Jason W. Ferguson

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.

The Dartmouth Stellar Evolution Database

The ever-expanding depth and quality of photometric and spectroscopic observations of stellar populations increase the need for theoretical models in regions of age-composition parameter space that are largely unexplored at present. Stellar evolution models that employ the most advanced physics and cover a wide range of compositions are needed to extract the most information from current observations of both resolved and unresolved stellar populations. The Dartmouth Stellar Evolution Database is a collection of stellar evolution tracks and isochrones that spans a range of [Fe/H] from –2.5 to +0.5, [α/Fe] from –0.2 to +0.8 (for [Fe/H] ≤ 0) or +0.2 (for [Fe/H] > 0), and initial He mass fractions from Y = 0.245 to 0.40. Stellar evolution tracks were computed for masses between 0.1 and 4 M☉, allowing isochrones to be generated for ages as young as 250 Myr. For the range in masses where the core He flash occurs, separate He-burning tracks were computed starting from the zero age horizontal branch. The tracks and isochrones have been transformed to the observational plane in a variety of photometric systems including standard UBV(RI)C, Stromgren uvby, SDSS ugriz, 2MASS JHKs, and HST ACS/WFC and WFPC2. The Dartmouth Stellar Evolution Database is accessible through a Web site at http://stellar.dartmouth.edu/~models/ where all tracks, isochrones, and additional files can be downloaded.

Low-Temperature Opacities

Previous computations of low-temperature Rosseland and Planck mean opacities from Alexander & Ferguson areupdatedandexpanded.Thenewcomputationsincludeamorecompleteequationofstate(EOS)withmoregrain species and updated optical constants. Grains are now explicitly included in thermal equilibrium in the EOS calculation, which allows for a much wider range of grain compositions to be accurately included than was previously the case. The inclusion of high-temperature condensates such as Al2O3 and CaTiO3 significantly affects the total opacityoveranarrowrangeoftemperaturesbeforetheappearanceofthefirstsilicategrains.Thenewopacitytables are tabulated for temperatures ranging from 30,000 to 500 K with gas densities from 10 � 4 to 10 � 19 gc m � 3 .C omparisons with previous Rosseland mean opacity calculations are discussed. At high temperatures, the agreement with OPAL and Opacity Project is quite good. Comparisons at lower temperatures are more divergent as a result of differences in molecular and grain physics included in different calculations. The computation of Planck mean opacities performed with the opacity sampling method is shown to require a very large number of opacity sampling wavelength points; previously published results obtained with fewer wavelength points are shown to be significantly in error. Methods for requesting or obtaining the new tables are provided. Subject heading gs: atomic data — equation of state — methods: numerical — molecular data

The NEXTGEN Model Atmosphere Grid. II. Spherically Symmetric Model Atmospheres for Giant Stars with Effective Temperatures between 3000 and 6800 K

We present the extension of our NextGen model atmosphere grid to the regime of giant stars. The input physics of the models presented here is nearly identical to that of the NextGen dwarf atmosphere models; however, spherical geometry is used self-consistently in the model calculations (including the radiative transfer). We revisit the discussion of the effects of spherical geometry on the structure of the atmospheres and the emitted spectra and discuss the results of non-LTE calculations for a few selected models.

New solar composition: The problem with solar models revisited

We construct updated solar models with different sets of solar abundances, including the most recent determinations by Asplund et al. The latter work predicts a larger (~10%) solar metallicity compared to previous measurements by the same authors but significantly lower (~25%) than the recommended value from a decade ago by Grevesse & Sauval. We compare the results of our models with determinations of the solar structure inferred through helioseismology measurements. The model that uses the most recent solar abundance determinations predicts the base of the solar convective envelope to be located at R CZ = 0.724 R ☉ and a surface helium mass fraction of Y surf = 0.231. These results are in conflict with helioseismology data (R CZ = 0.713 ± 0.001 R ☉ and Y surf = 0.2485 ± 0.0035) at 5σ and 11σ levels, respectively. Using the new solar abundances, we calculate the magnitude by which radiative opacities should be modified in order to restore agreement with helioseismology. We find that a maximum change of ~15% at the base of the convective zone is required with a smooth decrease toward the core, where the change needed is ~5%. The required change at the base of the convective envelope is about half the value estimated previously. We also present the solar neutrino fluxes predicted by the new models. The most important changes brought about by the new solar abundances are the increase by ~10% in the predicted 13N and 15O fluxes that arise mostly due to the increase in the C and N abundances in the newly determined solar composition.

The ACS Survey of Galactic Globular Clusters. II. Stellar Evolution Tracks, Isochrones, Luminosity Functions, and Synthetic Horizontal-Branch Models

The ACS Survey of Galactic Globular Clusters, an HST Treasury Project, will deliver high-quality, homogeneous photometry of 65 globular clusters. This paper introduces a new collection of stellar evolution tracks and isochrones suitable for analyzing the ACS survey data. Stellar evolution models were computed at [Fe/H] = -2.5, -2.0, -1.5, -1.0, -0.5, and 0; [α/Fe] = -0.2, 0, 0.2, 0.4, 0.6, and 0.8; and three initial He abundances for masses from 0.1 to 1.8 M⊙ and ages from 2 to 15 Gyr. Each isochrone spans a wide range in luminosity, from MV ~ 14 up to the tip of the red giant branch. These are complemented by a set of He-burning tracks that extend from the zero-age horizontal branch to the onset of thermal pulsations on the asymptotic giant branch. In addition, a set of computer programs are provided that make it possible to interpolate the isochrones in [Fe/H], generate luminosity functions from the isochrones, and create synthetic horizontal-branch models. The tracks and isochrones have been converted to the observational plane with two different color-Teff transformations, one synthetic and one semiempirical, in ground-based B, V, and I, and F606W and F814W for both ACS WFC and WFPC2 systems. All models and programs presented in this paper are available at the Dartmouth Stellar Evolution Database and the Multimission Archive at the Space Telescope Science Institute.

Spectral models for solar‐scaled and α‐enhanced stellar populations

We present the first models allowing one to explore in a consistent way the influence of changes in the α-element-to-iron abundance ratio on the high-resolution spectral properties of evolving stellar populations. The models cover the wavelength range from 3000 A to 1.34 μm at a constant resolution of full width at half-maximum (FWHM) = 1 A and a sampling of 0.2 A, for overall metallicities in the range 0.005 � Z � 0.048 and for stellar population ages between 3 and 14 Gyr. These models are based on a recent library of synthetic stellar spectra and a new library of stellar evolutionary tracks, both computed for three different iron abundances ([Fe/H] =− 0.5, 0.0 and 0.2) and two different α-element-to-iron abundance ratios ([α/Fe] = 0.0 and 0.4). We expect our fully synthetic models to be primarily useful for evaluating the differential effect of changes in the α/Fe ratio on spectral properties such as broad-band colours and narrow spectral features. In addition, we assess the accuracy of absolute model predictions in two ways: first, by comparing the predictions of models for scaled-solar metal abundances ([α/Fe] = 0.0) to those of existing models based on libraries of observed stellar spectra; and secondly, by comparing the predictions of models for α-enhanced metal abundances ([α/Fe] = 0.4) to observed spectra of massive early-type galaxies in the Sloan Digital Sky Survey Data Release 4. We find that our models predict accurate strengths for those spectral indices that are strongly sensitive to the abundances of Fe and α elements. The predictions are less reliable for the strengths of other spectral features, such as those dominated by the abundances of C and N, as expected from the fact that the models do not yet allow one to explore the influence of these elements in an independent way. We conclude that our models are a powerful tool for extracting new information about the chemical properties of galaxies for which high-quality spectra have been gathered by modern surveys.

New asymptotic giant branch models for a range of metallicities

We present a new grid of stellar model calculations for stars on the Asymptotic Giant Branch between 1.0 and 6.0 M� . Our grid consists of 10 chemical mixtures with 5 metallicities between Z = 0.0005 and Z = 0.04, and with both solar-like and α-element enhanced metal ratios for each metallicity. We treat consistently the carbon-enhancement of the stellar envelopes by using opacity tables with varying C/O-ratio and by employing theoretical mass loss rates for carbon stars. The low temperature opacities have been calculated specifically for this project. For oxygen stars we use an empirical mass loss formalism. The third dredge-up is naturally obtained by including convective overshooting. Our models reach effective temperatures in agreement with earlier synthetic models, which included approximative carbon-enriched molecular opacities and show good agreement with empirically determined carbonstar lifetimes. A fraction of the models could be followed into the post-AGB phase, for which we provide models in a mass range supplementing previous post-AGB calculations. Our grid constitutes the most extensive set of AGB-models, calculated with the latest physical input data and treating carbon-enhancement due to the third dredge-up most consistently.

A LARGE STELLAR EVOLUTION DATABASE FOR POPULATION SYNTHESIS STUDIES. V. STELLAR MODELS AND ISOCHRONES WITH CNONa ABUNDANCE ANTICORRELATIONS

We present a new grid of stellar models and isochrones for old stellar populations, covering a large range of [Fe/H] values, for an heavy element mixture characterized by CNONa abundance anticorrelations as observed in Galactic globular cluster stars. The effect of this metal abundance pattern on the evolutionary properties of low-mass stars, from the main sequence to the horizontal branch phase, is analyzed. We perform comparisons between these new models, and our reference α-enhanced calculations, and discuss briefly implications for color-magnitude diagrams showing multiple main sequence or subgiant branches. A brief qualitative discussion of the effect of CN abundances on color-T eff transformations is also presented, highlighting the need to determine theoretical color transformations for the appropriate metal mixture, if one wants to interpret observations in the Stromgren system, or broadband filters blueward of the Johnson V band.

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.