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

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 Peptidoglycan-Binding Protein FimV Promotes Assembly of the Pseudomonas aeruginosa Type IV Pilus Secretin▿

The Pseudomonas aeruginosa inner membrane protein FimV is among several proteins of unknown function required for type IV pilus-mediated twitching motility, arising from extension and retraction of pili from their site of assembly in the inner membrane. The pili transit the periplasm and peptidoglycan (PG) layer, ultimately exiting the cell through the PilQ secretin. Although fimV mutants are nonmotile, they are susceptible to killing by pilus-specific bacteriophage, a hallmark of retractable surface pili. Here we show that levels of recoverable surface pili were markedly decreased in fimV pilT retraction-deficient mutants compared with levels in the pilT control, demonstrating that FimV acts at the level of pilus assembly. Levels of inner membrane assembly subcomplex proteins PilM/N/O/P were decreased in fimV mutants, but supplementation of these components in trans did not restore pilus assembly or motility. Loss of FimV dramatically reduced the levels of the PilQ secretin multimer through which pili exit the cell, in part due to decreased levels of PilQ monomers, while PilF pilotin levels were unchanged. Expression of pilQ in trans in the wild type or fimV mutants increased total PilQ monomer levels but did not alter secretin multimer levels or motility. PG pulldown assays showed that the N terminus of FimV bound PG in a LysM motif-dependent manner, and a mutant with an in-frame chromosomal deletion of the LysM motif had reduced motility, secretin levels, and surface piliation. Together, our data show that FimV's role in pilus assembly is to promote secretin formation and that this function depends upon its PG-binding domain

Long-term in vitro 2D-culture of SDHB and SDHD-related human paragangliomas and pheochromocytomas

The neuroendocrine tumours paraganglioma and pheochromocytoma (PPGLs) are commonly associated with succinate dehydrogenase (SDH) gene variants, but no human SDH-related PPGL-derived cell line has been developed to date. The aim of this study was to systematically explore practical issues related to the classical 2D-culture of SDH-related human paragangliomas and pheochromocytomas, with the ultimate goal of identifying a viable tumour-derived cell line. PPGL tumour tissue/cells (chromaffin cells) were cultured in a variety of media formulations and supplements. Tumour explants and dissociated primary tumour cells were cultured and stained with a range of antibodies to identify markers suitable for use in human PPGL culture. We cultured 62 PPGLs, including tumours with confirmed SDHB, SDHC and SDHD variants, as well as several metastatic tumours. Testing a wide range of basic cell culture media and supplements, we noted a marked decline in chromaffin cell numbers over a 4-8 week period but the persistence of small numbers of synaptophysin/tyrosine hydroxylase-positive chromaffin cells for up to 99 weeks. In cell culture, immunohistochemical staining for chromogranin A and neuron-specific enolase was generally negative in chromaffin cells, while staining for synaptophysin and tyrosine hydroxylase was generally positive. GFAP showed the most consistent staining of type II sustentacular cells. Of the media tested, low serum or serum-free media best sustained relative chromaffin cell numbers, while lactate enhanced the survival of synaptophysin-positive cells. Synaptophysin-positive PPGL tumour cells persist in culture for long periods but show little evidence of proliferation. Synaptophysin was the most consistent cell marker for chromaffin cells and GFAP the best marker for sustentacular cells in human PPGL cultures