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Multidimensional nucleosynthesis studies with hundreds of nuclei linked through thousands of nuclear processes are still computationally prohibitive. To date, most nucleosynthesis studies rely either on hydrostatic/hydrodynamic simulations in spherical symmetry, or on post-processing simulations using temperature and density versus time profiles directly linked to huge nuclear reaction networks. Parallel computing has been regarded as the main permitting factor of computationally intensive simulations. This paper explores the different pros and cons in the parallelization of stellar codes, providing recommendations on when and how parallelization may help in improving the performance of a code for astrophysical applications. We report on different parallelization strategies succesfully applied to the spherically symmetric, Lagrangian, implicit hydrodynamic code SHIVA, extensively used in the modeling of classical novae and type I X-ray bursts. Speed-up factors of 26 and 35 have been obtained with a parallelized version of SHIVA, in a 200-shell simulation of a type I X-ray burst carried out with two nuclear reaction networks: a reduced one, consisting of 324 isotopes and 1392 reactions, and a more extended network with 606 nuclides and 3551 nuclear interactions. Maximum speed-ups of 41 (324-isotope network) and 85 (606-isotope network), are also predicted for 200 cores, stressing that the number of shells of the computational domain constitutes an effective upper limit for the maximum number of cores that could be used in a parallel application.
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In the era of advanced electromagnetic and gravitational wave detectors, it has become increasingly important to effectively combine and study the impact of stellar evolution on binaries and dynamical systems of stars. Systematic studies dedicated to exploring uncertain parameters in stellar evolution are required to account for the recent observations of the stellar populations. We present a new approach to the commonly used Single-Star Evolution (SSE) fitting formulae, one that is more adaptable: Method of Interpolation for Single Star Evolution (METISSE). It makes use of interpolation between sets of pre-computed stellar tracks to approximate evolution parameters for a population of stars. We have used METISSE with detailed stellar tracks computed by the Modules for Experiments in Stellar Astrophysics (MESA), Bonn Evolutionary Code (BEC) and Cambridge STARS code. METISSE better reproduces stellar tracks computed using the STARS code compared to SSE, and is on average three times faster. Using stellar tracks computed with MESA and BEC, we apply METISSE to explore the differences in the remnant masses, the maximum radial expansion, and the main-sequence lifetime of massive stars. We find that different physical ingredients used in the evolution of stars, such as the treatment of radiation dominated envelopes, can impact their evolutionary outcome. For stars in the mass range 9 to 100 M$_odot$, the predictions of remnant masses can vary by up to 20 M$_odot$, while the maximum radial expansion achieved by a star can differ by an order of magnitude between different stellar models.
Stellar astrophysicists are increasingly taking into account the effects of orbiting companions on stellar evolution. New discoveries, many thanks to systematic time-domain surveys, have underlined the role of binary star interactions in a range of astrophysical events, including some that were previously interpreted as due uniquely to single stellar evolution. Here, we review classical binary phenomena such as type Ia supernovae, and discuss new phenomena such as intermediate luminosity transients, gravitational wave-producing double black holes, or the interaction between stars and their planets. Finally, we examine the reassessment of well-known phenomena in light of interpretations that include both single and binary stars, for example supernovae of type Ib and Ic or luminous blue variables. At the same time we contextualise the new discoveries within the framework and nomenclature of the corpus of knowledge on binary stellar evolution. The last decade has heralded an era of revival in stellar astrophysics as the complexity of stellar observations is increasingly interpreted with an interplay of single and binary scenarios. The next decade, with the advent of massive projects such as the Large Synoptic Survey Telescope, the Square Kilometre Array, the James Webb Space Telescope and increasingly sophisticated computational methods, will see the birth of an expanded framework of stellar evolution that will have repercussions in many other areas of astrophysics such as galactic evolution and nucleosynthesis.
We present our experience with the modernization on the GR-MHD code BHAC, aimed at improving its novel hybrid (MPI+OpenMP) parallelization scheme. In doing so, we showcase the use of performance profiling tools usable on x86 (Intel-based) architectures. Our performance characterization and threading analysis provided guidance in improving the concurrency and thus the efficiency of the OpenMP parallel regions. We assess scaling and communication patterns in order to identify and alleviate MPI bottlenecks, with both runtime switches and precise code interventions. The performance of optimized version of BHAC improved by $sim28%$, making it viable for scaling on several hundreds of supercomputer nodes. We finally test whether porting such optimizations to different hardware is likewise beneficial on the new architecture by running on ARM A64FX vector nodes.
We investigate the distribution of different classes of spectroscopically identified sources and theoretical models in the color-color diagrams (CCDs) combining the near-infrared (NIR) and mid-infrared (MIR) data to develop a method to classify Outer Galaxy sources detected with the Spitzer Space Telescope (hereafter Spitzer) SMOG survey in the IRAC 3.6 and 8.0 micrometer and MIPS 24 micrometer bands. We supplement the Spitzer data with the data from other satellite and ground-based surveys. The main goal of our study is to discover and characterize the population of intermediate- and low-mass young stellar objects (YSOs) in the Outer Galaxy and use it to study star formation in a significantly different environment than the Galaxy inside the solar circle. Since the YSOs can be confused with evolved stars in the MIR, these classes of objects need to be carefully separated. Here we present the initial results of our analysis using the Ks-[8.0] vs. Ks-[24] CCD as an example. The evolved stars separated from YSOs in the YSO selection process will be investigated in detail in the follow-up study.
Sub-stellar objects exhibit photometric variability, which is believed to be caused by a number of processes, such as magnetically-driven spots or inhomogeneous cloud coverage. Recent models have shown that turbulent flows and waves, including internal gravity waves, may play an important role in cloud evolution. The aim of this paper is to investigate the effect of IGW on dust nucleation and dust growth, and whether observations of the resulting cloud structures could be used to recover atmospheric density information. For a simplified atmosphere in two dimensions, we numerically solved the governing fluid equations to simulate the effect on dust nucleation and mantle growth as a result of the passage of an IGW. Furthermore, we derived an expression that relates the properties of the wave-induced cloud structures to observable parameters in order to deduce the atmospheric density. Numerical simulations show that the $rho, p, T$ variations caused by gravity waves lead to an increase of the nucleation rate by up to a factor 20, and an increase of the mantle growth rate by up to a factor 1.6, compared to their equilibrium values. An exploration of the wider parameter space shows that in absolute terms, the increase in nucleation due to IGW is stronger in cooler (T dwarfs) and TiO2-rich sub-stellar atmospheres. The relative increase, however, is greater in warmer (L dwarf) and TiO2-poor atmospheres due to conditions less suited for efficient nucleation at equilibrium. These variations lead to banded areas in which dust formation is much more pronounced, similar to the cloud structures observed on Earth. We show that IGW in the atmosphere of sub-stellar objects can produce banded clouds structures similar to that observed on Earth. We propose a method with which potential observations of banded clouds could be used to estimate the atmospheric density of sub-stellar objects.
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