2,541 research outputs found
Ab initio computation of the broadening of water rotational lines by molecular hydrogen
Theoretical cross sections for the pressure broadening by hydrogen of
rotational transitions of water are compared to the latest available
measurements in the temperature range 65-220 K. A high accuracy interaction
potential is employed in a full close coupling calculation. A good agreement
with experiment is observed above ~80 K while the sharp drop observed
experimentally at lower temperatures is not predicted by our calculations.
Possible explanations for this discrepancy include the failure of the impact
approximation and the possible role of ortho-to-para conversion of H2.Comment: 11 pages, 3 figures, accepted for publicatio
CN excitation and electron densities in diffuse molecular clouds
Utilising previous work by the authors on the spin-coupled rotational
cross-sections for electron-CN collisions, data for the associated rate
coefficients is presented. Data on rotational, fine-structure and
hyperfine-structure transition involving rotational levels up to =20 are
computed for temperatures in the range 10 -- 1000~K. Rates are calculated by
combining Born-corrected R-matrix calculations with the infinite-order-sudden
(IOS) approximation. The dominant hyperfine transitions are those with . For dipole-allowed transitions, electron-impact rates
are shown to exceed those for excitation of CN by para-H() by five
orders of magnitude. The role of electron collisions in the excitation of CN in
diffuse clouds, where local excitation competes with the cosmic microwave
background (CMB) photons, is considered. Radiative transfer calculations are
performed and the results compared to observations. These comparisons suggest
that electron density lies in the range ~cm for
typical physical conditions present in diffuse clouds.Comment: 6 pages, 8 figures, accepted 14/08/201
Electron-impact excitation of diatomic hydride cations I: HeH, CH, ArH
{\bf R}-matrix calculations combined with the adiabatic nuclei approximation
are used to compute electron-impact rotiational excitation rates for three
closed-shell diatomic cations, HeH, CH, ArH. Comparisons with
previous studies show that an improved treatment of threshold effects leads to
significant changes in the low temperature rates, furthermore the new
calculations suggest that excitation of CH is dominated by
transitions as is expected for cations with a large dipole moment. A model for
ArH excitation in the Crab Nebula is presented which gives results
consistent with the observations for electron densities in the range ~cm.Comment: MNRAS submitted 7 pages, 9 figure
Public international law controversies over land acquisition and lang grabbing: a socio-legal perspective
This articles is focused on legal controversies concerning forced land grabbing and land acquisition
Public international law debate concerning forced evictions in China
The aim of this study is to present legal and social controvercies concerning current scale of forced evictions in China. My particular attention is devoted to the controvercies in the sphere of public international law. At least three millions Chinese citizens are evicted each year to make space. Forced evictions are sometimes considered as the part of the issue of development-induced displacement and resettlement. The most important caues of development-induced displacement and resettlement in China include construction of dams and hydro-power plants, urban evictions and preparation of mega-events. The object of this study is to draw attention of the controvercies converning evictions in urban areas from the point of view of public international law
A new nonlocal thermodynamical equilibrium radiative transfer method for cool stars
Context: The solution of the nonlocal thermodynamical equilibrium (non-LTE)
radiative transfer equation usually relies on stationary iterative methods,
which may falsely converge in some cases. Furthermore, these methods are often
unable to handle large-scale systems, such as molecular spectra emerging from,
for example, cool stellar atmospheres.
Aims: Our objective is to develop a new method, which aims to circumvent
these problems, using nonstationary numerical techniques and taking advantage
of parallel computers.
Methods: The technique we develop may be seen as a generalization of the
coupled escape probability method. It solves the statistical equilibrium
equations in all layers of a discretized model simultaneously. The numerical
scheme adopted is based on the generalized minimum residual method.
Result:. The code has already been applied to the special case of the water
spectrum in a red supergiant stellar atmosphere. This demonstrates the fast
convergence of this method, and opens the way to a wide variety of
astrophysical problems.Comment: 13 pages, 9 figure
Weak maser emission of methyl formate toward Sagittarius B2(N) in the Green Bank Telescope PRIMOS Survey
A non-LTE radiative transfer treatment of cis-methyl formate (HCOOCH3)
rotational lines is presented for the first time using a set of theoretical
collisional rate coefficients. These coefficients have been computed in the
temperature range 5-30 K by combining coupled-channel scattering calculations
with a high accuracy potential energy surface for HCOOCH3-He. The results are
compared to observations toward the Sagittarius B2(N) molecular cloud using the
publicly available PRIMOS survey from the Green Bank Telescope. A total of 49
low-lying transitions of methyl formate, with upper levels below 25 K, are
identified. These lines are found to probe a presumably cold (~30 K),
moderately dense (~1e4 cm-3) and extended region surrounding Sgr B2(N). The
derived column density of ~4e14 cm-2 is only a factor of ~10 larger than the
column density of the trans conformer in the same source. Provided that the two
conformers have the same spatial distribution, this result suggests that
strongly non-equilibrium processes must be involved in their synthesis.
Finally, our calculations show that all detected emission lines with a
frequency below 30 GHz are (collisionally pumped) weak masers amplifying the
continuum of Sgr B2(N). This result demonstrates the importance and generality
of non-LTE effects in the rotational spectra of complex organic molecules at
centimetre wavelengths.Comment: 33 pages, 9 figures, accepted in The Astrophysical Journal (january 4
2014
The 15N-enrichment in dark clouds and Solar System objects
The line intensities of the fundamental rotational transitions of H13CN and
HC15N were observed towards two prestellar cores, L183 and L1544, and lead to
molecular isotopic ratios 140 6 14N/15N 6 250 and 140 6 14N/15N 6 360,
respectively. The range of values reflect genuine spatial variations within the
cores. A comprehensive analysis of the available measurements of the nitrogen
isotopic ratio in prestellar cores show that molecules carrying the nitrile
functional group appear to be systematically 15N-enriched com- pared to those
carrying the amine functional group. A chemical origin for the differential
15N-enhance- ment between nitrile- and amine-bearing interstellar molecules is
proposed. This sheds new light on several observations of Solar System objects:
(i) the similar N isotopic fractionation in Jupiter's NH3 and solar wind N+;
(ii) the 15N-enrichments in cometary HCN and CN (that might represent a direct
inter- stellar inheritance); and (iii) 15N-enrichments observed in organics in
primitive cosmomaterials. The large variations in the isotopic composition of
N-bearing molecules in Solar System objects might then simply reflect the
different interstellar N reservoirs from which they are originating
- …