Manuel A. Bautista

Photoionization and High-Density Gas

We present results of calculations using the XSTAR version 2 computer code. This code is loosely based on the XSTAR version 1 code, which has been available for public use for some time. However, it represents an improvement and update in several major respects, including atomic data, code structure, user interface, and improved physical description of ionization/excitation. In particular, it now is applicable to high-density situations in which significant excited atomic level populations are likely to occur. We describe the computational techniques and assumptions and present sample runs with particular emphasis on high-density situations.

Photoionization Modeling and the K Lines of Iron

We calculate the efficiency of iron K line emission and iron K absorption in photoionized models using a new set of atomic data. These data are more comprehensive than those previously applied to the modeling of iron K lines from photoionized gases and allow us to systematically examine the behavior of the properties of line emission and absorption as a function of the ionization parameter, density, and column density of model constant density clouds. We show that, for example, the net fluorescence yield for the highly charged ions is sensitive to the level population distribution produced by photoionization, and these yields are generally smaller than those predicted assuming the population is according to statistical weight. We demonstrate that the effects of the many strongly damped resonances below the K ionization thresholds conspire to smear the edge, thereby potentially affecting the astrophysical interpretation of absorption features in the 7‐9 keV energy band. We show that the

The XSTAR Atomic Database

We report on the XSTAR atomic database that contains a large quantity of atomic rates for use in spectral modeling of astrophysical plasmas. The database includes atomic energy levels, line wavelengths, radiative transition probabilities, electron impact excitation rates, photoionization cross section, recombination rate coefficients, electron impact ionization rates, and fluorescence and Auger yields. The species considered are all the ions of H, He, C, N, O, Ne, Mg, Si, S, Ar, Ca, Fe, and Ni. The database collects recent data from many sources including CHIANTI, TOPbase, ADAS, NIST, and the IRON Project. Two particular features of the database are the incorporation of photoionization cross sections from the Opacity Project for all levels of every ion, and state-specific recombination rates. State-specific collisional ionization and three-body recombination are also included. The collection of data is used to build excitation-ionization models of each ion for calculation of their spectra for electron densities of up to 1018 cm-3 and temperatures between 100 and 109 K. In addition, every ion model is designed to converge to LTE under appropriate conditions. These models and data are implemented in the photoionization modeling code XSTAR v.2.

Dielectronic Recombination Data for Dynamic Finite-Density Plasmas. I; Goals and Methodology

A programme is outlined for the assembly of a comprehensive dielectronic recombination database within the generalized collisional-radiative (GCR) framework. It is valid for modelling ions of elements in dynamic finite-density plasmas such as occur in transient astrophysical plasmas such as solar flares and in the divertors and high transport regions of magnetic fusion devices. The resolution and precision of the data are tuned to spectral analysis and so are sufficient for prediction of the dielectronic recombination contributions to individual spectral line emissivities. The fundamental data are structured according to the format prescriptions of the Atomic Data and Analysis Structure (ADAS) and the production of relevant GCR derived data for application is described and implemented following ADAS. The requirements on the dielectronic recombination database are reviewed and the new data are placed in context and evaluated with respect to older and more approximate treatments. Illustrative results validate the new high-resolution zero-density dielectronic recombination data in comparison with measurements made in heavy-ion storage rings utilizing an electron cooler. We also exemplify the role of the dielectronic data on GCR coefficient behaviour for some representative light and medium weight elements.

Modeling of iron K lines: Radiative and Auger decay data for Fe II-Fe IX

A detailed analysis of the radiative and Auger de-excitation channels of K-shell vacancy states in Fe ii-Fe ix has been carried out. Level energies, wavelengths, A-values, Auger rates and fluorescence yields have been calculated for the lowest fine-structure levels populated by photoionization of the ground state of the parent ion. Dierent branching ratios, namely K2/K1 ,K /K ,K LM/KLL, KMM/KLL, and the total K-shell fluorescence yields, !K, obtained in the present work have been compared with other theoretical data and solid-state measurements, finding good general agreement with the latter. The K2/K1 ratio is found to be sensitive to the excitation mechanism. From these comparisons it has been possible to estimate an accuracy of10% for the present transition probabilities.

Ionization structure and spectra of iron in gaseous nebulae

The emission spectra and the ionization structure of the low ionization stages of iron, Fe I-Fe IV, in gaseous nebulae are studied. This work includes (i) new atomic data for photoionization cross sections, total e-ion recombination rates, excitation collision strengths, and transition probabilities calculated under the Iron Project by the Ohio State atomic astrophysics group; (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 and shows some significant differences from previous models. The new model suggests a fully ionized H II region at densities on the order of 103 cm-3 and a dynamic partially ionized H II/H I region at densities of 105-107 cm-3. Photoionization models also indicate that the optical [O I] and [Fe II] emission originates in high-density partially ionized regions within ionization fronts, thereby confirming the general Fe II/O I correlation in H II regions that was determined in earlier studies. The gas-phase iron abundance in Orion is estimated from observed spectra, including recently observed [Fe IV] lines.

Properties of the ionized gas in HH 202 – II. Results from echelle spectrophotometry with Ultraviolet Visual Echelle Spectrograph

We present results of deep echelle spectrophotometry of the brightest knot of the Herbig– Haro object HH 202 in the Orion Nebula – HH 202-S – using the Ultraviolet Visual Echelle Spectrograph in the spectral range from 3100 to 10 400 A. The high spectral resolution of the observations has permitted to separate the component associated with the ambient gas from that associated with the gas flow. We derive electron densities and temperatures from different diagnostics for both components, as well as the chemical abundances of several ions and elements from collisionally excited lines, including the first determinations of Ca + and Cr + abundances in the Orion Nebula. We also calculate the He + ,C 2+ ,O + and O 2+ abundances from recombination lines. The difference between the O 2+ abundances determined from collisionally excited and recombination lines – the so-called abundance discrepancy factor – is 0.35 and 0.11 dex for the shock and nebular components, respectively. Assuming that the abundance discrepancy is produced by spatial variations in the electron temperature, we derive values of the temperature fluctuation parameter, t 2 , of 0.050 and 0.016 for the shock and nebular components, respectively. Interestingly, we obtain almost coincident t 2 values for both components from the analysis of the intensity ratios of He I lines. We find significant departures from case B predictions in the Balmer and Paschen flux ratios of lines of high principal quantum number n. We analyse the ionization structure of HH 202-S, finding enough evidence to conclude that the flow of HH 202-S has compressed the ambient gas inside the nebula trapping the ionization front. We measure a strong increase of the total abundances of nickel and iron in the shock component, the abundance pattern and the results of photoionization models for both components are consistent with the partial destruction of dust after the passage of the shock wave in HH 202-S.

Resonance-averaged Photoionization Cross Sections for Astrophysical Models

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-Fe 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.

K-Shell Photoabsorption of Oxygen Ions

Extensive calculations of the atomic data required for the spectral modeling of the K-shell photoabsorption of oxygen ions have been carried out in a multicode approach. The present level energies and wavelengths for the highly ionized species (electron occupancies 2 ≤ N ≤ 4) are accurate to within 0.5 eV and 0.02 A, respectively. For N > 4, lack of measurements, wide experimental scatter, and discrepancies among theoretical values are handicaps in reliable accuracy assessments. The radiative and Auger rates are expected to be accurate to 10% and 20%, respectively, except for transitions involving strongly mixed levels. Radiative and Auger dampings have been taken into account in the calculation of photoabsorption cross sections in the K-threshold region, leading to overlapping Lorentzian shaped resonances of constant widths that cause edge smearing. The behavior of the improved opacities in this region has been studied with the XSTAR modeling code using simple constant density slab models and is displayed for a range of ionization parameters.

Distance to multiple kinematic components of quasar outflows: Very large telescope observations of QSO 2359-1241 and SDSS J0318-0600

Using high-resolution Very Large Telescope spectra, we study the multi-component outflow systems of two quasars exhibiting intrinsic Fe II absorption (QSO 2359-1241 and SDSS J0318-0600). From the extracted ionic column densities and using photoionization modeling, we determine the gas density, total column density, and ionization parameter for several of the components. For each object, the largest column density component is also the densest, and all other components have densities of roughly 1/4 of that of the main component. We demonstrate that all the absorbers lie roughly at the same distance from the source. Further, we calculate the total kinetic luminosities and mass outflow rates of all components and show that these quantities are dominated by the main absorption component.