Bulletin of the American Physical Society
46th Annual Meeting of the APS Division of Atomic, Molecular and Optical Physics
Volume 60, Number 7
Monday–Friday, June 8–12, 2015; Columbus, Ohio
Session B5: Invited Session: Atomic and Molecular Astrophysics |
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Chair: Daniel Savin, Columbia University Room: Fairfield |
Tuesday, June 9, 2015 10:30AM - 11:00AM |
B5.00001: Measurements of electron-impact single and multiple ionization for ions of astrophysical interest Invited Speaker: Michael Hahn Accurate cross-section data for electron impact ionization (EII) are needed in order to interpret the spectra of collisionally ionized plasmas. In astrophysics, such plasmas are formed in stars, supernova remnants, galaxies, and galaxy clusters. Spectroscopic diagnostics of such plasmas rely on accurate ion balance calculations, which depend, in turn, on the underlying rates for EII and electron-ion recombination. Single ionization is usually the dominant EII process, but in some dynamic systems multiple ionization can become important. We have been carrying out EII measurements using the TSR storage ring located at the Max Planck Institut f\"ur Kernphysik in Heidelberg, Germany. Storage ring measurements are largely free of metastable contamination, resulting in unambiguous EII data. In order to guide theory, we have focused on providing at least one single ionization measurement for as many isoelectronic sequences as possible. To date, we have measured single ionization for ions from 13 isoelectronic sequences: Be-like sulfur, B-like magnesium, and F-like through K-like iron. For multiple ionization, there is no adequate theory capable of calculating the needed cross sections. All such data are based on experimental measurements and supplemented by semiempirical extrapolations. We have performed several measurements of double ionization for Fe ions. However, very little experimental data exist for multiple ionization and more is needed in order to generate reliable ionization balance calculations for dynamic plasmas. [Preview Abstract] |
Tuesday, June 9, 2015 11:00AM - 11:30AM |
B5.00002: Atomic collision processes for modelling cool star spectra Invited Speaker: Paul Barklem The abundances of chemical elements in cool stars are very important in many problems in modern astrophysics. They provide unique insight into the chemical and dynamical evolution of the Galaxy, stellar processes such as mixing and gravitational settling, the Sun and its place in the Galaxy, and planet formation, to name a just few examples. Modern telescopes and spectrographs measure stellar spectral lines with precision of order 1 per cent, and planned surveys will provide such spectra for millions of stars. However, systematic errors in the interpretation of observed spectral lines leads to abundances with uncertainties greater than 20 per cent. Greater precision in the interpreted abundances should reasonably be expected to lead to significant discoveries, and improvements in atomic data used in stellar atmosphere models play a key role in achieving such advances in precision. In particular, departures from the classical assumption of local thermodynamic equilibrium (LTE) represent a significant uncertainty in the modelling of stellar spectra and thus derived chemical abundances. Non-LTE modelling requires large amounts of radiative and collisional data for the atomic species of interest. I will focus on inelastic collision processes due to electron and hydrogen atom impacts, the important perturbers in cool stars, and the progress that has been made. I will discuss the impact on non-LTE modelling, and what the modelling tells us about the types of collision processes that are important and the accuracy required. More specifically, processes of fundamentally quantum mechanical nature such as spin-changing collisions and charge transfer have been found to be very important in the non-LTE modelling of spectral lines of lithium, oxygen, sodium and magnesium. [Preview Abstract] |
Tuesday, June 9, 2015 11:30AM - 12:00PM |
B5.00003: High Level Ab Initio Kinetics as a Tool for Astrochemistry Invited Speaker: Stephen Klippenstein We will survey the application of ab initio theoretical kinetics to reactions of importance to astrochemistry. Illustrative examples will be taken from our calculations for (i) interstellar chemistry, (ii) Titan's atmospheric chemistry, and (iii) the chemistry of extrasolar giant planets. The accuracy of various aspects of the calculations will be summarized including (i) the underlying ab initio electronic structure calculations, (ii) the treatment of the high pressure recombination process, and (iii) the treatment of the pressure dependence of the kinetics. The applications will consider the chemistry of phosphorous on giant planets, the kinetics of water dimerization, the chemistry of nitrogen on Titan's atmosphere, as well as various reactions of interstellar chemistry interest such as the recombination of OH with H, and O($^{\mathrm{3}}$P) reacting with C$_{\mathrm{2}}$H$_{\mathrm{5}}$, CH$_{\mathrm{2}}$, and CCS. [Preview Abstract] |
Tuesday, June 9, 2015 12:00PM - 12:30PM |
B5.00004: Nuclear Spin Dependent Chemistry of the Trihydrogen Cation in Diffuse Interstellar Clouds Invited Speaker: Kyle Crabtree The trihydrogen cation, H$_3^+$, long thought to be the species responsible for initiating ion-molecule chemistry in the interstellar medium, was first observed in interstellar clouds twenty years ago. Since its detection, this cation has been used to infer temperatures, densities, cloud sizes, and the local cosmic ray ionization rate. However, in diffuse molecular clouds the excitation temperature of its two nuclear spin modifications, ortho ($I = 3/2$) and para-H$_3^+$ ($I = 1/2$) is found to differ markedly from the cloud kinetic temperature inferred from the spin modifications of molecular hydrogen (H$_2$) in the same environment. A steady state analysis of the chemical kinetics of ortho and para-H$_3^+$ suggests that the interplay of thermalizing collisions with H$_2$ and nuclear spin dependent dissociative recombination with electrons may result in a nonthermal excitation temperature. Each of these processes is complex. Collisions between H$_3^+$ and H$_2$ must obey selection rules based on conservation of nuclear spin angular momentum, and the allowed spin conversion reactions, which proceed through the fluxional (H$_5^+)^*$ intermediate, each have different statistical weights and energetic requirements. Meanwhile, theoretical and experimental studies of H$_3^+$ electron recombination carried out over the past 40 years have yielded rates that span 4 orders of magnitude in range. We will present experimental measurements of the nuclear spin dependence of the reactions of H$_3^+$ with H$_2$ and with electrons, as well as astronomical observations of H$_3^+$ in diffuse molecular clouds and time-dependent chemical modeling of these environments. Astrochemical models incorporating the latest experimental data still do not satisfactorily explain the observed excitation temperature in diffuse molecular clouds, and point to the need for state-selective measurements of the H$_3^+$ electron recombination rate. [Preview Abstract] |
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