Theoretical astrophysics and plasma physics at RPC

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Multimass schemes for collisionless N-body simulations

Monthly Notices of the Royal Astronomical Society 387:4 (2008) 1719-1726

Authors:

M Zhang, J Magorrian

Abstract:

We present a general scheme for constructing Monte Carlo realizations of equilibrium, collisionless galaxy models with known distribution function (DF) f 0. Our method uses importance sampling to find the sampling DF f s that minimizes the mean-square formal errors in a given set of projections of the DF f 0. The result is a multimass N-body realization of the galaxy model in which 'interesting' regions of phase space are densely populated by lots of low-mass particles, increasing the effective N there, and less interesting regions by fewer, higher mass particles. As a simple application, we consider the case of minimizing the shot noise in estimates of the acceleration field for an N-body model of a spherical Hernquist model. Models constructed using our scheme easily yield a factor of ~100 reduction in the variance at the central acceleration field when compared to a traditional equal-mass model with the same number of particles. When evolving both models with a real N-body code, the diffusion coefficients in our model are reduced by a similar factor. Therefore, for certain types of problems, our scheme is a practical method for reducing the two-body relaxation effects, thereby bringing the N-body simulations closer to the collisionless ideal. © 2008 The Author. Journal compilation © 2008 RAS.
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Periastron Precession Measurements in Transiting Extrasolar Planetary Systems at the Level of General Relativity

(2008)

Authors:

András Pál, Bence Kocsis
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The Radial Velocity Experiment (RAVE): second data release

ArXiv 0806.0546 (2008)

Authors:

T Zwitter, A Siebert, U Munari, KC Freeman, A Siviero, FG Watson, JP Fulbright, RFG Wyse, R Campbell, GM Seabroke, M Williams, M Steinmetz, O Bienayme, G Gilmore, EK Grebel, A Helmi, JF Navarro, B Anguiano, C Boeche, D Burton, P Cass, J Dawe, K Fiegert, M Hartley, K Russell, L Veltz, J Bailin, J Binney, J Bland-Hawthorn, A Brown, W Dehnen, NW Evans, P Re Fiorentin, M Fiorucci, O Gerhard, B Gibson, A Kelz, K Kujken, G Matijevic, I Minchev, QA Parker, J Penarrubia, A Quillen, MA Read, W Reid, S Roeser, G Ruchti, R-D Scholz, MC Smith, R Sordo, E Tolstoi, L Tomasella, S Vidrih, E Wylie de Boer

Abstract:

We present the second data release of the Radial Velocity Experiment (RAVE), an ambitious spectroscopic survey to measure radial velocities (RVs) and stellar atmosphere parameters of up to one million stars using the 6dF multi-object spectrograph on the 1.2-m UK Schmidt Telescope of the Anglo-Australian Observatory (AAO). It is obtaining medium resolution spectra (median R=7,500) in the Ca-triplet region (8,410--8,795 \AA) for southern hemisphere stars in the magnitude range 9
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Disassembling the Galaxy with angle-action coordinates

ArXiv 0806.0319 (2008)

Authors:

Paul J McMillan, James J Binney

Abstract:

Angle-action coordinates are used to study the relic of an N-body simulation of a self-gravitating satellite galaxy that was released on a short-period orbit within the disc of the Galaxy. Satellite stars that lie within 1.5 kpc of the Sun are confined to a grid of patches in action space. As the relic phase-mixes for longer, the patches become smaller and more numerous. These patches can be seen even when the angle-action coordinates of an erroneous Galactic potential are used, but using the wrong potential displaces them. Diagnostic quantities constructed from the angle coordinates both allow the true potential to be identified, and the relic to be dated. Hence when the full phase space coordinates of large numbers of solar-neighbourhood stars are known, it should be possible to identify members of particular relics from the distribution of stars in an approximate action space. This would then open up the possibility of determining the time since the relic was disrupted and gaining better knowledge of the Galactic potential. The availability of angle-action coordinates for arbitrary potentials is the key to these developments. The paper includes a brief introduction to the torus technique used to generate them.
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Limitations of gyrokinetics on transport time scales

Plasma Physics and Controlled Fusion 50:6 (2008)

Authors:

FI Parra, PJ Catto

Abstract:

We present a new recursive procedure to find a full f electrostatic gyrokinetic equation correct to first order in an expansion of gyroradius over magnetic field characteristic length. The procedure provides new insights into the limitations of the gyrokinetic quasineutrality equation. We find that the ion distribution function must be known at least to second order in gyroradius over characteristic length to calculate the long wavelength components of the electrostatic potential self-consistently. Moreover, using the example of a steady-state -pinch, we prove that the quasineutrality equation fails to provide the axisymmetric piece of the potential even with a distribution function correct to second order. We also show that second order accuracy is enough if a more convenient moment equation is used instead of the quasineutrality equation. These results indicate that the gyrokinetic quasineutrality equation is not the most effective procedure to find the electrostatic potential if the long wavelength components are to be retained in the analysis. © 2008 IOP Publishing Ltd.
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