3,244 research outputs found
Effects of magnetic fields on the cosmic-ray ionization of molecular cloud cores
Low-energy cosmic rays are the dominant source of ionization for molecular
cloud cores. The ionization fraction, in turn, controls the coupling of the
magnetic field to the gas and hence the dynamical evolution of the cores. The
purpose of this work is to compute the attenuation of the cosmic-ray flux rate
in a cloud core taking into account magnetic focusing, magnetic mirroring, and
all relevant energy loss processes. We adopt a standard cloud model
characterized by a mass-to-flux ratio supercritical by a factor of about 2 to
describe the density and magnetic field distribution of a low-mass starless
core, and we follow the propagation of cosmic rays through the core along flux
tubes enclosing different amount of mass. We then extend our analysis to cores
with different mass-to-flux ratios. We find that mirroring always dominates
over focusing, implying a reduction of the cosmic-ray ionization rate by a
factor of about 2-3 over most of a solar-mass core with respect to the value in
the intercloud medium outside the core. For flux tubes enclosing larger masses
the reduction factor is smaller, since the field becomes increasingly uniform
at larger radii and lower densities. We also find that the cosmic-ray
ionization rate is further reduced in clouds with stronger magnetic field, e.g.
by a factor of about 4 for a marginally critical cloud. The magnetic field
threading molecular cloud cores affects the penetration of low-energy cosmic
rays and reduces the ionization rate by a factor 3-4 depending on the position
inside the core and the magnetization of the core.Comment: 7 pages, 7 figures, to be published in Astronomy and Astrophysic
Synchrotron emission in molecular cloud cores: the SKA view
Understanding the role of magnetic fields in star-forming regions is of
fundamental importance. In the near future, the exceptional sensitivity of SKA
will offer a unique opportunity to evaluate the magnetic field strength in
molecular clouds and cloud cores through synchrotron emission observations. The
most recent Voyager 1 data, together with Galactic synchrotron emission and
Alpha Magnetic Spectrometer data, constrain the flux of interstellar cosmic-ray
electrons between MeV and GeV, in particular in the
energy range relevant for synchrotron emission in molecular cloud cores at SKA
frequencies. Synchrotron radiation is entirely due to primary cosmic-ray
electrons, the relativistic flux of secondary leptons being completely
negligible. We explore the capability of SKA in detecting synchrotron emission
in two starless molecular cloud cores in the southern hemisphere, B68 and FeSt
1-457, and we find that it will be possible to reach signal-to-noise ratios of
the order of at the lowest frequencies observable by SKA ( MHz)
with one hour of integration.Comment: 5 pages, 4 figures, accepted by Astronomy & Astrophysic
Large scale GW calculations
We present GW calculations of molecules, ordered and disordered solids and
interfaces, which employ an efficient contour deformation technique for
frequency integration, and do not require the explicit evaluation of virtual
electronic states, nor the inversion of dielectric matrices. We also present a
parallel implementation of the algorithm which takes advantage of separable
expressions of both the single particle Green's function and the screened
Coulomb interaction. The method can be used starting from density functional
theory calculations performed with semi-local or hybrid functionals. We applied
the newly developed technique to GW calculations of systems of unprecedented
size, including water/semiconductor interfaces with thousands of electrons
Design of defect spins in piezoelectric aluminum nitride for solid-state hybrid quantum technologies
Spin defects in wide-band gap semiconductors are promising systems for the
realization of quantum bits, or qubits, in solid-state environments. To date,
defect qubits have only been realized in materials with strong covalent bonds.
Here, we introduce a strain-driven scheme to rationally design defect spins in
functional ionic crystals, which may operate as potential qubits. In
particular, using a combination of state-of-the-art ab-initio calculations
based on hybrid density functional and many-body perturbation theory, we
predicted that the negatively charged nitrogen vacancy center in piezoelectric
aluminum nitride exhibits spin-triplet ground states under realistic uni- and
bi-axial strain conditions; such states may be harnessed for the realization of
qubits. The strain-driven strategy adopted here can be readily extended to a
wide range of point defects in other wide-band gap semiconductors, paving the
way to controlling the spin properties of defects in ionic systems for
potential spintronic technologies.Comment: In press. 32 pages, 4 figures, 3 tables, Scientific Reports 201
Nonempirical Range-separated Hybrid Functionals for Solids and Molecules
Dielectric-dependent hybrid (DDH) functionals were recently shown to yield
accurate energy gaps and dielectric constants for a wide variety of solids, at
a computational cost considerably less than that of GW calculations. The
fraction of exact exchange included in the definition of DDH functionals
depends (self-consistently) on the dielectric constant of the material. Here we
introduce a range-separated (RS) version of DDH functionals where short and
long-range components are matched using system dependent, non-empirical
parameters. We show that RS DDHs yield accurate electronic properties of
inorganic and organic solids, including energy gaps and absolute ionization
potentials. Furthermore we show that these functionals may be generalized to
finite systems.Comment: In press. 13 pages, 7 figures, 8 tables, Physical Review B 201
A Finite-field Approach for Calculations Beyond the Random Phase Approximation
We describe a finite-field approach to compute density response functions,
which allows for efficient and calculations beyond
the random phase approximation. The method is easily applicable to density
functional calculations performed with hybrid functionals. We present results
for the electronic properties of molecules and solids and we discuss a general
scheme to overcome slow convergence of quasiparticle energies obtained from
calculations, as a function of the basis set used to represent
the dielectric matrix
Interstellar dust charging in dense molecular clouds: cosmic ray effects
The local cosmic-ray (CR) spectra are calculated for typical characteristic
regions of a cold dense molecular cloud, to investigate two so far neglected
mechanisms of dust charging: collection of suprathermal CR electrons and
protons by grains, and photoelectric emission from grains due to the UV
radiation generated by CRs. The two mechanisms add to the conventional charging
by ambient plasma, produced in the cloud by CRs. We show that the CR-induced
photoemission can dramatically modify the charge distribution function for
submicron grains. We demonstrate the importance of the obtained results for
dust coagulation: While the charging by ambient plasma alone leads to a strong
Coulomb repulsion between grains and inhibits their further coagulation, the
combination with the photoemission provides optimum conditions for the growth
of large dust aggregates in a certain region of the cloud, corresponding to the
densities between cm and
cm. The charging effect of CR is of generic nature, and therefore is
expected to operate not only in dense molecular clouds but also in the upper
layers and the outer parts of protoplanetary discs.Comment: accepted by Ap
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