428 research outputs found
Optical Spectroscopy of Galactic Cirrus Clouds: Extended Red Emission in the Diffuse Interstellar Medium
We present initial results from the first optical spectroscopic survey of
high latitude Galactic cirrus clouds. The observed shape of the cirrus spectrum
does not agree with that of scattered ambient Galactic starlight. This mismatch
can be explained by the presence of Extended Red Emission (ERE) in the diffuse
interstellar medium, as found in many other astronomical objects, probably
caused by photoluminescence of hydrocarbons. The integrated ERE intensity,
I_ERE \approx 1.2 x 10^{-5} erg s^{-1} cm^{-2} sr^{-1}, is roughly a third of
the scattered light intensity, consistent with recent color measurements of
diffuse Galactic light. The peak of the cirrus ERE (lambda_{0} \sim 6000 AA) is
shifted towards short (bluer) wavelengths compared to the ERE in sources
excited by intense ultraviolet radiation, such as HII regions (lambda_{0} sim
8000 AA); such a trend is seen in laboratory experiments on hydrogenated
amorphous carbon films.Comment: 7 pages, 2 figures. Accepted for publication in ApJ Letter
Mid-Infrared Spectropolarimetric Constraints on the Core-Mantle Interstellar Dust Model
In the framework of the silicate core-carbonaceous organic mantle
interstellar dust model, the bulk of the visual/near-IR extinction and the
entire polarization are from nonspherical and aligned core-mantle grains. The
3.4 micron C-H and 9.7 micron Si-O absorption features, respectively arising
from the hydrocarbon mantle and the amorphous silicate core, are expected to be
polarized to a modestly different degree. Spectropolarimetric observations
toward the same lines of sight both in the 3.4 micron region and in the 9.7
micron region would be of great value to test the core-mantle dust model. The
fact that the 3.4 micron feature is not polarized along the line of sight
toward the Galactic center source IRS 7 is not yet sufficient to reject the
core-mantle model due to the lack of spectropolarimetric observation of this
region in the 9.7 micron region.Comment: 11 pages, 2 figues, submitted to The Astrophysical Journal Letter
Master Equation for Hydrogen Recombination on Grain Surfaces
Recent experimental results on the formation of molecular hydrogen on
astrophysically relevant surfaces under conditions similar to those encountered
in the interstellar medium provided useful quantitative information about these
processes. Rate equation analysis of experiments on olivine and amorphous
carbon surfaces provided the activation energy barriers for the diffusion and
desorption processes relevant to hydrogen recombination on these surfaces.
However, the suitability of rate equations for the simulation of hydrogen
recombination on interstellar grains, where there might be very few atoms on a
grain at any given time, has been questioned. To resolve this problem, we
introduce a master equation that takes into account both the discrete nature of
the H atoms and the fluctuations in the number of atoms on a grain. The
hydrogen recombination rate on microscopic grains, as a function of grain size
and temperature, is then calculated using the master equation. The results are
compared to those obtained from the rate equations and the conditions under
which the master equation is required are identified.Comment: Latex document. 14 pages of text. Four associated figs in in PS
format on separate files that are "called-in" the LaTeX documen
Spatial Separation of the 3.29 micron Emission Feature and Associated 2 micron Continuum in NGC 7023
We present a new 0.9" resolution 3.29 micron narrowband image of the
reflection nebula NGC 7023. We find that the 3.29 micron IEF in NGC 7023 is
brightest in narrow filaments NW of the illuminating star. These filaments have
been seen in images of K', molecular hydrogen emission lines, the 6.2 and 11.3
micron IEFs, and HCO+. We also detect 3.29 micron emission faintly but
distinctly between the filaments and the star. The 3.29 micron image is in
contrast to narrowband images at 2.09, 2.14, and 2.18 micron, which show an
extended emission peak midway between the filaments and the star, and much
fainter emission near the filaments. The [2.18]-[3.29] color shows a wide
variation, ranging from 3.4-3.6 mag at the 2 micron continuum peak to 5.5 mag
in the filaments. We observe [2.18]-[3.29] to increase smoothly with increasing
distance from the star, up until the filament, suggesting that the main
difference between the spatial distributions of the 2 micron continuum and the
the 3.29 micron emission is related to the incident stellar flux. Our result
suggests that the 3.29 micron IEF carriers are likely to be distinct from, but
related to, the 2 micron continuum emitters. Our finding also imply that, in
NGC 7023, the 2 micron continuum emitters are mainly associated with HI, while
the 3.29 micron IEF carriers are primarily found in warm molecular hydrogen,
but that both can survive in HI or molecular hydrogen. (abridged)Comment: to appear in ApJ, including 1 table and 8 figures, high resolution
figures available at http://www.ast.cam.ac.uk/~jin/n7023
Diffusion-limited reactions on a two-dimensional lattice with binary disorder
Reaction-diffusion systems where transition rates exhibit quenched disorder
are common in physical and chemical systems. We study pair reactions on a
periodic two-dimensional lattice, including continuous deposition and
spontaneous desorption of particles. Hopping and desorption are taken to be
thermally activated processes. The activation energies are drawn from a binary
distribution of well depths, corresponding to `shallow' and `deep' sites. This
is the simplest non-trivial distribution, which we use to examine and explain
fundamental features of the system. We simulate the system using kinetic Monte
Carlo methods and provide a thorough understanding of our findings. We show
that the combination of shallow and deep sites broadens the temperature window
in which the reaction is efficient, compared to either homogeneous system. We
also examine the role of spatial correlations, including systems where one type
of site is arranged in a cluster or a sublattice. Finally, we show that a
simple rate equation model reproduces simulation results with very good
accuracy.Comment: 9 pages, 5 figure
The adsorption and desorption of ethanol ices from a model grain surface
Reflection absorption infrared spectroscopy (RAIRS) and temperature programed desorption (TPD) have been used to probe the adsorption and desorption of ethanol on highly ordered pyrolytic graphite (HOPG) at 98 K. RAIR spectra for ethanol show that it forms physisorbed multilayers on the surface at 98 K. Annealing multilayer ethanol ices (exposures > 50 L) beyond 120 K gives rise to a change in morphology before crystallization within the ice occurs. TPD shows that ethanol adsorbs and desorbs molecularly on the HOPG surface and shows four different species in desorption. At low coverage, desorption of monolayer ethanol is observed and is described by first-order kinetics. With increasing coverage, a second TPD peak is observed at a lower temperature, which is assigned to an ethanol bilayer. When the coverage is further increased, a second multilayer, less strongly bound to the underlying ethanol ice film, is observed. This peak dominates the TPD spectra with increasing coverage and is characterized by fractional-order kinetics and a desorption energy of 56.3 +/- 1.7 kJ mol(-1). At exposures exceeding 50 L, formation of crystalline ethanol is also observed as a high temperature shoulder on the TPD spectrum at 160 K. (c) 2008 American Institute of Physics
Molecular Hydrogen Formation on Ice Under Interstellar Conditions
The results of experiments on the formation of molecular hydrogen on low
density and high density amorphous ice surfaces are analyzed using a rate
equation model. The activation energy barriers for the relevant diffusion and
desorption processes are obtained. The more porous morphology of the low
density ice gives rise to a broader spectrum of energy barriers compared to the
high density ice. Inserting these parameters into the rate equation model under
steady state conditions we evaluate the production rate of molecular hydrogen
on ice-coated interstellar dust grains.Comment: 20 pages, 3 tables and 10 figures. Accepted to ApJ. Minor changes
made and adittional references adde
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