Global Alfven Wave Heating of the Magnetosphere of Young Stars

Excitation of a Global Alfven wave (GAW) is proposed as a viable mechanism to explain plasma heating in the magnetosphere of young stars. The wave and basic plasma parameters are compatible with the requirement that the dissipation length of GAWs be comparable to the distance between the shocked region at the star's surface and the truncation region in the accretion disk. A two-fluid magnetohydrodynamic plasma model is used in the analysis. A current carrying filament along magnetic field lines acts as a waveguide for the GAW. The current in the filament is driven by plasma waves along the magnetic field lines and/or by plasma crossing magnetic field lines in the truncated region of the disk of the accreting plasma. The conversion of a small fraction of the kinetic energy into GAW energy is sufficient to heat the plasma filament to observed temperatures.Comment: Submitted to ApJ, aheatf.tex, 2 figure

Ellerman bombs and UV bursts: transient events in chromospheric current sheets

Ellerman bombs (EBs) and UV bursts are both brightenings related to flux emergence regions and specifically to magnetic flux of opposite polarity that meet in the photosphere. These two reconnection-related phenomena, nominally formed far apart, occasionally occur in the same location and at the same time, thus challenging our understanding of reconnection and heating of the lower solar atmosphere. We consider the formation of an active region, including long fibrils and hot and dense coronal plasma. The emergence of a untwisted magnetic flux sheet, injected $2.5$~Mm below the photosphere, is studied as it pierces the photosphere and interacts with the preexisting ambient field. Specifically, we aim to study whether EBs and UV bursts are generated as a result of such flux emergence and examine their physical relationship. The Bifrost radiative magnetohydrodynamics code was used to model flux emerging into a model atmosphere that contained a fairly strong ambient field, constraining the emerging field to a limited volume wherein multiple reconnection events occur as the field breaks through the photosphere and expands into the outer atmosphere. Synthetic spectra of the different reconnection events were computed using the $1.5$D RH code and the fully 3D MULTI3D code. The formation of UV bursts and EBs at intensities and with line profiles that are highly reminiscent of observed spectra are understood to be a result of the reconnection of emerging flux with itself in a long-lasting current sheet that extends over several scale heights through the chromosphere. Synthetic diagnostics suggest that there are no compelling reasons to assume that UV bursts occur in the photosphere. Instead, EBs and UV bursts are occasionally formed at opposite ends of a long current sheet that resides in an extended bubble of cool gas.Comment: 10 pages, 8 figures, accepted by A&

Tkachenko polarons in vortex lattices

We analyze the properties of impurities immersed in a vortex lattice formed by ultracold bosons in the mean field quantum Hall regime. In addition to the effects of a periodic lattice potential, the impurity is dressed by collective modes with parabolic dispersion (Tkachenko modes). We derive the effective polaron model, which contains a marginal impurity-phonon interaction. The polaron spectral function exhibits a Lorentzian broadening for arbitrarily small wave vectors even at zero temperature, in contrast with the result for optical or acoustic phonons. The anomalous damping of Tkachenko polarons could be detected experimentally using momentum-resolved spectroscopy.Comment: 10 pages, 2 figure

Confined states and direction-dependent transmission in graphene quantum wells

We report the existence of confined massless fermion states in a graphene quantum well (QW) by means of analytical and numerical calculations. These states show an unusual quasi-linear dependence on the momentum parallel to the QW: their number depends on the wavevector and is constrained by electron-hole conversion in the barrier regions. An essential difference with non-relativistic electron states is a mixing between free and confined states at the edges of the free-particle continua, demonstrated by the direction-dependent resonant transmission across a potential well.Comment: Submitted to PR

Population expansion in the North African Late Pleistocene signalled by mitochondrial DNA haplogroup U6

Background <br/> The archaeology of North Africa remains enigmatic, with questions of population continuity versus discontinuity taking centre-stage. Debates have focused on population transitions between the bearers of the Middle Palaeolithic Aterian industry and the later Upper Palaeolithic populations of the Maghreb, as well as between the late Pleistocene and Holocene. <br/> Results Improved resolution of the mitochondrial DNA (mtDNA) haplogroup U6 phylogeny, by the screening of 39 new complete sequences, has enabled us to infer a signal of moderate population expansion using Bayesian coalescent methods. To ascertain the time for this expansion, we applied both a mutation rate accounting for purifying selection and one with an internal calibration based on four approximate archaeological dates: the settlement of the Canary Islands, the settlement of Sardinia and its internal population re-expansion, and the split between haplogroups U5 and U6 around the time of the first modern human settlement of the Near East. <br/> Conclusions <br/> A Bayesian skyline plot placed the main expansion in the time frame of the Late Pleistocene, around 20 ka, and spatial smoothing techniques suggested that the most probable geographic region for this demographic event was to the west of North Africa. A comparison with U6's European sister clade, U5, revealed a stronger population expansion at around this time in Europe. Also in contrast with U5, a weak signal of a recent population expansion in the last 5,000 years was observed in North Africa, pointing to a moderate impact of the late Neolithic on the local population size of the southern Mediterranean coast

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

Elastic properties of carbon nanotubes and their heterojunctions

Comprehensive studies on the modelling and numerical simulation of the mechanical behaviour under tension, bending and torsion of single-walled carbon nanotubes and their heterojunctions are performed. It is proposed to deduce the mechanical properties of the carbon nanotubes heterojunctions from the knowledge of the mechanical properties of the single-walled carbon nanotubes, which are their constituent key unit

Orthogonality catastrophe and Kondo effect in graphene

Anderson's orthogonality catastrophe in graphene, at energies close to the Dirac point, is analyzed. It is shown that, in clean systems, the orthogonality catastrophe is suppressed, due to the vanishing density of states at the Dirac point. In the presence of preexisting localized states at the Dirac energy, the orthogonality catastrophe shows similar features to those found in normal metals with a finite density of states at the Fermi level. The implications for the Kondo effect induced by magnetic impurities, and for the Fermi edge singularities in tunneling processes are also discussed.Comment: 7 pages, 7 figure