Modeling Brown Dwarfs, L Dwarfs, and T Dwarfs

In this brief contribution, I touch on a subset of recent efforts in spectral and opacity modeling aimed at improving our understanding of brown dwarfs, L dwarfs, and T dwarfs. I discuss theoretical calculations of the alkali line profiles, newly generated CrH opacities, new evidence for refractory rainout in T dwarfs from optical spectral measurements, and the distinction between brown dwarfs and planets.Comment: 9 pages, in newpasp.sty format, 4 figures. To be published in the Proceedings of IAU Symposium 211 entitled {\it Brown Dwarfs

Understanding Core-Collapse Supernovae

I summarize, in the form of an extended abstract, the ongoing efforts at the University of Arizona (and in collaboration) to understand core-collapse supernovae theoretically. Included are short discussions of 1D (SESAME) and 2D (VULCAN/2D) codes and results, as well as discussions of the possible role of rotation. Highlighted are recent developments in multi-dimensional radiation hydrodynamics and the essential physics of the neutrino-driven mechanism.Comment: to be published in the proceedings of the Twelfth Workshop on "Nuclear Astrophysics," a Tribute to an Explosive Astrophysicist, Wolfgang Hillebrandt, on the occasion of his 60th Birthday, held at the Ringberg Castle, Lake Tegernsee, Germany, March 22 - 27, 2004, eds. E. Muller and H.-Th. Jank

The Next U.S. Astronomy Decadal Survey

The U.S. astronomy decadal surveys have been models for advice to government on how to apportion resources to optimise the scientific return on national investments in facilities and manpower. The U.S. is now gearing up to conduct its 2020 survey and the results are likely to guide international astronomy far into the future. Here, I summarize the current strains in an otherwise world-leading program of ground- and space-based astronomical discovery and some of the issues that will be faced by the participants in this upcoming collective exercise

The Role of Dust Clouds in the Atmospheres of Brown Dwarfs

The new spectroscopic classes, L and T, are defined by the role of dust clouds in their atmospheres, the former by their presence and the latter by their removal and near absence. Moreover, the M to L and L to T transitions are intimately tied to the condensation and character of silicate and iron grains, and the associated clouds play pivotal roles in the colors and spectra of such brown dwarfs. Spanning the effective temperature range from $\sim$2200 K to $\sim$600 K, these objects are being found in abundance and are a new arena in which condensation chemistry and the optical properties of grains is assuming astronomical importance. In this short paper, I summarize the role played by such refractories in determining the properties of these "stars" and the complexities of their theoretical treatment.Comment: To be published in the proceedings of "Cosmic Dust - Near and Far," eds Th. Henning, E. Grun, J. Steinacker, (Astronomical Society of the Pacific), held September 8 - 12, 200

Scientific Return of Coronagraphic Exoplanet Imaging and Spectroscopy Using WFIRST

In this study, we explore and review the scientific potential for exoplanet characterization by a high-contrast optical coronagraph on WFIRST/AFTA. We suggest that the heterogeneity in albedo spectra and planet/star flux ratios as a function of orbital distance, planet mass, and composition expected for the giant exoplanets at separations from their primaries accessible to WFIRST will provide unique constraints on giant planet atmospheres, evolution, aerosol and cloud properties, and general theory. Such exoplanets are not merely extrapolations of Jupiter and Saturn, but are likely to occupy a rich continuum of varied behaviors. Each in themselves and jointly, optical spectra, photometry, and polarization measurements of a diverse population of giant exoplanets in the solar neighborhood has the potential to reveal a multitude of fundamental features of their gas-giant chemistry, atmospheres, and formation. Such a campaign will enrich our understanding of this class of planets beyond what is possible with even a detailed exploration of the giants in our own solar system, and will compliment ongoing studies of exoplanets in the infrared and on close-in orbits inaccessible to coronagraphy.Comment: A study outlining the scientific case for a high-contrast coronagraph on WFIRST in support of its exoplanet campaign, performed on behalf of the WFIRST/AFTA Science Definition Team and the Exo-S and Exo-C Science and Technology Definition Team

Spectra as Windows into Exoplanet Atmospheres

Understanding a planet's atmosphere is a necessary condition for understanding not only the planet itself, but also its formation, structure, evolution, and habitability, This puts a premium on obtaining spectra, and developing credible interpretative tools with which to retrieve vital planetary information. However, for exoplanets these twin goals are far from being realized. In this paper, I provide a personal perspective on exoplanet theory and remote sensing via photometry and low-resolution spectroscopy. Though not a review in any sense, this paper highlights the limitations in our knowledge of compositions, thermal profiles, and the effects of stellar irradiation, focussing on, but not restricted to, transiting giant planets. I suggest that the true function of the recent past of exoplanet atmospheric research has been not to constrain planet properties for all time, but to train a new generation of scientists that, by rapid trial and error, is fast establishing a solid future foundation for a robust science of exoplanets.Comment: Accepted to the Proceedings of the National Academy of Sciences, as part of a Special Feature Edition on Exoplanet

On the Systematics of Core-Collapse Explosions

Recent observations of supernovae, supernova remnants, and radio pulsars suggest that there are correlations between pulsar kicks and spins, infrared and gamma-ray line profiles, supernova polarizations, and ejecta debris fields. A framework is emerging in which explosion asymmetries play a central role. The new perspective meshes recent multi-dimensional theoretical investigations of the explosion mechanism with trends in $^{56}$Ni yields and explosion kinetic energies. These trends imply that the mass above which black holes form after collapse is $\sim$30 M$_{\odot}$ and that supernova explosion energies may vary by as much as a factor of four. In addition, new neutrino-matter opacity calculations reveal that the inner cores of protoneutron stars are more transparent than hitherto suspected. This may have consequences for the delayed neutrino-driven mechanism of explosion itself.Comment: 9 pages, LaTeX, no figures, to be published in the proceedings of the 9'th Workshop on Nuclear Astrophysics, held at the Ringberg Castle, Germany, March 23-29, 1998, ed. E. M\"uller & W. Hillebrand

Alkali Metals and the Color of Brown Dwarfs

I summarize some of the consequences for the optical and very-near-infrared spectra of T dwarfs (in particular) and brown dwarfs (in general) of their possible dominance by the neutral alkali metal lines. As a byproduct of this study, I estimate the true optical color of ``brown'' dwarfs.Comment: 8 pages, LaTeX, 5 figures, to be published in Ultracool Dwarfs, a Special Session of the I.A.U. General Assembly held August 12, 2000 in Manchester, England, eds. Hugh Jones and Iain Steele, Springer-Verlag: Lecture Notes in Physics serie

A New Algorithm for 2-D Transport for Astrophysical Simulations: I. General Formulation and Tests for the 1-D Spherical Case

We derive new equations using the mixed-frame approach for one- and two-dimensional (axisymmetric) time-dependent radiation transport and the associated couplings with matter. Our formulation is multi-group and multi-angle and includes anisotropic scattering, frequency(energy)-dependent scattering and absorption, complete velocity dependence to order v/c, rotation, and energy redistribution due to inelastic scattering. Hence, the "2D" realization is actually "6 1/2"-dimensional. The effects of radiation viscosity are automatically incorporated. Moreover, we develop Accelerated-Lambda-Iteration, Krylov subspace (GMRES), Discontinuous-Finite-Element, and Feautrier numerical methods for solving the equations and present the results of one-dimensional numerical tests of the new formalism. The virtues of the mixed-frame approach include simple velocity dependence with no velocity derivatives, straight characteristics, simple physical interpretation, and clear generalization to higher dimensions. Our treatment can be used for both photon and neutrino transport, but we focus on neutrino transport and applications to core-collapse supernova theory in the discussions and examples.Comment: 44 pages, 14 figure

A theoretical look at the direct detection of giant planets outside the Solar System

Astronomy is at times a science of unexpected discovery. When it is, and if we are lucky, new intellectual territories emerge to challenge our views of the cosmos. The recent indirect detections using high-precision Doppler spectroscopy of now more than one hundred giant planets orbiting more than one hundred nearby stars is an example of such rare serendipity. What has been learned has shaken our preconceptions, for none of the planetary systems discovered to date is like our own. However, the key to unlocking a planet's chemical, structural, and evolutionary secrets is the direct detection of the planet's light. I review the embryonic theory of the spectra, atmospheres, and light curves of irradiated giant planets and put this theory into the context of the many proposed astronomical campaigns to image them.Comment: pre-editorial, non-copyrighted version of Review Article just published in Nature. 5 figures, one in JPEG forma