162 research outputs found
Magnetic field generation in fully convective rotating spheres
Magnetohydrodynamic simulations of fully convective, rotating spheres with
volume heating near the center and cooling at the surface are presented. The
dynamo-generated magnetic field saturates at equipartition field strength near
the surface. In the interior, the field is dominated by small-scale structures,
but outside the sphere by the global scale. Azimuthal averages of the field
reveal a large-scale field of smaller amplitude also inside the star. The
internal angular velocity shows some tendency to be constant along cylinders
and is ``anti-solar'' (fastest at the poles and slowest at the equator).Comment: 12 pages, 11 figures, 2 tables, to appear in the 10 Feb issue of Ap
ENHANCING THE FINANCIAL PERFORMANCE OF SMALL MEAT PROCESSORS
The small firms examined produce meats in the State of Texas and emphasize such products as sausage, jerky, brisket, and fresh meats. The authors test hypotheses with the intent to identify operational factors associated with firm financial success. A quartile model and an econometric model are both used for this purpose. Results generally suggest important factors for firms to be profitable include product selection, pricing strategies, special equipment, and location.Agribusiness, Agricultural Finance,
A Physical Limit to the Magnetic Fields of T Tauri Stars
Recent estimates of magnetic field strengths in T Tauri stars yield values
--. In this paper, I present an upper limit to the
photospheric values of by computing the equipartition values for different
surface gravities and effective temperatures. The values of derived from
the observations exceed this limit, and I examine the possible causes for this
discrepancy
Flux-transport dynamos with Lorentz force feedback on differential rotation and meridional flow: Saturation mechanism and torsional oscillations
In this paper we discuss a dynamic flux-transport dynamo model that includes
the feedback of the induced magnetic field on differential rotation and
meridional flow. We consider two different approaches for the feedback:
meanfield Lorentz force and quenching of transport coefficients such as
turbulent viscosity and heat conductivity. We find that even strong feedback on
the meridional flow does not change the character of the flux-transport dynamo
significantly; however it leads to a significant reduction of differential
rotation. To a large degree independent from the dynamo parameters, the
saturation takes place when the toroidal field at the base of the convection
zone reaches between 1.2 an 1.5 T, the energy converted intomagnetic energy
corresponds to about 0.1 to 0.2% of the solar luminosity. The torsional
oscillations produced through Lorentz force feedback on differential rotation
show a dominant poleward propagating branch with the correct phase relation to
the magnetic cycle. We show that incorporating enhanced surface cooling of the
active region belt (as proposed by Spruit) leads to an equatorward propagating
branch in good agreement with observations.Comment: 15 pages, 12 figures, Accepted for publication in ApJ August 10
issue; corrected typos, corrected referenc
Simulations of turbulent convection in rotating young solar-like stars: Differential rotation and meridional circulation
We present the results of three-dimensional simulations of the deep
convective envelope of a young (10 Myr) one-solar-mass star, obtained with the
Anelastic Spherical Harmonic code. Since young stars are known to be faster
rotators than their main sequence counterparts, we have systematically studied
the impact of the stellar rotation speed, by considering stars spinning up to
five times as fast as the Sun. The aim of these nonlinear models is to
understand the complex interactions between convection and rotation. We discuss
the influence of the turbulence level and of the rotation rate on the intensity
and the topology of the mean flows. For all of the computed models, we find a
solar-type superficial differential rotation, with an equatorial acceleration,
and meridional circulation that exhibits a multicellular structure. Even if the
differential rotation contrast decreases only marginally for high rotation
rates, the meridional circulation intensity clearly weakens according to our
simulations. We have also shown that, for Taylor numbers above a certain
threshold (Ta>10^9), the convection can develop a vacillating behavior. Since
simulations with high turbulence levels and rotation rates exhibit strongly
cylindrical internal rotation profiles, we have considered the influence of
baroclinic effects at the base of the convective envelope of these young Suns,
to see whether such effect can modify the otherwise near cylindrical profiles
to produce more conical, solar-like profiles.Comment: 32 pages, 18 figures, 2 tables, to appear in Ap
Simulations of dynamo action in fully convective stars
We present three-dimensional nonlinear magnetohydrodynamic simulations of the
interiors of fully convective M-dwarfs. Our models consider 0.3 solar-mass
stars using the Anelastic Spherical Harmonic code, with the spherical
computational domain extending from 0.08-0.96 times the overall stellar radius.
Like previous authors, we find that fully convective stars can generate
kG-strength magnetic fields (in rough equipartition with the convective flows)
without the aid of a tachocline of shear. Although our model stars are
everywhere unstably stratified, the amplitudes and typical pattern sizes of the
convective flows vary strongly with radius, with the outer regions of the stars
hosting vigorous convection and field amplification while the deep interiors
are more quiescent. Modest differential rotation is established in hydrodynamic
calculations, but -- unlike in some prior work --strongly quenched in MHD
simulations because of the Maxwell stresses exerted by the dynamo-generated
magnetic fields. Despite the lack of strong differential rotation, the magnetic
fields realized in the simulations possess significant mean (axisymmetric)
components, which we attribute partly to the strong influence of rotation upon
the slowly overturning flows.Comment: Accepted to the ApJ. 20 pages (emulateapj), 4 color figures
compressed to low-resolution; higher-resolution equivalents are available at
http://lcd-www.colorado.edu/~brownim/browning_2007_mstars.pd
Convective Dynamos and the Minimum X-ray Flux in Main Sequence Stars
The objective of this paper is to investigate whether a convective dynamo can
account quantitatively for the observed lower limit of X-ray surface flux in
solar-type main sequence stars. Our approach is to use 3D numerical simulations
of a turbulent dynamo driven by convection to characterize the dynamic
behavior, magnetic field strengths, and filling factors in a non-rotating
stratified medium, and to predict these magnetic properties at the surface of
cool stars. We use simple applications of stellar structure theory for the
convective envelopes of main-sequence stars to scale our simulations to the
outer layers of stars in the F0--M0 spectral range, which allows us to estimate
the unsigned magnetic flux on the surface of non-rotating reference stars. With
these estimates we use the recent results of \citet{Pevtsov03} to predict the
level of X-ray emission from such a turbulent dynamo, and find that our results
compare well with observed lower limits of surface X-ray flux. If we scale our
predicted X-ray fluxes to \ion{Mg}{2} fluxes we also find good agreement with
the observed lower limit of chromospheric emission in K dwarfs. This suggests
that dynamo action from a convecting, non-rotating plasma is a viable
alternative to acoustic heating models as an explanation for the basal emission
level seen in chromospheric, transition region, and coronal diagnostics from
late-type stars.Comment: ApJ, accepted, 30 pages with 7 figure
X-rays in the Orion Nebula Cluster: Constraints on the origins of magnetic activity in pre-main sequence stars
A recent Chandra/ACIS observation of the Orion Nebula Cluster detected 1075
sources (Feigelson et al. 2002), providing a uniquely large and well-defined
sample to study the dependence of magnetic activity on bulk properties for
stars descending the Hayashi tracks. The following results are obtained: (1)
X-ray luminosities L_t in the 0.5-8 keV band are strongly correlated with
bolometric luminosity with = -3.8 for stars with masses 0.7<M<2
Mo, an order of magnitude below the main sequence saturation level; (2) the
X-ray emission drops rapidly below this level in some or all stars with 2<M<3
Mo; (3) the presence or absence of infrared circumstellar disks has no apparent
relation to X-ray levels; and (4) X-ray luminosities exhibit a slight rise as
rotational periods increase from 0.4 to 20 days. This last finding stands in
dramatic contrast to the strong anticorrelation between X-rays and period seen
in main sequence stars.
The absence of a strong X-ray/rotation relationship in PMS stars, and
particularly the high X-ray values seen in some very slowly rotating stars, is
a clear indication that the mechanisms of magnetic field generation differ from
those operating in main sequence stars. The most promising possibility is a
turbulent dynamo distributed throughout the deep convection zone, but other
models such as alpha-Omega dynamo with `supersaturation' or relic core fields
are not immediately excluded. The drop in magnetic activity in
intermediate-mass stars may reflect the presence of a significant radiative
core. The evidence does not support X-ray production in large-scale star-disk
magnetic fields.Comment: 51 pages, 8 figures. To appear in the Astrophysical Journa
The Variability and Rotation of Pre-main Sequence Stars in IC 348: Does Intracluster Environment Influence Stellar Rotation?
A variability study of the young cluster IC 348 at Van Vleck Observatory has
been extended to a total of seven years. Twelve new periodic stars have been
found in the last two years, bringing the total discovered by this program to
40. In addition, we confirm 16 of the periods reported by others and resolve
some discrepancies. The total number of known rotation periods in the cluster,
from all studies has now reached 70. This is sufficient to demonstrate that the
parent population of K5-M2 stars is rotationally indistinguishable from that in
the Orion Nebula Cluster even though their radii are 20% smaller and they would
be expected to spin about twice as fast if angular momentum were conserved. The
median radius and, therefore, inferred age of the IC 348 stars actually closely
matches that of NGC 2264, but the stars spin significantly more slowly. This
suggests that another factor besides mass and age plays a role in establishing
the rotation properties within a cluster and we suggest that it is environment.
If disk locking were to persist for longer times in less harsh environments,
because the disks themselves persist for longer times, it could explain the
generally slower rotation rates observed for stars in this cluster, whose
earliest type star is of class B5. We have also obtained radial velocities, the
first for PMS stars in IC348, and v sin i measurements for 30 cluster stars to
assist in the study of rotation and as an independent check on stellar radii.
Several unusual variable stars are discussed; in some or all cases their
behavior may be linked to occultations by circumstellar material. A strong
correlation exists between the range of photometric variability and the slope
of the spectral energy distribution in the infrared. Nineteen of the 21 stars
with I ranges exceeding 0.4 mag show infrared evidence for circumstellar disks.Comment: Accepted for publication in A
X-ray emission from young brown dwarfs in the Orion Nebula Cluster
We use the sensitive X-ray data from the Chandra Orion Ultradeep Project
(COUP) to study the X-ray properties of 34 spectroscopically-identified brown
dwarfs with near-infrared spectral types between M6 and M9 in the core of the
Orion Nebula Cluster. Nine of the 34 objects are clearly detected as X-ray
sources. The apparently low detection rate is in many cases related to the
substantial extinction of these brown dwarfs; considering only the BDs with
mag, nearly half of the objects (7 out of 16) are detected in
X-rays. Our 10-day long X-ray lightcurves of these objects exhibit strong
variability, including numerous flares. While one of the objects was only
detected during a short flare, a statistical analysis of the lightcurves
provides evidence for continuous (`quiescent') emission in addition to flares
for all other objects. Of these, the M9 brown dwarf COUP 1255 = HC 212
is one of the coolest known objects with a clear detection of quiescent X-ray
emission. The X-ray properties (spectra, fractional X-ray luminosities, flare
rates) of these young brown dwarfs are similar to those of the low-mass stars
in the ONC, and thus there is no evidence for changes in the magnetic activity
around the stellar/substellar boundary, which lies at M6 for ONC
sources. Since the X-ray properties of the young brown dwarfs are also similar
to those of M6--M9 field stars, the key to the magnetic activity in very cool
objects seems to be the effective temperature, which determines the degree of
ionization in the atmosphere.Comment: accepted for ApJS, COUP special issu
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