520 Astronomie und zugeordnete Wissenschaften
Refine
Year of publication
Document Type
- Postprint (23) (remove)
Language
- English (23)
Is part of the Bibliography
- yes (23)
Keywords
- magnetic fields (3)
- MHD (2)
- Sun: activity (2)
- Sun: coronal mass ejections (CMEs) (2)
- X-rays: binaries (2)
- instabilities (2)
- relativistic jets (2)
- stars: abundances (2)
- Auger–Meitner (1)
- Be (1)
- Coster–Kronig (1)
- FLASH (1)
- MU-M (1)
- NEXAFS (1)
- Sun: flares (1)
- Sun: magnetic fields (1)
- Supernovae (1)
- Weibel instability (1)
- X-ray (1)
- X-rays: ISM (1)
- accretion (1)
- astronomical databases: miscellaneous (1)
- bidirectional reflectance (1)
- binaries: spectroscopic (1)
- biological physics (1)
- conduction (1)
- coronal mass ejections (1)
- coronal mass ejections (CMEs) (1)
- data analysis (1)
- data fusion (1)
- diffusion (1)
- disaster management (1)
- exposed water ice (1)
- filaments (1)
- first passage time (1)
- gene regulatory networks (1)
- generalization (1)
- geovisualization (1)
- global jets (1)
- helical magnetic fields (1)
- history and philosophy of astronomy (1)
- hydrodynamics (1)
- ice absorption features (1)
- interstellar ice (1)
- kinetic instabilities (1)
- kink instability (1)
- magnetic field generation (1)
- magnetohydrodynamics (1)
- methods: numerical (1)
- murchison meteorite (1)
- natural hazards (1)
- nucleobases (1)
- particle acceleration (1)
- particle-in-cell simulations (1)
- photoelectron (1)
- planetary nebulae: general (1)
- planetary nebulae: individual (A 30) (1)
- pulsars: individual (SXP 1062) (1)
- radiation (1)
- reaction cascade (1)
- resolution infrared-spectroscopy (1)
- schematic maps (1)
- solar coronal mass ejections (1)
- solar storm (1)
- spectra (1)
- stars: AGB and post-AGB (1)
- stars: Wolf-Rayet (1)
- stars: atmospheres (1)
- stars: early-type (1)
- stars: emission-line (1)
- stars: individual (HD 164492C, CPD –57 ◦ 3509, HD 54879, β CMa, ε CMa) (1)
- stars: magnetic fields (1)
- stars: mass loss (1)
- stars: mass-loss (1)
- stars: massive stars (1)
- stars: stellar evolution (1)
- stellar coronal mass ejections (1)
- sulfur (1)
- sun (1)
- sun: activity (1)
- sun: photosphere (1)
- sun: sunspots (1)
- surface (1)
- techniques: image processing (1)
- thiouracil (1)
- young stellar objects (1)
Institute
We present results on ultrafast gas electron diffraction (UGED) experiments with femtosecond resolution using the MeV electron gun at SLAC National Accelerator Laboratory. UGED is a promising method to investigate molecular dynamics in the gas phase because electron pulses can probe the structure with a high spatial resolution. Until recently, however, it was not possible for UGED to reach the relevant timescale for the motion of the nuclei during a molecular reaction. Using MeV electron pulses has allowed us to overcome the main challenges in reaching femtosecond resolution, namely delivering short electron pulses on a gas target, overcoming the effect of velocity mismatch between pump laser pulses and the probe electron pulses, and maintaining a low timing jitter. At electron kinetic energies above 3 MeV, the velocity mismatch between laser and electron pulses becomes negligible. The relativistic electrons are also less susceptible to temporal broadening due to the Coulomb force. One of the challenges of diffraction with relativistic electrons is that the small de Broglie wavelength results in very small diffraction angles. In this paper we describe the new setup and its characterization, including capturing static diffraction patterns of molecules in the gas phase, finding time-zero with sub-picosecond accuracy and first time-resolved diffraction experiments. The new device can achieve a temporal resolution of 100 fs root-mean-square, and sub-angstrom spatial resolution. The collimation of the beam is sufficient to measure the diffraction pattern, and the transverse coherence is on the order of 2 nm. Currently, the temporal resolution is limited both by the pulse duration of the electron pulse on target and by the timing jitter, while the spatial resolution is limited by the average electron beam current and the signal-to-noise ratio of the detection system. We also discuss plans for improving both the temporal resolution and the spatial resolution.
Recent studies have claimed the existence of very massive stars (VMS) up to 300 M⊙ in the local Universe. As this finding may represent a paradigm shift for the canonical stellar upper-mass limit of 150 M⊙, it is timely to discuss the status of the data, as well as the far-reaching implications of such objects. We held a Joint Discussion at the General Assembly in Beijing to discuss (i) the determination of the current masses of the most massive stars, (ii) the formation of VMS, (iii) their mass loss, and (iv) their evolution and final fate. The prime aim was to reach broad consensus between observers and theorists on how to identify and quantify the dominant physical processes.
Hazards and accessibility
(2018)
The assessment of natural hazards and risk has traditionally been built upon the estimation of threat maps, which are used to depict potential danger posed by a particular hazard throughout a given area. But when a hazard event strikes, infrastructure is a significant factor that can determine if the situation becomes a disaster. The vulnerability of the population in a region does not only depend on the area’s local threat, but also on the geographical accessibility of
the area. This makes threat maps by themselves insufficient for supporting real-time decision-making, especially for those tasks that involve the use of the road network, such as management of relief operations, aid distribution, or planning of evacuation routes, among others. To overcome this problem, this paper proposes a multidisciplinary approach divided in two parts. First, data fusion of satellite-based threat data and open infrastructure data from OpenStreetMap, introducing a threat-based routing service. Second, the visualization of this data through cartographic generalization and schematization. This emphasizes critical areas along roads in a simple way and allows users to visually evaluate the impact natural hazards may have on infrastructure. We develop and illustrate this methodology with a case study of landslide threat for an area in Colombia.
To understand the evolution and morphology of planetary nebulae, a detailed knowledge of their central stars is required. Central stars that exhibit emission lines in their spectra, indicating stellar mass-loss allow to study the evolution of planetary nebulae in action. Emission line central stars constitute about 10 % of all central stars. Half of them are practically hydrogen-free Wolf-Rayet type central stars of the carbon sequence, [WC], that show strong emission lines of carbon and oxygen in their spectra. In this contribution we address the weak emission-lines central stars (wels). These stars are poorly analyzed and their hydrogen content is mostly unknown. We obtained optical spectra, that include the important Balmer lines of hydrogen, for four weak emission line central stars. We present the results of our analysis, provide spectral classification and discuss possible explanations for their formation and evolution.
We present 3D zero-beta ideal MHD simulations of the solar flare/CME event that occurred in Active Region 11060 on 2010 April 8. The initial magnetic configurations of the two simulations are stable nonlinear force-free field and unstable magnetic field models constructed by Su et al. (2011) using the flux rope insertion method. The MHD simulations confirm that the stable model relaxes to a stable equilibrium, while the unstable model erupts as a CME. Comparisons between observations and MHD simulations of the CME are also presented.
X-ray observations of young Planetary Nebulæ (PNe) have revealed diffuse emission in extended regions around both H-rich and H-deficient central stars. In order to also repro-duce physical properties of H-deficient objects, we have, at first, extended our time-dependent radiation-hydrodynamic models with heat conduction for such conditions. Here we present some of the important physical concepts, which determine how and when a hot wind-blown bubble forms. In this study we have had to consider the, largely unknown, evolution of the CSPN, the slow (AGB) wind, the fast hot-CSPN wind, and the chemical composition. The main conclusion of our work is that heat conduction is needed to explain X-ray properties of wind-blown bubbles also in H-deficient objects.
The nucleus of Hen 2-428 is a short orbital period (4.2 h) spectroscopic binary, whose status as potential supernovae type Ia progenitor has raised some controversy in the literature. We present preliminary results of a thorough analysis of this interesting system, which combines quantitative non-local thermodynamic (non-LTE) equilibrium spectral modelling, radial velocity analysis, multi-band light curve fitting, and state-of-the art stellar evolutionary calculations. Importantly, we find that the dynamical system mass that is derived by using all available He II lines does not exceed the Chandrasekhar mass limit. Furthermore, the individual masses of the two central stars are too small to lead to an SN Ia in case of a dynamical explosion during the merger process.
Ammonium salts are a reservoir of nitrogen on a cometary nucleus and possibly on some asteroids
(2020)
The measured nitrogen-to-carbon ratio in comets is lower than for the Sun, a discrepancy which could be alleviated if there is an unknown reservoir of nitrogen in comets. The nucleus of comet 67P/Churyumov-Gerasimenko exhibits an unidentified broad spectral reflectance feature around 3.2 micrometers, which is ubiquitous across its surface. On the basis of laboratory experiments, we attribute this absorption band to ammonium salts mixed with dust on the surface. The depth of the band indicates that semivolatile ammonium salts are a substantial reservoir of nitrogen in the comet, potentially dominating over refractory organic matter and more volatile species. Similar absorption features appear in the spectra of some asteroids, implying a compositional link between asteroids, comets, and the parent interstellar cloud.
Interplay of coupling and common noise at the transition to synchrony in oscillator populations
(2016)
There are two ways to synchronize oscillators: by coupling and by common forcing, which can be pure noise. By virtue of the Ott-Antonsen ansatz for sine-coupled phase oscillators, we obtain analytically tractable equations for the case where both coupling and common noise are present. While noise always tends to synchronize the phase oscillators, the repulsive coupling can act against synchrony, and we focus on this nontrivial situation. For identical oscillators, the fully synchronous state remains stable for small repulsive coupling; moreover it is an absorbing state which always wins over the asynchronous regime. For oscillators with a distribution of natural frequencies, we report on a counter-intuitive effect of dispersion (instead of usual convergence) of the oscillators frequencies at synchrony; the latter effect disappears if noise vanishes.
SXP 1062 is an exceptional case of a young neutron star in a wind-fed high-mass X-ray binary associated with a supernova remnant. A unique combination of measured spin period, its derivative, luminosity and young age makes this source a key probe for the physics of accretion and neutron star evolution. Theoretical models proposed to explain the properties of SXP 1062 shall be tested with new data.