellar systems containing on the
order of 100,000 stars. Due to the high stellar densities in the cores of these
clusters, close encounters and even physical collisions between stars are
inevitable. These dynamical interactions can produce exotic types of single and
binary stars that are extremely rare in the galactic field, but which may be
important to the dynamical evolution of their host clusters. A common feature
of these dynamically-formed stellar populations is that many of their members
are relatively hot, and thus bright in the far-ultraviolet (FUV) waveband. In
this short review, I describe how space-based FUV observations are being used
to find and study these populations.Comment: 15 pages, 6 figures; invited "Brief Review" for Modern Physics
Letters
The ACS Virgo Cluster Survey III. Chandra and HST Observations of Low-Mass X-Ray Binaries and Globular Clusters in M87
The ACIS instrument on board the Chandra X-ray Observatory has been used to
carry out the first systematic study of low-mass X-ray binaries (LMXBs) in M87.
We identify 174 X-ray point-sources, of which ~150 are likely LMXBs. This LMXB
catalog is combined with deep F475W and F850LP images taken with ACS on HST to
examine the connection between LMXBs and globular clusters in M87. Of the 1688
globular clusters in our catalog, f_X = 3.6 +- 0.5% contain a LMXB and we find
that the metal-rich clusters are 3 +- 1 times more likely to harbor a LMXB than
their metal-poor counterparts. In agreement with previous findings for other
galaxies, we find that brighter, more metal-rich clusters are more likely to
contain a LMXB. For the first time, however, we are able to demonstrate that
the probability, p_X, that a given cluster will contain a LMXB depends
sensitively on the dynamical properties of the host cluster. Specifically, we
use the HST images to measure the half-light radius, concentration index and
central density, \rho_0, for each globular, and define a parameter, \Gamma,
which is related to the tidal capture and binary-neutron star exchange rate.
Our preferred form for p_X is then p_X \propto \Gamma \rho_0^{-0.42\pm0.11}
(Z/Z_{\odot})^{0.33\pm0.1}. We argue that if the form of p_X is determined by
dynamical processes, then the observed metallicity dependence is a consequence
of an increased number of neutron stars per unit mass in metal-rich globular
clusters. Finally, we find no compelling evidence for a break in the luminosity
distribution of resolved X-ray point sources. Instead, the LMXB luminosity
function is well described by a power law with an upper cutoff at L_X ~ 10^39
erg/s. (abridged)Comment: 23 pages, 21 figures. Accepted for publication in ApJ. Also available
at http://www.physics.rutgers.edu/~pcote/acs/publications.htm
Initial Reports of the Deep Sea Drilling Project, vol. 85
Covering Leg 85 of the cruises of the Drilling Vessel Glomar Challenger Los Angeles, California, to Honolulu, Hawaii March-April 1982.
Includes six chapters:
1. INTRODUCTION: BACKGROUND AND EXPLANATORY NOTES, DEEP SEA DRILLING PROJECT LEG 85, CENTRAL EQUATORIAL PACIFIC
2. SITE 571
3. SITE 572
4. SITE 573
5. SITE 574
6. SITE 57
Traffic agents for improving QoS in mixed infrastructure and ad hoc modes wireless LAN
As an important complement to infrastructured wireless networks, mobile ad hoc networks (MANET) are more flexible in providing wireless access services, but more difficult in meeting different quality of service (QoS) requirements for mobile customers. Both infrastructure and ad hoc network structures are supported in wireless local area networks (WLAN), which can offer high data-rate wireless multimedia services to the mobile stations (MSs) in a limited geographical area. For those out-of-coverage MSs, how to effectively connect them to the access point (AP) and provide QoS support is a challenging issue. By mixing the infrastructure and the ad hoc modes in WLAN, we propose in this paper a new coverage improvement scheme that can identify suitable idle MSs in good service zones as traffic agents (TAs) to relay traffic from those out-of-coverage MSs to the AP. The service coverage area of WLAN is then expanded. The QoS requirements (e.g., bandwidth) of those MSs are considered in the selection process of corresponding TAs. Mathematical analysis, verified by computer simulations, shows that the proposed TA scheme can effectively reduce blocking probability when traffic load is light
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