7,513 research outputs found
The chemistry of vibrationally excited H2 in the interstellar medium
The internal energy available in vibrationally excited H2 molecules can be
used to overcome or diminish the activation barrier of various chemical
reactions of interest for molecular astrophysics. In this article we
investigate in detail the impact on the chemical composition of interstellar
clouds of the reactions of vibrationally excited H2 with C+, He+, O, OH, and
CN, based on the available chemical kinetics data. It is found that the
reaction of H2 (v>0) and C+ has a profound impact on the abundances of some
molecules, especially CH+, which is a direct product and is readily formed in
astronomical regions with fractional abundances of vibrationally excited H2,
relative to ground state H2, in excess of 10^(-6), independently of whether the
gas is hot or not. The effects of these reactions on the chemical composition
of the diffuse clouds zeta Oph and HD 34078, the dense PDR Orion Bar, the
planetary nebula NGC 7027, and the circumstellar disk around the B9 star HD
176386 are investigated through PDR models. We find that formation of CH+ is
especially favored in dense and highly FUV illuminated regions such as the
Orion Bar and the planetary nebula NGC 7027, where column densities in excess
of 10^(13) cm^(-2) are predicted. In diffuse clouds, however, this mechanism is
found to be not efficient enough to form CH+ with a column density close to the
values derived from astronomical observations.Comment: accepted for publication in the Astrophysical Journal; 9 pages, 7
figure
The chemistry of compact planetary nebulae
We report high-sensitivity millimetre observations of several molecular
species (13CO, HCN, HNC, CN, HCO+ and N2H+) in a sample of compact planetary
nebulae. Some species such as HCO+ and CN are particularly abundant compared to
envelopes around AGB stars or even interstellar clouds. We have estimated the
following average values for the column densities ratios: CN/HCN~2.6,
HCO+/HCN~0.5, and HNC/HCN~0.4. Thus, the chemical composition of the molecular
envelopes in these compact PNe appears somewhat intermediate between the
composition of proto-PNe (such as CRL 2688 or CRL 618) and well evolved PNe
(such as the Ring, M4--9, or the Helix). From observations of the CO
isotopomers, we have estimated that the 12C/13C ratio is in the range 10 ~<
12C/13C ~< 40. These values are below those expected from standard asymptotic
giant branch models and suggest non-standard mixing processes. The observed
molecular abundances are compared to very recent modelling work, and we
conclude that the observations are well explained, in general terms, by
time-dependent gas-phase chemical models in which the ionization rate is
enhanced by several orders of magnitude with respect to the average
interstellar value. Thus, our observations confirm that the chemistry in the
neutral shells of PNe is essentially governed by the high energy radiation from
the hot central stars. The complexity of the chemical processes is increased by
numerous factors linked to the properties of the central star and the geometry
and degree of clumpiness of the envelope. Several aspects of the PN chemistry
that remains to be understood are discussed within the frame of the available
chemical models.Comment: 9 pages, 3 figures. "In press" in Astronomy and Astrophysic
On the master equation approach to diffusive grain-surface chemistry: the H, O, CO system
We have used the master equation approach to study a moderately complex
network of diffusive reactions occurring on the surfaces of interstellar dust
particles. This network is meant to apply to dense clouds in which a large
portion of the gas-phase carbon has already been converted to carbon monoxide.
Hydrogen atoms, oxygen atoms, and CO molecules are allowed to accrete onto dust
particles and their chemistry is followed. The stable molecules produced are
oxygen, hydrogen, water, carbon dioxide (CO2), formaldehyde (H2CO), and
methanol (CH3OH). The surface abundances calculated via the master equation
approach are in good agreement with those obtained via a Monte Carlo method but
can differ considerably from those obtained with standard rate equations.Comment: 13 pages, 5 figure
On the Higher bit Version of Approximate Inhomogeneous Short Integer Solution Problem
We explore a bitwise modification in Ajtai\u27s one-way function. Our main contribution is to define the higher-bit approximate inhomogeneous short integer solution (ISIS) problem and prove its reduction to the ISIS problem. In this new instance, our main idea is to discard low-weighted bits to gain compactness.
As an application, we construct a bitwise version of a hash-and-sign signature in the random oracle model whose security relies on the (Ring)-LWE and (Ring)-ISIS assumptions. Our scheme is built from the hash-and-sign digital signature scheme based on the relaxed notion of approximate trapdoors introduced by Chen, Genise and Mukherjee (2019). Their work can be interpreted as a bitwise optimization of the work of Micciancio and Peikert (2012). We extend this idea and apply our technique to the scheme by discarding low-weighted bits in the public key. Our modification brings improvement in the public key size but also in the signature size when used in the right setting.
However, constructions based on the higher-bit approximate ISIS save memory space at the expense of loosening security. Parameters must be set in regards with this trade-off
Spin-density-wave instabilities in the organic conductor (TMTSF)_2ClO_4: Role of anion ordering
We study the spin-density-wave instabilities in the quasi-one-dimensional
conductor (TMTSF)_2ClO_4. The orientational order of the anions ClO_4 doubles
the unit cell and leads to the presence of two electrnic bands at the Fermi
level. From the Ginzburg-Landau expansion of the free energy, we determine the
low-temperature phase diagram as a function of the strength of the Coulomb
potential due to the anions. Upon increasing the anion potential, we first find
a SDW phase corresponding to an interband pairing. This SDW phase is rapidly
supressed, the metallic phase being then stable down to zero temperature. The
SDW instability is restored when the anion potential becomes of the order of
the hopping amplitude. The metal-SDW transition corresponds to an intraband
pairing which leaves half of the Fermi surface metallic. At lower temperature,
a second transition, corresponding to the other intraband pairing, takes place
and opens a gap on the whole Fermi surface. We discuss the consequences of our
results for the experimental phase diagram of (TMTSF)_2ClO_4 at high magnetic
field.Comment: 13 pages, 10 figures, Version 2 with minor correction
Reduction of chemical networks. I. The case of molecular clouds
We present a new method to analyse and reduce chemical networks and apply
this technique to the chemistry in molecular clouds. Using the technique, we
investigated the possibility of reducing the number of chemical reactions and
species in the UMIST 95 database simultaneously. In addition, we did the same
reduction but with the ``objective technique'' in order to compare both
methods. We found that it is possible to compute the abundance of carbon
monoxide and fractional ionisation accurately with significantly reduced
chemical networks in the case of pure gas-phase chemistry. For gas-grain
chemistry involving surface reactions reduction is not worthwhile. Compared to
the ``objective technique'' our reduction method is more effective but more
time-consuming as well.Comment: 15 pages, 7 postscript figures, accepted for publication in A&
Thermal Properties of Heavy Fermion Compound YbP
Low-temperature specific heat and its field-dependence up to 16 T was
measured in a stoichiometric single crystal of YbP. A sharp peak was observed
at {\it T} = 0.53 K in zero magnetic field. Application of external
field seems to induce a new magnetic phase above 11 T. The field dependence of
the transition temperature in the high-field phase is different from that of
the low field phase. The linear coefficient of the electronic specific heat is
estimated as 120 mJ/mole K from low temperature specfic heat, suggesting
heavy Fermion state in YbP.Comment: to be published in J.Phys.Soc.Jpn on May, 200
Growth of epitaxially oriented Ag nanoislands on air-oxidized Si(111)-(7x7) surfaces: Influence of short range order on the substrate
Clean Si(111)-(7{x7) surfaces, followed by air-exposure, have been
investigated by reflection high energy electron diffraction (RHEED) and
scanning tunneling microscopy (STM). Fourier transforms (FTs) of STM images
show the presence of short range (7x7) order on the air-oxidized surface.
Comparison with FTs of STM images from a clean Si(111)-(7x7) surface shows that
only the 1/7th order spots are present on the air-oxidized surface. The oxide
layer is ~ 2-3 nm thick, as revealed by cross-sectional transmission electron
microscopy (XTEM). Growth of Ag islands on these air-oxidized Si(111)-(7x7)
surfaces has been investigated by in-situ RHEED and STM and ex-situ XTEM and
scanning electron microscopy. Ag deposition at room temperature leads to the
growth of randomly oriented Ag islands while preferred orientation evolves when
Ag is deposited at higher substrate temperatures. For deposition at 550{\deg}C
face centered cubic Ag nanoislands grow with a predominant epitaxial
orientation [1 -1 0]Ag || [1 -1 0]Si, (111)Ag || (111)Si along with its twin
[-1 1 0]Ag || [1 -1 0]Si, (111)Ag || (111)Si, as observed for epitaxial growth
of Ag on Si(111) surfaces. The twins are thus rotated by a 180{\deg} rotation
of the Ag unit cell about the Si [111] axis. It is intriguing that Ag
nanoislands follow an epitaxial relationship with the Si(111) substrate in
spite of the presence of a 2-3 nm thick oxide layer between Ag and Si.
Apparently the short range order on the oxide surface influences the
crystallographic orientation of the Ag nanoislands.Comment: 10 figure
On the Ionisation Fraction in Protoplanetary Disks I: Comparing Different Reaction Networks
We calculate the ionisation fraction in protostellar disk models using a
number of different chemical reaction networks, including gas-phase and
gas-grain reaction schemes. The disk models we consider are conventional
alpha-disks, which include viscous heating and radiative cooling. The primary
source of ionisation is assumed to be X-ray irradiation from the central star.
We consider a number of gas-phase chemical networks. In general we find that
the simple models predict higher fractional ionisation levels and more
extensive active zones than the more complex models. When heavy metal atoms are
included the simple models predict that the disk is magnetically active
throughout. The complex models predict that extensive regions of the disk
remain magnetically uncoupled even with a fractional abundance of magnesium of
10(-8). The addition of submicron sized grains with a concentration of 10(-12)
causes the size of the dead zone to increase dramatically for all kinetic
models considered. We find that the simple and complex gas-grain reaction
schemes agree on the size and structure of the resulting dead zone. We examine
the effects of depleting the concentration of small grains as a crude means of
modeling the growth of grains during planet formation. We find that a depletion
factor of 10(-4) causes the gas-grain chemistry to converge to the gas-phase
chemistry when heavy metals are absent. 10(-8) is required when magnesium is
included. This suggests that efficient grain growth and settling will be
required in protoplanetary disks, before a substantial fraction of the disk
mass in the planet forming zone between 1 - 10 AU becomes magnetically active
and turbulent.Comment: 21 pages, 23 figures, accepted for publication in A & A Includes
correction to our implementation of the Umebayashi-Nakano reaction networ
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