750 research outputs found
Magnetic inflation and stellar mass. V. Intensification and saturation of M-dwarf absorption lines with Rossby number
In young Sun-like stars and field M-dwarf stars, chromospheric and coronal magnetic activity indicators such as HÎą, X-ray, and radio emission are known to saturate with low Rossby number (Ro lesssim 0.1), defined as the ratio of rotation period to convective turnover time. The mechanism for the saturation is unclear. In this paper, we use photospheric Ti i and Ca i absorption lines in the Y band to investigate magnetic field strength in M dwarfs for Rossby numbers between 0.01 and 1.0. The equivalent widths of the lines are magnetically enhanced by photospheric spots, a global field, or a combination of the two. The equivalent widths behave qualitatively similar to the chromospheric and coronal indicators: we see increasing equivalent widths (increasing absorption) with decreasing Ro and saturation of the equivalent widths for Ro lesssim 0.1. The majority of M dwarfs in this study are fully convective. The results add to mounting evidence that the magnetic saturation mechanism occurs at or beneath the stellar photosphere.Published versio
Statistical mechanics of permanent random atomic and molecular networks: Structure and heterogeneity of the amorphous solid state
Under sufficient permanent random covalent bonding, a fluid of atoms or small
molecules is transformed into an amorphous solid network. Being amorphous,
local structural properties in such networks vary across the sample. A natural
order parameter, resulting from a statistical-mechanical approach, captures
information concerning this heterogeneity via a certain joint probability
distribution. This joint probability distribution describes the variations in
the positional and orientational localization of the particles, reflecting the
random environments experienced by them, as well as further information
characterizing the thermal motion of particles. A complete solution, valid in
the vicinity of the amorphous solidification transition, is constructed
essentially analytically for the amorphous solid order parameter, in the
context of the random network model and approach introduced by Goldbart and
Zippelius [Europhys. Lett. 27, 599 (1994)]. Knowledge of this order parameter
allows us to draw certain conclusions about the stucture and heterogeneity of
randomly covalently bonded atomic or molecular network solids in the vicinity
of the amorphous solidification transition. Inter alia, the positional aspects
of particle localization are established to have precisely the structure
obtained perviously in the context of vulcanized media, and results are found
for the analogue of the spin glass order parameter describing the orientational
freezing of the bonds between particles.Comment: 31 pages, 5 figure
Angular and energy dependence of cross sections for orbital 1 excitations
The main features of the cross sections of low-lying orbital
excitations with in heavy deformed nuclei are studied in RPA on
the example of Gd. The dependence of the DWBA E2 and M1 cross sections
on the scattering angle and incident
electron energy MeV is analyzed in PWBA. The cross section is
larger for M1 than for E2 transitions at any angle if MeV. The
longitudinal (Coulomb) C2 excitation dominates the E2 response for . Only transverse M1 and E2 excitations compete for
and the former one is dominant for fm.
The M1 response is almost purely orbital up to fm even in
backward scattering. Qualitative PWBA estimates based on the -dependence of
the form factors alone are not able to predict some important features of the
cross sections stemming from the strong magnetic and orbital
character of the studied 1 excitations. The expectation for M1 over E2
dominance in backward scattering should not be extended to higher momentum
transfers and incident energies.Comment: Latex, 28 pages, 12 postscript figures included using uufile
Pre-breakup magmatism on the Vøring margin: Insight from new sub-basalt imaging and results from Ocean Drilling program hole 642E
Highlights
⢠Sub-basalt imaging improvement on the Vøring Margin
⢠Definition of a new seismic facies unit: the Lower Series Flows
⢠Significant organic carbon content within the melting crustal segment
⢠Apectodinium augustum marker for the PETM is reworked into the Lower Series Flows
⢠The Lower Series Flows, early Eocene in age, predate the Vøring Margin breakup
Abstract
Improvements in sub-basalt imaging combined with petrological and geochemical observations from the Ocean Drilling Program (ODP) Hole 642E core provide new constraints on the initial breakup processes at the Vøring Margin. New and reprocessed high quality seismic data allow us to identify a new seismic facies unit which we define as the Lower Series Flows. This facies unit is seismically characterized by wavy to continuous subparallel reflections with an internal disrupted and hummocky shape. Drilled lithologies, which we correlate to this facies unit, have been interpreted as subaqueous flows extruding and intruding into wet sediments. Locally, the top boundary of this facies unit is defined as a negative in polarity reflection, and referred as the K-Reflection. This reflection can be correlated with the spatial extent of pyroclastic deposits, emplaced during transitional shallow marine to subaerial volcanic activities during the rift to drift transition. The drilled Lower Series Flows consist of peraluminous, cordierite bearing peperitic basaltic andesitic to dacitic flows interbedded with thick volcano-sedimentary deposits and intruded sills. The peraluminous geochemistry combined with available C (from calcite which fills vesicles and fractures), Sr, Nd, and Pb isotopes data point towards upper crustal rock-mantle magma interactions with a significant contribution of organic carbon rich pelagic sedimentary material during crustal anatexis. From biostratigraphic analyses, Apectodinium augustum was found in the The Lower Series Flows. This species is a marker for the Paleocene â Eocene Thermal Maximum (PETM). However, the absence of very low carbon isotope values (from bulk organic matter), that characterize the PETM, imply that A.augustum was reworked into the early Eocene sediments of this facies unit which predate the breakup time of the Vøring Margin.
Finally, a plausible conceptual emplacement model for the Lower Series Flows facies unit is proposed. This model comprises several stages: (1) the emplacement of subaqueous peperitic basaltic andesitic flows intruding and/or extruding wet sediments; (2) a subaerial to shallow marine volcanism and extrusion of dacitic flows; (3) a proto-breakup phase with intense shallow marine to subaerial explosive volcanism responsible for pyroclastic flow deposits which can be correlated with the seismic K-Reflection and (4) the main breakup stage with intense transitional tholeiitic MORB-type volcanism and large subsidence concomitant with the buildup of the Seaward Dipping Reflector wedge
Hydrogen-poor superluminous stellar explosions
Supernovae (SNe) are stellar explosions driven by gravitational or
thermonuclear energy, observed as electromagnetic radiation emitted over weeks
or more. In all known SNe, this radiation comes from internal energy deposited
in the outflowing ejecta by either radioactive decay of freshly-synthesized
elements (typically 56Ni), stored heat deposited by the explosion shock in the
envelope of a supergiant star, or interaction between the SN debris and
slowly-moving, hydrogen-rich circumstellar material. Here we report on a new
class of luminous SNe whose observed properties cannot be explained by any of
these known processes. These include four new SNe we have discovered, and two
previously unexplained events (SN 2005ap; SCP 06F6) that we can now identify as
members. These SNe are all ~10 times brighter than SNe Ia, do not show any
trace of hydrogen, emit significant ultra-violet (UV) flux for extended periods
of time, and have late-time decay rates which are inconsistent with
radioactivity. Our data require that the observed radiation is emitted by
hydrogen-free material distributed over a large radius (~10^15 cm) and
expanding at high velocities (>10^4 km s^-1). These long-lived, UV-luminous
events can be observed out to redshifts z>4 and offer an excellent opportunity
to study star formation in, and the interstellar medium of, primitive distant
galaxies.Comment: Accepted to Nature. Press embargoed until 2011 June 8, 18:00 U
Lattice Boltzmann Simulations of Liquid Crystal Hydrodynamics
We describe a lattice Boltzmann algorithm to simulate liquid crystal
hydrodynamics. The equations of motion are written in terms of a tensor order
parameter. This allows both the isotropic and the nematic phases to be
considered. Backflow effects and the hydrodynamics of topological defects are
naturally included in the simulations, as are viscoelastic properties such as
shear-thinning and shear-banding.Comment: 14 pages, 5 figures, Revte
A conformational change in the helicase core is necessary but not sufficient for RNA unwinding by the DEAD box helicase YxiN
Cooperative binding of ATP and RNA to DEAD-box helicases induces the closed conformation of their helicase core, with extensive interactions across the domain interface. The bound RNA is bent, and its distortion may constitute the first step towards RNA unwinding. To dissect the role of the conformational change in the helicase core for RNA unwinding, we characterized the RNA-stimulated ATPase activity, RNA unwinding and the propensity to form the closed conformer for mutants of the DEAD box helicase YxiN. The ATPase-deficient K52Q mutant forms a closed conformer upon binding of ATP and RNA, but is deficient in RNA unwinding. A mutation in motif III slows down the catalytic cycle, but neither affects the propensity for the closed conformer nor its global conformation. Hence, the closure of the cleft in the helicase core is necessary but not sufficient for RNA unwinding. In contrast, the G303A mutation in motif V prevents a complete closure of the inter-domain cleft, affecting ATP binding and hydrolysis and is detrimental to unwinding. Possibly, the K52Q and motif III mutants still introduce a kink into the backbone of bound RNA, whereas G303A fails to kink the RNA substrate
Fluctuations of elastic interfaces in fluids: Theory and simulation
We study the dynamics of elastic interfaces-membranes-immersed in thermally
excited fluids. The work contains three components: the development of a
numerical method, a purely theoretical approach, and numerical simulation. In
developing a numerical method, we first discuss the dynamical coupling between
the interface and the surrounding fluids. An argument is then presented that
generalizes the single-relaxation time lattice-Boltzmann method for the
simulation of hydrodynamic interfaces to include the elastic properties of the
boundary. The implementation of the new method is outlined and it is tested by
simulating the static behavior of spherical bubbles and the dynamics of bending
waves. By means of the fluctuation-dissipation theorem we recover analytically
the equilibrium frequency power spectrum of thermally fluctuating membranes and
the correlation function of the excitations. Also, the non-equilibrium scaling
properties of the membrane roughening are deduced, leading us to formulate a
scaling law describing the interface growth, W^2(L,T)=L^3 g[t/L^(5/2)], where
W, L and T are the width of the interface, the linear size of the system and
the temperature respectively, and g is a scaling function. Finally, the
phenomenology of thermally fluctuating membranes is simulated and the frequency
power spectrum is recovered, confirming the decay of the correlation function
of the fluctuations. As a further numerical study of fluctuating elastic
interfaces, the non-equilibrium regime is reproduced by initializing the system
as an interface immersed in thermally pre-excited fluids.Comment: 15 pages, 11 figure
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