The magnitude of viscous dissipation in strongly stratified two-dimensional convection

Convection in astrophysical systems must be maintained against dissipation. Although the effects of dissipation are often assumed to be negligible, theory suggests that in strongly stratified convecting fluids, the dissipative heating rate can exceed the luminosity carried by convection. Here we explore this possibility using a series of numerical simulations. We consider two-dimensional numerical models of hydrodynamic convection in a Cartesian layer under the anelastic approximation and demonstrate that the dissipative heating rate can indeed exceed the imposed luminosity. We establish a theoretical expression for the ratio of the dissipative heating rate to the luminosity emerging at the upper boundary, in terms only of the depth of the layer and the thermal scale height. In particular, we show that this ratio is independent of the diffusivities and confirm this with a series of numerical simulations. Our results suggest that dissipative heating may significantly alter the internal dynamics of stars and planets.Comment: 8 pages, 5 figures, accepted for publication in ApJ Letter

Modeling the Rise of Fibril Magnetic Fields in Fully Convective Stars

Many fully convective stars exhibit a wide variety of surface magnetism, including starspots and chromospheric activity. The manner by which bundles of magnetic field traverse portions of the convection zone to emerge at the stellar surface is not especially well understood. In the Solar context, some insight into this process has been gleaned by regarding the magnetism as consisting partly of idealized thin flux tubes (TFT). Here, we present the results of a large set of TFT simulations in a rotating spherical domain of convective flows representative of a 0.3 solar-mass, main-sequence star. This is the first study to investigate how individual flux tubes in such a star might rise under the combined influence of buoyancy, convection, and differential rotation. A time-dependent hydrodynamic convective flow field, taken from separate 3D simulations calculated with the anelastic equations, impacts the flux tube as it rises. Convective motions modulate the shape of the initially buoyant flux ring, promoting localized rising loops. Flux tubes in fully convective stars have a tendency to rise nearly parallel to the rotation axis. However, the presence of strong differential rotation allows some initially low latitude flux tubes of moderate strength to develop rising loops that emerge in the near-equatorial region. Magnetic pumping suppresses the global rise of the flux tube most efficiently in the deeper interior and at lower latitudes. The results of these simulations aim to provide a link between dynamo-generated magnetic fields, fluid motions, and observations of starspots for fully convective stars.Comment: 20 pages, 15 figures, accepted to Astrophysical Journa

Differential Rotation and Magnetism in Simulations of Fully Convective Stars

Stars of sufficiently low mass are convective throughout their interiors, and so do not possess an internal boundary layer akin to the solar tachocline. Because that interface figures so prominently in many theories of the solar magnetic dynamo, a widespread expectation had been that fully convective stars would exhibit surface magnetic behavior very different from that realized in more massive stars. Here I describe how recent observations and theoretical models of dynamo action in low-mass stars are partly confirming, and partly confounding, this basic expectation. In particular, I present the results of 3--D MHD simulations of dynamo action by convection in rotating spherical shells that approximate the interiors of 0.3 solar-mass stars at a range of rotation rates. The simulated stars can establish latitudinal differential rotation at their surfaces which is solar-like at ``rapid'' rotation rates (defined within) and anti-solar at slower rotation rates; the differential rotation is greatly reduced by feedback from strong dynamo-generated magnetic fields in some parameter regimes. I argue that this ``flip'' in the sense of differential rotation may be observable in the near future. I also briefly describe how the strength and morphology of the magnetic fields varies with the rotation rate of the simulated star, and show that the maximum magnetic energies attained are compatible with simple scaling arguments.Comment: 9 pages, 2 color figures, to appear in Proc. IAU Symposium 271, "Astrophysical Dynamics: from Stars to Galaxies

Magnetic processes in astrophysics: theory, simulations, experiments

Copyright © 2014 Taylor & Francis. This is an Accepted Manuscript of an book review published by Taylor & Francis in Geophysical & Astrophysical Fluid Dynamics on 21 October 2014, available online: http://www.tandfonline.com/10.1080/03091929.2014.964919Book Review Magnetic processes in astrophysics: theory, simulations, experiments, by Gunther Rudiger, Rainer Hollerbach, and Leonid L. Kitchatinov, Wiley-VCH Verlag GmbH & Co. KGaA, Boschstr. 12, 69469 Weinheim, Germany, 2013, 356 pp., hardcover (E-book also available) (ISBN 978-3-527-41034-7

Theoretical limits on magnetic field strengths in low-mass stars

Observations have suggested that some low-mass stars have larger radii than predicted by 1-D structure models. Some theoretical models have invoked very strong interior magnetic fields (of order 1 MG or more) as a possible cause of such large radii. Whether fields of that strength could in principle by generated by dynamo action in these objects is unclear, and we do not address the matter directly. Instead, we examine whether such fields could remain in the interior of a low mass object for a significant time, and whether they would have any other obvious signatures. First, we estimate timescales for the loss of strong fields by magnetic buoyancy instabilities. We consider a range of field strengths and simple morphologies, including both idealized flux tubes and smooth layers of field. We confirm some of our analytical estimates using thin flux tube magnetohydrodynamic (MHD) simulations of the rise of buoyant fields in a fully-convective M-dwarf. Separately, we consider the Ohmic dissipation of such fields. We find that dissipation provides a complementary constraint to buoyancy: while small-scale, fibril fields might be regenerated faster than they rise, the dissipative heating associated with such fields would in some cases greatly exceed the luminosity of the star. We show how these constraints combine to yield limits on the internal field strength and morphology in low-mass stars. In particular, we find that for stars of 0.3 solar masses, no fields in flux tubes stronger than about 800 kG are simultaneously consistent with both constraints.Comment: 19 pages, 10 figures, accepted to Ap

Inferring physical conditions in interstellar clouds of H_2

We have developed a code that models the formation, destruction, radiative transfer, and vibrational/rotational excitation of H_2 in a detailed fashion. We discuss how such codes, together with FUSE observations of H_2 in diffuse and translucent lines of sight, may be used to infer various physical parameters. We illustrate the effects of changes in the major physical parameters (UV radiation field, gas density, metallicity), and we point out the extent to which changes in one parameter may be mirrored by changes in another. We provide an analytic formula for the molecular fraction, f_H2, as a function of cloud column density, radiation field, and grain formation rate of H_2. Some diffuse and translucent lines of sight may be concatenations of multiple distinct clouds viewed together. Such situations can give rise to observables that agree with the data, complicating the problem of uniquely identifying one set of physical parameters with a line of sight. Finally, we illustrate the application of our code to an ensemble of data, such as the FUSE survey of H_2 in the Large and Small Magellanic Clouds (LMC/SMC), in order to constrain the elevated UV radiation field intensity and reduced grain formation rate of H_2 in those low- metallicity environments.Comment: 33 pages (aastex, manuscript), 9 figures (3 color). accepted to Ap