Strong field dynamics with ultrashort electron wave packet replicas

We investigate theoretically electron dynamics under a VUV attosecond pulse train which has a controlled phase delay with respect to an additional strong infrared laser field. Using the strong field approximation and the fact that the attosecond pulse is short compared to the excited electron dynamics, we arrive at a minimal analytical model for the kinetic energy distribution of the electron as well as the photon absorption probability as a function of the phase delay between the fields. We analyze the dynamics in terms of electron wave packet replicas created by the attosecond pulses. The absorption probability shows strong modulations as a function of the phase delay for VUV photons of energy comparable to the binding energy of the electron, while for higher photon energies the absorption probability does not depend on the delay, in line with the experimental observations for helium and argon, respectively.Comment: 14 pages, 8 figure

Herschel GASPS spectral observations of T Tauri stars in Taurus: unraveling far-infrared line emission from jets and discs

At early stages of stellar evolution young stars show powerful jets and/or outflows that interact with protoplanetary discs and their surroundings. Despite the scarce knowledge about the interaction of jets and/or outflows with discs, spectroscopic studies based on Herschel and ISO data suggests that gas shocked by jets and/or outflows can be traced by far-IR (FIR) emission in certain sources. We want to provide a consistent catalogue of selected atomic ([OI] and [CII]) and molecular (CO, OH, and H$_{2}$O) line fluxes observed in the FIR, separate and characterize the contribution from the jet and the disc to the observed line emission, and place the observations in an evolutionary picture. The atomic and molecular FIR (60-190 $\rm \mu m$) line emission of protoplanetary discs around 76 T Tauri stars located in Taurus are analysed. The observations were carried out within the Herschel key programme Gas in Protoplanetary Systems (GASPS). The spectra were obtained with the Photodetector Array Camera and Spectrometer (PACS). The sample is first divided in outflow and non-outflow sources according to literature tabulations. With the aid of archival stellar/disc and jet/outflow tracers and model predictions (PDRs and shocks), correlations are explored to constrain the physical mechanisms behind the observed line emission. The much higher detection rate of emission lines in outflow sources and the compatibility of line ratios with shock model predictions supports the idea of a dominant contribution from the jet/outflow to the line emission, in particular at earlier stages of the stellar evolution as the brightness of FIR lines depends in large part on the specific evolutionary stage. [Abridged Abstract]Comment: 37 pages, 27 figures, accepted for publication in A&

Frictional state evolution during normal stress perturbations probed with ultrasonic waves

Fault normal stress changes dynamically during earthquake rupture; however, the impact of these changes on dynamic frictional strength is poorly understood. Here we report on a laboratory study to investigate the effect of normal stress perturbations on the friction of westerly granite surfaces sheared under normal stresses of 1-25 MPa. We measure changes in surface friction and elastic properties, using acoustic waves, for step changes in normal stress of 1–50% and shearing velocities of 1-100 μm/s. We demonstrate that transmitted elastic wave amplitude is a reliable proxy for the real contact area at the fault interface at steady state. For step increases in normal stress, wave amplitude increases immediately and then continues to increase during elastic shear loading to a peak value from which it decreases as fault slip rate increases. Friction changes in a similar fashion, showing an inelastic increase over a characteristic shear displacement that is independent of loading rate. Perturbations in normal stress during shear cause excursions in the frictional slip rate that must be accounted for in order to accurately predict the evolution of fault strength and elastic properties. Our work improves understanding of induced seismicity and triggered earthquakes with particular focus on simulating static triggering and stress transfer phenomena using rate-and-state frictional formulations in earthquake rupture models

Density correlations in ultracold atomic Fermi gases

We investigate density fluctuations in a coherent ensemble of interacting fermionic atoms. Adapting the concept of full counting statistics, well-known from quantum optics and mesoscopic electron transport, we study second-order as well as higher-order correlators of density fluctuations. Using the mean-field BCS state to describe the whole interval between the BCS limit and the BEC limit, we obtain an exact expression for the cumulant-generating function of the density fluctuations of an atomic cloud. In the two-dimensional case, we obtain a closed analytical expression. Poissonian fluctuations of a molecular condensate on the BEC side are strongly suppressed on the BCS side. The size of the fluctuations in the BCS limit is a direct measure of the pairing potential. We also discuss the BEC-BCS crossover of the third cumulant and the temperature dependence of the second cumulant.Comment: 4 pages, 4 figures. To appear in Phys. Rev. A. New calculation of the bin statistics of a free Bose gas; updated and extended bibliograph

Efficient Multiphysics Design Workflow of Synchronous Reluctance Motors

This paper proposes a new design strategy for Synchronous Reluctance machines, with cooperative design in the two environments SyR-e and Motor-CAD. The paper proposes to use the open-source SyR-e for initial, equation based design of the machine. Then, the design is validated and refined in Motor-CAD, in multiple physical domains. This synergy complements both design environments and turns into a comprehensive design package, not yet available in the literature, assembling accessible design equations, magnetic and mechanical FEA and drive operating profiles evaluation to the trademark thermal analysis of Motor-CAD. The cooperative design strategy is described in the paper with reference to the case of a Pure Synchronous Reluctance motor prototype for vehicular tractio

Using a physics-informed neural network and fault zone acoustic monitoring to predict lab earthquakes

Predicting failure in solids has broad applications including earthquake prediction which remains an unattainable goal. However, recent machine learning work shows that laboratory earthquakes can be predicted using micro-failure events and temporal evolution of fault zone elastic properties. Remarkably, these results come from purely data-driven models trained with large datasets. Such data are equivalent to centuries of fault motion rendering application to tectonic faulting unclear. In addition, the underlying physics of such predictions is poorly understood. Here, we address scalability using a novel Physics-Informed Neural Network (PINN). Our model encodes fault physics in the deep learning loss function using time-lapse ultrasonic data. PINN models outperform data-driven models and significantly improve transfer learning for small training datasets and conditions outside those used in training. Our work suggests that PINN offers a promising path for machine learning-based failure prediction and, ultimately for improving our understanding of earthquake physics and prediction

Nitric Acid Particles in Cold Thick Ice Clouds Observed at Global Scale: Link with Lightning, Temperature, and Upper Tropospheric Water Vapor

Signatures of nitric acid particles (NAP) in cold thick ice clouds have been derived from satellite observations. Most NAP are detected in the Tropics (9 to 20% of clouds with T less than 202.5 K). Higher occurrences were found in the rare mid-latitudes very cold clouds. NAP occurrence increases as cloud temperature decreases and NAP are more numerous in January than July. Comparisons of NAP and lightning distributions show that lightning is the main source of the NOx, which forms NAP in cold clouds. Qualitative comparisons of NAP with upper tropospheric humidity distributions suggest that NAP play a role in the dehydration of the upper troposphere when the tropopause is colder than 195K

The Transiting Exocomets in the HD 172555 System

The Earth is thought to have formed dry, in a part of the Solar Nebula deficient in organic material, and to have acquired its organics and water through bombardment by minor bodies. Observations of this process in well-dated systems can provide insight into the probable origin and composition of the bombarding parent bodies. Transiting cometary activity has previously been reported in Ca II for the late-A member of the 241 Myr old Pictoris Moving Group member, HD 172555(Kiefer et al. 2014). We present HST STIS and COS spectra of HD 172555 demonstrating that the star has chromospheric emission and variable in falling gas features in transitions of silicon and carbon ions at times when no Fe II absorption is seen in the UV data, and no Ca II absorption is seen in contemporary optical spectra. The lack of CO absorption and stable gas absorption at the system velocity is consistent with the absence of a cold Kuiper belt analog (Riviere-Marichalar et al. 2012) in this system. The presence of infall in some species at one epoch and others at different epochs suggests that, like Pictoris, there may be more than one family of exocomets. If perturbed into star-grazing orbits by the same mechanism as for Pic, these data suggest that the wide planet frequency among A-early F stars in the PMG is at least 37.5, well above the frequency estimated for young moving groups independent of host star spectral type