Electron-Ion Recombination Rate Coefficients and Photoionization Cross Sections for Astrophysically Abundant Elements VI. Ni II

We present the first detailed ab initio quantum mechanical calculations for total and state-specific recombination rate coefficients for e + Ni III --> Ni II. These rates are obtained using a unified treatment for total electron-ion recombination that treats the nonresonant radiative recombination and the resonant dielectronic recombination in a self-consistent unified manner in the close coupling approximation. Large-scale calculations are carried out using a 49-state wavefunction expansion from core configurations 3d^8, 3d^74s, and 3d^64p that permits the inclusion of prominent dipole allowed core transitions. These extensive calculations for the recombination rates of Ni II required hundreds of CPU hours on the Cray T90. The total recombination rate coefficients are provided for a wide range of temperature. The state-specific recombination rates for 532 bound states of doublet and quartet symmetries, and the corresponding photoionization cross sections for leaving the core in the ground state, are presented. Present total recombination rate coefficients differ considerably from the currently used data in astrophysical models.Comment: ApJ Suppl. (submitted), 4 figure

Electron-Ion Recombination of Fe V

Relevant data is available at: http://www.astronomy.ohio-state.edu/~nahar/nahar_radiativeatomicdata/index.htmlIonization balance in the low-ionization stages of iron is important in a variety of astrophysical plasmas. Accurate recombination rates are needed but are difficult to compute in a detailed quantum mechanical treatment. The main difficulty lies in the complexity of strong electron-electron correlation arising due to the open 3d shell. We present the total electron-ion recombination rate coefficients for the process, e + Fe VI → Fe V, obtained in an ab initio manner. Large-scale computations are carried out in the close coupling (CC) approximation using the R-matrix method employing a unified treatment that considers the infinite number of states of the recombined ion and incorporates both the radiative recombination (RR) and the dielectronic recombination (DR) processes in a self-consistent manner. It involves calculations of detailed photoionization cross sections, σ_PI, with autoionizing resonances of a large number of bound states with n ≤ n_max, such as 1054 bound states for the present case of Fe V. The number also equals the total number of state-specific recombination rates obtained for the ion. The high-n states are treated through the DR theory by Bell & Seaton. The same wavefunction expansion is employed in all photoionization/recombination calculations, thereby ensuring self-consistency. Rates are presented at a wide range of temperature for all practical applications. Present rates for total recombination differ considerably from currently used values obtained using simpler approximations. Application of the new recombination rate coefficients and photoionization cross sections for the ionization structure of iron in planetary nebulae show that under typical conditions the relative fractions of Fe V and Fe VI change by nearly a factor of 2.This work was supported partially by the NSF (AST 98-70089) and the NASA Astrophysics Data Program

Electron-ion recombination of Fe IV

Relevant data is available at: http://www.astronomy.ohio-state.edu/~nahar/nahar_radiativeatomicdata/index.htmlAb initio calculations are presented for total and partial recombination cross sections and rate coefficients for e + Fe V → Fe IV, employing a unified treatment that incorporates both the radiative and dielectronic recombinations (RR and DR) in a self-consistent and accurate manner. The theoretical treatment is based on the close-coupling approximation using the R-matrix method. Recombination calculations for heavy atoms such as Fe IV, with 3d open-shell ground configuration, require an extensive eigenfunction expansion for the target ion, as well as a carefully optimized basis set of electron-ion bound configurations that represent short range correlation effects. The large-scale calculations are preceded by electron scattering and photoionization calculations using a 31-term eigenfunction expansion dominated by the configurations 3d^4, 3d^3 4s, and 3d^3 4p of Fe V. Photorecombination cross sections and DR collision strengths are thereby obtained, including individual photorecombination cross sections for a large number of bound states of Fe IV that couple to the ground state 3d^4 ^5 D of the target ion Fe V—all possible bound states up to n = 10 (740 LS terms of Fe IV). The cross sections include autoionizing resonances, also up to the n = 10 complex, accounting for the unified (RR + DR) contribution into the n ≤ 10 (low-n) bound states of Fe IV. Recombination into the high-n states, 10 ≤ n ≤ ∞, is obtained through the DR collision strengths for the corresponding series of resonances in the electron-ion continua. The convergence of the close-coupling expansion, with respect to the target ion states and the (electron plus ion) correlation functions is discussed with reference to other highly complex atomic systems. Maxwellian average at a range of temperatures yields the total rate coefficient, as well as partial contributions directly to state-specific recombination rate coefficients. The new close-coupling rates differ considerably from those heretofore obtained from simpler approximations. We expect the present data to be of importance in the modeling of astrophysical and laboratory plasmas where iron is often a prominent constituent. [S1050-2947(98)06912-1]The work was supported by a NSF grant for the Iron Project (Grant No. PHY-9421898), and by NASA Grant No. NAGW-3315 and NAS-32643

Electron-Ion Recombination of Neutral Iron

Relevant data is available at: http://www.astronomy.ohio-state.edu/~nahar/nahar_radiativeatomicdata/index.htmlThe total and state-specific electron-ion recombination rate coefficients are obtained for Fe I. The calculations are carried out using a new ab initio method that incorporates both the radiative and the dielectronic recombination processes in an unified and self-consistent manner. The computations employ the close coupling approximation and the R-matrix method from atomic collision theory. A 52 state close coupling eigenfunction expansion dominated by the states of the ground 3d^6 4s and excited 3d^7, 3d^6 4p, 3d^5 4s^2, and 3d^5 4s4p configurations of Fe II are used in the present calculations. The important electron correlation and radiation damping effects are included via explicit coupling of autoionization and radiative channels. This is the first detailed atomic calculation for the recombination rates for Fe I. The present rates are considerably higher than the radiative recombination rates being used currently in the low-temperature region, T ≤ 10^4 K, whereas they are about 4 times lower than those given by the Burgess general formula for dielectronic recombination at higher temperatures. The implications of the new recombination rate coefficients and photoionization cross sections for Fe I on the ionization structure of iron in the cold neutral interstellar medium are studied. It is found that the ratio of Fe II to Fe I obtained with the new atomic data increases by a factor of about 3-30 over previous calculations.The work has been supported by NSF grant PHY- 9421898 and NASA grants NAGW-3315 and NAS-32643

Atomic Processes in Planetary Nebulae and H II Regions

Spectroscopic studies of Planetary Nebulae (PNe) and H {\sc ii} regions have driven much development in atomic physics. In the last few years the combination of a generation of powerful observatories, the development of ever more sophisticated spectral modeling codes, and large efforts on mass production of high quality atomic data have led to important progress in our understanding of the atomic spectra of such astronomical objects. In this paper I review such progress, including evaluations of atomic data by comparisons with nebular spectra, detection of spectral lines from most iron-peak elements and n-capture elements, observations of hyperfine emission lines and analysis of isotopic abundances, fluorescent processes, and new techniques for diagnosing physical conditions based on recombination spectra. The review is directed toward atomic physicists and spectroscopists trying to establish the current status of the atomic data and models and to know the main standing issues.Comment: 9 pages, 1 figur

Physical Conditions in Quasar Outflows: VLT Observations of QSO 2359-1241

We analyze the physical conditions of the outflow seen in QSO 2359-1241 (NVSS J235953-124148), based on high resolution spectroscopic VLT observations. This object was previously studied using Keck/HIRES data. The main improvement over the HIRES results is our ability to accurately determine the number density of the outflow. For the major absorption component, level population from five different Fe II excited level yields n_H=10^4.4 cm^-3 with less than 20% scatter. We find that the Fe ii absorption arises from a region with roughly constant conditions and temperature greater than 9000 K, before the ionization front where temperature and electron density drop. Further, we model the observed spectra and investigate the effects of varying gas metalicities and the spectral energy distribution of the incident ionizing radiation field. The accurately measured column densities allow us to determine the ionization parameter log(U) = -2.4 and total column density of the outflow (log(N_H) = 20.6 cm^-2). Combined with the number density finding, these are stepping stones towards determining the mass flux and kinetic luminosity of the outflow, and therefore its importance to AGN feedback processes.Comment: 21 pages, 3 figures (accepted for publication in the ApJ