14,017 research outputs found
Efficiency of radial transport of ices in protoplanetary disks probed with infrared observations: the case of CO
The efficiency of radial transport of icy solid material from outer disk to
the inner disk is currently unconstrained. Efficient radial transport of icy
dust grains could significantly alter the composition of the gas in the inner
disk. Our aim is to model the gaseous CO abundance in the inner disk and
use this to probe the efficiency of icy dust transport in a viscous disk.
Features in the simulated CO spectra are investigated for their dust flux
tracing potential. We have developed a 1D viscous disk model that includes gas
and grain motions as well as dust growth, sublimation and freeze-out and a
parametrisation of the CO chemistry. The thermo-chemical code DALI was used
to model the mid-infrared spectrum of CO, as can be observed with
JWST-MIRI. CO ice sublimating at the iceline increases the gaseous CO
abundance to levels equal to the CO ice abundance of , which
is three orders of magnitude more than the gaseous CO abundances of observed by Spitzer. Grain growth and radial drift further increase
the gaseous CO abundance. A CO destruction rate of at least
s is needed to reconcile model prediction with observations. This rate
is at least two orders of magnitude higher than the fastest known chemical
destruction rate. A range of potential physical mechanisms to explain the low
observed CO abundances are discussed. Transport processes in disks can have
profound effects on the abundances of species in the inner disk. The
discrepancy between our model and observations either suggests frequent shocks
in the inner 10 AU that destroy CO, or that the abundant midplane CO is
hidden from our view by an optically thick column of low abundance CO in to
the disk surface XDR/PDR. Other molecules, such as CH or NH, can give
further handles on the rate of mass transport.Comment: Accepted for publication in A&A, 18 pages, 13 figures, abstract
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Formation of the Wink Sink, A Salt Dissolution and Collapse Feature, Winkler County, Texas
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Towards a Coulomb gas of instantons in the SO(4)xU(1) Higgs model on R_4
The Higgs model on is extended by a term so
that the action receives a nonvanishing contribution from the interactions of
2-instantons and 3-instantons, and can be expressed as the inverse of the
Laplacian on in terms of the mutual distances of the instantons. The
one-instanton solutions of both the basic and the extended models have been
studied in detail numerically.Comment: 29 pages LaTeX, 4 Figures available from authors on reques
Designing protein ÎČ-sheet surfaces by Z-score optimization
Studies of lattice models of proteins have suggested that the appropriate energy expression for protein design may include nonthermodynamic terms to accommodate negative design concerns. One method, developed in lattice model studies, maximizes a quantity known as the "Z-score," which compares the lowest energy sequence whose ground state structure is the target structure to an ensemble of random sequences. Here we show that, in certain circumstances, the technique can be applied to real proteins. The resulting energy expression is used to design the ÎČ-sheet surfaces of two real proteins. We find experimentally that the designed proteins are stable and well folded, and in one case is even more thermostable than the wild type
Tracking environmental trends in the Great Bay Estuarine System through comparisons of historical and present-day green and red algal community structure and nutrient content
Monitoring macroalgae populations is an effective means of detecting long term water quality changes in estuarine systems. To investigate the environmental status of New Hampshireâs Great Bay National Estuarine Research Reserve, this study assessed the abundance/distribution of macrophytes, particularly Gracilaria and Ulva species, relative to eutrophication patterns; compared historical (1970s-1990s) and current algal biomass/cover at several sites; and compared Ulva and Gracilaria tissue N/P content to ambient and historical levels. Ulva and Gracilaria biomass/cover have increased significantly at several sites. Cover by Ulva species, at seasonal maxima, was over 90 times the value recorded in the 1970s at Lubberland Creek, and exceeded 50% at all sites in the upper estuary. Gracilaria cover was greater than 25% at Depot Road in the upper estuary, whereas the historical measure was 1%. Sequencing of ITS2, rbcL and CO1 revealed the presence of previously undetected Ulva and Gracilaria species, including Gracilaria vermiculophylla (Ohmi) Papenfuss, an invasive species of Asian origin. Gracilaria vermiculophylla has exceeded G. tikvahiae as the dominant Gracilaria species in Great Bay. Historical voucher specimen screening suggests G. vermiculophylla was introduced as recently as 2003. Nitrogen and phosphorus levels are elevated in the estuary. We should expect continued seasonal nuisance algal blooms
Generation of Circular Polarization of the Cosmic Microwave Background
The standard cosmological model, which includes only Compton scattering
photon interactions at energy scales near recombination, results in zero
primordial circular polarization of the cosmic microwave background. In this
paper we consider a particular renormalizable and gauge-invariant standard
model extension coupling photons to an external vector field via a Chern-Simons
term, which arises as a radiative correction if gravitational torsion couples
to fermions. We compute the transport equations for polarized photons from a
Boltzmann-like equation, showing that such a coupling will source circular
polarization of the microwave background. For the particular coupling
considered here, the circular polarization effect is always negligible compared
to the rotation of the linear polarization orientation, also derived using the
same formalism. We note the possibility that limits on microwave background
circular polarization may probe other photon interactions and related
fundamental effects such as violations of Lorentz invariance.Comment: 20 pages. Revised version includes an explicit calculation of gauge
invariance. Text reorganized to improve clarity, and references adde
Directionally asymmetric self-assembly of cadmium sulfide nanotubes using porous alumina nanoreactors: Need for chemohydrodynamic instability at the nanoscale
We explore nanoscale hydrodynamical effects on synthesis and self-assembly of
cadmium sulfide nanotubes oriented along one direction. These nanotubes are
synthesized by horizontal capillary flow of two different chemical reagents
from opposite directions through nanochannels of porous anodic alumina which
are used primarily as nanoreactors. We show that uneven flow of different
chemical precursors is responsible for directionally asymmetric growth of these
nanotubes. On the basis of structural observations using scanning electron
microscopy, we argue that chemohydrodynamic convective interfacial instability
of multicomponent liquid-liquid reactive interface is necessary for sustained
nucleation of these CdS nanotubes at the edges of these porous nanochannels
over several hours. However, our estimates clearly suggest that classical
hydrodynamics cannot account for the occurrence of such instabilities at these
small length scales. Therefore, we present a case which necessitates further
investigation and understanding of chemohydrodynamic fluid flow through
nanoconfined channels in order to explain the occurrence of such interfacial
instabilities at nanometer length scales.Comment: 26 pages, 6 figures; http://www.iiserpune.ac.in/researchhighlight
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