Herzen University

Saint Petersburg, Russia

Herzen University

Saint Petersburg, Russia
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Vertogradov V.D.,Herzen University
Journal of Physics: Conference Series | Year: 2016

The gravitational collapse of generalised Vaidya spacetime is considered. It is known that the endstate of gravitational collapse, as to whether a black hole or a naked singularity is formed, depends on the mass function M(v,r). Here we give conditions for the mass function which corresponds to the equation of the state P = αρ where α ∈ (0,] and according to these conditions we obtain either a black hole or a naked singularity at the endstate of gravitational collapse. Also we give conditions for the mass function when the singularity is gravitationally strong. Also we provide the metric which is the analogue of Vaidya metric in case of rotation.

Dadonova A.V.,Herzen University | Devdariani A.Z.,Saint Petersburg State University
Journal of Physics: Conference Series | Year: 2012

Exchange interaction leads to the formation of gerade and ungerade states of temporary molecules (quasimolecules) formed during the H- + H slow collisions. The work deals with the radiation produced by optical transitions between those states. The main characteristics involved in the description of optical transitions in quasimolecules, i.e., energy terms, an optical dipole transition moments, have been calculated in the frame of zero-range potentials model. The main feature of calculations is that the results can be expressed analytically in closed forms via the Lambert W function.

Lind K.,European Southern Observatory | Lind K.,Max Planck Institute for Astrophysics | Asplund M.,Max Planck Institute for Astrophysics | Barklem P.S.,Uppsala University | Belyaev A.K.,Herzen University
Astronomy and Astrophysics | Year: 2011

Neutral sodium is a minority species in the atmospheres of late-type stars, and line formation in local thermodynamic equilibrium (LTE) is often a poor assumption, in particular for strong lines. We present an extensive grid of non-LTE calculations for several Na  I lines in cool stellar atmospheres, including metal-rich and metal-poor dwarfs and giants. For the first time, we constructed a Na model atom that incorporates accurate quantum mechanical calculations for collisional excitation and ionisation by electrons as well as collisional excitation and charge exchange reactions with neutral hydrogen. Similar to Li I, the new rates for hydrogen impact excitation do not affect the statistical equilibrium calculations, while charge exchange reactions have a small but non-negligible influence. The presented LTE and non-LTE curves-of-growth can be interpolated to obtain non-LTE abundances and abundance corrections for arbitrary stellar parameter combinations and line strengths. The typical corrections for weak lines are -0.1.-0.2 dex, whereas saturated lines may overestimate the abundance in LTE by more than 0.5 dex. The non-LTE Na abundances appear very robust with respect to uncertainties in the input collisional data. © ESO 2011.

Belyaev A.K.,Herzen University | Barklem P.S.,Uppsala University | Dickinson A.S.,Northumbria University | Gadea F.X.,CNRS Laboratory for Quantum Chemistry and Physics
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2010

Full quantum-scattering calculations are reported for low-energy near-threshold inelastic collision cross sections for H+Na. The calculations include transitions between all levels up to and including the ionic state (ion-pair production) for collision energies from the threshold up to 10 eV. These results are important for astrophysical modeling of spectra in stellar atmospheres. Results for the 3s-3p excitation are carefully examined using three different quantum chemistry input data sets, and large differences are found near the threshold. The differences are found to be predominantly due to differences in the radial coupling rather than potentials and are also found not to relate to differences in couplings in a simple manner. In fact, of the three input couplings, the two that are most similar give the cross sections with the largest differences. The 3s-3p cross sections show orbiting resonances which have been seen in earlier studies, while Feshbach resonances associated with closed channels were also found to be present in the low-energy cross sections for some transitions. © 2010 The American Physical Society.

Sayfutyarova E.R.,Moscow State University | Buchachenko A.A.,Moscow State University | Yakovleva S.A.,Herzen University | Belyaev A.K.,Herzen University
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2013

Charge-transfer cold Yb++ Rb collision dynamics is investigated theoretically using high-level ab initio potential energy curves, dipole moment functions, and nonadiabatic coupling matrix elements. Within the scalar-relativistic approximation, the radiative transitions from the entrance A1Σ+ to the ground X1Σ+ state are found to be the only efficient charge-transfer pathway. The spin-orbit coupling does not open other efficient pathways, but alters the potential energy curves and the transition dipole moment for the A-X pair of states. The radiative, as well as the nonradiative, charge-transfer cross sections calculated within the 10 -3-10 cm-1 collision energy range exhibit all features of the Langevin ion-atom collision regime, including a rich structure associated with centrifugal barrier tunneling (orbiting) resonances. Theoretical rate coefficients for two Yb isotopes agree well with those measured by immersing Yb+ ions in an ultracold Rb ensemble in a hybrid trap. Possible origins of discrepancy in the product distributions and relations to previously studied similar processes are discussed. © 2013 American Physical Society.

Belyaev A.K.,Uppsala University | Belyaev A.K.,University Paul Sabatier | Lebedev O.V.,Herzen University
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2011

The branching classical trajectory method for inelastic atomic collision processes is proposed. The approach is based on two features: (i) branching of a classical trajectory in a nonadiabatic region and (ii) the nonadiabatic transition probability formulas particularly adapted for a classical trajectory treatment. In addition to transition probabilities and inelastic cross sections, the proposed approach allows one to calculate incoming and outgoing currents. The method is applied to inelastic Na + H collisions providing the results in reasonable agreement with full quantum calculations. © 2011 American Physical Society.

Tacconi M.,University of Rome La Sapienza | Gianturco F.A.,University of Rome La Sapienza | Belyaev A.K.,Herzen University
Physical Chemistry Chemical Physics | Year: 2011

Collisions between Ca cations and Rb atoms are computed within a quantum approach that generates the most relevant potential energy curves from accurate ab initio methods and carries out the low-energy scattering calculations by including nonadiabatic and spin-orbit coupling terms. The cross sections are obtained at relative energies typical for the likely arrangements of Rb atoms in a Magneto-Optical Trap overlapped with a Coulomb Crystal of Ca cations. The dominant nonadiabatic process is clearly identified and the efficiency of the nonadiabatic coupling terms which lead to the charge-exchange process is discussed. © the Owner Societies 2011.

Belyaev A.K.,Herzen University
Journal of Physics: Conference Series | Year: 2012

Modeling of formation of spectral lines in stellar atmospheres requires accurate data on inelastic collision processes. The only source of this information is theoretical calculations mainly performed within the standard adiabatic Born-Oppenheimer approach. It is shown that the conventional application of this approach encounters severe difficulties, in particular, due to nonzero couplings in the asymptotic region. This results in the fact that the conventional applications do not provide convergences, which are necessary for calculations of inelastic cross sections, giving infinite cross sections. The proposed remedy is the quantum reprojection method within the standard adiabatic Born-Oppenheimer approach. The method takes nonvanishing asymptotic nonadiabatic couplings into account and distinguishes asymptotic currents in molecular-state channels and in atomic-state channels. The method is demonstrated for the example of low-energy inelastic Li + Na collisions for which the conventional application of the Born-Oppenheimer approach fails.

Belyaev A.K.,Herzen University
Physical Review A - Atomic, Molecular, and Optical Physics | Year: 2015

Quantum chemical results for the lowest four potentials of the HeH molecule and the corresponding rotational and radial nonadiabatic coupling matrix elements are reported. Close-coupling calculations of the integral cross sections for the excitation processes H(1s)+He→H(2s,2p)+He are performed on this basis. The calculated cross sections are in a reasonable agreement with the experimental data. © 2015 American Physical Society.

Grib A.A.,Herzen University | Pavlov Y.V.,Russian Academy of Sciences
Astroparticle Physics | Year: 2011

Scattering of particles in the gravitational field of Kerr black holes is considered. It is shown that scattering energy of particles in the centre of mass system can obtain very large values not only for extremal black holes but also for nonextremal ones existing in Nature. This can be used for explanation of still unresolved problem of the origin of ultrahigh energy cosmic rays observed in Auger experiment. Extraction of energy after the collision is investigated. It is shown that due to the Penrose process the energy of the particle escaping the hole at infinity can be large. Contradictions in the problem of getting high energetic particles escaping the black hole are resolved. © 2010 Elsevier B.V. All rights reserved.

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