On inelastic hydrogen atom collisions in stellar atmospheres

The influence of inelastic hydrogen atom collisions on non-LTE spectral line formation has been, and remains to be, a significant source of uncertainty for stellar abundance analyses, due to the difficulty in obtaining accurate data for low-energy atomic collisions either experimentally or theoretically. For lack of a better alternative, the classical "Drawin formula" is often used. Over recent decades, our understanding of these collisions has improved markedly, predominantly through a number of detailed quantum mechanical calculations. In this paper, the Drawin formula is compared with the quantum mechanical calculations both in terms of the underlying physics and the resulting rate coefficients. It is shown that the Drawin formula does not contain the essential physics behind direct excitation by H atom collisions, the important physical mechanism being quantum mechanical in character. Quantitatively, the Drawin formula compares poorly with the results of the available quantum mechanical calculations, usually significantly overestimating the collision rates by amounts that vary markedly between transitions.Comment: 9 pages, 6 figures, accepted for A&

Electronic structure, reactivity, and spectroscopy of dihydrides of group-IB metals

International audienceAtomic pseudopotentials and highly correlated wave functions, including spin-orbit interactions, have been used to evaluate the electronic structure, stability, and spectroscopy of triatomic molecule MH2, with a metal M belonging to group IB (Cu, Ag, and Au). CuH2 and AuH2 have been recently observed by IR spectroscopy in solid hydrogen and bending anharmonic wave numbers have been assigned to these two systems. The AgH2 molecule has not been detected nor experimentally characterized, despite several theoretical works arguing on its stability. Our results confirm that the MH2 radicals have a metastable bent ground state separated from the dissociation into [M+H-2] ground state by barriers which have been evaluated to 1.43, 0.78, and 0.80 eV, for Cu, Ag, and Au compounds, respectively. These barriers are calculated smaller than in previous determinations but still large enough to stabilize the MH2 systems. Spectroscopic data are calculated for these radicals

Inelastic excitation and charge transfer processes for oxygen in collision with H atoms

International audiencePotential energy functions of the OH molecule are investigated from small to large inter-atomic distances R. The electronic problem is treated using an efficient Full Configuration Interaction (Full CI) approach that avoids orbital jumps found usually in multi-configuration self-consistent-field followed by multi-reference configuration interaction calculations of excited states. The calculations are performed for all the doublet, quartet, and sextet OH molecular states, up to the O(2p34s 3S) + H(1s 2S) asymptote, and for the lowest O− + H+ and O+ + H− ionic states. Inter-atomic distances, ranging from 0.5 Å to 20 Å, are spanned with a very small step in order to describe accurately the avoided crossings between the adiabatic potential energy functions. The accuracy of the potentials at small and large R values is analyzed. These Full CI calculations provide for the first time a global description of the 40 lowest molecular states of OH, well suited for dynamical calculations. The resulting potentials are used to obtain first estimates of cross sections and rate coefficients for different inelastic processes through the multichannel approach. This method, based on a Landau-Zener formalism taking into account the ionic-covalent avoided crossings at large distances, gives reliable results for the most intense transitions. It is shown that the largest rate coefficients correspond to mutual neutralization and ion-pair production processes

Inelastic processes in oxygen–hydrogen collisions

International audienceNew accurate theoretical rate coefficients for (de)-excitation and charge transfer in low-energy O + H, O+ + H− and O− + H+ collisions are reported. The calculations of cross-sections and rate coefficients are performed by means of the quantum probability current method, using full configuration interaction ab initio electronic structure calculations that provide a global description of all 43 lowest molecular states from short to asymptotic internuclear distances. Thus, both long- and short-range non-adiabatic regions are taken into account for the first time. All the doublet, quartet and sextet OH molecular states, with excitation energy asymptotes up to 12.07 eV, as well as the two lowest ionic states with the asymptotes O−H+ and O+H− are treated. Calculations are performed for the collision energy range 0.01–100eV and the temperature range 1 000–10 000 K. The mechanisms underlying the processes are analysed: it is shown that the largest rate coefficients, with values exceeding 10−8 cm3 s−1, are due to ionic–covalent interactions present at large internuclear distances, while short-range interactions play an important role for rates with moderate values involved in (de)-excitation processes. As a consequence, a comparison of the present data with previously published results shows that differences of up to several orders of magnitude exist for rate coefficients with moderate values. It is worth pointing out the relatively large rate coefficients for triplet–quintuplet oxygen transitions, as well as for transitions between the O(2p33s5So) and O(2p33p5P) levels of the oxygen triplet and H(n = 2) levels. The calculated data are important for modelling stellar spectra, leading to accurate oxygen abundances

Inelastic Mg+H collision data for non-LTE applications in stellar atmospheres

Rate coefficients for inelastic Mg+H collisions are calculated for all transitions between the lowest seven levels and the ionic state (charge transfer), namely Mg(3s2 1S, 3s3p 3P, 3s3p 1P, 3s4s 3S, 3s4s 1S, 3s3d 1D, 3s4p 3P)+H(1s) and Mg+(3s 2S)+H−. The rate coefficients are based on cross-sections from full quantum scattering calculations, which are themselves based on detailed quantum chemical calculations for the MgH molecule. The data are needed for non-LTE applications in cool astrophysical environments, especially cool stellar atmospheres, and are presented for a temperature range of 500−8000 K. From consideration of the sensitivity of the cross-sections to various uncertainties in the calculations, most importantly input quantum chemical data and the numerical accuracy of the scattering calculations, a measure of the possible uncertainties in the rate coefficients is estimated

Model estimates of inelastic calcium-hydrogen collision data for non-LTE stellar atmospheres modeling

International audienceAims. Inelastic processes in low-energy Ca + H and Ca+ + H- collisions are treated for the states from the ground state up to the ionicstate with the aim to provide rate coeffcients needed for non-LTE modeling of Ca in cool stellar atmospheres.Methods. The electronic molecular structure was determined using a recently proposed model approach that is based on an asymptoticmethod. Nonadiabatic nuclear dynamics were treated by means of multichannel formulas, based on the Landau-Zener model fornonadiabatic transition probabilities.Results. The cross sections and rate coeffcients for inelastic processes in Ca + H and Ca+ + H- collisions were calculated for alltransitions between 17 low-lying covalent states plus the ionic state. It is shown that the highest rate coeffcient values correspond tothe excitation, de-excitation, ion-pair formation, and mutual neutralization processes involving the Ca(4s5s 1;3S) and the ionic Ca+ +H- states. The next group with the second highest rate coeffcients includes the processes involving the Ca(4s5p 1;3P), Ca(4s4d 1;3D),and Ca(4s4p 1P) states. The processes from these two groups are likely to be important for non-LTE modeling

Impact of new atomic data for the formation of the Mg I 4571 AA line in benchmark stars

info:eu-repo/semantics/publishe

Impact of new atomic data for NLTE studies of neutral magnesium in cool stars: I. Test cases with a simple Mg I model atom

info:eu-repo/semantics/nonPublishe