IRIS: A Generic Three-Dimensional Radiative Transfer Code

We present IRIS, a new generic three-dimensional (3D) spectral radiative transfer code that generates synthetic spectra, or images. It can be used as a diagnostic tool for comparison with astrophysical observations or laboratory astrophysics experiments. We have developed a 3D short-characteristic solver that works with a 3D nonuniform Cartesian grid. We have implemented a piecewise cubic, locally monotonic, interpolation technique that dramatically reduces the numerical diffusion effect. The code takes into account the velocity gradient effect resulting in gradual Doppler shifts of photon frequencies and subsequent alterations of spectral line profiles. It can also handle periodic boundary conditions. This first version of the code assumes Local Thermodynamic Equilibrium (LTE) and no scattering. The opacities and source functions are specified by the user. In the near future, the capabilities of IRIS will be extended to allow for non-LTE and scattering modeling. IRIS has been validated through a number of tests. We provide the results for the most relevant ones, in particular a searchlight beam test, a comparison with a 1D plane-parallel model, and a test of the velocity gradient effect. IRIS is a generic code to address a wide variety of astrophysical issues applied to different objects or structures, such as accretion shocks, jets in young stellar objects, stellar atmospheres, exoplanet atmospheres, accretion disks, rotating stellar winds, cosmological structures. It can also be applied to model laboratory astrophysics experiments, such as radiative shocks produced with high power lasers.Comment: accepted for publication in A&A; 17 pages, 9 figures, 2 table

Non-LTE Models and Theoretical Spectra of Accretion Disks in Active Galactic Nuclei. III. Integrated Spectra for Hydrogen-Helium Disks

We have constructed a grid of non-LTE disk models for a wide range of black hole mass and mass accretion rate, for several values of viscosity parameter alpha, and for two extreme values of the black hole spin: the maximum-rotation Kerr black hole, and the Schwarzschild (non-rotating) black hole. Our procedure calculates self-consistently the vertical structure of all disk annuli together with the radiation field, without any approximations imposed on the optical thickness of the disk, and without any ad hoc approximations to the behavior of the radiation intensity. The total spectrum of a disk is computed by summing the spectra of the individual annuli, taking into account the general relativistic transfer function. The grid covers nine values of the black hole mass between M = 1/8 and 32 billion solar masses with a two-fold increase of mass for each subsequent value; and eleven values of the mass accretion rate, each a power of 2 times 1 solar mass/year. The highest value of the accretion rate corresponds to 0.3 Eddington. We show the vertical structure of individual annuli within the set of accretion disk models, along with their local emergent flux, and discuss the internal physical self-consistency of the models. We then present the full disk-integrated spectra, and discuss a number of observationally interesting properties of the models, such as optical/ultraviolet colors, the behavior of the hydrogen Lyman limit region, polarization, and number of ionizing photons. Our calculations are far from definitive in terms of the input physics, but generally we find that our models exhibit rather red optical/UV colors. Flux discontinuities in the region of the hydrogen Lyman limit are only present in cool, low luminosity models, while hotter models exhibit blueshifted changes in spectral slope.Comment: 20 pages, 31 figures, ApJ in press, spectral models are available for downloading at http://www.physics.ucsb.edu/~blaes/habk

Modelling ultraviolet-line diagnostics of stars, the ionized and the neutral interstellar medium in star-forming galaxies

We combine state-of-the-art models for the production of stellar radiation and its transfer through the interstellar medium (ISM) to investigate ultraviolet-line diagnostics of stars, the ionized and the neutral ISM in star-forming galaxies. We start by assessing the reliability of our stellar population synthesis modelling by fitting absorption-line indices in the ISM-free ultraviolet spectra of 10 Large-Magellanic-Cloud clusters. In doing so, we find that neglecting stochastic sampling of the stellar initial mass function in these young ($\sim10$-100 Myr), low-mass clusters affects negligibly ultraviolet-based age and metallicity estimates but can lead to significant overestimates of stellar mass. Then, we proceed and develop a simple approach, based on an idealized description of the main features of the ISM, to compute in a physically consistent way the combined influence of nebular emission and interstellar absorption on ultraviolet spectra of star-forming galaxies. Our model accounts for the transfer of radiation through the ionized interiors and outer neutral envelopes of short-lived stellar birth clouds, as well as for radiative transfer through a diffuse intercloud medium. We use this approach to explore the entangled signatures of stars, the ionized and the neutral ISM in ultraviolet spectra of star-forming galaxies. We find that, aside from a few notable exceptions, most standard ultraviolet indices defined in the spectra of ISM-free stellar populations are prone to significant contamination by the ISM, which increases with metallicity. We also identify several nebular-emission and interstellar-absorption features, which stand out as particularly clean tracers of the different phases of the ISM.Comment: 27 pages, 21 figures. Accepted for publication in MNRA

Non-LTE Models and Theoretical Spectra of Accretion Disks in Active Galactic Nuclei

We present self-consistent models of the vertical structure and emergent spectrum of AGN accretion disks. The central object is assumed to be a supermassive Kerr black hole. We demonstrate that NLTE effects and the effects of a self-consistent vertical structure of a disk play a very important role in determining the emergent radiation, and therefore should be taken into account. In particular, NLTE models exhibit a largely diminished H I Lyman discontinuity when compared to LTE models, and the He II discontinuity appears strongly in emission for NLTE models. Consequently, the number of ionizing photons in the He II Lyman continuum predicted by NLTE disk models is by 1 - 2 orders of magnitude higher than that following from the black-body approximation. This prediction has important implications for ionization models of AGN broad line regions, and for models of the intergalactic radiation field and the ionization of helium in the intergalactic medium.Comment: 11 pages; 2 postscript figures; LaTeX, AASPP4 macro; to appear in the Astrophysical Journal (Letters

Theoretical Interpretation of the Measurements of the Secondary Eclipses of TrES-1 and HD209458b

We calculate the planet-star flux-density ratios as a function of wavelength from 0.5 microns to 25 microns for the transiting extrasolar giant planets TrES-1 and HD209458b and compare them with the recent Spitzer/IRAC-MIPS secondary eclipse data in the 4.5, 8.0, and 24 micron bands. With only three data points and generic calibration issues, detailed conclusions are difficult, but inferences regarding atmospheric composition, temperature, and global circulation can be made. Our models reproduce the observations reasonably well, but not perfectly, and we speculate on the theoretical consequences of variations around our baseline models. One preliminary conclusion is that we may be seeing in the data indications that the day side of a close-in extrasolar giant planet is brighter in the mid-infrared than its night side, unlike Jupiter and Saturn. This correspondence will be further tested when the data anticipated in other Spitzer bands are acquired, and we make predictions for what those data may show.Comment: 15 pages, including 3 color figures, submitted to the Astrophysical Journa

Theory for the Secondary Eclipse Fluxes, Spectra, Atmospheres, and Light Curves of Transiting Extrasolar Giant Planets

We have created a general methodology for calculating the wavelength-dependent light curves of close-in extrasolar giant planets (EGPs) as they traverse their orbits. Focussing on the transiting EGPs HD189733b, TrES-1, and HD209458b, we calculate planet/star flux ratios during secondary eclipse and compare them with the Spitzer data points obtained so far in the mid-infrared. We introduce a simple parametrization for the redistribution of heat to the planet's nightside, derive constraints on this parameter (P_n), and provide a general set of predictions for planet/star contrast ratios as a function of wavelength, model, and phase. Moreover, we calculate average dayside and nightside atmospheric temperature/pressure profiles for each transiting planet/P_n pair with which existing and anticipated Spitzer data can be used to probe the atmospheric thermal structure of severely irradiated EGPs. We find that the baseline models do a good job of fitting the current secondary eclipse dataset, but that the Spitzer error bars are not yet small enough to discriminate cleanly between all the various possibilities.Comment: 14 figures, 7 text pages (in two-column emulateapj format); Accepted to the Ap.J. June 26, 2006; one cosmetic change made to astro-ph version

A Grid of Relativistic, non-LTE Accretion Disk Models for Spectral Fitting of Black Hole Binaries

Self-consistent vertical structure models together with non-LTE radiative transfer should produce spectra from accretion disks around black holes which differ from multitemperature blackbodies at levels which may be observed. High resolution, high signal-to-noise observations warrant spectral modeling which both accounts for relativistic effects, and treats the physics of radiative transfer in detail. In Davis et al. (2005) we presented spectral models which accounted for non-LTE effects, Compton scattering, and the opacities due to ions of abundant metals. Using a modification of this method, we have tabulated spectra for black hole masses typical of Galactic binaries. We make them publicly available for spectral fitting as an Xspec model. These models represent the most complete realization of standard accretion disk theory to date. Thus, they are well suited for both testing the theory's applicability to observed systems and for constraining properties of the black holes, including their spins.Comment: 7 pages, emulate ApJ, accepted to Ap

Theoretical Spectra and Light Curves of Close-in Extrasolar Giant Planets and Comparison with Data

We present theoretical atmosphere, spectral, and light-curve models for extrasolar giant planets (EGPs) undergoing strong irradiation for which {\it Spitzer} planet/star contrast ratios or light curves have been published (circa June 2007). These include HD 209458b, HD 189733b, TrES-1, HD 149026b, HD 179949b, and $\upsilon$ And b. By comparing models with data, we find that a number of EGP atmospheres experience thermal inversions and have stratospheres. This is particularly true for HD 209458b, HD 149026b, and $\upsilon$ And b. This finding translates into qualitative changes in the planet/star contrast ratios at secondary eclipse and in close-in EGP orbital light curves. Moreover, the presence of atmospheric water in abundance is fully consistent with all the {\it Spitzer} data for the measured planets. For planets with stratospheres, water absorption features invert into emission features and mid-infrared fluxes can be enhanced by a factor of two. In addition, the character of near-infrared planetary spectra can be radically altered. We derive a correlation between the importance of such stratospheres and the stellar flux on the planet, suggesting that close-in EGPs bifurcate into two groups: those with and without stratospheres. From the finding that TrES-1 shows no signs of a stratosphere, while HD 209458b does, we estimate the magnitude of this stellar flux breakpoint. We find that the heat redistribution parameter, P$_n$, for the family of close-in EGPs assumes values from $\sim$0.1 to $\sim$0.4. This paper provides a broad theoretical context for the future direct characterization of EGPs in tight orbits around their illuminating stars.Comment: Accepted to Ap. J., provided here in emulateapj format: 28 pages, 8 figures, many with multiple panel