Stochastic Acceleration in Relativistic Parallel Shocks

(abridged) We present results of test-particle simulations on both the first and the second order Fermi acceleration at relativistic parallel shock waves. We consider two scenarios for particle injection: (i) particles injected at the shock front, then accelerated at the shock by the first order mechanism and subsequently by the stochastic process in the downstream region; and (ii) particles injected uniformly throughout the downstream region to the stochastic process. We show that regardless of the injection scenario, depending on the magnetic field strength, plasma composition, and the employed turbulence model, the stochastic mechanism can have considerable effects on the particle spectrum on temporal and spatial scales too short to be resolved in extragalactic jets. Stochastic acceleration is shown to be able to produce spectra that are significantly flatter than the limiting case of particle energy spectral index -1 of the first order mechanism. Our study also reveals a possibility of re-acceleration of the stochastically accelerated spectrum at the shock, as particles at high energies become more and more mobile as their mean free path increases with energy. Our findings suggest that the role of the second order mechanism in the turbulent downstream of a relativistic shock with respect to the first order mechanism at the shock front has been underestimated in the past, and that the second order mechanism may have significant effects on the form of the particle spectra and its evolution.Comment: 14 pages, 11 figures (9 black/white and 2 color postscripts). To be published in the ApJ (accepted 6 Nov 2004

Direct evidence that maltose transport activity Is affected by the lipid composition of brewer’s yeast

A brewer’s yeast strain was grown with maltose as sole carbon source under strictly anaerobic conditions with and without ergosterol and /or unsaturated fatty acid (Tween 80) supplements. Under all these conditions the MALx1 genes for maltose transporters were strongly expressed during growth. The fatty acid unsaturation indices of growing and stationary phase yeast were increased from about 20% to 56–69% by supplementation with Tween 80. Ergosterol contents were increased up to at least 4- fold by supplementation with ergosterol and Tween 80. Maltose transport activity measured at 20°C was not increased by supplementation with Tween 80 alone, but was increased 2-fold and 3-fold, respectively, in growing and stationary phase yeast by supplementation with ergosterol together with Tween 80. The stimulation of maltose transport by ergosterol was greater when the transport was measured at temperatures (10°C and 0°C) lower than 20°C. The results show that proper function of maltose transporters requires adequate amounts of ergosterol in the yeast. This effect may partly explain the low maltose (and maltotriose) uptake rates both in the second half of brewery fermentations, when the sterol content of yeast has fallen, and when fresh wort is pitched with sterol-deficient cropped yeast.Finnish brewing and malting industry (PBL) ; Leonardo da Vinci programme ; Fundação para a Ciência e a Tecnologia (FCT

Myenteric Neurons Do Not Replicate in Small Intestine Under Normal Physiological Conditions in Adult Mouse

BACKGROUND & AIMS: The enteric nervous system (ENS) is the largest part of the peripheral nervous system; moreover, abnormal ENS development and function are associated with multiple human pathologies. Data from several groups suggest that under normal physiological conditions in adult animals, enteric nerve cells do not replicate. A study by Kulkarni et al in 2017 challenged this view and proposed that nearly 70% of enteric neurons in the myenteric ganglia are born in 1 week The authors of this study suggested that differences in DNA labelling times and DNA denaturation conditions might explain discrepancies with previous reports. Previous studies were carried out using different conditions and labelling techniques in various regions of the gastrointestinal tract; thus, conclusions have remained elusive. METHODS: Here, we have eliminated those variables by analyzing the whole small intestine using the reagents and conditions that Kulkarni et al used. To exclude variables related to immunohistochemistry, we carried out parallel experiments with "click chemistry"-based detection of DNA replication. RESULTS: Although proliferation was readily detected in the epithelium, we found no evidence of neuronal replication in the myenteric ganglia. CONCLUSIONS: We conclude that within 1 week under normal physiological conditions, myenteric neurons in the small intestine do not replicate.Peer reviewe

Splitting times of doubly quantized vortices in dilute Bose-Einstein condensates

Recently, the splitting of a topologically created doubly quantized vortex into two singly quantized vortices was experimentally investigated in dilute atomic cigar-shaped Bose-Einstein condensates [Y. Shin et al., Phys. Rev. Lett. 93, 160406 (2004)]. In particular, the dependency of the splitting time on the peak particle density was studied. We present results of theoretical simulations which closely mimic the experimental set-up. Contrary to previous theoretical studies, claiming that thermal excitations are the essential mechanism in initiating the splitting, we show that the combination of gravitational sag and time dependency of the trapping potential alone suffices to split the doubly quantized vortex in time scales which are in good agreement with the experiments. We also study the dynamics of the resulting singly quantized vortices which typically intertwine--especially, a peculiar vortex chain structure appears for certain parameter values.Comment: 5 pages, 5 figure

Comparison of mean-field theories for vortices in trapped Bose-Einstein condensates

We compute structures of vortex configurations in a harmonically trapped Bose-Einstein condensed atom gas within three different gapless self-consistent mean-field theories. Outside the vortex core region, the density profiles for the condensate and the thermal gas are found to differ only by a few percent between the Hartree-Fock-Bogoliubov-Popov theory and two of its recently proposed gapless extensions. In the core region, however, the differences in the density profiles are substantial. The structural differences are reflected in the energies of the quasiparticle states localized near the vortex core. Especially, the predictions for the energy of the lowest quasiparticle excitation differ considerably between the theoretical models investigated.Comment: 4 pages, 2 figure

Influence of Supercurrents on Low-Temperature Thermopower in Mesoscopic N/S Structures

The thermopower of mesoscopic normal metal/superconductor structures has been measured at low temperatures. Effect of supercurrent present in normal part of the structure was studied in two cases: when it was created by applied external magnetic field and when it was applied directly using extra superconducting electrodes. Temperature and magnetic field dependencies of thermopower are compared to the numerical simulations based on the quasiclassical theory of the superconducting proximity effect.Comment: 21 pages, 12 figures. To be published in the proceedings of the ULTI conference organized in Lammi, Finland (2006

Particle acceleration in thick parallel shocks with high compression ratio

We report studies on first-order Fermi acceleration in parallel modified shock waves with a large scattering center compression ratio expected from turbulence transmission models. Using a Monte Carlo technique we have modeled particle acceleration in shocks with a velocity ranging from nonrelativistic to ultrarelativistic and a thickness extending from nearly steplike to very wide structures exceeding the particle diffusion length by orders of magnitude. The nonrelativistic diffusion approximation is found to be surprisingly accurate in predicting the spectral index of a thick shock with large compression ratio even in the cases involving relativistic shock speeds.Comment: 4 pages, 2 figures, accepted to A&

Toeplitz operators on the unit ball with locally integrable symbols

We study the boundedness of Toeplitz operators T-psi with locally integrable symbols on weighted harmonic Bergman spaces over the unit ball of R-n. Generalizing earlier results for analytic function spaces, we derive a general sufficient condition for the boundedness of T-psi in terms of suitable averages of its symbol. We also obtain a similar "vanishing" condition for compactness. Finally, we show how these results can be transferred to the setting of the standard weighted Bergman spaces of analytic functions.Peer reviewe

Turbulence transmission in parallel modified shocks using ray tracing

We apply a semi-classical approach of handling waves as quasiparticle gas in a slowly varying flow -- analogous to ray tracing -- to calculate the Alfven wave transmission parameters, the resulting cross-helicity of the waves and the scattering-centre compression ratio, for cases where the shock thickness is large enough for the turbulent waves in the plasma to see the transition of the background flow parameters as smooth and slowly varying. For nonrelativistic shocks the wave transmission produces similar effects on the downstream turbulence and the scattering-centre compression ratio as does the transmission through a step shock: the downstream Alfven waves propagate predominantly towards the shock in the local plasma frame and, thus, the scattering-centre compression ratio is larger than the gas compression ratio. For thick relativistic shocks, however, we find qualitative differences with respect to the step-shock case: for low-Alfvenic-Mach-number shocks the downstream waves propagate predominantly away from the shock, and the scattering-centre compression ratio is lower than that of the gas. Thus, when taken into account, the Alfven wave transmission can decrease the efficiency of the first-order Fermi acceleration in a thick relativistic shock.Comment: 6 pages, 5 figures, accepted to A&