4,390 research outputs found
Mixed Quantum/Classical Theory of Rotationally and Vibrationally Inelastic Scattering in Space-fixed and Body-fixed Reference Frames
We formulated the mixed quantum/classical theory for rotationally and vibrationally inelastic scattering process in the diatomic molecule + atom system. Two versions of theory are presented, first in the space-fixed and second in the body-fixed reference frame. First version is easy to derive and the resultant equations of motion are transparent, but the state-to-state transition matrix is complex-valued and dense. Such calculations may be computationally demanding for heavier molecules and/or higher temperatures, when the number of accessible channels becomes large. In contrast, the second version of theory requires some tedious derivations and the final equations of motion are rather complicated (not particularly intuitive). However, the state-to-state transitions are driven by real-valued sparse matrixes of much smaller size. Thus, this formulation is the method of choice from the computational point of view, while the space-fixed formulation can serve as a test of the body-fixed equations of motion, and the code. Rigorous numerical tests were carried out for a model system to ensure that all equations, matrixes, and computer codes in both formulations are correct
Role, importance and vulnerability of top predators on the Great Barrier Reef: a review
The purpose of this review is to evaluate the ecological role of predators on the Great Barrier Reef (GBR), their vulnerability to human activities and their contribution to ecosystem and economic values. Marine systems around the world are under increasing pressure, from the localised anthropogenic impacts of fishing and terrestrial run-off to the global pressures of climate change. There is concern over exploitation and declining numbers and biomass of large marine predators, worldwide and on the GBR. Understanding the role of predation and the consequences of predator loss is a priority for managers. To better understand the link between the protection of exploited fish stocks, the enhancement of the GBR’s overall resilience and the maintenance of ecosystem structure and function, this review seeks to answer the following questions:ID: 181
The solar type protostar IRAS16293-2422: new constraints on the physical structure
Context: The low mass protostar IRAS16293-2422 is a prototype Class 0 source
with respect to the studies of the chemical structure during the initial phases
of life of Solar type stars. Aims: In order to derive an accurate chemical
structure, a precise determination of the source physical structure is
required. The scope of the present work is the derivation of the structure of
IRAS16293-2422. Methods: We have re-analyzed all available continuum data
(single dish and interferometric, from millimeter to MIR) to derive accurate
density and dust temperature profiles. Using ISO observations of water, we have
also reconstructed the gas temperature profile. Results: Our analysis shows
that the envelope surrounding IRAS16293-2422 is well described by the Shu
"inside-out" collapsing envelope model or a single power-law density profile
with index equal to 1.8. In contrast to some previous studies, our analysis
does not show evidence of a large (>/- 800 AU in diameter) cavity. Conclusions:
Although IRAS16293-2422 is a multiple system composed by two or three objects,
our reconstruction will be useful to derive the chemical structure of the large
cold envelope surrounding these objects and the warm component, treated here as
a single source, from single-dish observations of molecular emission
New quantum chemical computations of formamide deuteration support a gas-phase formation of this prebiotic molecule
Based on recent work, formamide might be a potentially very important
molecule in the emergence of terrestrial life. Although detected in the
interstellar medium for decades, its formation route is still debated, whether
in the gas phase or on the dust grain surfaces. Molecular deuteration has
proven to be, in other cases, an efficient way to identify how a molecule is
synthesised. For formamide, new published observations towards the
IRAS16293-2422 B hot corino show that its three deuterated forms have all the
same deuteration ratio, 2--5%, and that this is a factor 3--8 smaller than that
measured for H2CO towards the IRAS16293-2422 protostar. Following a previous
work on the gas-phase formamide formation via the reaction NH2 + H2CO -> HCONH2
+ H, we present here new calculations of the rate coefficients for the
production of monodeuterated formamide through the same reaction, starting from
monodeuterated NH2 or H2CO. Some misconceptions regarding our previous
treatment of the reaction are also cleared up. The results of the new
computations show that, at the 100 K temperature of the hot corino, the rate of
deuteration of the three forms is the same, within 20%. On the contrary, the
reaction between non-deuterated species proceeds three times faster than that
with deuterated ones. These results confirm that a gas-phase route for the
formation of formamide is perfectly in agreement with the available
observations.Comment: MNRAS in pres
Gas phase formation of the prebiotic molecule formamide: insights from new quantum computations
New insights into the formation of interstellar formamide, a species of great
relevance in prebiotic chemistry, are provided by electronic structure and
kinetic calculations for the reaction NH2 + H2CO -> NH2CHO + H. Contrarily to
what previously suggested, this reaction is essentially barrierless and can,
therefore, occur under the low temperature conditions of interstellar objects
thus providing a facile formation route of formamide. The rate coefficient
parameters for the reaction channel leading to NH2CHO + H have been calculated
to be A = 2.6x10^{-12} cm^3 s^{-1}, beta = -2.1 and gamma = 26.9 K in the range
of temperatures 10-300 K. Including these new kinetic data in a refined
astrochemical model, we show that the proposed mechanism can well reproduce the
abundances of formamide observed in two very different interstellar objects:
the cold envelope of the Sun-like protostar IRAS16293-2422 and the molecular
shock L1157-B2. Therefore, the major conclusion of this Letter is that there is
no need to invoke grain-surface chemistry to explain the presence of formamide
provided that its precursors, NH2 and H2CO, are available in the gas-phase.Comment: MNRAS Letters, in pres
Galaxy Peculiar Velocities and Infall onto Groups
We perform statistical analyses to study the infall of galaxies onto groups
and clusters in the nearby Universe. The study is based on the UZC and SSRS2
group catalogs and peculiar velocity samples. We find a clear signature of
infall of galaxies onto groups over a wide range of scales 5 h^{-1} Mpc<r<30
h^{-1} Mpc, with an infall amplitude on the order of a few hundred kilometers
per second. We obtain a significant increase in the infall amplitude with group
virial mass (M_{V}) and luminosity of group member galaxies (L_{g}). Groups
with M_{V}<10^{13} M_{\odot} show infall velocities V_{infall} \simeq 150 km
s^{-1} whereas for M_{V}>10^{13} M_{\odot} a larger infall is observed,
V_{infall} \simeq 200 km s^{-1}. Similarly, we find that galaxies surrounding
groups with L_{g}<10^{15} L_{\odot} have V_{infall} \simeq 100 km s^{-1},
whereas for L_{g}>10^{15} L_{\odot} groups, the amplitude of the galaxy infall
can be as large as V_{infall} \simeq 250 km s^{-1}. The observational results
are compared with the results obtained from mock group and galaxy samples
constructed from numerical simulations, which include galaxy formation through
semianalytical models. We obtain a general agreement between the results from
the mock catalogs and the observations. The infall of galaxies onto groups is
suitably reproduced in the simulations and, as in the observations, larger
virial mass and luminosity groups exhibit the largest galaxy infall amplitudes.
We derive estimates of the integrated mass overdensities associated with groups
by applying linear theory to the infall velocities after correcting for the
effects of distance uncertainties obtained using the mock catalogs. The
resulting overdensities are consistent with a power law with \delta \sim 1 at r
\sim 10 h^{-1}Mpc.Comment: 25 pages, 10 figure
Ionization toward the high-mass star-forming region NGC 6334 I
Context. Ionization plays a central role in the gas-phase chemistry of
molecular clouds. Since ions are coupled with magnetic fields, which can in
turn counteract gravitational collapse, it is of paramount importance to
measure their abundance in star-forming regions. Aims. We use spectral line
observations of the high-mass star-forming region NGC 6334 I to derive the
abundance of two of the most abundant molecular ions, HCO+ and N2H+, and
consequently, the cosmic ray ionization rate. In addition, the line profiles
provide information about the kinematics of this region. Methods. We present
high-resolution spectral line observations conducted with the HIFI instrument
on board the Herschel Space Observatory of the rotational transitions with Jup
> 5 of the molecular species C17O, C18O, HCO+, H13CO+, and N2H+. Results. The
HCO+ and N2H+ line profiles display a redshifted asymmetry consistent with a
region of expanding gas. We identify two emission components in the spectra,
each with a different excitation, associated with the envelope of NGC 6334 I.
The physical parameters obtained for the envelope are in agreement with
previous models of the radial structure of NGC 6334 I based on submillimeter
continuum observations. Based on our new Herschel/HIFI observations, combined
with the predictions from a chemical model, we derive a cosmic ray ionization
rate that is an order of magnitude higher than the canonical value of 10^(-17)
s-1. Conclusions. We find evidence of an expansion of the envelope surrounding
the hot core of NGC 6334 I, which is mainly driven by thermal pressure from the
hot ionized gas in the region. The ionization rate seems to be dominated by
cosmic rays originating from outside the source, although X-ray emission from
the NGC 6334 I core could contribute to the ionization in the inner part of the
envelope.Comment: This paper contains a total of 10 figures and 3 table
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