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Register a new userSelfgravitating disks in binary systems: an SPH approach -- I. Implementation of the code and reliability tests
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The study of the stability of massive gaseous disks around a star in a non-isolated context is not a trivial issue and becomes a more complicated task for disks hosted by binary systems. The role of self-gravity is thought to be significant, whenever the ratio of the disk to the star mass is non-negligible. To tackle these issues we implemented, tested and applied our own Smoothed Particle Hydrodynamics (SPH) algorithm. The code (named GaSPH) passed various quality tests and shows good performances, so to be reliably applied to the study of disks around stars accounting for self-gravity. This work aims to introduce and describe the algorithm, making some performance and stability tests. It constitutes the first part of a series of studies in which self-gravitating disks in binary systems will be let evolve in larger environments such as Open Clusters.
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Starspots are thought to be regions of locally strong magnetic fields, similar to sunspots, and they can generate photometric brightness modulations. To deduce stellar and spot properties, such as spot emergence and decay rates, we implement computational code for starspot modeling. It is implemented with an adaptive parallel tempering algorithm and an importance sampling algorithm for parameter estimation and model selection in the Bayesian framework. For evaluating the performance of the code, we apply it to synthetic light curves produced with 3 spots. The light curves are specified in the spot parameters, such as the radii, intensities, latitudes, longitudes, and emergence/decay durations. The spots are circular with specified radii and intensities relative to the photosphere, and the stellar differential rotation coefficient is also included in the light curves. As a result, stellar and spot parameters are uniquely deduced. The number of spots is correctly determined: the 3-spot model is preferable because the model evidence is much greater than that of 2-spot models by orders of magnitude and more than that of 4-spot model by a more modest factor, whereas the light curves are produced to have 2 or 1 local minimum during one equatorial rotation period by adjusting the values of longitude. The spot emergence and decay rates can be estimated with error less than an order of magnitude, considering the difference of the number of spots.
We show that the standard model for geometrically thin accretion disks (alpha-disks) leads to inconsistencies if selfgravity plays a role. This problem arises from the parametrization of viscosity in terms of local sound velocity and vertical disk scale height. A viscosity prescription based on turbulent flows at the critical effective Reynolds number allows for consistent models of thin selfgravitating disks, and recovers the alpha-disk solution as the limiting case of negligible selfgravity. We suggest that such selfgravitating disks may explain the observed spectra of protoplanetary disks and yield a natural explanation for the radial motions inferred from the observed metallicity gradients in disk galaxies.
We present an implementation of an adaptive ray tracing (ART) module in the Athena hydrodynamics code that accurately and efficiently handles the radiative transfer involving multiple point sources on a three-dimensional Cartesian grid. We adopt a recently proposed parallel algorithm that uses non-blocking, asynchronous MPI communications to accelerate transport of rays across the computational domain. We validate our implementation through several standard test problems including the propagation of radiation in vacuum and the expansions of various types of HII regions. Additionally, scaling tests show that the cost of a full ray trace per source remains comparable to that of the hydrodynamics update on up to $sim 10^3$ processors. To demonstrate application of our ART implementation, we perform a simulation of star cluster formation in a marginally bound, turbulent cloud, finding that its star formation efficiency is $12%$ when both radiation pressure forces and photoionization by UV radiation are treated. We directly compare the radiation forces computed from the ART scheme with that from the M1 closure relation. Although the ART and M1 schemes yield similar results on large scales, the latter is unable to resolve the radiation field accurately near individual point sources.
We present EvoL, the new release of the Padova N-body code for cosmological simulations of galaxy formation and evolution. In this paper, the basic Tree + SPH code is presented and analysed, together with an overview on the software architectures. EvoL is a flexible parallel Fortran95 code, specifically designed for simulations of cosmological structure formation on cluster, galactic and sub-galactic scales. EvoL is a fully Lagrangian self-adaptive code, based on the classical Oct-tree and on the Smoothed Particle Hydrodynamics algorithm. It includes special features such as adaptive softening lengths with correcting extra-terms, and modern formulations of SPH and artificial viscosity. It is designed to be run in parallel on multiple CPUs to optimize the performance and save computational time. We describe the code in detail, and present the results of a number of standard hydrodynamical tests.
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Ruslan F. Gabbasov
, Jaime Klapp-Escribano
, Joel Suarez-Cansino andn Leonardo Di G. Sigalotti
2013
The method of Smoothed Particle Hydrodynamics (SPH) has been widely studied and implemented for a large variety of problems, ranging from astrophysics to fluid dynamics and elasticity problems in solids. However, the method is known to have several deficiencies and discrepancies in comparison with traditional mesh-based codes. In particular, there has been a discussion about its ability to reproduce the Kelvin-Helmholtz Instability in shearing flows. Several authors reported that they were able to reproduce correctly the instability by introducing some improvements to the algorithm. In this contribution, we compare the results of Read et al. (2010) implementation of the SPH algorithm with the original Gadget-2 N-body/SPH code.
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