703 research outputs found
On the Ionisation of Warm Opaque Interstellar Clouds and the Intercloud Medium
In this paper we use a number of observations to construct an integrated
picture of the ionisation in the interiors of quiescent warm opaque
interstellar clouds and in the intercloud medium (ICM) outside dense HII
regions and hot dilute bubbles. Our main conclusion is that within 1kpc
of the sun the ionisation rate of hydrogen per unit volume in both the
interiors of such clouds and in the ICM is independent of the local density of
neutral hydrogen, and varies with position by less than 20 per cent.
These conclusions strongly favour the decaying neutrino hypothesis for the
ionisation of the interstellar medium in these regions.
Our analysis is based on a variety of observations, of which the most
remarkable is the discovery by Spitzer and Fitzpatrick (1993) that, in the four
slowly moving clouds along the line of sight to the halo star HD93521, the
column densities of both SII and CII, which individually range over a
factor 4, are proportional to the column density of HI to within 20
per cent. This proportionality is used to show that the free electrons exciting
the CII to CII are located mainly in the interiors of the clouds, rather
than in their skins, despite the large opacity of the clouds to Lyman continuum
radiation. The same conclusion also follows more unambiguously from the low
value of the H flux in this direction which was found by Reynolds
(1996) in unpublished observations.
These results are then used, in conjunction with observations of three pulsar
parallaxes and dispersion measures, and with data on HeI, NII and OI line
emissions, to constrain the ionisation of H, He, N and O and the flux of Lyman
continuum photons from O stars in the ICM.Comment: 16 pages, no figures, Latex fil
Factors that delay active construction projects and potential solutions
The South Carolina Department of Transportation constantly pursues more effective ways to complete their projects on time. There are 6 major reasons why construction projects have been delayed in South Carolina. This project was chosen to identify the reasons why delays have occurred to active construction projects and explore potential solutions to these problems
The Plant-Associated Microbe Gene Ontology (PAMGO) Consortium: community development of new Gene Ontology terms describing biological processes involved in microbe-host interactions
All microbes that form beneficial, neutral, or pathogenic associations with hosts face similar challenges. They must physically adhere to and/or gain entry to host tissues; they must avoid, suppress, or tolerate host defenses; they must acquire nutrients from the host and successfully multiply. Microbes that associate with hosts come from many kingdoms of life and include bacteria, fungi, oomycetes, and nematodes. The increasing numbers of full genome sequences from these diverse microbes provide the opportunity to discover common mechanisms by which the microbes forge and maintain intimate associations with host organisms. However, cross-genome analyses have been hindered by lack of a universal vocabulary for describing biological processes involved in the interplay between microbes and their hosts. The Plant-Associated Microbe Gene Ontology (PAMGO) Consortium has been working for three years as an official interest group of the Gene Ontology (GO) Consortium to develop well-defined GO terms that describe many of the biological processes common to diverse plant- and animal-associated microbes. Creating these terms, over 700 at this time, has required a synthesis of diverse points of view from many research communities. The use of these terms in genome annotation will allow cross-genome searches for genes with common function (without demand for sequence similarity) and also improve the interpretation of data from high-throughput microarray and proteomic analyses. This article, and the more focused mini-reviews that make up this supplement to BMC Microbiology, describe the development and use of these terms
Effects of correlated turbulent velocity fields on the formation of maser lines
The microturbulent approximation of turbulent motions is widely used in
radiative transfer calculations. Mainly motivated by its simple computational
application it is probably in many cases an oversimplified treatment of the
dynamical processes involved. This aspect is in particular important in the
analysis of maser lines, since the strong amplification of radiation leads to a
sensitive dependence of the radiation field on the overall velocity structure.
To demonstrate the influence of large scale motions on the formation of maser
lines we present a simple stochastic model which takes velocity correlations
into account. For a quantitative analysis of correlation effects, we generate
in a Monte Carlo simulation individual realizations of a turbulent velocity
field along a line of sight. Depending on the size of the velocity correlation
length we find huge deviations between the resulting random profiles in respect
of line shape, intensity and position of single spectral components. Finally,
we simulate the emission of extended maser sources. A qualitative comparison
with observed masers associated with star forming regions shows that our model
can reproduce the observed general spectral characteristics. We also
investigate shortly, how the spectra are effected when a systematic velocity
field (simulating expansion) is superposed on the fluctuations. Our results
convincingly demonstrate that hydrodynamical motions are of great importance
for the understanding of cosmic masers.Comment: Accepted for publication in A&A. 8 pages, 8 figure
High signal-to-noise ratio observations and the ultimate limits of precision pulsar timing
We demonstrate that the sensitivity of high-precision pulsar timing
experiments will be ultimately limited by the broadband intensity modulation
that is intrinsic to the pulsar's stochastic radio signal. That is, as the peak
flux of the pulsar approaches that of the system equivalent flux density,
neither greater antenna gain nor increased instrumental bandwidth will improve
timing precision. These conclusions proceed from an analysis of the covariance
matrix used to characterise residual pulse profile fluctuations following the
template matching procedure for arrival time estimation. We perform such an
analysis on 25 hours of high-precision timing observations of the closest and
brightest millisecond pulsar, PSR J0437-4715. In these data, the standard
deviation of the post-fit arrival time residuals is approximately four times
greater than that predicted by considering the system equivalent flux density,
mean pulsar flux and the effective width of the pulsed emission. We develop a
technique based on principal component analysis to mitigate the effects of
shape variations on arrival time estimation and demonstrate its validity using
a number of illustrative simulations. When applied to our observations, the
method reduces arrival time residual noise by approximately 20%. We conclude
that, owing primarily to the intrinsic variability of the radio emission from
PSR J0437-4715 at 20 cm, timing precision in this observing band better than 30
- 40 ns in one hour is highly unlikely, regardless of future improvements in
antenna gain or instrumental bandwidth. We describe the intrinsic variability
of the pulsar signal as stochastic wideband impulse modulated self-noise
(SWIMS) and argue that SWIMS will likely limit the timing precision of every
millisecond pulsar currently observed by Pulsar Timing Array projects as larger
and more sensitive antennae are built in the coming decades.Comment: 16 pages, 9 figures, accepted for publication in MNRAS. Updated
version: added DOI and changed manuscript to reflect changes in the final
published versio
VLBI Imaging of Water Maser Emission from the Nuclear Torus of NGC 1068
We have made the first VLBI synthesis images of the H2O maser emission
associated with the central engine of the Seyfert galaxy NGC 1068. Emission
extends about +/-300 km/s from the systemic velocity. Images with
submilliarcsecond angular resolution show that the red-shifted emission lies
along an arc to the northwest of the systemic emission. (The blue-shifted
emission has not yet been imaged with VLBI.) Based on the maser velocities and
the relative orientation of the known radio jet, we propose that the maser
emission arises on the surface of a nearly edge-on torus, where physical
conditions are conducive to maser action. The visible part of the torus is
axially thick, with comparable height and radius. The velocity field indicates
sub-Keplerian differential rotation around a central mass of about 1e7 Msun
that lies within a cylindrical radius of about 0.65 pc. The estimated
luminosity of the central engine is about 0.5 of the Eddington limit. There is
no detectable compact radio continuum emission near the proposed center of the
torus (T_B< 5e6 K on size scales of about 0.1 pc), so that the observed
flat-spectrum core cannot be direct self-absorbed synchrotron radiation.Comment: 12 pages, 4 figures. To appear in ApJ Part 2. Also available at
http://www.physics.ucsb.edu/~vlbiweb
Programmed cell death in host-symbiont associations, viewed through the Gene Ontology
Manipulation of programmed cell death (PCD) is central to many host microbe interactions. Both plant and animal cells use PCD as a powerful weapon against biotrophic pathogens, including viruses, which draw their nutrition from living tissue. Thus, diverse biotrophic pathogens have evolved many mechanisms to suppress programmed cell death, and mutualistic and commensal microbes may employ similar mechanisms. Necrotrophic pathogens derive their nutrition from dead tissue, and many produce toxins specifically to trigger programmed cell death in their hosts. Hemibiotrophic pathogens manipulate PCD in a most exquisite way, suppressing PCD during the biotrophic phase and stimulating it during the necrotrophic phase. This mini-review will summarize the mechanisms that have evolved in diverse microbes and hosts for controlling PCD and the Gene Ontology terms developed by the Plant-Associated Microbe Gene Ontology (PAMGO) Consortium for describing those mechanisms
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