Dmitri A. Verner

Atomic data for astrophysics. II. New analytic fits for photoionization cross sections of atoms and ions

We present a complete set of analytic fits to the nonrelativistic photoionization cross sections for the ground states of atoms and ions of elements from H through Si, and S, Ar, Ca, and Fe. Near the ionization thresholds, the fits are based on the Opacity Project theoretical cross sections interpolated and smoothed over resonances. At higher energies, the fits reproduce calculated Hartree-Dirac-Slater photoionization cross sections. {copyright} {ital 1996 The American Astronomical Society.}

Atomic Data for Astrophysics. I. Radiative Recombination Rates for H-like, He-like, Li-like, and Na-like Ions over a Broad Range of Temperature

We present new calculations and analytic fits to the rates of radiative recombination toward H-like, He-like, Li-like, and Na-like ions of all elements from H through Zn ({ital Z}=30). The fits are valid over a wide range of temperature, from 3 K to 10{sup 9} K. {copyright} {ital 1996 The American Astronomical Society.}

Physical Conditions in Low-Ionization Regions of the Orion Nebula

ABSTRACTWe reexamine the spectroscopic underpinnings of recent suggestions that [O I ] and[Fe II ] lines from the Orion H region are produced in gas where the iron-carryinggrains have been destroyed and the electron density is surprisingly high. Our newobservations show that previous detections of [O I ] 5577 were dominated by telluricemission. Our limits are consistent with a moderate density (≈ 10 4 cm −3 ) photoionizedgas. We show that a previously proposed model of the Orion H II region reproducesthe observed [O I ] and [Fe II ] spectrum. These lines are fully consistent with formationin a dusty region of moderate density.Subject headings: ISM: H II regions — ISM: abundances — ISM: atoms — ISM:individual (Orion Nebula)1. IntroductionThe Orion Nebula is the defining blister H II region (Zuckerman 1973; Balick, Gammon, &Hjellming 1974). A star cluster ionizes the skin of the molecular cloud, causing an expansion away 1 Based in part on observations made with the NASA/ESAHubble Space Telescope, obtained at the Space TelescopeScience Institute, which is operated by AURA, Inc., under NASA contract NAS5-26555

Far-ultraviolet absorption spectra of quasars: How to find missing hot gas and metals

We show that some high-redshift QSO absorption systems that reveal only the H I Lyman series lines at wavelengths visible from the ground may be a new class of ultra-high-ionization metal line systems, with metal lines in the far-UV region which is now being explored with satellites. At high temperatures or in intense radiation fields metal systems will not show the usual C IV absorption, and O VI will become the most prominent metal absorber. At still higher ionization, O VI also becomes weak, and the strongest metal lines are from Ne VIII, Mg X, and Si XII, which have doublets in the range 500-800 angstrom. Hence very high ionization metal systems will not show metal lines in existing spectra. Recent X-ray observations show that galaxy halos contain hot gas, so we predict that far-UV spectra of QSOs will also show this gas.

Charge Transfer between Neutral Atoms and Highly Ionized Species: Implications for ISO Observations

We estimate rate coefficients for charge transfer between neutral hydrogen and helium and moderately to highly ionized heavy elements. Although charge transfer does not have much influence on hot collisionally ionized plasmas, its effects on photoionized plasmas can be profound. We present several photoionization models that illustrate the significant effect of charge transfer on the far-infrared lines detected by ISO.

Photoionization models of QSO's intervening absorption clouds

A new sufficiently compact code LINESPEC is described which is designed to determine the kinetic (collisional+photo) ionization equilibrium in a slab of a hot (T≥104K) rarefied gas, and to simulate synthetic absorption-line spectra formed due to passing continuous radiation (from a quasar) through the slab. Eighty-six resonant absorption lines of ions and atoms of H, He, C, N, O, Ne, Mg, Al, Si, S, Ca, Fe are included with wavelengths in the range 912≤λ≤5000 Å. the behaviour and observability of various lines are analysed as a function of intensity of the ionizing radiation and kinetic plasma temperature (for a power-law spectrum of ionizing photons,Nph(E) ∞E−γ with γ=1.5;E is photon energy). For the purely collisional ionization (Nph=0), the spectrum contains various combinations of absorption lines of hydrogen and/or of atoms and ions of other elements, the relative intensities of the lines being strongly temperaturedependent. If the ionizing radiation is intense enough, the Lα line dominates. In a wide parameter range Lα may be the only line visible in the UV spectral range.

Quasars, the early universe, and plasma simulations

Quasars are among the most luminous objects in the universe, and the most distant for which we can obtain good spectroscopy. These spectra carry with them information about the formation of the first large structures in the universe and the first generations of star formation in newly born galaxies. The spectrum of a quasar has strong broad emission lines, formed by a non-equilibrium low-density photoionized gas. This spectrum can best be interpreted by reference to large-scale simulations of the full environment. This in turn is strongly coupled to many basic plasma physics questions. Here we discuss the current status of quasar research, numerical plasma simulations, and the atomic database.

Ionization equilibrium and spectrum formation in QSOs' absorption regions

A new and rather compact code LINESPEC is developed to calculate the absorption line spectra formed due to passage of continuous radiation (from a quasar) through a layer of a hot (T≥104 K) and rarefied (N≪1014 cm−3) plasma in the state of steady kinetic (collisional+photo) ionization equilibrium. Illustrative calculations show that the spectra are very sensitive toT and to the ratio of number densities of ionizing photons and particles.