290 research outputs found
Modelling turbulent fluxes due to thermal convection in rectilinear shearing flow
We revisit a phenomenological description of turbulent thermal convection
along the lines proposed originally by Gough (1965) in which eddies grow solely
by extracting energy from the unstably stratified mean state and are
subsequently destroyed by internal shear instability. This work is part of an
ongoing investigation for finding a procedure to calculate the turbulent fluxes
of heat and momentum in the presence of a shearing background flow in stars.Comment: 2 pages, 1 figure, accepted for publication in IAU Symposium 271
"Astrophysical Dynamics: From Galaxies to Stars", Nice, 201
Helioseismic detection of deep meridional flow
Steady meridional flow makes no first-order perturbation to the frequencies
of helioseismic normal modes. It does, however, Doppler shift the local
wavenumber, thereby distorting the eigenfunctions. For high-degree modes, whose
peaks in a power spectrum are blended into continuous ridges, the effect of the
distortion is to shift the locations of those ridges. From this blended
superposition of modes, one can isolate oppositely directed wave components
with the same local horizontal wavenumber and measure a frequency difference
which can be safely used to infer the subsurface background flow. But such a
procedure fails for the components of the more-deeply-penetrating low-degree
modes that are not blended into ridges. Instead, one must analyze the spatial
distortions explicitly. With a simple toy model, we illustrate one method by
which that might be accomplished by measuring the spatial variation of the
oscillation phase. We estimate that by this procedure it might be possible to
infer meridional flow deep in the solar convection zone.Comment: 23 pages, 9 color figures, submitted to the Astrophysical Journa
Prospects for Measuring Differential Rotation in White Dwarfs Through Asteroseismology
We examine the potential of asteroseismology for exploring the internal
rotation of white dwarf stars. Data from global observing campaigns have
revealed a wealth of frequencies, some of which show the signature of
rotational splitting. Tools developed for helioseismology to use many solar
p-mode frequencies for inversion of the rotation rate with depth are adapted to
the case of more limited numbers of modes of low degree. We find that the small
number of available modes in white dwarfs, coupled with the similarity between
the rotational-splitting kernels of the modes, renders direct inversion
unstable. Accordingly, we adopt what we consider to be plausible functional
forms for the differential rotation profile; this is sufficiently restrictive
to enable us to carry out a useful calibration. We show examples of this
technique for PG 1159 stars and pulsating DB white dwarfs. Published frequency
splittings for white dwarfs are currently not accurate enough for meaningful
inversions; reanalysis of existing data can provide splittings of sufficient
accuracy when the frequencies of individual peaks are extracted via
least-squares fitting or multipeak decompositions. We find that when mode
trapping is evident in the period spacing of g modes, the measured splittings
can constrain dOmega/dr.Comment: 26 pages, 20 postscript figures. Accepted for publication in The
Astrophysical Journa
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