195 research outputs found
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
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