5,697 research outputs found
Deuterium fractionation on interstellar grains studied with the direct master equation approach
We have studied deuterium fractionation on interstellar grains with the use
of an exact method known as the direct master equation approach. We consider
conditions pertinent to dense clouds at late times when the hydrogen is mostly
in molecular form and a large portion of the gas-phase carbon has already been
converted to carbon monoxide. Hydrogen, oxygen and deuterium atoms, as well as
CO molecules, are allowed to accrete on to dust particles and react there to
produce various stable molecules. The surface abundances, as well as the
abundance ratios between deuterated and normal isotopomers, are compared with
those calculated with the Monte Carlo approach. We find that the agreement
between the Monte Carlo and the direct master equation methods can be made as
close as desired. Compared with previous examples of the use of the direct
master equation approach, our present method is much more efficient. It should
now be possible to run large-scale gas-grain models in which the diffusive dust
chemistry is handled `exactly'.Comment: 7 pages, 3 figure
From Prestellar to Protostellar Cores II. Time Dependence and Deuterium Fractionation
We investigate the molecular evolution and D/H abundance ratios that develop
as star formation proceeds from a dense-cloud core to a protostellar core, by
solving a gas-grain reaction network applied to a 1-D radiative hydrodynamic
model with infalling fluid parcels. Spatial distributions of gas and ice-mantle
species are calculated at the first-core stage, and at times after the birth of
a protostar. Gas-phase methanol and methane are more abundant than CO at radii
AU in the first-core stage, but gradually decrease with time,
while abundances of larger organic species increase. The warm-up phase, when
complex organic molecules are efficiently formed, is longer-lived for those
fluid parcels in-falling at later stages. The formation of unsaturated carbon
chains (warm carbon-chain chemistry) is also more effective in later stages;
C, which reacts with CH to form carbon chains, increases in abundance
as the envelope density decreases. The large organic molecules and carbon
chains are strongly deuterated, mainly due to high D/H ratios in the parent
molecules, determined in the cold phase. We also extend our model to simulate
simply the chemistry in circumstellar disks, by suspending the 1-D infall of a
fluid parcel at constant disk radii. The species CHOCH and HCOOCH
increase in abundance in yr at the fixed warm temperature; both
also have high D/H ratios.Comment: accepted to ApJ. 55 pages, 7 figures, 3 table
Electron attachment rates for PAH anions in the ISM and dark molecular clouds: dependence on their chemical properties
CONTEXT: The attachment of free electrons to polycondensed aromatic ring
molecules (PAHs) is studied for the variety of these molecules with different
numbers of condensed rings and over a broad range of electron temperatures,
using a multichannel quantum scattering approach. The calculations of the
relevant cross sections are used in turn to model the corresponding attachment
rates for each of the systems under study, and these rates are parametrized as
a function of temperature using a commonly employed expression for two-body
processes in the interstellar medium (ISM). AIM: The scope of this work is to
use first principles to establish the influence of chemical properties on the
efficiency of the electron-attachment process for PAHs. METHODS: Quantum
multichannel scattering methods are employed to generate the relevant cross
sections, hence the attachment rates, using integral elastic cross sections
computed over a broad range of relevant energies, from threshold up to 1000 K
and linking the attachment to low-energy resonant collisions. RESULTS: The
rates obtained for the present molecules are found to markedly vary within the
test ensemble of the present work and to be lower than the earlier values used
for the entire class of PAHs anions, when modelling their evolutions in ISM
environments. The effects of such differences on the evolutions of chemical
networks that include both PAH and PAH- species are analysed in some detail and
related to previous calculations.Comment: accepted to be published on A&
Sarma phase in relativistic and non-relativistic systems
We investigate the stability of the Sarma phase in two-component fermion
systems in three spatial dimensions. For this purpose we compare
strongly-correlated systems with either relativistic or non-relativistic
dispersion relation: relativistic quarks and mesons at finite isospin density
and spin-imbalanced ultracold Fermi gases. Using a Functional Renormalization
Group approach, we resolve fluctuation effects onto the corresponding phase
diagrams beyond the mean-field approximation. We find that fluctuations induce
a second order phase transition at zero temperature, and thus a Sarma phase, in
the relativistic setup for large isospin chemical potential. This motivates the
investigation of the cold atoms setup with comparable mean-field phase
structure, where the Sarma phase could then be realized in experiment. However,
for the non-relativistic system we find the stability region of the Sarma phase
to be smaller than the one predicted from mean-field theory. It is limited to
the BEC side of the phase diagram, and the unitary Fermi gas does not support a
Sarma phase at zero temperature. Finally, we propose an ultracold quantum gas
with four fermion species that has a good chance to realize a zero-temperature
Sarma phase.Comment: version published in Phys.Lett.B; 10 pages, 5 figure
LIINUS/SERPIL: a design study for interferometric imaging spectroscopy at the LBT
LIINUS/SERPIL is a design study to augment LBTs interferometric beam combiner
camera LINC-NIRVANA with imaging spectroscopy. The FWHM of the interferometric
main beam at 1.5 micron will be about 10 mas, offering unique imaging and
spectroscopic capabilities well beyond the angular resolution of current 8-10m
telescopes. At 10 mas angular scale, e.g., one resolution element at the
distance of the Galactic Center corresponds to the average diameter of the
Pluto orbit (79 AU), hence the size of the solar system. Taking advantage of
the LBT interferometric beam with an equivalent maximum diameter of 23 m,
LIINUS/SERPIL is an ideal precursor instrument for (imaging) spectrographs at
extremely large full aperture telescopes. LIINUS/SERPIL will be built upon the
LINC-NIRVANA hardware and LIINUS/SERPIL could potentially be developed on a
rather short timescale. The study investigates several concepts for the optical
as well as for the mechanical design. We present the scientific promises of
such an instrument together with the current status of the design study.Comment: 12 pages, SPIE conference proceeding, Orlando, 200
Electron-attachment rates for carbon-rich molecules in protoplanetary atmospheres: the role of chemical differences
The formation of anionic species in the interstellar medium from interaction
of linear molecules containing carbon, nitrogen and hydrogen as atomic
components (polyynes) with free electrons in the environment is modelled via a
quantum treatment of the collision dynamics. The ensuing integral cross
sections are employed to obtain the corresponding attachment rates over a broad
range of temperatures for the electrons. The calculations unequivocally show
that a parametrization form often employed for such rates yields a broad range
of values that turn out to be specific for each molecular species considered,
thus excluding using a unique set for the whole class of polyynes.Comment: accepted to be published on MNRA
Methanol in the sky with diamonds
The present of gas phase methanol in dense interstellar molecular clouds was established by radio detection of its rotational emission lines. However, the position, width, and profile of a absorption band near 1470 cm(exp -1) in the IR spectra of many dense molecular clouds strongly suggests that solid methanol is an important component of interstellar ices. In an attempt to better constrain the identification of 1470 cm(exp -1) feature, we began a program to search for other characteristic absorption bands of solid state methanol in the spectra of objects known to produce this band. One such feature is now identified in the spectra of several dense molecular clouds and its position, width, and profile fit well with those of laboratory H2O:CH3OH ices. Thus, the presence of methanol-bearing ices in space is confirmed
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