247 research outputs found
Quiescent X-ray emission from an evolved brown dwarf ?
I report on the X-ray detection of Gl569Bab. During a 25ksec Chandra
observation the binary brown dwarf is for the first time spatially separated in
X-rays from the flare star primary Gl569A. Companionship to Gl569A constrains
the age of the brown dwarf pair to ~300-800 Myr. The observation presented here
is only the second X-ray detection of an evolved brown dwarf. About half of the
observing time is dominated by a large flare on Gl569Bab, the remainder is
characterized by weak and non-variable emission just above the detection limit.
This emission -- if not related to the afterglow of the flare -- represents the
first detection of a quiescent corona on a brown dwarf, representing an
important piece in the puzzle of dynamos in the sub-stellar regime.Comment: to appear in ApJ
Magnetic diffusivity tensor and dynamo effects in rotating and shearing turbulence
The turbulent magnetic diffusivity tensor is determined in the presence of
rotation or shear. The question is addressed whether dynamo action from the
shear-current effect can explain large-scale magnetic field generation found in
simulations with shear. For this purpose a set of evolution equations for the
response to imposed test fields is solved with turbulent and mean motions
calculated from the momentum and continuity equations. The corresponding
results for the electromotive force are used to calculate turbulent transport
coefficients. The diagonal components of the turbulent magnetic diffusivity
tensor are found to be very close together, but their values increase slightly
with increasing shear and decrease with increasing rotation rate. In the
presence of shear, the sign of the two off-diagonal components of the turbulent
magnetic diffusion tensor is the same and opposite to the sign of the shear.
This implies that dynamo action from the shear--current effect is impossible,
except perhaps for high magnetic Reynolds numbers. However, even though there
is no alpha effect on the average, the components of the alpha tensor display
Gaussian fluctuations around zero. These fluctuations are strong enough to
drive an incoherent alpha--shear dynamo. The incoherent shear--current effect,
on the other hand, is found to be subdominant.Comment: 12 pages, 13 figures, improved version, accepted by Ap
Rotation and Activity in Mid-M to L Dwarfs
We analyze rotation velocities and chromospheric (H-alpha) activity, derived
from multi-year, high-resolution spectra, in 56 mid-M to L dwarfs. Rotational
velocities are found to increase from mid-M to L. This is consistent with a
lengthening of spin-down timescale with later type, though in the L types the
trend may also be a function of stellar age. From M5 to M8.5, a saturation-type
rotation-activity relation is seen, similar to that in earlier types. However,
the saturation velocity in our case is much higher, at about 12 km/s. A sharp
drop in activity is observed at about M9, with later types showing little or no
H-alpha emission, in spite of rapid rotation. This may be due to the very high
resistivities in the predominantly neutral atmospheres of these cool objects.Comment: 7 pages, 4 figures, to be published in proceedings of 'Cool Stars and
the Sun 12
Waves, Coriolis force and the dynamo effect
Dynamo activity caused by waves in a rotating magneto-plasma is investigated.
In astrophysical environments such as accretion disks and at sufficiently small
spatial scales, the Hall effect is likely to play an important role. It is
shown that a combination of the Coriolis force and Hall effect can produce a
finite -effect by generating net helicity in the small scales. The
shear/ion-cyclotron normal mode of the Hall plasma is the dominant contributor
to the dynamo action for short scale motions.Comment: 8 pages, 3 figures, ApJ (in press
Quiescent Radio Emission from Southern Late-type M Dwarfs and a Spectacular Radio Flare from the M8 Dwarf DENIS 1048-3956
We report the results of a radio monitoring program conducted at the
Australia Telescope Compact Array to search for quiescent and flaring emission
from seven nearby Southern late-type M and L dwarfs. Two late-type M dwarfs,
the M7 V LHS 3003 and the M8 V DENIS 1048-3956, were detected in quiescent
emission at 4.80 GHz. The observed emission is consistent with optically thin
gyrosynchrotron emission from mildly relativistic (~1-10 keV) electrons with
source densities n_e ~ 10 G magnetic fields. DENIS
1048-3956 was also detected in two spectacular, short-lived flares, one at 4.80
GHz (peak f_nu = 6.0+/-0.8 mJy) and one at 8.64 GHz (peak f_nu = 29.6+/-1.0
mJy) approximately 10 minutes later. The high brightness temperature (T_B >~
10^13 K), short emission period (~4-5 minutes), high circular polarization
(~100%), and apparently narrow spectral bandwidth of these events imply a
coherent emission process in a region of high electron density (n_e ~
10^11-10^12 cm^-3) and magnetic field strength (B ~ 1 kG). If the two flare
events are related, the apparent frequency drift in the emission suggests that
the emitting source either moved into regions of higher electron or magnetic
flux density; or was compressed, e.g., by twisting field lines or gas motions.
The quiescent fluxes from the radio-emitting M dwarfs violate the Gudel-Benz
empirical radio/X-ray relations, confirming a trend previously noted by Berger
et al. (abridged)Comment: 28 pages, 8 figures, accepted for publication in Ap
Dynamical evolution of neutrino--cooled accretion disks: detailed microphysics, lepton-driven convection, and global energetics
We present a detailed, two dimensional numerical study of the microphysical
conditions and dynamical evolution of accretion disks around black holes when
neutrino emission is the main source of cooling. Such structures are likely to
form after the gravitational collapse of massive rotating stellar cores, or the
coalescence of two compact objects in a binary (e.g., the Hulse--Taylor
system). The physical composition is determined self consistently by
considering two regimes: neutrino--opaque and neutrino--transparent, with a
detailed equation of state which takes into account neutronization, nuclear
statistical equilibrium of a gas of free nucleons and alpha particles,
blackbody radiation and a relativistic Fermi gas of arbitrary degeneracy.
Various neutrino emission processes are considered, with electron/positron
capture onto free nucleons providing the dominant contribution to the cooling
rate. We find that important temporal and spatial scales, related to the
optically thin/optically thick transition are present in the disk, and manifest
themselves clearly in the energy output in neutrinos. This transition produces
an inversion of the lepton gradient in the innermost regions of the flow which
drives convective motions, and affects the density and disk scale height radial
profiles. The electron fraction remains low in the region close to the black
hole, and if preserved in an outflow, could give rise to heavy element
nucleosynthesis. Our specific initial conditions arise from the binary merger
context, and so we explore the implications of our results for the production
of gamma ray bursts.Comment: 26 pages, 12 figures, to appear in Ap
Overcoming the Back Reaction on Turbulent Motions in the Presence of Magnetic Fields
Standard magnetohydrodynamic theories, such as the mean field dynamo theory,
have been criticized when the back reaction of the magnetic field on turbulent
motions is neglected. For the dynamo, this back reaction has been argued to
suppress the turbulent motions required for optimal mean field production. Here
it is suggested that if the magnetic field is spatially intermittent, for
example residing in flux tubes, the back reaction on turbulent flows may be
significantly reduced.Comment: Accepted to Physical Review Letters, 15 pages, plain Te
Blockade of T-cell activation by dithiocarbamates involves novel mechanisms of inhibition of nuclear factor of activated T cells.
Dithiocarbamates (DTCs) have recently been reported as powerful inhibitors of NF-kappaB activation in a number of cell types. Given the role of this transcription factor in the regulation of gene expression in the inflammatory response, NF-kappaB inhibitors have been suggested as potential therapeutic drugs for inflammatory diseases. We show here that DTCs inhibited both interleukin 2 (IL-2) synthesis and membrane expression of antigens which are induced during T-cell activation. This inhibition, which occurred with a parallel activation of c-Jun transactivating functions and expression, was reflected by transfection experiments at the IL-2 promoter level, and involved not only the inhibition of NF-kappaB-driven reporter activation but also that of nuclear factor of activated T cells (NFAT). Accordingly, electrophoretic mobility shift assays (EMSAs) indicated that pyrrolidine DTC (PDTC) prevented NF-kappaB, and NFAT DNA-binding activity in T cells stimulated with either phorbol myristate acetate plus ionophore or antibodies against the CD3-T-cell receptor complex and simultaneously activated the binding of AP-1. Furthermore, PDTC differentially targeted both NFATp and NFATc family members, inhibiting the transactivation functions of NFATp and mRNA induction of NFATc. Strikingly, Western blotting and immunocytochemical experiments indicated that PDTC promoted a transient and rapid shuttling of NFATp and NFATc, leading to their accelerated export from the nucleus of activated T cells. We propose that the activation of an NFAT kinase by PDTC could be responsible for the rapid shuttling of the NFAT, therefore transiently converting the sustained transactivation of this transcription factor that occurs during lymphocyte activation, and show that c-Jun NH2-terminal kinase (JNK) can act by directly phosphorylating NFATp. In addition, the combined inhibitory effects on NFAT and NF-KB support a potential use of DTCs as immunosuppressants
On the rotational evolution of solar- and late-type stars, its magnetic origins, and the possibility of stellar gyrochronology
We propose a simple interpretation of the rotation period data for solar- and
late-type stars. The open cluster and Mt. Wilson star observations suggest that
rotating stars lie primarily on two sequences, initially called I and C. Some
stars lie in the intervening gap. These sequences, and the fractional numbers
of stars on each sequence evolve systematically with cluster age, enabling us
to construct crude rotational isochrones allowing `stellar gyrochronology', a
procedure, upon improvement, likely to yield ages for individual field stars.
The age and color dependences of the sequences allow the identification of the
underlying mechanism, which appears to be primarily magnetic. The majority of
solar- and late-type stars possess a dominant Sun-like, or Interface magnetic
field, which connects the convective envelope both to the radiative interior of
the star and to the exterior where winds can drain off angular momentum. These
stars spin down Skumanich-style. An age-decreasing fraction of young G, K, and
M stars, which are rapid rotators, possess only a Convective field which is not
only inefficient in depleting angular momentum, but also incapable of coupling
the surface convection zone to the inner radiative zone, so that only the outer
zone is spun down, and on an exponential timescale. These stars do not yet
possess large-scale dynamos. The large-scale magnetic field associated with the
dynamo, apparently created by the shear between the decoupled radiative and
convective zones, (re)couples the convective and radiative zones and drives a
star from the Convective to the Interface sequence through the gap on a
timescale that increases as stellar mass decreases. (Abstract is truncated
here.)Comment: 32 pages, 6 figures (4 in color), accepted by Ap
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