53,534 research outputs found
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
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
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
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
The unusual distribution of molecular gas and star formation in Arp 140
We investigate the atomic and molecular interstellar medium and star
formation of NGC 275, the late-type spiral galaxy in Arp 140, which is
interacting with NGC 274, an early-type system. The atomic gas (HI)
observations reveal a tidal tail from NGC 275 which extends many optical radii
beyond the interacting pair. The HI morphology implies a prograde encounter
between the galaxy pair approximately 1.5 x 10**8 years ago. The Halpha
emission from NGC 275 indicates clumpy irregular star-formation, clumpiness
which is mirrored by the underlying mass distribution as traced by the Ks-band
emission. The molecular gas distribution is striking in its anti-correlation
with the {HII regions. Despite the evolved nature of NGC 275's interaction and
its barred potential, neither the molecular gas nor the star formation are
centrally concentrated. We suggest that this structure results from stochastic
star formation leading to preferential consumption of the gas in certain
regions of the galaxy. In contrast to the often assumed picture of interacting
galaxies, NGC 275, which appears to be close to merger, does not display
enhanced or centrally concentrated star formation. If the eventual merger is to
lead to a significant burst of star formation it must be preceded by a
significant conversion of atomic to molecular gas as at the current rate of
star formation all the molecular gas will be exhausted by the time the merger
is complete.Comment: 13 paper, accepted my Monthly Notices of the Royal Astronomical
Societ
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