Depositional Environment of Coral–Rudist Associations in the Upper Cretaceous Cardenas Formation (Central Mexico)

In the Cardenas Formation (central Mexico), a 175 m thick sedimentary sequence of Maastrichtian age was analyzed with respect to its palaeontology and sedimentology. A wide variety of lithological and palaeontological features characterize this sequence comprising unfossiliferous and fossil-bearing sand- and siltstones, and diverse rudist and coral–rudist associations in carbonate or mixed carbonate/clastic lithologies. A total of 24 rudist and coral–rudist associations are exposed in the investigated section, which are grouped into 5 limestone units. Radiolitid assemblages, coral–rudist reefs, coral-domi­na­ted reefs, and hippuritid-dominated reefs are present. The stacking pattern of these reef intervals indicates a general transgressive trend through the entire section. Smaller-scale facies trends could be distinguished within each limestone unit, comprising deepening-upward sequences, defined by a shoreface–calcareous algae–radiolitid–marl facies transition, and shallowing-upward sequences defi­ned by a hippuritid–actaeonellid–coral/rudist facies transition. This cyclic sedimentation pattern is obscured by an episodic input of clastic sediments derived from the uplifting Sierra Madre Oriental, which in turn triggered either the development or decline of reefs

The VLT-FLAMES Tarantula Survey XVII. Physical and wind properties of massive stars at the top of the main sequence

The evolution and fate of very massive stars (VMS) is tightly connected to their mass-loss properties. Their initial and final masses differ significantly as a result of mass loss. VMS have strong stellar winds and extremely high ionising fluxes, which are thought to be critical sources of both mechanical and radiative feedback in giant Hii regions. However, how VMS mass-loss properties change during stellar evolution is poorly understood. In the framework of the VLT-Flames Tarantula Survey (VFTS), we explore the mass-loss transition region from optically thin O to denser WNh star winds, thereby testing theoretical predictions. To this purpose we select 62 O, Of, Of/WN, and WNh stars, an unprecedented sample of stars with the highest masses and luminosities known. We perform a spectral analysis of optical VFTS as well as near-infrared VLT/SINFONI data using the non-LTE radiative transfer code CMFGEN to obtain stellar and wind parameters. For the first time, we observationally resolve the transition between optically thin O and optically thick WNh star winds. Our results suggest the existence of a kink between both mass-loss regimes, in agreement with recent MC simulations. For the optically thick regime, we confirm the steep dependence on the Eddington factor from previous theoretical and observational studies. The transition occurs on the MS near a luminosity of 10^6.1Lsun, or a mass of 80...90Msun. Above this limit, we find that - even when accounting for moderate wind clumping (with f = 0.1) - wind mass-loss rates are enhanced with respect to standard prescriptions currently adopted in stellar evolution calculations. We also show that this results in substantial helium surface enrichment. Based on our spectroscopic analyses, we are able to provide the most accurate ionising fluxes for VMS known to date, confirming the pivotal role of VMS in ionising and shaping their environments.Comment: Accepted for publication in A&A, 19 pages, 14 figures, 6 tables, (74 pages appendix, 68 figures, 4 tables

Quantum and semiclassical study of magnetic anti-dots

We study the energy level structure of two-dimensional charged particles in inhomogeneous magnetic fields. In particular, for magnetic anti-dots the magnetic field is zero inside the dot and constant outside. Such a device can be fabricated with present-day technology. We present detailed semiclassical studies of such magnetic anti-dot systems and provide a comparison with exact quantum calculations. In the semiclassical approach we apply the Berry-Tabor formula for the density of states and the Borh-Sommerfeld quantization rules. In both cases we found good agreement with the exact spectrum in the weak magnetic field limit. The energy spectrum for a given missing flux quantum is classified in six possible classes of orbits and summarized in a so-called phase diagram. We also investigate the current flow patterns of different quantum states and show the clear correspondence with classical trajectories.Comment: 14 pages, 13 figure

Triplet Dispersion in CuGeO_3: Perturbative Analysis

We reconsider the 2d model for CuGeO_3 introduced previously (Phys. Rev. Lett. 79, 163 (1997)). Using a computer aided perturbation method based on flow equations we expand the 1-triplet dispersion up to 10th order. The expansion is provided as a polynom in the model parameters. The latter are fixed by fitting the theoretical result to experimental data obtained by INS. For a dimerization delta = 0.08(1) we find an excellent agreement with experiment. This value is at least 2 to 3 times higher than values deduced previously from 1d chain approaches. For the intrachain frustration alpha_0 we find a smaller value of 0.25(3). The existence of interchain frustration conjectured previously is confirmed by the analysis of temperature dependent susceptibility.Comment: 8 pages, 10 figures, submitted to Phys. Rev.

Accelerator experiments with soft protons and hyper-velocity dust particles: application to ongoing projects of future X-ray missions

We report on our activities, currently in progress, aimed at performing accelerator experiments with soft protons and hyper-velocity dust particles. They include tests of different types of X-ray detectors and related components (such as filters) and measurements of scattering of soft protons and hyper-velocity dust particles off X-ray mirror shells. These activities have been identified as a goal in the context of a number of ongoing space projects in order to assess the risk posed by environmental radiation and dust and qualify the adopted instrumentation with respect to possible damage or performance degradation. In this paper we focus on tests for the Silicon Drift Detectors (SDDs) used aboard the LOFT space mission. We use the Van de Graaff accelerators at the University of T\"ubingen and at the Max Planck Institute for Nuclear Physics (MPIK) in Heidelberg, for soft proton and hyper-velocity dust tests respectively. We present the experimental set-up adopted to perform the tests, status of the activities and some very preliminary results achieved at present time.Comment: Proceedings of SPIE, Vol. 8443, Paper No. 8443-24, 201

The LOFT Ground Segment

LOFT, the Large Observatory For X-ray Timing, was one of the ESA M3 mission candidates that completed their assessment phase at the end of 2013. LOFT is equipped with two instruments, the Large Area Detector (LAD) and the Wide Field Monitor (WFM). The LAD performs pointed observations of several targets per orbit (~90 minutes), providing roughly ~80 GB of proprietary data per day (the proprietary period will be 12 months). The WFM continuously monitors about 1/3 of the sky at a time and provides data for about ~100 sources a day, resulting in a total of ~20 GB of additional telemetry. The LOFT Burst alert System additionally identifies on-board bright impulsive events (e.g., Gamma-ray Bursts, GRBs) and broadcasts the corresponding position and trigger time to the ground using a dedicated system of ~15 VHF receivers. All WFM data are planned to be made public immediately. In this contribution we summarize the planned organization of the LOFT ground segment (GS), as established in the mission Yellow Book 1 . We describe the expected GS contributions from ESA and the LOFT consortium. A review is provided of the planned LOFT data products and the details of the data flow, archiving and distribution. Despite LOFT was not selected for launch within the M3 call, its long assessment phase (> 2 years) led to a very solid mission design and an efficient planning of its ground operations.Comment: Proc. SPIE 9144, Space Telescopes and Instrumentation 2014: Ultraviolet to Gamma Ray, 91446