ALBERT

Beschreibung: 3-D full waveform inversion (FWI) of seismic wavefields is routinely implemented with explicit time-stepping simulators. A clear advantage of explicit time stepping is the avoidance of solving large-scale implicit linear systems that arise with frequency domain formulations. However, FWI using explicit time stepping may require a very fine time step and (as a consequence) significant computational resources and run times. If the computational challenges of wavefield simulation can be effectively handled, an FWI scheme implemented within the frequency domain utilizing only a few frequencies, offers a cost effective alternative to FWI in the time domain. We have therefore implemented a 3-D FWI scheme for elastic wave propagation in the Fourier domain. To overcome the computational bottleneck in wavefield simulation, we have exploited an efficient Krylov iterative solver for the elastic wave equations approximated with second and fourth order finite differences. The solver does not exploit multilevel preconditioning for wavefield simulation, but is coupled efficiently to the inversion iteration workflow to reduce computational cost. The workflow is best described as a series of sequential inversion experiments, where in the case of seismic reflection acquisition geometries, the data has been laddered such that we first image highly damped data, followed by data where damping is systemically reduced. The key to our modelling approach is its ability to take advantage of solver efficiency when the elastic wavefields are damped. As the inversion experiment progresses, damping is significantly reduced, effectively simulating non-damped wavefields in the Fourier domain. While the cost of the forward simulation increases as damping is reduced, this is counterbalanced by the cost of the outer inversion iteration, which is reduced because of a better starting model obtained from the larger damped wavefield used in the previous inversion experiment. For cross-well data, it is also possible to launch a successful inversion experiment without laddering the damping constants. With this type of acquisition geometry, the solver is still quite effective using a small fixed damping constant. To avoid cycle skipping, we also employ a multiscale imaging approach, in which frequency content of the data is also laddered (with the data now including both reflection and cross-well data acquisition geometries). Thus the inversion process is launched using low frequency data to first recover the long spatial wavelength of the image. With this image as a new starting model, adding higher frequency data refines and enhances the resolution of the image. FWI using laddered frequencies with an efficient damping schemed enables reconstructing elastic attributes of the subsurface at a resolution that approaches half the smallest wavelength utilized to image the subsurface. We show the possibility of effectively carrying out such reconstructions using two to six frequencies, depending upon the application. Using the proposed FWI scheme, massively parallel computing resources are essential for reasonable execution times.

Beschreibung: The mass outflows in T Tauri stars (TTS) are thought to be an effective mechanism to remove angular momentum during the pre-main-sequence contraction of a low-mass star. The most powerful winds are observed at the FUor stage of stellar evolution. V1331 Cyg has been considered as a TTS at the pre-FUor stage. We analyse high-resolution spectra of V1331 Cyg collected in 1998–2007 and 20-d series of spectra taken in 2012. For the first time the photospheric spectrum of the star is detected and stellar parameters are derived: spectral type G7–K0 IV, mass 2.8 M , radius 5 R , v sin i  〈 6 km s –1 . The photospheric spectrum is highly veiled, but the amount of veiling is not the same in different spectral lines, being lower in weak transitions and much higher in strong transitions. The Fe  ii 5018, Mg  i 5183, K  i 7699 and some other lines of metals are accompanied by a ‘shell’ absorption at radial velocity of about –240 km s –1 . We show that these absorptions form in the post-shock gas in the jet, i.e. the star is seen though its jet. The P Cyg profiles of Hα and Hβ indicate the terminal wind velocity of about 500 km s –1 , which vary on time-scales from several days to years. A model of the stellar wind is developed to interpret the observations. The model is based on calculation of hydrogen spectral lines using the radiative transfer code torus . The observed Hα and Hβ line profiles and their variability can be well reproduced with a stellar wind model, where the mass-loss rate and collimation (opening angle) of the wind are variable. The changes of the opening angle may be induced by small variability in magnetization of the inner disc wind. The mass-loss rate is found to vary within (6–11) 10 –8 M yr –1 , with the accretion rate of 2.0 10 –6 M yr –1 .

Beschreibung: 〈span〉〈div〉Summary〈/div〉Simulation of acoustic wave propagation in the Laplace-Fourier (LF) domain, with a spatially uniform mesh, can be computationally demanding especially in areas with large velocity contrasts. To improve efficiency and convergence, we use 3D second- and fourth-order velocity-pressure finite difference (FD) discontinuous meshes (DM). Our DM algorithm can use any spatial discretization ratio between meshes. We evaluate direct and iterative parallel solvers for computational speed, memory requirements and convergence. Benchmarks in realistic 3D models and topographies show more efficient and stable results for DM with direct solvers than uniform mesh results with iterative solvers.〈/span〉

Beschreibung: 〈span〉〈div〉SUMMARY〈/div〉We introduce a new approach for locating earthquakes using arrival times derived from waveforms. The most costly computational step of the algorithm scales as the number of stations in the active seismographic network. In this approach, a variation on existing grid search methods, a series of full waveform simulations are conducted for all receiver locations, with sources positioned successively at each station. The travel time field over the region of interest is calculated by applying a phase picking algorithm to the numerical wavefields produced from each simulation. An event is located by subtracting the stored travel time field from the arrival time at each station. This provides a shifted and time-reversed travel time field for each station. The shifted and time-reversed fields all approach the origin time of the event at the source location. The mean or median value at the source location thus approximates the event origin time. Measures of dispersion about this mean or median time at each grid point, such as the sample standard error and the average deviation, are minimized at the correct source position. Uncertainty in the event position is provided by the contours of standard error defined over the grid. An application of this technique to a synthetic data set indicates that the approach provides stable locations even when the travel times are contaminated by additive random noise containing a significant number of outliers and velocity model errors. It is found that the waveform-based method out-performs one based upon the eikonal equation for a velocity model with rapid spatial variations in properties due to layering. A comparison with conventional location algorithms in both a laboratory and field setting demonstrates that the technique performs at least as well as existing techniques.〈/span〉

Beschreibung: 〈span〉〈div〉SUMMARY〈/div〉Simulation of acoustic wave propagation in the Laplace–Fourier (LF) domain, with a spatially uniform mesh, can be computationally demanding especially in areas with large velocity contrasts. To improve efficiency and convergence, we use 3-D second- and fourth-order velocity-pressure finite difference (FD) discontinuous meshes (DM). Our DM algorithm can use any spatial discretization ratio between meshes. We evaluate direct and iterative parallel solvers for computational speed, memory requirements and convergence. Benchmarks in realistic 3-D models and topographies show more efficient and stable results for DM with direct solvers than uniform mesh results with iterative solvers.〈/span〉

Beschreibung: The secondary of the famous young binary RW Aur is much less studied than the primary. To rectify this shortcoming, we present here the results of UBVRIJHK photometric, VRI polarimetric and optical spectral observations of RW Aur B. The star demonstrates chaotic brightness variations in the optical band, with irregular, short (∼1-d) dimmings with an amplitude ΔV up to 1.3 mag. The dimmings are accompanied by an increase in the linear polarization (up to 3 per cent in the I band), presumably as a result of the scattering of stellar radiation by dust in the circumstellar disc, which means that RW Aur B can be classified as a UX Ori-type star. We conclude that the observed excess emission at λ ≲ 0.45 μm and longwards of ~2 μm as well as the variability of fluxes and profiles of the H i, He i and Na i D emission lines are due to the accretion process. At the same time, emission components of Ca ii lines indicate that RW Aur B has a powerful chromosphere. Assuming solar elemental abundances, we find the following parameters for the star: Teff = 4100–4200 K, AV = 0.6 ± 0.1 (out of the dimming events), L* ≈ 0.6 ${ m L}_odot ,, R_* approx 1.5$ ${ m R}_odot ,, Mapprox 0.85$ M$_odot ,, dot{M}_{ m acc}lt 5imes 10^{-9}$ M⊙ yr−1. Finally, we discuss possible reasons for the different levels of accretion activity of the RW Aur binary components and present arguments in favour of the fact that the components are gravitationally bound.

Beschreibung: Classical T Tauri stars with ages of less than 10 Myr possess accretion discs. Magnetohydrodynamic processes at the boundary between the disc and the stellar magnetosphere control the accretion and ejections gas flows. We carried out a long series of simultaneous spectroscopic and photometric observations of the classical T Tauri stars, RY Tauri and SU Aurigae, with the aim to quantify the accretion and outflow dynamics at time-scales from days to years. It is shown that dust in the disc wind is the main source of photometric variability of these stars. In RY Tau, we observed a new effect: during events of enhanced outflow, the circumstellar extinction becomes lower. The characteristic time of changes in outflow velocity and stellar brightness indicates that the obscuring dust is near the star. The outflow activity in both stars is changing on a time-scale of years. Periods of quiescence in the variability of the Hα profile were observed during the 2015-2016 period in RY Tau and during the 2016-2017 period in SU Aur. We interpret these findings in the framework of the magnetospheric accretion model, and we discuss how the global stellar magnetic field can influence the long-term variations of the outflow activity.