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The unified method for total electron-ion recombination is extended to study the dielectronic satellite (DES) lines. These lines, formed from radiative decay of autoionizing states, are highly sensitive temperature diagnostics of astrophysical and laboratory plasma sources. The computation of the unified recombination rates is based on the relativistic Breit-Pauli R-matrix method and close coupling approximation. Extending the theoretical formulation developed earlier we present recombination rate coefficients for the 22 satellite lines of KLL complexes of helium-like Fe XXV and Ni XXVII. The isolated resonance approximation, commonly used throughout plasma modeling, treats these resonances essentially as bound features except for dielectronic capture into, and autoionization out of, these levels. A line profile or cross section shape is often assumed. On the other hand, by including the coupling between the autoionizing and continuum channels, the unified method gives the intrinsic spectrum of DES lines which includes not only the energies and strengths, but also the natural line or cross section shapes. A formulation is presented to derive autoionization rates from unified resonance strengths and enable correspondence with the isolated resonance approximation. While the rates compare very well with existing rates for the strong lines to <20%, the differences for weaker DES lines are larger. We also illustrate the application of the present results to the analysis of K ALPHA complexes observed in high-temperature X-ray emission spectra of Fe XXV and Ni XXVII. There are considerable differences with previous results in the total KLL intensity for Fe XXV at temperatures below the temperature of maximum abundance in coronal equilibrium. (Abbreviated Abstract), Comment: 21 pages, 5 figures, to appear in Physica Scripta

New, more detailed calculations of the emission spectra of the He-like K ALPHA complex of calcium, iron and nickel have been carried out using data from both distorted-wave and R-matrix calculations. The value of the GD ratio (an extended definition of the G ratio that accounts for the effect of resolved and unresolved satellite lines) is significantly enhanced at temperatures below the temperature of He-like maximum abundance. Furthermore it is shown that satellite lines are important contributors to the GD ratio such that GD/G>1 at temperatures well above the temperature of maximum abundance. These new calculations demonstrate, with an improved treatment of the KLn (n>=3) satellite lines, that K ALPHA satellite lines need to be included in models of He like spectra even at relatively high temperatures. The excellent agreement between spectra and line ratios calculated from R-matrix and distorted-wave data also confirms the validity of models based on distorted-wave data for highly charged systems, provided the effect of resonances are taken into account as independent processes., Comment: 12 pages, 5 figures, to appear in Monthly Notices of the Royal Astronomical Society. The definitive version is available at www.blackwell-synergy.com

We report the discovery of strong soft X-ray emission lines and a hard continuum above 2 keV in the Narrow-Line Seyfert 1 galaxy Mrk 335 during an extremely low X-ray flux state. Mrk 335 was observed for 22 ks by XMM-Newton in July 2007 as a Target of Opportunity to examine it in its X-ray low-flux state, which was discovered with Swift. Long-term light curves suggest that this is the lowest flux state this AGN has ever been seen in. However, Mrk 335 is still sufficiently bright that its X-ray properties can be studied in detail. The X-ray continuum spectrum is very complex and requires several components to model. Statistically, partial covering and blurred reflection models work well. We confirm the presence of a strong narrow Fe line at 6.4 keV. High-resolution spectroscopy with the XMM-RGS reveals strong, soft X-ray emission lines not detected in previous, higher signal-to-noise, XMM-Newton observations, such as: highly ionized Fe lines, O VII, Ne IX and Mg XI lines. The optical/UV fluxes are similar to those previously measured with Swift. Optical spectroscopy taken in 2007 September do not show any changes to optical spectra obtained 8 years earlier., Comment: Accepted for publication in ApJ; 26 pages, 10 figues, in press, ApJ 681 (July 01, 2008); Updated version with corrections made by the editor

The General Spectral Modeling (GSM) code employs the quasi-static approximation, a standard, low-density methodology that assumes the ionization balance is separable from a determination of the excited-state populations that give rise to the spectra. GSM also allows for some states to be treated only as contributions to effective rates. While these two approximations are known to be valid at low densities, this work investigates using such methods to model high-density, non-LTE emission spectra and determines at what point the approximations break down by comparing to spectra produced by the LANL code ATOMIC which makes no such approximations. As both approximations are used by other astrophysical and low-density modeling codes, the results should be of broad interest. He-like K$\alpha$ emission spectra are presented for Ni, Fe, and Ca, in order to gauge the effect of both approximations employed in GSM. This work confirms that at and above the temperature of maximum abundance of the He-like ionization stage, the range of validity for both approximations is sufficient for modeling the low- and moderate-density regimes one typically finds in astrophysical and magnetically confined fusion plasmas. However, a breakdown does occur for high densities; we obtain quantitative limits that are significantly higher than previous works. This work demonstrates that, while the range of validity for both approximations is sufficient to predict the density-dependent quenching of the z line, the approximations break down at higher densities. Thus these approximations should be used with greater care when modeling high-density plasmas such as those found in inertial confinement fusion and electromagnetic pinch devices., Comment: Accepted by Physical Review A (http://pra.aps.org/). 11 pages + LANL cover, 5 figures. Will update citation information as it becomes available. Abbreviated abstract is listed here

It is shown that above the temperature of maximum abundance, recombination rates into the excited states of He-like ions that are calculated using earlier, more approximate methods differ markedly from rates obtained from recent distorted-wave and R-Matrix calculations (unified recombination rate coefficients) for Ca, Fe, and Ni. The present rates lead to G ratios that are greatly lower than those resulting from the more approximate rates in previous works, by up to a factor of six at high electron temperatures. Excellent agreement between the distorted-wave and the R-Matrix rates, as well as excellent agreement in the G ratios calculated from them, provides support for the accuracy of these new values which have a broad applicability to the modelling and interpreting of X-ray spectra from a variety of astrophysical and laboratory sources., Comment: 11 pages including LANL cover, 6 figures. Minor typos corrected to conform to the version that will appear in print in MNRAS. The definitive version is available at http://www.blackwellsynergy.com. Specifically http://www.blackwell-synergy.com/doi/abs/10.1111/j.1365-2966.2007.12410.x

Codes to compute mean opacities and radiative accelerations for arbitrary chemical mixtures using the Opacity Project recently revised data have been restructured in a client--server architecture and transcribed as a subroutine library. This implementation increases efficiency in stellar modelling where element stratification due to diffusion processes is depth dependent, and thus requires repeated fast opacity reestimates. Three user modes are provided to fit different computing environments, namely a web browser, a local workstation and a distributed grid., Comment: 5 pages, 1 figure

We present the first results from the "Low Energy Detector" payload of the "Solar X-ray Spectrometer (SOXS)" mission, which was launched onboard the GSAT-2 Indian spacecraft on 08 May 2003 by the GSLV-D2 rocket to study solar flares. The SOXS Low Energy Detector (SLD) payload was designed, developed, and fabricated by the Physical Research Laboratory (PRL) in collaboration with the Space Application Centre (SAC), Ahmedabad and the ISRO Satellite Centre (ISAC), Bangalore of Indian Space Research Organization (ISRO). The energy ranges of the Si PIN and CZT detectors are 4 - 25 keV and 4 - 56 keV respectively. The Si PIN provides sub-keV energy resolution while the CZT reveals ~1.7 keV energy resolution throughout the energy range. The high sensitivity and sub-keV energy resolution of the Si PIN detector allows measuring the intensity, peak energy, and the equivalent width of the Fe-line complex at approximately 6.7 keV as a function of time in all ten M-class flares studied in this investigation. The peak energy (Ep) of the Fe-line feature varies between 6.4 and 6.7 keV with increasing in temperature from 9 to 58 MK. We found that the equivalent width (w) of the Fe-line feature increases exponentially with temperature up to 30 MK and then increases very slowly up to 40 Mk. It remains between 3.5 and 4 keV in the temperature range of 30 - 45 MK. We compare our measurements of w with calculations made earlier by various investigators and propose that these measurements may improve theoretical models. We interpret the variation of both Ep and w with temperature as due to the changes in the ionization and recombination conditions in the plasma during the flare interval and as a consequence the contribution from different ionic emission lines also varies., Comment: 21 pages, 12 figures, Solar Physics (in press)

Helium-like ions provide the most important X-ray spectral diagnostics in high temperature fusion and astrophysical plasmas. We previously presented computed collision strengths for O~VII including relativistic fine structure, levels up to the $n=4$ complex and radiation damping of autoionizing resonances. We have extended this work to other He-like ions (N, Ne, Mg, Al, Si, S, Ca). The calculations are carried out using the Breit-Pauli R-matrix (BPRM) method with a 31-level eigenfunction expansion. Collision strengths for the principal lines important in X-ray plasma diagnostics, w, x, y and z, corresponding to the 4 transitions to the ground level 1s^2(^1S_0) <- 1s2p(^1P^o_1), 1s2p(^3P^o_2), 1s2p(^3P^o_1), 1s2s(^3S_1), are explicitly shown. We find the effect of radiation damping to be significant for the forbidden transitions in heavier He-like ions, which should affect the diagnostic line ratios. We extrapolated the collision strengths to their values at infinite energy using the Burgess-Tully extrapolation technique. This is required to calculate the Maxwellian average collision strengths at high temperature. We show that the coupling between dipole allowed and inter-combination transitions affects increasingly the effective collision strengths for the n ^1S_0 - n' ^3P_1 transition as the charge of the ion increases. This clearly affects the treatment of the extrapolation toward the infinite energy point of the collision strength. This work is carried out as part of the Iron Project-RmaX Network., Comment: 13 pages, 6 figures, submitted to J.Phys.B

Efforts to benchmark astrophysical observations with X-ray laboratory measurements have been stymied by observed and measured differences of up to a factor of two in the ratio '3s/3d' of Fe XVII lines at ~17 \AA and ~15 \AA respectively. Using the electron distribution function (EDF) as a new physical parameter, we compute the Fe XVII line ratios and account for these differences. Based on large-scale relativistic close coupling calculations using the Breit-Pauli R-matrix method, revealing the precise effect of resonances in collisional excitation, we employ collisional-radiative models using cross sections convolved with both the Gaussian and the Maxwellian EDF. Comparison with astrophysical observations and laboratory measurements demonstrates that (a) the 3s/3d line ratio depends not only on the EDF but also on the electron temperature/energy of the source, (b) plasma conditions in experimental measurements and astrophysical observations may be quite different, and (c) departure from a Maxwellian should manifest itself in, and be used as a diagnostics of, particle distributions in plasmas., Comment: 5 pages, 2 figures, submitted to MNRAS

Based on new calculations we reconfirm the low and high density limits on the forbidden fine structure line ratio [O II] I(3729)/I(3726): lim_{N_ e} --> 0} = 1.5 and lim_{N_ e} --> \infty} = 0.35. Employing [O II] collision strengths calculated using the Breit-Pauli R-matrix method we rule out any significant deviation due to relativistic effects from these canonical values. The present results are in substantial agreement with older calculations by Pradhan (1976) and validate the extensive observational analysis of gaseous nebulae by Copetti and Writzel (2002) and Wang et al (2004) that reach the same conclusions. The present theoretical results and the recent observational analyses differ significantly from the calculations by MacLaughlin and Bell (1998) and Keenan et al (1999). The new maxwellian averaged effective collision strengths are presented for the 10 transitions among the first 5 levels to enable computations of [O II] line ratios., Comment: Submitted to MNRAS (Letters), 4 pages, 2 figures, 1 table

As an extension of the method by Bahcall et al. (2004) to investigate the time dependence of the fine structure constant, we describe an approach based on new observations of forbidden line multiplets from different ionic species. We obtain optical spectra of fine structure transitions in [Ne III], [Ne V], [O III], [OI], and [SII] multiplets from a sample of 14 Seyfert 1.5 galaxies in the low-z range 0.035 < z < 0.281. Each source and each multiplet is independently analyzed to ascertain possible errors. Averaging over our sample, we obtain a conservative value alpha^2(t)/\alpha^2(0) = 1.0030+-0.0014. However, our sample is limited in size and our fitting technique simplistic as we primarily intend to illustrate the scope and strengths of emission line studies of the time variation of the fine structure constant. The approach can be further extended and generalized to a "many-multiplet emission line method" analogous in principle to the corresponding method using absorption lines. With that aim, we note that the theoretical limits on emission line ratios of selected ions are precisely known, and provide well constrained selection criteria. We also discuss several other forbidden and allowed lines that may constitute the basis for a more rigorous study using high-resolution instruments on the next generation of 8 m class telescopes., Comment: 20 pages, 4 figures, sumbitted to AJ

We present new calculations for transition probabilities of Fe IV, with much more extensive datasets than heretofore available. The large-scale close coupling R-matrix calculations yield 1,798 LS bound states with n $\leq$ 11 and $l \leq$ 9, and corresponding transition probabilities for 138,121 dipole allowed transitions in the form of oscillator strengths $f$, line strengths $S$, and A-coeffficients for a variety of applications. This represents the largest R-matrix dataset in LS coupling for any ion under either the Opacity Project or the Iron Project. Through algebraic transformation of the LS multiplets, a total of 712,120 dipole allowed fine structure transitions for Fe IV are obtained. Observed transition energies are used together with the energy independent line strengths to derive the $f$- and the $A$-values; the adopted algorithm used calculated energies for the remainder. Present results show significantly better accuracy for the important low-lying states than previous calculations. Monochromatic and mean opacities for Fe IV are computed and compared with those obtained using the Opacity Project data. We find differences which could have important consequences for several astrophysical applications involving low ionization stages of iron., Comment: 10 pages, 1 figire, submitted to Astronomy and Astrophysics

Time dependent numerical simulations of the K ALPHA complex of Fe XXV are carried out as a function of temperature/density/radiation field variations in high-temperature astrophysical and laboratory plasmas. In addition to several well known features, the transient and steady state spectra reveal the effects due to (a) time-dependent thermal and non-thermal radiation fields, (b) photo and collisional excitation and ionization, and (c) high densities, on the `quartet' of principal w,x,y,z lines, and dielectronic satellites. The highly detailed models show precisely how, assuming a temporal-temperature correlation, the X-ray intensity varies between 6.6 - 6.7 keV and undergoes a `spectral inversion' in the w and z line intensities, characterizing an ionization or a recombination dominated plasma. The dielectronic satellite intensities are the most temperature dependent features, but insensitive to density variations, and significantly contribute to the K ALPHA complex for T < 6.7 keV leading to asymmetric profiles. The 6.7 keV K ALPHA complex should be a potential diagnostic of X-ray flares in AGN, afterglows in gamma-ray bursts, and other non-equilibrium sources with the high-resolution measurements possible from the upcoming mission Astro-E2. It is also shown that high electron densities attenuate the line intensities in simulations relevant to laboratory plasmas, such as in inertial confinement fusion, laser, or magnetic Z-pinch devices., Comment: 13 pages, 4 figures, 1 postscript coversheet. This is a preprint of an article accepted for publication in the Monthly Notices of the Royal Astronomical Society

We investigate the effect of resonances in photoionization and recombination cross sections computed using the R-matrix method. Photoionization and recombination rates derived from high-resolution cross sections for oxygen ions are compared with earlier works with less resolution and accuracy, such as in the widely used Opacity Project data. We find significant differences in photoionization rates for O II metastable states, averaged over Planck functions corresponding to ionizing radiation fields, with respect to the intrinsic accuracy of the calculations and improved resolution. Furthermore, for highly charged ions other physical effects are also important. Recombination rate coefficients, averaged over a Maxwellian distribution, are extremely sensitive to the position and resolution of near-threshold resonances, and radiation damping, in (e + O VII) --> O VI + hnu. Surprisingly however, the effect on the monochromatic and the mean Rosseland and Planck bound-free opacities is relatively small, but may be potentially significant., Comment: 11 pages, 4 figures, Journal of Physics B

(Abridge) Near-infrared spectroscopy is used to study the kinematics and excitation mechanisms of the H2 and [FeII] gas in a sample of AGN. The H2 lines are unresolved in all objects in which they were detected while the [FeII] lines have widths implying gas velocities of up to 650 km/s. This suggests that, very likely, the H2 and [FeII] emission does not originate from the same parcel of gas. Molecular H2 were detected in 90% of the sample, including PG objects, indicating detectavel amounts of molecular material even in objects with low levels of circumnuclear starburst activity. The data favors thermal excitation for the H2 lines. Indeed, in NGC3227, Mrk766, NGC4051 and NGC4151, the molecular emission is found to be purely thermal. This result is also confirmed by the rather similar vibrational and rotational temperatures in the objects for which they were derived. [FeII] lines are detected in all of the AGN. The [FeII] 1.254mu/Pa-beta ratio is compatible with excitation of the [FeII] by the active nucleus, but in Mrk 766 it implies a stellar origin. A correlation between H2/Br-gamma and [FeII]/Pa-beta is found. We confirm that it is a useful diagnostic tool in the NIR to separate emitting line objects by their level of nuclear activity. X-ray excitation models are able to explain the observed H2 and part of the [FeII] emission. Most likely, a combination of X-ray heating, shocks driven by the radio jet, and circumnuclear star formation contributes, in different proportions, to the H2 and [FeII] emission. In most of our spectra, the [FeII] 1.257mu/1.644mu ratio is found to be 30% lower than the intrinsic value based on current atomic data. This implies either than the extinction towards the [FeII] emitting clouds is very similar in most objects or there are possible inaccuracies in the A-values in the [FeII] transitions., Comment: 18 pages, 6 figures, Accepted for publication in Astronomy & Astrophysics

We present theoretical iron emission line strengths for physical conditions typical of Active Galactic Nuclei with Broad-Line Regions. The non-local thermodynamic equilibrium (NLTE) models include a new and extensive treatment of radiative transfer in the FeIII ion, complementing the FeII emission line strengths predicted in our earlier works. We also briefly present preliminary results for the FeI emission from AGN using a reduced atom model. We can satisfactorily reproduce the empirical UV FeIII emission line template of Vestergaard & Wilkes (2001) for the prototypical narrow-line Seyfert 1 galaxy I Zw 1, both in terms of the general FeIII flux distribution and the relative strength of the FeIII and FeII emission. However, a number of detailed features are still not matched; the most prominent example is the strongest single FeIII feature observed in the I Zw 1 spectrum, UV47: it is predicted to be strong only in models suppressing Fe-H charge exchange reactions. We examine the role of variations in cloud turbulent velocity and iron abundance and carry out Monte Carlo simulations to demonstrate the effect of uncertainties in atomic data on the computed spectra., Comment: 30 pages, 11 eps figures Submitted to ApJ

Ongoing activities under an international collaboration of atomic physicists and astrophysicists under the Iron Project and the RmaX Project, with applications to X-ray astronomy, are briefly described., Comment: 4 pgs., 1 fig., in Proceedings of IAU XXV Joint Discussion No. 17 "Atomic Data For X-Ray Astronomy", details at http://www.astronomy.ohio-state.edu/~pradhan/Iau/iau.html

A unified scheme using the R-matrix method has been developed for electron-ion recombination subsuming heretofore separate treatments of radiative and dielectronic recombination (RR and DR). The ab initio coupled channel approach unifies resonant and non-resonant phenomena, and enables a general and self-consistent treatment of photoionization and electron-ion recombination employing idential wavefunction expansion. Detailed balance takes account of interference effects due to resonances in cross sections, calculated explicitly for a large number of recombined (e+ion) bound levels over extended energy regions. The theory of DR by Bell and Seaton is adapted for high-n resonances in the region below series limits. The R-matrix method is employed for (A) partial and total photoionization and photorecombination cross sections of (e+ion) bound levels, and (B) DR and (e+ion) scattering cross sections. Relativistic effects and fine structure are considered in the Breit-Pauli approximation. Effects such as radiation damping may be taken into account where necessary. Unfiied recombination cross sections are in excellent agreement with measurements on ion storage rings to about 10-20%. In addition to high accuracy, the strengths of the method are: (I) both total and level-specific cross sections and rate coefficients are obtained, and (II) a single (e+ion) recombination rate coefficient for any given atom or ion is obtained over the entire temperature range of practical importance in laboratory and astrophysical plasmas, (III) self-consistent results are obtained for photoionization and recombination; comprehensive datasets have been computed for over 50 atoms and ions. Selected data are presented for iron ions., Comment: 33 pages, 13 figures, Review in "Radiation Processes In Physics and Chemistry", Elsevier (in press). Postscript file with higher resolution figures at http://www.astronomy.ohio-state.edu/~pradhan/pr.ps

Aimed at ionization balance and spectral analysis of UV and X-ray sources, we present self-consistent sets of photoionization cross sections, recombination cross sections, and rate coefficients for Li-like O VI and He-like O VII. Relativistic fine structure is considered through the Breit-Pauli R-matrix (BPRM) method in the close coupling approximation, implementing the unified treatment for total electron-ion recombination subsuming both radiative and di-electronic recombination processes. Self-consistency is ensured by using an identical wavefunction expansion for the inverse processes of photoionization and photo-recombination. Radiation damping of resonances, important for H-like and He-like core ions, is included. Compared to previous LS coupling results without radiative decay of low-n (<= 10) resonances, the presents results show significant reduction in O VI recombination rates at high temperatures. In addition to the total rates, level-specific photoionization cross sections and recombination rates are presented for all fine structure levels n (lSLJ) up to n <= 10, to enable accurate computation of recombination-cascade matrices and spectral formation of prominent UV and X-ray lines such as the 1032,1038 A doublet of O VI, and the `triplet' forbidden, intercombination, and resonance X-ray lines of O VII at 22.1, 21.8, and 21.6 \ang respectively. Altogether, atomic parameters for 98 levels of O VI and 116 fine structure levels of O VII are theoretically computed. These data should provide a reasonably complete set of photoionization and recombination rates in collisional or radiative equilibrium., Comment: 33 pages, 8 figures, submitted to ApJS

An extensive set of fine structure levels and corresponding transition probabilities for allowed and forbidden transitions in Fe XVII is presented. A total of 490 bound energy levels of Fe XVII of total angular momenta 0 <= J <= 7 of even and odd parities with 2 <= n <= 10, 0 <= l <= 8, 0 <= L <= 8, and singlet and triplet multiplicities, are obtained. They translate to over 2.6 x 10^4 allowed (E1) transitions that are of dipole and intercombination type, and about 3000 forbidden transitions that include electric quadrupole (E2), magnetic dipole (M1), electric octopole (E3), and magnetic quadrupole (M2) type representing the most detailed calculations to date for the ion. Oscillator strengths f, line strengths S, and coefficients A of spontaneous emission for the E1 type transitions are obtained in the relativistic Breit-Pauli R-matrix approximation. A valus for the forbidden transitions are obtained from atomic structure calculations using codes SUPERSTRUCTURE and GRASP. The energy levels are identified in spectroscopic notation with the help of a newly developed level identification algorithm. Nearly all 52 spectroscopically observed levels have been identified, their binding energies agreeing within 1% with our calculation. Computed transition probabilities are compared with other calculations and measurement. The effect of 2-body magnetic terms and other interactions is discussed. Present data set enhances by more than an order of magnitude the heretofore available data for the transition probabilities of Fe XVII., Comment: Submitted to Astron. Astrophys., 13 pages : Radiative data for Ne-like Iron for X-ray and EUV applications

The K-alpha resonance complexes in oxygen ions O I - O VI are theoretically computed and resonance oscillator strengths and wavelengths are presented. The highly resolved photoionization cross sections, with relativistic fine structure, are computed in the coupled channel approximation using the Breit-Pauli R-matrix method. A number of strong K-alpha resonances are found to be appreciable, with resonance oscillator strengths f_r > 0.1. The K-alpha resonance wavelengths of O I-O VI lie in a relatively narrow wavelength range 22 - 23.5 A, and the X-ray opacity in this region should therefore be significantly affected by K --> L transitions in oxygen. The results should be useful in the interpretation of soft X-ray spectra observed from Chandra and XMM-Newton., Comment: Monthly Notices of Roy. Astro. Soc. (in press), 10 pgs. 1 figure

A comprehensive study of relativistic and resonance effects in electron impact excitation of (e+Fe XVII) is carried out using the BPRM method in the relativistic close coupling approximation. Two sets of eigenfunction expansions are employed; first, up to the n = 3 complex corresponding 37 fine-structure levels (37CC) from 21 LS terms; second, up to the n = 4 corresponding to 89 fine-structure levels (89CC) from 49 LS terms. In contrast to previous works, the 37CC and the 89CC collision strengths exhibit considerable differences. Denser and broader resonances due to n = 4 are present in the 89CC results both above and {\it below} the 37 thresholds, thus significantly affecting the collision strengths for the primary X-ray and EUV transitions within the first 37 n = 3 levels. Extensive study of other effects on the collision strengths is also reported: (i) electric and magnetic multipole transitions E1, E2, E3 and M1, M2, (ii) J-partial wave convergence of dipole and non-dipole transitions, (iii) high energy behaviour compared to other approximations. Theortical results are benchmarked against experiments to resolve longstanding discrepancies -- collision strengths for the three prominent X-ray lines 3C, 3D and 3E at 15.014, 15.265, and 15.456 \AA are in good agreement with two independent measurements on Electron-Beam-Ion-Traps (EBIT). Finally, line ratios from a collisional-radiative model using the new collisional rates are compared with observations from stellar coronae and EBITs to illustrate potential applications in laboratory and astrophysical plasmas., Comment: 36 pages, 14 figures

We present theoretical FeII emission line strengths for physical conditions typical of Active Galactic Nuclei with Broad-Line Regions. The FeII line strengths were computed with a precise treatment of radiative transfer using extensive and accurate atomic data from the Iron Project. Excitation mechanisms for the FeII emission included continuum fluorescence, collisional excitation, self-fluorescence amoung the FeII transitions, and fluorescent excitation by Lyman-alpha and Lyman-beta. A large FeII atomic model consisting of 827 fine structure levels (including states to E ~ 15 eV) was used to predict fluxes for approximately 23,000 FeII transitions, covering most of the UV, optical, and IR wavelengths of astrophysical interest. Spectral synthesis for wavelengths from 1600 Angstroms to 1.2 microns is presented. Applications of present theoretical templates to the analysis of observations are described. In particular, we discuss recent observations of near-IR FeII lines in the 8500 Angstrom -- 1 micron region which are predicted by the Lyman-alpha fluorescence mechanism. We also compare our UV spectral synthesis with an empirical iron template for the prototypical, narrow-line Seyfert galaxy I Zw 1. The theoretical FeII template presented in this work should also applicable to a variety of objects with FeII spectra formed under similar excitation conditions, such as supernovae and symbiotic stars., Comment: 33 pages, 15 postscript figures

Theoretical differential oscillator strengths related to monochromatic opacity due to K- and L-shell absorption from oxygen and iron ions are directly compared with the Chandra and Xmm-Newton spectra of Seyfert~1 galaxy MCG--6-30-15. We compute the highly resolved continuum with resonances due to O I - VI and Fe XVI. It is found that the KLn (n --> infty) resonance series limits in O VI, and the lowest LMM resonance in Fe XVI, lie at the prominent 17.5 A break in the observed spectra. We also calculate and identify, for the first time, the observed gap in spectral flux at 22-23 A in the Chandra spectra due to K --> L resonant absorption features from all O-ions O I - VI, potentially leading to abundance determination of several or all ionization stages; similar signature gaps may be predicted due to other elements. The precise atomic parameters are computed in the relativistic close coupling approximation using the Breit-Pauli R-matrix method. The new X-ray opacities might possibly distinguish between models of a dusty warm absorber, and/or gravitational redshift and broadening due to a massive black hole in MCG--6-30-15., Comment: 16 pages, 3 figures. Fig. 2 is in color, available from http://www.astronomy.ohio-state.edu/~oelgoetz/xop-mcg6.html

The primary X-ray diagnostic lines in He-like ions are mainly excited by electron impact from the ground level to the n = 2 levels, but at high temperatures n > 2 levels are also excited. In order to describe the atomic processes more completely collision strengths are computed for OVII including for the first time all of the following: (i) relativistic fine structure, (ii) levels up to the n = 4, and (iii) radiation damping of autoionizing resonances. The calculations are carried out using the Breit-Pauli R-matrix (BPRM) method with a 31-level eigenfunction expansion. Resonance structures in collision strengths are delineated in detail up to the n = 4 thresholds. For highly charged He-like ions radiation damping of autoionizing resonances is known to be significant. We investigate this effect in detail and find that while resonances are discernibly damped radiatively as the series limit n --> infty is approached from below, the overall effect on effective cross sections and rate coefficients is found to be very small. Collision strengths for the principal lines important in X-ray plasma diagnostics, w,x,y and z, corresponding to the 4 transitions to the ground level 1s^2 (^1S_0) <-- 1s2p (^1P^o_1), 1s2p (^3P^o_2), 1s2p (^3P^o_1), 1s2s (^3S_1), are explicitly shown. It is found that the effective collision strength of the forbidden z-line is up to a factor of 4 higher at T < 10^6 K than previous values. This is likely to be of considerable importance in the diagnostics of photoionized astrophysical plasmas. Significant differences are also found with previous works for several other transitions. This work is carried out as part of the Iron Project-RmaX Network., Comment: 16 pages, 8 figures (3 in color), submitted to Journal of Physics B

New theoretical results from large-scale relativistic close coupling calculations reveal the precise effect of resonances in collisional excitation of x-ray lines of Ne-like Fe XVII. Employing the Breit-Pauli R-matrix method and a 89-level eigenfunction expansion including up to n = 4 levels shows significant resonance enhancement of the collision strengths of forbidden and intercombination transitions. The present results differ from all previous calculations, heretofore without detailed resonance structures, and should help resolve longstanding discrepancies. In particular, the present line ratios of three benchmark diagnostic lines 3C, 3D, and 3E at 15.014, 15.265, and 15.456 A respectively, are in excellent agreement with two independent measurements on Electron-Beam-Ion-Traps [Laming et al, Astrophys. J, 545, L161 (2000) and Brown et al, Astrophys. J, 502, 1015 (1998)]. The strong energy dependence due to resonances in these and other cross sections is demonstrated for the first time. It is of general importance and strongly manifests itself in x-ray plasma diagnostics., Comment: Phys. Rev. Letters (in press), 6 pages, 2 figures in color

It is shown that the dielectronic satellites (DES) dominate X-ray spectral formation in the 6.7 keV K-alpha complex of Fe XXV at temperatures below that of maximum abundance in collisional ionization equilibrium T_m. Owing to their extreme temperature sensitivity the DES are excellent spectral diagnostics for T < T_m in photoionized, colllisional, or hybrid plasmas; whereas the forbidden, intercombination, and resonance lines of Fe XXV are not. A diagnostic line ratio GD(T) is defined including the DES and the lines, with parameters from new relativistic atomic calculations. The DES absorption resonance strengths may be obtained from differential oscillator strengths to possibly yield the Fe XXIV/Fe XXV column densities. The DES contribution to highly ionized Fe should be of interest for models of redward broadening of K-alpha features, ionized accretion discs, accretion flows, and K-alpha temporal-temperature variability in AGN., Comment: 5 pages, 4 Figs., submitted to MNRAS. High-resolution, full-size figures are available from http://www.astronomy.ohio-state.edu/~pradhan

Relativistic close coupling calculations are reported for unified electronic recombination of (e + C IV) including non-resonant and resonant recombination processes, radiative and dielectronic recombination (RR and DR). Detailed comparison of the theoretical unified results with two recent experiments on ion storage rings (Mannervik etal. [1] and Schippers etal. [2]) shows very good agreement in the entire measured energy region 2s -- 2p with 2pnl resonances. The results benchmark theory and experiments to uncertainties of ~15%, and show that the resonant and the background cross sections are not an incoherent sum of separate RR and DR contributions. The limiting values of the DR cross sections, as n ---> infinity, are shown to correspond to those due to electron impact excitation (EIE) at the 2Po_(1/2,3/2) fine structure thresholds, delineated for the first time. The near-threshold 2s 2S_(1/2) - 2p 2Po_(1/2,3/2) EIE cross sections are also compared with recent experimental measurements. The demonstrated threshold fine structure and resonance effects should be of general importance in excitation and recombination of positive ions., Comment: 7 pages, 3 figures; accepted in Phys. Rev. Letters, no change from submitted version

Relativistic and channel coupling effects in photoionization and unified electronic recombination of Fe XVII are demonstrated with an extensive 60-level close coupling calculation using the Breit-Pauli R-matrix method. Photoionization and (e + ion) recombination calculations are carried out for the total and the level-specific cross sections, including the ground and several hundred excited bound levels of Fe XVII (up to fine structure levels with n = 10). The unified (e + ion) recombination calculations for (e + Fe XVIII --> Fe XVII) include both the non-resonant and resonant recombination (`radiative' and `dielectronic recombination' -- RR and DR). The low-energy and the high energy cross sections are compared from: (i) a 3-level calculation with 2s^2p^5 (^2P^o_{1/2,3/2}) and 2s2p^6 (^2S_{1/2}), and (ii) the first 60-level calculation with \Delta n > 0 coupled channels with spectroscopic 2s^2p^5, 2s2p^6, 2s^22p^4 3s, 3p, 3d, configurations, and a number of correlation configurations. Strong channel coupling effects are demonstrated throughout the energy ranges considered, in particular via giant photoexcitation-of-core (PEC) resonances due to L-M shell dipole transition arrays 2p^5 --> 2p^4 3s, 3d in Fe XIII that enhance effective cross sections by orders of magnitude. Comparison is made with previous theoretical and experimental works on photoionization and recombination that considered the relatively small low-energy region (i), and the weaker \Delta n = 0 couplings. While the 3-level results are inadequate, the present 60-level results should provide reasonably complete and accurate datasets for both photoionization and (e + ion) recombination of Fe~XVII in laboratory and astrophysical plasmas., Comment: 19 pages, 8 figures, Phys. Rev. A (submitted)

Recently X-ray photoabsorption in KLL resonances of O VI was predicted [Pradhan, Astrophys.J. Lett. 545, L165 (2000)], and detected by the Chandra X-ray Observatory [Lee et al, Astrophys. J. {\it Lett.}, submitted]. The required resonance oscillator strengths f_r, are evaluated in terms of the differential oscillator strength df/de that relates bound and continuum absorption. We present the f_r values from radiatively damped and undamped photoionization cross sections for Li-like C,O, and Fe calculated using relativistic close coupling Breit-Pauli R-matrix method. The KLL resonances of interest here are: 1s2p (^3P^o) 2s [^4P^o_{1/2,3/2}, ^2P^o_{1/2,3/2}] and 1s2p (^1P^o) 2s [^2P^o_{1/2,3/2}]. The KLL photoabsorption resonances in Fe XXIV are fully resolved up to natural autoionization profiles for the first time. It is demonstrated that the undamped f_r independently yield the resonance radiative decay rates, and thereby provide a precise check on the resolution of photoionization calculations in general. The predicted photoabsorption features should be detectable by the X-ray space observatories and enable column densities in highly ionized astrophysical plasmas to be determined from the calculated f_r. The dielectronic satellites may appear as redward broadening of resonances lines in emission and absorption., Comment: 9 pages, 2 figurs, Phys. Rev. A, Rapid Communication (submitted)

Unified recombination cross sections and rates are computed for (e + Fe XVIII) --> Fe XVII including non-resonant and resonant (radiative and di-electronic recombination, RR and DR) processes in an ab initio manner with relativistic fine structure. The highly resolved theoretical cross sections exhibit considerably more resonance structures than observed in the heavy-ion storage ring measurements at Heidelberg, Germany. Nonetheless, the detailed resonance complexes agree well with experiment, and the unfiied rates agree with the sum of experimentally derived DR and theoretical RR rates to ~ 20%, within experimental or theoretical uncertainties. Theoretical results may provide estimates of field ionization of rydberg levels close to the DR peak, and non-resonant background contirbutions, particularly close to the RR peak as E --> 0. More generally, the unified results avoid the physical and practical problems in astrophysical models inherent in the separation of electronic recombination into RR and DR on the one hand, and further subdivision into low-energy Delta n = 0 DR and high-energy Delta n > 0 DR in photoionized and collisionally ionized X-ray plasmas., Comment: 16 pages, 2 figures

It is shown that photoabsorption via autoionizing resonances may be appreciable and used for abundance analysis. Analogous to spectral lines, the `resonance oscillator strength' f_r may be defined and evaluated in terms of the differential oscillator strength df/d(epsilon) that relates bound and continuum absorption. X-ray photoabsorption in KLL (1s2s2p) resonances of O VI is investigated using highly resolved relativistic photoionization cross sections with fine structure. It is found that f_r is comparable to that for UV dipole transition in O VI (2s - 2p) and the X-ray (1s^2 ^1S_0 - 1s2p ^1P^o_1) transition in O VII. The dominant O VI(KLL) components lie at 22.05 and 21.87 Angstroms. These predicted absorption features should be detectable by the Chandra X-Ray Observatory (CXO) and the X-Ray Multi-Mirror Mission (XMM). The combined UV/X-ray spectra of O VI/O VII should yield valuable information on the ionization structure and abundances in sources such as the `warm absorber' region of active galactic nuclei and the hot intergalactic medium. Some general implications of resonant photoabsorption are addressed., Comment: Astrophys. J. Letters (in press), 9 pages, 3 figures

Photoionization and recombination cross sections and rate coefficients are calculated for Li-like Fe XXIV and He-like Fe XXV using the Breit-Pauli R-matrix (BPRM) method. A complete set of total and level-specific parameters is obtained to enable X-ray photoionization and spectral modeling. The ab initio calculations for the unified (e + ion) recombination rate coefficients include both the non-resonant and the resonant recombination (radiative and di-electronic recombination, RR and DR, respectively) for (e + Fe XXV) -> Fe XXIV and (e + Fe XXVI) -> Fe XXV. The level specific rates are computed for all fine structure levels up to n = 10, enabling accurate computation of recombination-cascade matrices and effective rates for the X-ray lines. The total recombination rate coefficients for both Fe XXIV and Fe XXV differ considerably, by several factors, from the sum of RR and DR rates currently used to compute ionization fractions in astrophysical models. As the photoionization/recombination calculations are carried out using an identical eigenfunction expansion, the cross sections for both processes are theoretically self-consistent; the overall uncertainty is estimated to be about 10-20%. All data for Fe XXIV and Fe XXV (and also for H-like Fe XXVI, included for completeness) are available electronically., Comment: 31 pages, 10fugs

Relativistic atomic structure calculations for electric dipole E1, electric quadrupole E2 and magnetic dipole M1 transition probabilities among the first 80 fine-structure levels of Fe VI, dominated by configurations 3d^3, 3d^24s, and 3d^24p, are carried out using the Breit-Pauli version of the code Superstructure. Experimental energies are used to improve the accuracy of these transition probabilities. Employing the 80-level collision-radiative (CR) model with these dipole and forbidden transition probabilities, and Iron Project R-matrix collisional data, we present a number of [Fe VI] line ratios applicable to spectral diagnostics of photoionized H II regions. It is shown that continuum fluorescent excitation needs to be considered in CR models in order to interpret the observed line ratios of optical [Fe VI] lines in planetary nebulae NGC 6741, IC 351, and NGC 7662. The analysis leads to parametrization of line ratios as function of, and as constraints on, the electron density and temperature, as well as the effective radiation temperature of the central source and a geometrical dilution factor. The spectral diagnostics may also help ascertain observational uncertainties. The method may be generally applicable to other objects with intensive background radiation fields, such as novae and active galactic nuclei. The extensive new Iron Project radiative and collisional calculations enable a consistent analysis of many line ratios for the complex iron ions., Comment: 25 pages, 8 figures, submitted to Astron.Astrophys. Suppl.Ser

The first complete set of unified cross sections and rate coefficients are calculated for photoionization and recombination of He- and Li-like ions using the relativistic Breit-Pauli R-matrix method. We present total, unified (e + ion) recombination rate coefficients for (e + C VI ---> C V) and (e + C V \longrightarrow C IV) including fine structure. Level-specific recombination rate coefficients up to the n = 10 levels are also obtained for the first time; these differ considerably from the approximate rates currently available. Applications to recombination-cascade coefficients in X-ray spectral models of K-alpha emission from the important He-like ions is pointed out. The overall uncertainty in the total recombination rates should not exceed 10-20%. Ionization fractions for Carbon are recomputed in the coronal approximation using the new rates. The present (e + ion) recombination rate coefficients are compared with several sets of available data, including previous LS coupling results, and `experimentally derived' rate coefficients. The role of relativistic fine structure, resolution of resonances, radiation damping, and interference effects is discussed. Two general features of recombination rates are noted: (i) the non-resonant (radiative recombination) peak as E,T ---> 0, and the (ii) the high-T resonant (di-electronic recombination) peak., Comment: 38 pages, 10 figures, submitted to Astrophys. J. (Suppl)

Recent advances in theoretical atomic physics have enabled large-scale calculation of atomic parameters for a variety of atomic processes with high degree of precision. The development and application of these methods is the aim of the Iron Project. At present the primary focus is on collisional processes for all ions of iron, Fe I- FeXXVI, and other iron-peak elements; new work on radiative processes has also been initiated. Varied applications of the Iron Project work to X-ray astronomy are discussed, and more general applications to other spectral ranges are pointed out. The IP work forms the basis for more specialized projects such as the RMaX Project, and the work on photoionization/recombination, and aims to provide a comprehensive and self-consistent set of accurate collsional and radiative cross sections, and transition probabilities, within the framework of relativistic close coupling formulation using the Breit-Pauli R-Matrix method. An illustrative example is presented of how the IP data may be utilised in the formation of X-ray spectra of the K$\alpha$ complex at 6.7 keV from He-like Fe XXV., Comment: 7 pagee, 2 figures. In Proceedings of the Symposium on Atomic Data Needs For X-Ray Astronomy, NASA GSFC, Dec. 16-17, 1999

An extensive set of dipole-allowed, intercombination, and forbidden transition probabilities for Fe V is presented. The Breit-Pauli R-matrix (BPRM) method is used to calculate 1.46 x 10^6 oscillator strengths for the allowed and intercombination E1 transitions among 3,865 fine-structure levels dominated by configuration complexes with n <= 10 and l <= 9. These data are complemented by an atomic structure configuration interaction (CI) calculation using the SUPERSTRUCTURE program for 362 relativistic quadrupole (E2) and magnetic dipole (M1) transitions among 65 low-lying levels dominated by the 3d^4 and 3d^ 4s configurations. Procedures have been developed for the identification of the large number of fine-structure levels and transitions obtained through the BPRM calculations. The target ion Fe VI is represented by an eigenfunction expansion of 19 fine-structure levels of 3d^3 and a set of correlation configurations. Fe V bound levels are obtained with angular and spin symmetries SL\pi and J\pi of the (e + Fe VI) system such that 2S+1 = 5,3,1, L <= 10, J <= 8 of even and odd parities. The completeness of the calculated dataset is verified in terms of all possible bound levels belonging to relevant LS terms and transitions in correspondence with the LS terms. The fine-structure averaged relativistic values are compared with previous Opacity Project LS coupling data and other works. The 362 forbidden transition probabilities considerably extend the available data for the E2 and M1 transtions, and are in good agreement with those computed by Garstang for the 3d^4 transitions., Comment: 19 pages, 1 figure. This paper marks the beginning of a large-scale effort of ab initio atomic calculations that should eventually lead to re-calculation of accurate iron opacities. Astron. Astrophys. Suppl. Ser. (in press)

Ab initio theoretical calculations are reported for the electric (E1) dipole allowed and intercombination fine structure transitions in Fe V using the Breit-Pauli R-matrix (BPRM) method. We obtain 3865 bound fine structure levels of Fe V and $1.46 x 10^6$ oscillator strengths, Einstein A-coefficients and line strengths. In addition to the relativistic effects, the intermediate coupling calculations include extensive electron correlation effects that represent the complex configuration interaction (CI). Fe V bound levels are obtained with angular and spin symmetries $SL\pi$ and $J\pi$ of the (e + Fe VI) system such that $2S+1$ = 5,3,1, $L \leq$ 10, $J \leq 8$. The bound levels are obtained as solutions of the Breit-Pauli (e + ion) Hamiltonian for each $J\pi$, and are designated according to the `collision' channel quantum numbers. A major task has been the identification of these large number of bound fine structure levels in terms of standard spectroscopic designations. A new scheme, based on the analysis of quantum defects and channel wavefunctions, has been developed. The identification scheme aims particularly to determine the completeness of the results in terms of all possible bound levels for applications to analysis of experimental measurements and plasma modeling. An uncertainty of 10-20% for most transitions is estimated., Comment: 31 pages, 1 figure, Physica Scripta (in press)

Fluorescent excitation of spectral lines is demonstrated as a function of temperature-luminosity and the distance of the emitting region from the central stars of planetary nebulae. The electron densities and temperatures are determined, and the method is exemplified through a detailed analysis of spectral observations of a high excitation PN, NGC 6741, observed by Hyung and Aller(1997). Fluorescence should also be important in the determination of element abundances. It is suggested that the method could be generally applied to determine or constrain the luminosity and the region of spectral emission in other intensively radiative sources such as novae, supernovae, and active galactic nuclei., Comment: 5 pages, 4 figures (fig.4 in color), ApJ (in press)

Photoionization cross sections and recombination rate coefficients are presented for the L-shell ground state fine structure levels $2s^22p^4 \ ^3P_{2,0,1}$ of Fe~XIX. Several sets of calculations including relativistic effects are carried out: (i) Breit-Pauli R-matrix (BPRM), (ii) Relativistic Distorted Wave (RDW), and (iii) a semi-relativistic calculation. Non-relativistic LS coupling calculations are also done for comparison. The BPRM calculations employ a configuration interaction target representation for Fe~XX consisting of 12 LS terms (23-fine structure levels), as in the recently reported BPRM calculations by Donnelly et al (MNRAS, 307, 595, 1999). The background cross sections in all three sets of present calculations agree with one another, but differ considerably from those of Donnelly et al. Owing to much more extensive resonance structures in the present BPRM calculations, the sum of the corresponding recombination rate coefficients for the $^3P_{2,0,1}$ levels are up to 50% higher than the LS rates at low temperarures but comparable for higher temperatures; in contrast to the results of Donnelly et al who obtained the LS rates to be higher than their BPRM results by about a factor of 2. Reasons for these discrepancies are discussed., Comment: 7 pages, 3 figures, MNRAS, In Press

Relativistic and resonance effects in the photoionization of Mg-like Fe~XV are investigated using the Breit-Pauli R-matrix Method (BPRM) at near-threshold and intermediate energies, complemented by the Relativistic Random Phase Approximation (RRPA) and multi-channel quantum defect theory in the energy region up to the L-shell ionization thresholds. The cross sections exhibit extensive resonance structures that considerably enhance the effective photoionization of Fe~XV. These results should be of general interest in photoionization modeling of X-ray sources observed by space observatories., Comment: 4 pages, 4 figs., Phys. Rev. A (Rapid Communications, In Press), X-ray Photoionization, Inner-shell Resonant Processes

We have calculated FeII emission line strengths for Active Galactic Nuclei Broad-Line Regions using precise radiative transfer and Iron Project atomic data. We improve the treatment of all previously considered excitation mechanisms for the FeII emission, continuum fluorescence, collisional excitation, fluorescence by self-overlap among the iron lines, and fluorescent excitation by Lyman-alpha. We demonstrate that Lyman-alpha fluorescence is of fundamental importance in determining the strength of the FeII emission. In addition to enhancing the ultraviolet and optical FeII flux, Lyman-alpha fluorescence also results in significant near-infrared FeII emission in the 8500-9500 Angstrom wavelength range. New observations are suggested to probe this effect in strong FeII emitting quasars., Comment: 12 pages, 2 encapsulated Postscript figures. To be Published in The Astrophysical Journal Letters

We present ground state photoionization cross sections of atoms and ions averaged over resonance structures for photoionization modeling of astrophysical sources. The detailed cross sections calculated in the close-coupling approximation using the R-matrix method, with resonances delineated at thousands of energies, are taken from the Opacity Project database TOPbase and the Iron Project, including new data for the low ionization stages of iron Fe I--V. The resonance-averaged cross sections are obtained by convolving the detailed cross sections with a Gaussian distribution over the autoionizing resonances. This procedure is expected to minimize errors in the derived ionization rates that could result from small uncertainties in computed positions of resonances, while preserving the overall resonant contribution to the cross sections in the important near threshold regions. The detailed photoionization cross sections at low photon energies are complemented by new relativistic distorted-wave calculations for Z<= 12, and from central-field calculations for Z>12 at high energies, including inner-shell ionization. The effective cross sections are then represented by a small number of points that can be readily interpolated linearly for practical applications; a Fortran subroutine and data are available. The present numerically averaged cross sections are compared with analytic fits that do not accurately represent the effective cross sections in regions dominated by resonances., Comment: 23 pages, 5 embedded figures (eps), submitted ApJS

The emission spectra and the ionization structure of the low ionization stages of iron, Fe I--IV, in gaseous nebulae are studied. This work includes: (i) new atomic data: photoionization cross sections, total e-ion recombination rates, excitation collision strengths, and transition probabilities; (ii) detailed study of excitation mechanisms for the [Fe II], [Fe III], and [Fe IV] emission, and spectroscopic analysis of the observed IR, optical, and UV spectra; (iii) study of the physical structure and kinematics of the nebulae and their ionization fronts. Spectral analysis of the well observed Orion nebula is carried out as a test case, using extensive collisional-radiative and photoionization models. It is shown that the [Fe II] emission from the Orion nebula is predominantly excited via electron collisions in high density partially ionized zones; radiative fluorescence is relatively less effective. Further evidence for high density zones is derived from the [O I] and [Ni II] spectral lines, as well as from the kinematic measurements of ionic species in the nebula. The ionization structure of iron in Orion is modeled using the newly calculated atomic data, showing some significant differences from previous models. The new model suggests a fully ionized H II region at densities on the order of $10^3$ cm$^{-3}$, and a dynamic partially ionized H II/H I region at densities of $10^5-10^7$ \cm3. Photoionization models also indicate that the optical [O I] and [Fe II] emission originates in high density partially ionized regions within ionization fronts. The gas phase iron abundance in Orion is estimated from observed spectra., Comment: AAS LaTex, 60 pages 18 figures. Astrophysical Journal. in press

We discuss implications of Hopkins Ultraviolet Telescope (HUT) detections of C III lambda 977 and N III lambda 990 emission from the narrow-line region of the Seyfert 2 galaxy NGC 1068. In their discovery paper Kriss et al. showed that the unexpectedly great strength of these lines implies that the emitting gas must be shock-heated if the lines are collisionally excited. Here we investigate other processes which excite these lines in photoionization equilibrium. Recombination, mainly dielectronic, and continuum fluorescence are strong contributors to the line. The resulting intensities are sensitive to the velocity field of the emitting gas and require that the turbulence be of the same order of magnitude as the observed line width. We propose optical observations that will decide whether the gas is collisionally or radiatively heated.

Employing new atomic data and a 15 level collisional-radiative model, it is shown that population inversion, and consequent laser action, can be achieved in far-infrared (FIR) lines (C II) 157 micrometer, (N III) 58 micrometer, (O IV) 26 micrometer, (Ne VI) 7.6 micrometer, and (Mg VIII) 3 micrometer. Coupling of the lasting transition 2s(sup 2)2p((sup 2)P(sup o sub 3/2)-(sup 2)P(sup o sub 1/2)) to higher energy levels of the 2s2p(sup 2) configuration in boron-like ions provides the dominant pumping flux. Lasing column lengths are found to be l approximately equal 10(exp -4)-1.0 pcs, at T(sub e) approximately equal 10(exp 4)-10(exp 6) K, N(sub e) approximately equal 10(exp 4)-10(exp 10)/cu cm, conditions and sizes prevalent in broad-line regions of active-galactic nuclei and novae. Detectable emission fluxes are estimated. Lasing activity in the source could significantly affect abundance determinations. The FIR line emission should be readily observable with the upcoming Infrared Space Observatory.

The Bloom and Pradhan (1992) results on the excitation rates for transitions in O IV matrix are used to derive emission-line ratios near 1400 A, applicable to gaseous nebulae. The results are compared with the values measured from high-resolution spectra obtained by the IUE satellite.

New calculations are reported for electron excitation collision strengths, rate coefficients, transition probabilities, and line ratios for the astrophysically important optical and UV lines in S II. The collision strengths are calculated in the close coupling approximation using the R-matrix method. The present calculations are more extensive than previous ones, including all transitions among the 12 lowest LS terms and the corresponding 28 fine-structure levels in the collisional-radiative model for S II. While the present rate coefficients for electron impact excitation are within 10-30 percent of the previous values for the low-lying optical transitions employed as density diagnostics of H II regions and nebulae, the excitation rates for the UV transitions 4S super 0 sub 3/2 - 4Psub 1/2,3/2,5/2 differ significantly from earlier calculations, by up to factor of 2. We describe temperature and density sensitive flux ratios for a number of UV lines. The present UV results are likely to be of interest in a more accurate interpretation of S II emission from the Io plasma torus in the magnetosphere of Jupiter, as well as other UV sources observed from the IUE, ASTRO 1, and the HST.

New and improved collision strengths and Maxwellian-averaged rate coefficients for temperatures between 1000 and 40,000 K are presented for all the prominent transitions in the spectra of C II, N III, and O IV. The collision strengths show extensive autoionization structures that are delineated in detail and which enhance the rate coefficients for several transitions by a considerable amount. Maxwellian-averaged collision strengths are calculated for all possible fine-structure transitions along the states included in the eigenfunction expansion of the target ion. All calculations are carried out in the close-coupling approximation using the R-matrix method as adapted for the Opacity Project.