Search results for "Astrophysics"
showing 10 items of 8341 documents
Riemann Solvers in General Relativistic Hydrodynamics
1999
Our contribution concerns with the numerical solution of the 3D general relativistic hydrodynamical system of equations within the framework of the 3+1 formalism. We summarize the theoretical ingredients which are necessary in order to build up a numerical scheme based on the solution of local Riemann problems. Hence, the full spectral decomposition of the Jacobian matrices of the system, i.e., the eigenvalues and the right and left eigenvectors, is explicitly shown. An alternative approach consists in using any of the special relativistic Riemann solvers recently developed for describing the evolution of special relativistic flows. Our proposal relies on a local change of coordinates in te…
"Mariage des Maillages": A new numerical approach for 3D relativistic core collapse simulations
2004
We present a new 3D general relativistic hydrodynamics code for simulations of stellar core collapse to a neutron star, as well as pulsations and instabilities of rotating relativistic stars. It uses spectral methods for solving the metric equations, assuming the conformal flatness approximation for the three-metric. The matter equations are solved by high-resolution shock-capturing schemes. We demonstrate that the combination of a finite difference grid and a spectral grid can be successfully accomplished. This "Mariage des Maillages" (French for grid wedding) approach results in high accuracy of the metric solver and allows for fully 3D applications using computationally affordable resour…
History and results of the Riga dynamo experiments
2008
On 11 November 1999, a self-exciting magnetic eigenfield was detected for the first time in the Riga liquid sodium dynamo experiment. We report on the long history leading to this event, and on the subsequent experimental campaigns which provided a wealth of data on the kinematic and the saturated regime of this dynamo. The present state of the theoretical understanding of both regimes is delineated, and some comparisons with other laboratory dynamo experiments are made.
Looking through the photoionisation wake: Vela X-1 at $\varphi_\mathrm{orb} \approx 0.75$ with Chandra/HETG
2021
The Supergiant X-ray binary Vela X-1 represents one of the best astrophysical sources to investigate the wind environment of a O/B star irradiated by an accreting neutron star. Previous studies and hydrodynamic simulations of the system revealed a clumpy environment and the presence of two wakes: an accretion wake surrounding the compact object and a photoionisation wake trailing it along the orbit. Our goal is to conduct, for the first time, high-resolution spectroscopy on Chandra/HETG data at the orbital phase $\varphi_\mathrm{orb} \approx 0.75$, when the line of sight is crossing the photoionisation wake. We aim to conduct plasma diagnostics, inferring the structure and the geometry of t…
Core-Collapse Supernovae: Reflections and Directions
2012
Core-collapse supernovae are among the most fascinating phenomena in astrophysics and provide a formidable challenge for theoretical investigation. They mark the spectacular end of the lives of massive stars and, in an explosive eruption, release as much energy as the sun produces during its whole life. A better understanding of the astrophysical role of supernovae as birth sites of neutron stars, black holes, and heavy chemical elements, and more reliable predictions of the observable signals from stellar death events are tightly linked to the solution of the long-standing puzzle how collapsing stars achieve to explode. In this article our current knowledge of the processes that contribute…
Accretion-ejection connection in the young brown dwarf candidate ISO-Cha1 217
2014
As the number of observed brown dwarf outflows is growing it is important to investigate how these outflows compare to the well studied jets from young stellar objects. A key point of comparison is the relationship between outflow and accretion activity and in particular the ratio between the mass outflow and accretion rates ($\dot{M}_{out}$/$\dot{M}_{acc}$). The brown dwarf candidate ISO-ChaI 217 was discovered by our group, as part of a spectro-astrometric study of brown dwarfs, to be driving an asymmetric outflow with the blue-shifted lobe having a position angle of $\sim$ 20$^{\circ}$. The aim here is to further investigate the properties of ISO-ChaI 217, the morphology and kinematics o…
Science Objectives for an X-Ray Microcalorimeter Observing the Sun
2010
We present the science case for a broadband X-ray imager with high-resolution spectroscopy, including simulations of X-ray spectral diagnostics of both active regions and solar flares. This is part of a trilogy of white papers discussing science, instrument (Bandler et al. 2010), and missions (Bookbinder et al. 2010) to exploit major advances recently made in transition-edge sensor (TES) detector technology that enable resolution better than 2 eV in an array that can handle high count rates. Combined with a modest X-ray mirror, this instrument would combine arcsecondscale imaging with high-resolution spectra over a field of view sufficiently large for the study of active regions and flares,…
The atmospheric structure and fundamental parameters of the red supergiants AH Sco, UY Sct and KW Sgr
2013
We present the atmospheric structure and the fundamental properties of the red supergiants (RSGs) AH Sco, UY Sct, and KW Sgr based on VLTI/AMBER observations. We carried out spectro-interferometric observations of AH Sco, UY Sct, and KW Sgr in the near-infrared K band with the VLTI/AMBER instrument, and compared the data to a new grid of hydrostatic PHOENIX model atmospheres. In our visibility data, we observe molecular layers of water and CO in extended atmospheres. For a uniform disk modeling, we observe size increases at the water band of 10% to 25% and at the CO bandheads of 20%-35% with respect to the near continuum bandpass. The PHOENIX atmosphere models predict the spectra and the co…
Surface imaging of cool evolved stars in the era of the ELT
2019
Cool evolved stars are the main source of chemical enrichment of the interstellar medium. Understanding their mass loss offers a unique opportunity to study the cycle of matter. We discuss interferometric studies and their comparison to latest state-of-the-art dynamic model atmospheres. They show broad agreement for asymptotic giant branch stars. For red supergiants, however, current models cannot explain observed extensions by far, pointing to missing physical processes in their models, and uncertainties in our general understanding of mass loss. We present ongoing imaging and time-series observations that may provide the strongest constraint and may help to identify missing dynamic proces…
Performance Analysis of the SO/PHI Software Framework for On-board Data Reduction
2019
The Polarimetric and Helioseismic Imager (PHI) is the first deep-space solar spectropolarimeter, on-board the Solar Orbiter (SO) space mission. It faces: stringent requirements on science data accuracy, a dynamic environment, and severe limitations on telemetry volume. SO/PHI overcomes these restrictions through on-board instrument calibration and science data reduction, using dedicated firmware in FPGAs. This contribution analyses the accuracy of a data processing pipeline by comparing the results obtained with SO/PHI hardware to a reference from a ground computer. The results show that for the analyzed pipeline the error introduced by the firmware implementation is well below the requirem…