Ekaterina M. Verner

Cloudy 90: Numerical simulation of plasmas and their spectra

ABSTRACT CLOUDY is a large‐scale spectral synthesis code designed to simulate fully physical conditions within an astronomical plasma and then predict the emitted spectrum. Here we describe version 90 (C90) of the code, paying particular attention to changes in the atomic database and numerical methods that have affected predictions since the last publicly available version, C84. The computational methods and uncertainties are outlined together with the direction future development will take. The code is freely available and is widely used in the analysis and interpretation of emission‐line spectra. Web access to the Fortran source for CLOUDY, its documentation Hazy, and an independent electronic form of the atomic database is also described.

Numerical Simulations of Fe II Emission Spectra

This paper describes the techniques that we have used to incorporate a large-scale model of the Fe+ ion and resulting Fe II emission into CLOUDY, a spectral synthesis code designed to simulate conditions within a plasma and model the resulting spectrum. We describe the numerical methods we use to determine the level populations, mutual line overlap fluorescence, collisional effects, and the heating-cooling effects of the atom on its environment. As currently implemented, the atom includes the lowest 371 levels (up to 11.6 eV) and predicts intensities of 68,635 lines. We describe our data sources, which include the most recent transition probabilities and collision strengths. Although we use detailed fits to temperature-dependent collision strengths where possible, in many cases the uncertain approximation is the only source for collision data. The atom is designed to be readily expanded to include more levels and to incorporate more accurate sets of collision and radiative data as computers grow faster and the atomic databases expand. We present several test cases showing that the atom goes to LTE in the limits of high particle and radiation densities. We give an overview of general features of the Fe II spectra as their dependencies on the basic parameters of our models (density, flux, microturbulent velocity, the Fe abundance, and Lyα pumping). Finally, we discuss several applications to active galactic nuclei to illustrate the diagnostic power of the Fe II spectrum and make some predictions for UV observations.

High-Resolution Spectroscopy of Faint Emission Lines in the Orion Nebula

We present high-resolution spectrophotometric observations of the Orion Nebula, made with the Cassegrain echelle spectrograph on the Blanco 4 m telescope at Cerro Tololo Inter-American Observatory (CTIO). The resolution and signal-to-noise ratio make it possible to identify 444 emission lines in the 3498-7468 ? range, down to 104 times fainter than H?. We present a detailed atlas of these emission lines along with an analysis of the associated errors. This data set is used to study the velocity field in the Orion Nebula. The forbidden lines split into two distinct groups. The low-ionization group has ions with an ionization potential less than 20 eV. Lines of these ions, [O I], [N I], [Ni II], and [Fe II], have recession velocities, relative to the hydrogen lines, of +10 to +15 km s-1. There is a sharp change to the second, high-ionization group, which includes lines of ions with ionization potentials larger than 20 eV, namely, [S II], [O II], [N II], and [Fe III]. These lines have velocities around +3 km s-1, with a slight trend of decreasing velocity with the increasing ionization potential. This is consistent with previously proposed dynamical models in which lines of ions with different ionization potentials originate at different distances from the ionizing stars. Significant acceleration appears to take place across the narrow region where Fe2+ exists. Across this region the gas receives an acceleration of ~ 2.5 ? 10-5 cm s-2. This provides a constraint on hydrodynamical models. We set a limit He II 4686/H? < 7 ? 10-5, which in turn sets a limit to the intensity of the ionizing continuum at energies higher than 54 eV. Modern stellar atmospheres predict a continuum that is far stronger than is present in the region near ?1 Ori C.

Continuum Pumping of [Fe II] in the Orion Nebula

This paper presents detailed comparisons between numerical simulations of Fe II emission spectra and recent high-resolution and signal-to-noise spectra of the Orion Nebula. We have identi—ed 40 (Fe II) lines in the spectrum, allowing extensive comparisons between theory and observations. The identi—ca- tions are based on predictions of a realistic model of the Fe II atom, which includes the lowest 371 levels (all levels up to 11.6 eV). We investigate the dependence of the spectrum on electron density and on pumping by the stellar continuum. Orion is important because it provides a relatively simple environ- ment in which to test complex simulations. We have identi—ed the pumping routes that are responsible for the observed emission. Our theoretical model of Fe II emission is in good agreement with the obser- vational data. Subject headings: atomic dataH II regionsISM: individual (Orion Nebula) ¨ line: formation ¨ line: identi—cation

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

Luminosity Indicators in Dusty Photoionized Environments

The luminosity of the central source in ionizing radiation is an essential parameter in a photoionized environment and is one of the most fundamental physical quantities one can measure. We outline a method of determining the luminosity for any emission-line region using only infrared data. In dusty environments, grains compete with hydrogen in absorbing continuum radiation. Grains produce infrared emission, and hydrogen produces recombination lines. We have computed a very large variety of photoionization models, using ranges of abundances, grain mixtures, ionizing continua, densities, and ionization parameters. The conditions were appropriate for such diverse objects as H ii regions, planetary nebulae, starburst galaxies, and the narrow- and broad-line regions of active nuclei. The ratio of the total thermal grain emission relative to Hβ (IR/Hβ) is the primary indicator of whether the cloud behaves as a classical Stromgren sphere (a hydrogen-bounded nebula) or whether grains absorb most of the incident continuum (a dust-bounded nebula). We find two global limits: when IR/Hβ < 100, infrared recombination lines determine the source luminosity in ionizing photons; when IR/Hβ 100, the grains act as a bolometer to measure the luminosity.