The evolution of the jet from Herbig Ae star HD 163296 from 1999 to 2011

Young A and B stars, the so-called Herbig Ae/Be stars (HAeBe), are surrounded by an active accretion disk and drive outflows. We study the jet HH 409, which is launched from the HAeBe star HD 163296, using new and archival observations from Chandra and HST/STIS. In X-rays we can show that the central source is not significantly extended. The approaching jet, but not the counter-jet, is detected in Ly alpha. In addition, there is red-shifted Ly alpha emission extended in the same direction as the jet, that is also absent in the counter-jet. We can rule out an accretion or disk-wind origin for this feature. In the optical we find the knots B and B2 in the counter-jet. Knot B has been observed previously, so we can derive its proper motion of 0.37+-0.01 arcsec/yr. Its electron density is 3000/cm^3, thus the cooling time scale is a few months only, so the knot needs to be reheated continuously. The shock speed derived from models of H alpha and forbidden emission lines (FELs) decreased from 50 km/s in 1999 to 30 km/s in 2011 because the shock front loses energy as it travels along the jet. Knot B2 is observed at a similar position in 2011 as knot B was in 1999, but shows a lower ionization fraction and higher mass loss rate, proving variations in the jet launching conditions.Comment: 14 pages, 8 figures, accepted by A&

The Ekman-Hartmann layer in MHD Taylor-Couette flow

We study magnetic effects induced by rigidly rotating plates enclosing a cylindrical MHD Taylor-Couette flow at the finite aspect ratio $H/D=10$. The fluid confined between the cylinders is assumed to be liquid metal characterized by small magnetic Prandtl number, the cylinders are perfectly conducting, an axial magnetic field is imposed \Ha \approx 10, the rotation rates correspond to \Rey of order $10^2-10^3$. We show that the end-plates introduce, besides the well known Ekman circulation, similar magnetic effects which arise for infinite, rotating plates, horizontally unbounded by any walls. In particular there exists the Hartmann current which penetrates the fluid, turns into the radial direction and together with the applied magnetic field gives rise to a force. Consequently the flow can be compared with a Taylor-Dean flow driven by an azimuthal pressure gradient. We analyze stability of such flows and show that the currents induced by the plates can give rise to instability for the considered parameters. When designing an MHD Taylor-Couette experiment, a special care must be taken concerning the vertical magnetic boundaries so they do not significantly alter the rotational profile.Comment: 9 pages, 6 figures; accepted to PR

Dynamics of a Heisenberg spin chain in the quantum critical regime: NMR experiment versus effective field theory

A comprehensive comparison between the magnetic field- and temperature-dependent low frequency spin dynamics in the antiferromagnetic spin-1/2 Heisenberg chain (AFHC) system copper pyrazine dinitrate, probed via the 13C-nuclear magnetic resonance (NMR) relaxation rate 1/T1, and the field theoretical approach in the Luttinger liquid (LL) regime has been performed. We have found a very good agreement between the experiment and theory in the investigated temperature and field range. Our results demonstrate how strongly the quantum critical point affects the spin dynamics of Heisenberg spin chain compounds.Comment: 5 pages, 3 figure

Chandra observation of Cepheus A: The diffuse emission of HH 168 resolved

X-ray emission from massive stellar outflows has been detected in several cases. We present a Chandra observation of HH 168 and show that the soft X-ray emission from a plasma of 0.55 keV within HH 168 is diffuse. The X-ray emission is observed on two different scales: Three individual, yet extended, regions are embedded within a complex of low X-ray surface brightness. Compared to the bow shock the emission is displaced against the outflow direction. We show that there is no significant contribution from young stellar objects (YSOs) and discuss several shock scenarios that can produce the observed signatures. We establish that the X-ray emission of HH 168 is excited by internal shocks in contrast to simple models, which expect the bow shock to be the most X-ray luminous.Comment: 8 pages, 5 figures, accepted for publication in A&

Structural, magnetic, electric, dielectric, and thermodynamic properties of multiferroic GeV4S8

The lacunar spinel GeV4S8 undergoes orbital and ferroelectric ordering at the Jahn-Teller transition around 30 K and exhibits antiferromagnetic order below about 14 K. In addition to this orbitally driven ferroelectricity, lacunar spinels are an interesting material class, as the vanadium ions form V4 clusters representing stable molecular entities with a common electron distribution and a well-defined level scheme of molecular states resulting in a unique spin state per V4 molecule. Here we report detailed x-ray, magnetic susceptibility, electrical resistivity, heat capacity, thermal expansion, and dielectric results to characterize the structural, electric, dielectric, magnetic, and thermodynamic properties of this interesting material, which also exhibits strong electronic correlations. From the magnetic susceptibility, we determine a negative Curie-Weiss temperature, indicative for antiferromagnetic exchange and a paramagnetic moment close to a spin S = 1 of the V4 molecular clusters. The low-temperature heat capacity provides experimental evidence for gapped magnon excitations. From the entropy release, we conclude about strong correlations between magnetic order and lattice distortions. In addition, the observed anomalies at the phase transitions also indicate strong coupling between structural and electronic degrees of freedom. Utilizing dielectric spectroscopy, we find the onset of significant dispersion effects at the polar Jahn-Teller transition. The dispersion becomes fully suppressed again with the onset of spin order. In addition, the temperature dependencies of dielectric constant and specific heat possibly indicate a sequential appearance of orbital and polar order.Comment: 15 pages, 9 figure

Huygens' Principle for the Klein-Gordon equation in the de Sitter spacetime

In this article we prove that the Klein-Gordon equation in the de Sitter spacetime obeys the Huygens' principle only if the physical mass $m$ of the scalar field and the dimension $n\geq 2$ of the spatial variable are tied by the equation $m^2=(n^2-1)/4$. Moreover, we define the incomplete Huygens' principle, which is the Huygens' principle restricted to the vanishing second initial datum, and then reveal that the massless scalar field in the de Sitter spacetime obeys the incomplete Huygens' principle and does not obey the Huygens' principle, for the dimensions $n=1,3$, only. Thus, in the de Sitter spacetime the existence of two different scalar fields (in fact, with m=0 and $m^2=(n^2-1)/4$), which obey incomplete Huygens' principle, is equivalent to the condition $n=3$ (in fact, the spatial dimension of the physical world). For $n=3$ these two values of the mass are the endpoints of the so-called in quantum field theory the Higuchi bound. The value $m^2=(n^2-1)/4$ of the physical mass allows us also to obtain complete asymptotic expansion of the solution for the large time. Keywords: Huygens' Principle; Klein-Gordon Equation; de Sitter spacetime; Higuchi Boun