Propagation in 3D spiral-arm cosmic-ray source distribution models and secondary particle production using PICARD

We study the impact of possible spiral-arm distributions of Galactic cosmic-ray sources on the flux of various cosmic-ray nuclei throughout our Galaxy. We investigate model cosmic-ray spectra at the nominal position of the sun and at different positions within the Galaxy. The modelling is performed using the recently introduced numerical cosmic ray propagation code \textsc{Picard}. Assuming non-axisymmetric cosmic ray source distributions yields new insights on the behaviour of primary versus secondary nuclei. We find that primary cosmic rays are more strongly confined to the vicinity of the sources, while the distribution of secondary cosmic rays is much more homogeneous compared to the primaries. This leads to stronger spatial variation in secondary to primary ratios when compared to axisymmetric source distribution models. A good fit to the cosmic-ray data at Earth can be accomplished in different spiral-arm models, although leading to decisively different spatial distributions of the cosmic-ray flux. This results in very different cosmic ray anisotropies, where even a good fit to the data becomes possible. Consequently, we advocate directions to seek best fit propagation parameters that take into account the higher complexity introduced by the spiral-arm structure on the cosmic-ray distribution. We specifically investigate whether the flux at Earth is representative for a large fraction of the Galaxy. The variance among possible spiral-arm models allows us to quantify the spatial variation of the cosmic-ray flux within the Galaxy in presence of non-axisymmetric source distributions.Comment: 38 pages, 16 figures, accepted for publication in Astroparticle Physic

Efficient numerical methods for Anisotropic Diffusion of Galactic Cosmic Rays

Anisotropic diffusion is imperative in understanding cosmic ray diffusion across the Galaxy, the heliosphere, and the interplay of cosmic rays with the Galactic magnetic field. This diffusion term contributes to the highly stiff nature of the cosmic ray transport equation. To conduct numerical simulations of time-dependent cosmic ray transport, implicit integrators (namely, Crank-Nicolson (CN)) have been traditionally favoured over the CFL-bound explicit integrators in order to be able to take large step sizes. We propose exponential methods to treat the linear anisotropc diffusion equation in the presence of advection and time-independent and time-dependent sources. These methods allow us to take even larger step sizes that can substantially speed-up the simulations whilst generating highly accurate solutions. In or subsequent work, we will use these exponential solvers in the Picard code to study anisotropic cosmic ray diffusion and we will consider additional physical processes such as continuous momentum losses and reacceleration.Comment: The 38th International Cosmic Ray Conference (ICRC2023

Investigation of the recombination of the retarded shell of "born-again" CSPNe by time-dependent radiative transfer models

A standard planetary nebula stays more than 10 000 years in the state of a photoionized nebula. As long as the timescales of the most important ionizing processes are much smaller, the ionization state can be characterized by a static photoionization model and simulated with codes like CLOUDY (Ferland et al. 1998). When the star exhibits a late Helium flash, however, its ionizing flux stops within a very short period. The star then re-appears from itsopaque shell after a few years (or centuries) as a cold giant star without any hard ionizing photons. Describing the physics of such behavior requires a fully time-dependent radiative transfer model. Pollacco (1999), Kerber et al. (1999) and Lechner & Kimeswenger (2004) used data of the old nebulae around V605 Aql and V4334 Sgr to derive a model of the pre-outburst state of the CSPN in a static model. Their argument was the long recombination time scale for such thin media. With regard to these models Schoenberner (2008) critically raised the question whether a significant change in the ionization state (and thus the spectrum) has to be expected after a time of up to 80 years, and whether static models are applicable at all.Comment: (3 pages, 1 figure, to appear in proceedings of the IAU Symposium 283: "Planetary Nebulae: An Eye to the Future", Eds.: A. Manchado, L. Stanghellini and D. Schoenberner; presenting author: Stefan Kimeswenger

Numerical investigation of the turbulent ISM

In this work we introduced basic turbulence theory into the framework of the interstellar medium. In many cases turbulence simulations are applied to the interstellar medium (ISM) merely because it is a medium, where extremely high Reynolds numbers are actually realised, and the parameters of the ISM are only taken into account as far as they are needed for the turbulence research. Here, however, we investigated the basic turbulence properties, while at the same time we modelled the properties of the ISM as thoroughly as possible. The important point is that there are many physical processes going on in the ISM, which should be incorporated in the corresponding simulations. These processes reachfrom external influences of the radiation field originating from hot stars to the internal interaction of the particles culminating in the intricate chemistry of the molecular cloud medium. Each of the different phases of the ISM has its own dominant processes to be taken into account for a realistic modelling...thesi

Numerical investigation of the turbulent ISM

The main topic of this work is the investigation of the interstellar medium with regard to large-scale turbulent fluctuations. For an appropriate description of the interstellar gas we utilised a MHD model with additional heating and cooling processes included. Apart from this also the influence of chemical reactions was studied. In this work we developed a numerical model for the interstellar medium using a numerical solver which suppresses artificial oscillations near the omnipresent shock structures in the highly compressible gas. After a thorough analysis of the numerical scheme with particular emphasis on the solenoidality of the magnetic field we applied the numerics to the investigation of different phases of the medium. This way, we were able to confirm the current turbulence theories and also find a match of the simulation results to actual observations