Kinematic and Thermal Structure at the onset of high-mass star formation

We want to understand the kinematic and thermal properties of young massive gas clumps prior to and at the earliest evolutionary stages of high-mass star formation. Do we find signatures of gravitational collapse? Do we find temperature gradients in the vicinity or absence of infrared emission sources? Do we find coherent velocity structures toward the center of the dense and cold gas clumps? To determine kinematics and gas temperatures, we used ammonia, because it is known to be a good tracer and thermometer of dense gas. We observed the NH$_3$(1,1) and (2,2) lines within seven very young high-mass star-forming regions with the VLA and the Effelsberg 100m telescope. This allows us to study velocity structures, linewidths, and gas temperatures at high spatial resolution of 3-5$"$, corresponding to $\sim$0.05 pc. We find on average cold gas clumps with temperatures in the range between 10 K and 30 K. The observations do not reveal a clear correlation between infrared emission peaks and ammonia temperature peaks. We report an upper limit for the linewidth of $\sim$1.3 km s$^{-1}$, at the spectral resolution limit of our VLA observation. This indicates a relatively low level of turbulence on the scale of the observations. Velocity gradients are present in almost all regions with typical velocity differences of 1 to 2 km s$^{-1}$ and gradients of 5 to 10 km s$^{-1}$ pc$^{-1}$. These velocity gradients are smooth in most cases, but there is one exceptional source (ISOSS23053), for which we find several velocity components with a steep velocity gradient toward the clump centers that is larger than 30 km s$^{-1}$ pc$^{-1}$. This steep velocity gradient is consistent with recent models of cloud collapse. Furthermore, we report a spatial correlation of ammonia and cold dust, but we also find decreasing ammonia emission close to infrared emission sources.Comment: 20 pages, 10 figure

Discovery of > 200 RR Lyrae Variables in M62: An Oosterhoff I Globular Cluster with a Predominantly Blue HB

We report on the discovery of a large number of RR Lyrae variable stars in the moderately metal-rich Galactic globular cluster M62 (NGC 6266), which places it among the top three most RR Lyrae-rich globular clusters known. Likely members of the cluster in our studied field, from our preliminary number counts, include about 130 fundamental-mode (RRab) pulsators, with = 0.548 d, and about 75 first-overtone (RRc) pulsators, with = 0.300 d. The average periods and the position of the RRab variables with well-defined light curves in the Bailey diagram both suggest that the cluster is of Oosterhoff type I. However, the morphology of the cluster's horizontal branch (HB) is strikingly similar to that of the Oosterhoff type II globular cluster M15 (NGC 7078), with a dominant blue HB component and a very extended blue tail. Since M15 and M62 differ in metallicity by about one dex, we conclude that metallicity, at a fixed HB type, is a key parameter determining the Oosterhoff status of a globular cluster and the position of its variables in the Bailey diagram.Comment: 5 pages, 4 figures. ApJ Letters, in pres

Fragmentation and dynamical collapse of the starless high-mass star-forming region IRDC18310-4

Aims: We study the fragmentation and dynamical properties of a massive starless gas clump at the onset of high-mass star formation. Methods: Based on Herschel continuum data we identify a massive gas clump that remains far-infrared dark up to 100mum wavelengths. The fragmentation and dynamical properties are investigated by means of Plateau de Bure Interferometer and Nobeyama 45m single-dish spectral line and continuum observations. Results: The massive gas reservoir fragments at spatial scales of ~18000AU in four cores. Comparing the spatial extent of this high-mass region with intermediate- to low-mass starless cores from the literature, we find that linear sizes do not vary significantly over the whole mass regime. However, the high-mass regions squeeze much more gas into these similar volumes and hence have orders of magnitude larger densities. The fragmentation properties of the presented low-to high-mass regions are consistent with gravitational instable Jeans fragmentation. Furthermore, we find multiple velocity components associated with the resolved cores. Recent radiative transfer hydrodynamic simulations of the dynamic collapse of massive gas clumps also result in multiple velocity components along the line of sight because of the clumpy structure of the regions. This result is supported by a ratio between viral and total gas mass for the whole region <1. Conclusions: This apparently still starless high-mass gas clump exhibits clear signatures of early fragmentation and dynamic collapse prior to the formation of an embedded heating source. A comparison with regions of lower mass reveals that the linear size of star-forming regions does not necessarily have to vary much for different masses, however, the mass reservoirs and gas densities are orders of magnitude enhanced for high-mass regions compared to their lower-mass siblings.Comment: 11 pages, 10 figures, accepted to Astronomy and Astrophysics, high-resolution version with all figures included can be found at http://www.mpia.de/homes/beuther/papers.htm

Coulomb induced positive current-current correlations in normal conductors

In the white-noise limit current correlations measured at different contacts of a mesoscopic conductor are negative due to the antisymmetry of the wave function (Pauli principle). We show that current fluctuations at capacitive contacts induced via the long range Coulomb interaction as consequence of charge fluctuations in the mesoscopic sample can be {\it positively} correlated. The positive correlations are a consequence of the extension of the wave-functions into areas near both contacts. As an example we investigate in detail a quantum point contact in a high magnetic field under conditions in which transport is along an edge state.Comment: Revtex, 4 pages includes 2 figure

Tubular structures of GaS

In this Brief Report we demonstrate, using density-functional tight-binding theory, that gallium sulfide (GaS) tubular nanostructures are stable and energetically viable. The GaS-based nanotubes have a semiconducting direct gap which grows towards the value of two-dimensional hexagonal GaS sheet and is in contrast to carbon nanotubes largely independent of chirality. We further report on the mechanical properties of the GaS-based nanotubes

Geochemical comparison of K-T boundaries from the Northern and Southern Hemispheres

Closely spaced (cm-scale) traverses through the K-T boundary at Stevns Klint (Denmark), Woodside Creek (New Zealand) and a new Southern Hemisphere site at Richards Bay (South Africa) were subjected to trace element and isotopic (C, O, Sr) investigation. Intercomparison between these data-sets, and correlation with the broad K-T database available in the literature, indicate that the chemistry of the boundary clays is not globally constant. Variations are more common than similarities, both of absolute concentrations, and interelement ratios. For example, the chondrite normalized platinum-group elements (PGE) patterns of Stevns Klint are not like those of Woodside Creek, with the Pt/Os ratios showing the biggest variation. These differences in PGE patterns are difficult to explain by secondary alteration of a layer that was originally chemically homogeneous, especially for elements of such dubious crustal mobility as Os and Ir. The data also show that enhanced PGE concentrations, with similar trends to those of the boundary layers, occur in the Cretaceous sediments below the actual boundary at Stevns Klint and all three the New Zealand localities. This confirms the observations of others that the geochemistry of the boundary layers apparently does not record a unique component. It is suggested that terrestrial processes, eg. an extended period of Late Cretaceous volcanism can offer a satisfactory explanation for the features of the K-T geochemical anomaly. Such models would probably be more consistent with the observed stepwise, or gradual, palaeontological changes across this boundary, than the instant catastrophe predicated by the impact theory

Impact of the inherent periodic structure on the effective medium description of left-handed and related meta-materials

We study the frequency dependence of the effective electromagnetic parameters of left-handed and related meta-materials of the split ring resonator and wire type. We show that the reduced translational symmetry (periodic structure) inherent to these meta-materials influences their effective electromagnetic response. To anticipate this periodicity, we formulate a periodic effective medium model which enables us to distinguish the resonant behavior of electromagnetic parameters from effects of the periodicity of the structure. We use this model for the analysis of numerical data for the transmission and reflection of periodic arrays of split ring resonators, thin metallic wires, cut wires as well as the left-handed structures. The present method enables us to identify the origin of the previously observed resonance/anti-resonance coupling as well as the occurrence of negative imaginary parts in the effective permittivities and permeabilities of those materials. Our analysis shows that the periodicity of the structure can be neglected only for the wavelength of the electromagnetic wave larger than 30 space periods of the investigated structure.Comment: 23 pages, 14 figure

Remnant Fermi surface in the presence of an underlying instability in layered 1T-TaS_2

We report high resolution angle-scanned photoemission and Fermi surface (FS) mapping experiments on the layered transition-metal dichalcogenide 1T-TaS_2 in the quasi commensurate (QC) and the commensurate (C) charge-density-wave (CDW) phase. Instead of a nesting induced partially removed FS in the CDW phase we find a pseudogap over large portions of the FS. This remnant FS exhibits the symmetry of the one-particle normal state FS even when passing from the QC-phase to the C-phase. Possibly, this Mott localization induced transition represents the underlying instability responsible for the pseudogapped FS

RR Lyrae Stars in the Andromeda Halo from Deep Imaging with the Advanced Camera for Surveys

We present a complete census of RR Lyrae stars in a halo field of the Andromeda galaxy. These deep observations, taken as part of a program to measure the star formation history in the halo, spanned a period of 41 days with sampling on a variety of time scales, enabling the identification of short and long period variables. Although the long period variables cannot be fully characterized within the time span of this program, the enormous advance in sensitivity provided by the Advanced Camera for Surveys on the Hubble Space Telescope allows accurate characterization of the RR Lyrae population in this field. We find 29 RRab stars with a mean period of 0.594 days, 25 RRc stars with a mean period of 0.316 days, and 1 RRd star with a fundamental period of 0.473 days and a first overtone period of 0.353 days. These 55 RR Lyrae stars imply a specific frequency S_RR=5.6, which is large given the high mean metallicity of the halo, but not surprising given that these stars arise from the old, metal-poor tail of the distribution. This old population in the Andromeda halo cannot be clearly placed into one of the Oosterhoff types: the ratio of RRc/RRabc stars is within the range seen in Oosterhoff II globular clusters, the mean RRab period is in the gap between Oosterhoff types, and the mean RRc period is in the range seen in Oosterhoff I globular clusters. The periods of these RR Lyraes suggest a mean metallicity of [Fe/H]=-1.6, while their brightness implies a distance modulus to Andromeda of 24.5+/-0.1, in good agreement with the Cepheid distance.Comment: 15 pages, latex. Accepted for publication in The Astronomical Journa