76 research outputs found
The role of damped Alfven waves on magnetospheric accretion models of young stars
We examine the role of Alfven wave damping in heating the plasma in the
magnetic funnels of magnetospheric accretion models of young stars. We study
four different damping mechanisms of the Alfven waves: nonlinear, turbulent,
viscous-resistive and collisional. Two different possible origins for the
Alfven waves are discussed: 1) Alfven waves generated at the surface of the
star by the shock produced by the infalling matter; and 2) Alfven waves
generated locally in the funnel by the Kelvin-Helmholtz instability. We find
that, in general, the damping lengths are smaller than the tube length. Since
thermal conduction in the tube is not efficient, Alfven waves generated only at
the star's surface cannot heat the tube to the temperatures necessary to fit
the observations. Only for very low frequency Alfven waves ~10^{-5} the ion
cyclotron frequency, is the viscous-resistive damping length greater than the
tube length. In this case, the Alfven waves produced at the surface of the star
are able to heat the whole tube. Otherwise, local production of Alfven waves is
required to explain the observations. The turbulence level is calculated for
different frequencies for optically thin and thick media. We find that
turbulent velocities varies greatly for different damping mechanisms, reaching
\~100 km s^{-1} for the collisional damping of small frequency waves.Comment: 29 pages, 12 figures, to appear in The Astrophysical Journa
Heat stress in a temperate climate leads to adapted sensor-based behavioral patterns of dairy cows
Most research on heat stress has focused on (sub)tropical climates. The effects of higher ambient temperatures on the daily behavior of dairy cows in a maritime and temperate climate are less studied. With this retrospective observational study, we address that gap by associating the daily time budgets of dairy cows in the Netherlands with daily temperature and temperature-humidity index (THI) variables. During a period of 4 years, cows on 8 commercial dairy farms in the Netherlands were equipped with neck and leg sensors to collect data from 4,345 cow lactations regarding their daily time budget. The time spent eating, ruminating, lying, standing, and walking was recorded. Individual cow data were divided into 3 data sets: (1) lactating cows from 5 farms with a conventional milking system (CMS) and pasture access, (2) lactating cows from 3 farms with an automatic milking system (AMS) without pasture access, and (3) dry cows from all 8 farms. Hourly environment temperature and relative humidity data from the nearest weather station of the Dutch National Weather Service was used for THI calculation for each farm. Based on heat stress thresholds from previous studies, daily mean temperatures were grouped into 7 categories: 0 = (<0°C), 1 = (0â12°C, reference category), 2 = (12â16°C), 3 = (16â20°C), 4 = (20â24°C), 5 = (24â28°C), and 6 = (â„28°C). Temperature-humidity index values were grouped as follows: 0 = (THI <30), 1 = (THI 30â56, reference category), 2 = (THI 56â60), 3 = (THI 60â64), 4 = (THI 64â68), 5 = (THI 68â72) and 6 = (THI â„72). To associate daily mean temperature and THI with sensor-based behavioral parameters of dry cows and of lactating cows from AMS and CMS farms, we used generalized linear mixed models. In addition, associations between sensor data and other climate variables, such as daily maximum and minimum temperature, and THI were analyzed. On the warmest days, eating time decreased in the CMS group by 92 min/d, in the AMS group by 87 min/d, and in the dry group by 75 min/d compared with the reference category. Lying time decreased in the CMS group by 36 min/d, in the AMS group by 56 min/d, and in the dry group by 33 min/d. Adaptation to daily temperature and THI was already noticeable from a mean temperature of 12°C or a mean THI of 56 or above, when dairy cows started spending less time lying and eating and spent more time standing. Further, rumination time decreased, although only in dry cows and cows on AMS farms. With higher values for daily mean THI and temperature, walking time decreased as well. These patterns were very similar for temperature and THI variables. These results show that dairy cows in temperate climates begin to adapt their behavior at a relatively low mean environmental temperature or THI. In the temperate maritime climate of the Netherlands, our results indicate that daily mean temperature suffices to study the effects of behavioral adaptation to heat stress in dairy cows
A holin and an endopeptidase are essential for chitinolytic protein secretion in <i>Serratia marcescens</i>
Pathogenic bacteria adapt to their environment and manipulate the biochemistry of hosts by secretion of effector molecules. Serratia marcescens is an opportunistic pathogen associated with healthcare-acquired infections and is a prolific secretor of proteins, including three chitinases (ChiA, ChiB, and ChiC) and a chitin binding protein (Cbp21). In this work, genetic, biochemical, and proteomic approaches identified genes that were required for secretion of all three chitinases and Cbp21. A genetic screen identified a holin-like protein (ChiW) and a putative l-alanyl-d-glutamate endopeptidase (ChiX), and subsequent biochemical analyses established that both were required for nonlytic secretion of the entire chitinolytic machinery, with chitinase secretion being blocked at a late stage in the mutants. In addition, live-cell imaging experiments demonstrated bimodal and coordinated expression of chiX and chiA and revealed that cells expressing chiA remained viable. It is proposed that ChiW and ChiX operate in tandem as components of a protein secretion system used by gram-negative bacteria
Turbulence-driven Polar Winds from T Tauri Stars Energized by Magnetospheric Accretion
Pre-main-sequence stars are observed to be surrounded by both accretion flows
and some kind of wind or jet-like outflow. Recent work by Matt and Pudritz has
suggested that if classical T Tauri stars exhibit stellar winds with mass loss
rates about 0.1 times their accretion rates, the wind can carry away enough
angular momentum to keep the stars from being spun up unrealistically by
accretion. This paper presents a preliminary set of theoretical models of
accretion-driven winds from the polar regions of T Tauri stars. These models
are based on recently published self-consistent simulations of the Sun's
coronal heating and wind acceleration. In addition to the convection-driven MHD
turbulence (which dominates in the solar case), we add another source of wave
energy at the photosphere that is driven by the impact of plasma in neighboring
flux tubes undergoing magnetospheric accretion. This added energy, determined
quantitatively from the far-field theory of MHD wave generation, is sufficient
to produce T Tauri-like mass loss rates of at least 0.01 times the accretion
rate. While still about an order of magnitude below the level required for
efficient angular momentum removal, these are the first self-consistent models
of T Tauri winds that agree reasonably well with a range of observational mass
loss constraints. The youngest modeled stellar winds are supported by Alfven
wave pressure, they have low temperatures ("extended chromospheres"), and they
are likely to be unstable to the formation of counterpropagating shocks and
clumps far from the star.Comment: 19 pages (emulateapj style), 13 figures, ApJ, in press (v. 689,
December 10, 2008
The Enigmatic Esx Proteins:Looking Beyond Mycobacteria
Bacteria export proteins across membranes using a range of transport machineries. Type VII secretion systems (T7SSs), originally described in mycobacteria, are now known to be widespread across diverse bacterial phyla. Recent studies have characterized secretion components and mechanisms of type VII secretion in pathogenic and environmental bacteria. A variety of functions have been attributed to T7SS substrates, including interactions with eukaryotes and with other bacteria. Here, we evaluate the growing body of knowledge on T7SSs, with focus on the nonmycobacterial systems, reviewing their phylogenetic distribution, structure and function in diverse settings
Functional regionalisation of spatial interaction data: an evalution of some suggested strategies
Three alternative approaches (the functional distance approach, the iterative proportional fitting based procedure and the intramax procedure) to the functional regionalisation of spatial interaction data are compared and evaluated in this paper. After a general discussion of the basic properties of each approach, the detailed processes that are involved in each case are illustrated by reference to a worked example. The last section of the paper presents the findings obtained by each of these approaches as a result of empirical studies of Greater London, Merseyside, and the Netherlands. The study as a whole identifies a number of ambiguities in the existing literature which point to the need for a more rigorous examination of current methodology. They also suggest that more attention needs to be given to the role that is played by aggregation procedures as against data transformation procedures in the evaluation process. Although each user must decide which of the three methods best suits his particular purposes, the results obtained by means of the intramax procedure seem generally more readily interpretable than those obtained by either of the other two methods.
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