Equivalence of the Ehrenfest Theorem and the Fluid-rotor Model for Mixed Quantum/Classical Theory of Collisional Energy Transfer

The theory of two seemingly different quantum/classical approaches to collisional energy transfer and ro-vibrational energy flow is reviewed: a heuristic fluid-rotor method, introduced earlier to treat recombination reactions[M. Ivanov and D. Babikov, J. Chem. Phys.134, 144107 (Year: 2011)10.1063/1.3576103], and a more rigorous method based on the Ehrenfest theorem. It is shown analytically that for the case of a diatomic molecule + quencher these two methods are entirely equivalent. Notably, they both make use of the average moment of inertia computed as inverse of average of inverse of the distributed moment of inertia. Despite this equivalence, each of the two formulations has its own advantages, and is interesting on its own. Numerical results presented here illustrate energy and momentum conservation in the mixed quantum/classical approach and open opportunities for computationally affordable treatment of collisional energy transfer

Dynamo Effects Near The Transition from Solar to Anti-Solar Differential Rotation

Numerical MHD simulations play increasingly important role for understanding mechanisms of stellar magnetism. We present simulations of convection and dynamos in density-stratified rotating spherical fluid shells. We employ a new 3D simulation code for the solution of a physically consistent anelastic model of the process with a minimum number of parameters. The reported dynamo simulations extend into a "buoyancy-dominated" regime where the buoyancy forcing is dominant while the Coriolis force is no longer balanced by pressure gradients and strong anti-solar differential rotation develops as a result. We find that the self-generated magnetic fields, despite being relatively weak, are able to reverse the direction of differential rotation from anti-solar to solar-like. We also find that convection flows in this regime are significantly stronger in the polar regions than in the equatorial region, leading to non-oscillatory dipole-dominated dynamo solutions, and to concentration of magnetic field in the polar regions. We observe that convection has different morphology in the inner and at the outer part of the convection zone simultaneously such that organized geostrophic convection columns are hidden below a near-surface layer of well-mixed highly-chaotic convection. While we focus the attention on the buoyancy-dominated regime, we also demonstrate that conical differential rotation profiles and persistent regular dynamo oscillations can be obtained in the parameter space of the rotation-dominated regime even within this minimal model.Comment: Published in the Astrophysical Journa

An integral test of inelastic scattering cross sections using measured neutron spectra from thick shells of Ta, W, Mo, and Be

Integral test of inelastic scattering cross sections using measured neutron spectra from thick shells of Ta, W, Mo, and B

The stellar mass-accretion rate relation in T Tauri stars and brown dwarfs

Recent observations show a strong correlation between stellar mass and accretion rate in young stellar and sub-stellar objects, with the scaling $\dot{M}_{acc} \propto M_*^2$ holding over more than four orders of magnitude in accretion rate. We explore the consequences of this correlation in the context of disk evolution models. We note that such a correlation is not expected to arise from variations in disk angular momentum transport efficiency with stellar mass, and suggest that it may reflect a systematic trend in disk initial conditions. In this case we find that brown dwarf disks initially have rather larger radii than those around more massive objects. By considering disk evolution, and invoking a simple parametrization for a shut-off in accretion at the end of the disk lifetime, we show that such models predict that the scatter in the stellar mass-accretion rate relationship should increase with increasing stellar mass, in rough agreement with current observations.Comment: 4 pages, 2 figures. Accepted for publication in ApJ Letter