M. Flock^1,2, S. Fromang^1,2, M. González^2,3 and B. Commerçon⁴

¹CEA, Irfu, SAp Centre de Saclay 91191 Gif-sur-Yvette France
²UMR AIM, CEA-CNRS-Univ. Paris Diderot, Centre de Saclay, 91191 Gif-sur-Yvette, France
³Université Paris Diderot, Sorbonne Paris Cité, AIM, UMR 7158, CEA, CNRS, 91191 Gif-sur-Yvette, France
⁴Laboratoire de radioastronomie, UMR 8112 du CNRS, École normale supérieure et Observatoire de Paris, 24 rue Lhomond, 75231 Paris Cedex 05, France

Aims. Our aim is to study the thermal and dynamical evolution of protoplanetary discs in global simulations, including the physics of radiation transfer and magneto-hydrodynamic turbulence caused by the magneto-rotational instability.
Methods. We have developed a radiative transfer method based on the flux-limited diffusion approximation that includes frequency dependent irradiation by the central star. This hybrid scheme is implemented in the PLUTO code. The focus of our implementation is on the performance of the radiative transfer method. Using an optimized Jacobi preconditioned BiCGSTAB solver, the radiative module is three times faster than the magneto-hydrodynamic step for the disc set-up we consider. We obtain weak scaling efficiencies of 70% up to 1024 cores.
Results. We present the first global 3D radiation magneto-hydrodynamic simulations of a stratified protoplanetary disc. The disc model parameters were chosen to approximate those of the system AS 209 in the star-forming region Ophiuchus. Starting the simulation from a disc in radiative and hydrostatic equilibrium, the magneto-rotational instability quickly causes magneto-hydrodynamic turbulence and heating in the disc. We find that the turbulent properties are similar to that of recent locally isothermal global simulations of protoplanetary discs. For example, the rate of angular momentum transport α is a few times 10^-3. For the disc parameters we use, turbulent dissipation heats the disc midplane and raises the temperature by about 15% compared to passive disc models. The vertical temperature profile shows no temperature peak at the midplane as in classical viscous disc models. A roughly flat vertical temperature profile establishes in the optically thick region of the disc close to the midplane. We reproduce the vertical temperature profile with viscous disc models for which the stress tensor vertical profile is flat in the bulk of the disc and vanishes in the disc corona.
Conclusions. The present paper demonstrates for the first time that global radiation magneto-hydrodynamic simulations of turbulent protoplanetary discs are feasible with current computational facilities. This opens up the window to a wide range of studies of the dynamics of the inner parts of protoplanetary discs, for which there are significant observational constraints.

Reference
Flock M, Fromang S, González M and Commerçon B (2013) Radiation magnetohydrodynamics in global simulations of protoplanetary discs. Astronomy & Astrophysics 560:A43.
[doi:10.1051/0004-6361/201322451]
Reproduced with permission © ESO

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Althea V. Moorhead^a, Paul A. Wiegert^b and William J. Cooke^c

^aGeocent LLC, Jacobs ESSSA Group, Marshall Space Flight Center, Huntsville, Alabama 35812
^bDepartment of Physics and Astronomy, The University of Western Ontario, London N6A3K7, Canada
^cNASA Meteoroid Environment Office, Marshall Space Flight Center, Huntsville, Alabama 35812

Long-period comet C/2013 A1 (Siding Spring) will experience a close encounter with Mars on 2014 Oct 19. As of 2013 Oct 21, the distance of closest approach between the two is projected to be between 89,000 km and 173,000 km, with a nominal value of 131,000 km. Thus, a collision between the comet and the planet has been ruled out, but the comet’s coma may very well envelop Mars and its man-made satellites. We present a simple analytic model of the dust component of cometary comae that describes the spatial distribution of cometary dust and meteoroids and their size distribution. We find that this model successfully reproduces, to within an order of magnitude, particle fluxes measured by spacecraft Giotto in the coma of 1P/Halley and by spacecraft Stardust in the coma of 81P/Wild 2. We apply our analytic model to C/2013 A1 (Siding Spring) and compute the expected total fluence of potentially damaging particles at Mars at the time of closest approach between the two bodies; we obtain a nominal fluence of 0.15 particles per square meter. We conduct numerical simulations of particle ejection from the comet’s nucleus and compare the resulting spatial distribution with that of our analytic model, and conclude that our spherically symmetric analytic model is adequate for order-of-magnitude fluence estimates.

Reference
Moorhead AV, Wiegert PA andCooke WJ (in press) The meteoroid fluence at Mars due to comet C/2013 A1 (Siding Spring). Icarus
[doi:10.1016/j.icarus.2013.11.028]
Copyright Elsevier

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