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

Simulations of core convection and resulting dynamo action in rotating A-type stars

We present the results of 3--D nonlinear simulations of magnetic dynamo action by core convection within A-type stars of 2 solar masses, at a range of rotation rates. We consider the inner 30% by radius of such stars, with the spherical domain thereby encompassing the convective core and a portion of the surrounding radiative envelope. The compressible Navier-Stokes equations, subject to the anelastic approximation, are solved to examine highly nonlinear flows that span multiple scale heights, exhibit intricate time dependence, and admit magnetic dynamo action. Small initial seed magnetic fields are found to be amplified greatly by the convective and zonal flows. The central columns of strikingly slow rotation found in some of our progenitor hydrodynamic simulations continue to be realized in some simulations to a lesser degree, with such differential rotation arising from the redistribution of angular momentum by the nonlinear convection and magnetic fields. We assess the properties of the magnetic fields thus generated, the extent of convective penetration, the magnitude of the differential rotation, and the excitation of gravity waves within the radiative envelope.Comment: Talk at IAU Symposium 224: The A-Star Puzzle. 6 pages, 3 figures, 2 in color, compressed with appreciable loss of qualit

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

Reversible signal transmission in an active mechanical metamaterial

Mechanical metamaterials are designed to enable unique functionalities, but are typically limited by an initial energy state and require an independent energy input to function repeatedly. Our study introduces a theoretical active mechanical metamaterial that incorporates a biological reaction mechanism to overcome this key limitation of passive metamaterials. Our material allows for reversible mechanical signal transmission, where energy is reintroduced by the biologically motivated reaction mechanism. By analysing a coarse grained continuous analogue of the discrete model, we find that signals can be propagated through the material by a travelling wave. Analysis of the continuum model provides the region of the parameter space that allows signal transmission, and reveals similarities with the well-known FitzHugh-Nagumo system. We also find explicit formulae that approximate the effect of the timescale of the reaction mechanism on the signal transmission speed, which is essential for controlling the material.Comment: 20 pages, 7 figure

Accuracy of Physical Activity Monitors in Persons with Class III Obesity

Background Small, wearable monitors are widely used to assess physical activity (PA) in obesity treatment programs ranging from lifestyle interventions to post-bariatric surgical programs. Although wearable monitors can overcome the recall biases often associated with self-reports, the accuracy of these devices may be impacted by anthropometric measures, mode of PA, and wear location. Thus, it is important to examine the accuracy of objective PA monitors during commonly performed activities such as walking. Methods Fifteen individuals with class III obesity completed a self-paced 6-minute walk while wearing the StepWatch 3 (SW3), Omron, Digiwalker (DW), SenseWear Pro 2 Armband (SWA), and Fitbit objective PA monitors. Simultaneously, energy expenditure (EE) was measured using a portable indirect calorimeter. Height, weight, hip circumference, and waist circumference were also measured. Monitor values for step counts and Calories were compared to hand tally counts and indirect calorimetry (IC), respectively. Results Step-counting percent errors (PE) were not significantly different among the SW3 (PE=0.56%), Omron (PE=5.53%), and Fitbit (PE=4.33%). The DW significantly undercounted steps by 28% (p=0.037). The SWA overestimated EE by 71.6% (p=0.003), while the Fitbit’s 10% overestimate did not differ significantly from IC (p=0.114). Conclusion Objective monitors are useful for step counting and estimating energy expenditure, but consideration should be given to device accuracy when selecting evaluative tools for the bariatric population

Farming—It\u27s So Citified: An Urban Agriculture Marketing Campaign

The marketing campaign of the inaugural West Virginia Urban Agriculture Conference, a project between West Virginia State University (WVSU) and West Virginia University Extension Services and partnering agencies, was a tremendous success. The WVSU communications team developed an innovative, character-driven campaign combining urban and rural elements for a unique visual messaging strategy. The cornerstone of the campaign was Chicken Stu, the official spokes-chicken of the conference, who shared his journey from the farm to the city via social media. The campaign generated buzz, surpassed goals, and illustrates the impact of innovative social media marketing in promoting Extension initiatives

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