36,655 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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The 50-horsepower solar-powered irrigation facility located near Gila Bend, Arizona
The 50 horsepower solar powered irrigation facility near Gila Bend, Arizona which includes a Rankine cycle demonstrates the technical feasibility of solar powered pumping. The design of a facility specifically for the irrigation farmer using the technology that has been developed over the last four years is proposed
Interstellar grain mantles
Interstellar molecular grain mantles are an important component of the interstellar dust inside dense molecular clouds as evidenced by the detection of absorption bands at 2.97, 3.08, 4.61, 6.0 and 6.8 microns. Mantles may also be the precursors of more complex grain mantles in the diffuse interstellar medium. The molecular composition of these icy grain mantles were calculated employing gas phase as well as grain surface reactions. The calculated mixtures consist mainly of the molecules H2O, H2CO, N2, CO, O2, H2O2, NH2, and their deuterated counterparts in varying ratios. The exact compositions depend strongly on the physical conditions in the gas phase. The absorption spectra of H2O with other molecules was studied in the laboratory. Optical constants were determined for a few selected mixtures. Extinction and polarization cross sections across the 3 micron ice band were calculated. A comparison with the observations towards BN shows that the low frequency wing observed on this feature is due to absorption by a mixture of H2O and other molecules rather than scattering by large, pure H2O ice grains
The Dark Matter Contribution to Galactic Diffuse Gamma Ray Emission
Observations of diffuse Galactic gamma ray emission (DGE) by the Fermi Large
Area Telescope (LAT) allow a detailed study of cosmic rays and the interstellar
medium. However, diffuse emission models of the inner Galaxy underpredict the
Fermi-LAT data at energies above a few GeV and hint at possible
non-astrophysical sources including dark matter (DM) annihilations or decays.
We present a study of the possible emission components from DM using the
high-resolution Via Lactea II N-body simulation of a Milky Way-sized DM halo.
We generate full-sky maps of DM annihilation and decay signals that include
modeling of the adiabatic contraction of the host density profile, Sommerfeld
enhanced DM annihilations, -wave annihilations, and decaying DM. We compare
our results with the DGE models produced by the Fermi-LAT team over different
sky regions, including the Galactic center, high Galactic latitudes, and the
Galactic anti-center. This work provides possible templates to fit the
observational data that includes the contribution of the subhalo population to
DM gamma-ray emission, with the significance depending on the
annihilation/decay channels and the Galactic regions being considered.Comment: Published by PR
High accuracy calculations of the rotation-vibration spectrum of H
Calculation of the rotation-vibration spectrum of H3+, as well as of its
deuterated isotopologues, with near-spectroscopic accuracy requires the
development of sophisticated theoretical models, methods, and codes. The
present paper reviews the state-of-the-art in these fields. Computation of
rovibrational states on a given potential energy surface (PES) has now become
standard for triatomic molecules, at least up to intermediate energies, due to
developments achieved by the present authors and others. However, highly
accurate Born--Oppenheimer energies leading to highly accurate PESs are not
accessible even for this two-electron system using conventional electronic
structure procedures e.g., configuration-interaction or coupled-cluster
techniques with extrapolation to the complete basis set limit). For this
purpose highly specialized techniques must be used, e.g., those employing
explicitly correlated Gaussians and nonlinear parameter optimizations. It has
also become evident that a very dense grid of \ai\ points is required to obtain
reliable representations of the computed points extending from the minimum to
the asymptotic limits. Furthermore, adiabatic, relativistic, and QED correction
terms need to be considered to achieve near-spectroscopic accuracy during
calculation of the rotation-vibration spectrum of H3+. The remaining and most
intractable problem is then the treatment of the effects of non-adiabatic
coupling on the rovibrational energies, which, in the worst cases, may lead to
corrections on the order of several \cm. A promising way of handling this
difficulty is the further development of effective, motion- or even
coordinate-dependent, masses and mass surfaces. Finally, the unresolved
challenge of how to describe and elucidate the experimental pre-dissociation
spectra of H and its isotopologues is discussed.Comment: Topical review to be published in J Phys B: At Mol Opt Phy
Interactions of keV sterile neutrinos with matter
A sterile neutrino with mass of several keV is a well-motivated dark-matter
candidate, and it can also explain the observed velocities of pulsars via
anisotropic emission of sterile neutrinos from a cooling neutron star. We
discuss the interactions of such relic particles with matter and comment on the
prospects of future direct detection experiments. A relic sterile neutrino can
interact, via sterile-active mixing, with matter fermions by means of
electroweak currents, with the final state containing a relativistic active
neutrino. The recoil momentum impacted onto a matter fermion is determined by
the sterile neutrino mass and is enough to ionize atoms and flip the spins of
nuclei. While this suggests a possibility of direct experimental detection, we
calculate the rates and show that building a realistic detector of the required
size would be a daunting challenge.Comment: 5 pages, 1 figur
The effect of disorder on the free-energy for the Random Walk Pinning Model: smoothing of the phase transition and low temperature asymptotics
We consider the continuous time version of the Random Walk Pinning Model
(RWPM), studied in [5,6,7]. Given a fixed realization of a random walk Y$ on
Z^d with jump rate rho (that plays the role of the random medium), we modify
the law of a random walk X on Z^d with jump rate 1 by reweighting the paths,
giving an energy reward proportional to the intersection time L_t(X,Y)=\int_0^t
\ind_{X_s=Y_s}\dd s: the weight of the path under the new measure is exp(beta
L_t(X,Y)), beta in R. As beta increases, the system exhibits a
delocalization/localization transition: there is a critical value beta_c, such
that if beta>beta_c the two walks stick together for almost-all Y realizations.
A natural question is that of disorder relevance, that is whether the quenched
and annealed systems have the same behavior. In this paper we investigate how
the disorder modifies the shape of the free energy curve: (1) We prove that, in
dimension d larger or equal to three 3, the presence of disorder makes the
phase transition at least of second order. This, in dimension larger or equal
to 4, contrasts with the fact that the phase transition of the annealed system
is of first order. (2) In any dimension, we prove that disorder modifies the
low temperature asymptotic of the free energy.Comment: 18 page
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