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We present observations of the 51 Ophiuchi circumstellar disk made with the Keck interferometer operating in nulling mode at N band. We model these data simultaneously with VLTI-MIDI visibility data and a Spitzer IRS spectrum using a variety of optically thin dust cloud models and an edge-on optically thick disk model. We find that single-component optically thin disk models and optically thick disk models are inadequate to reproduce the observations, but an optically thin two-component disk model can reproduce all of the major spectral and interferometric features. Our preferred disk model consists of an inner disk of blackbody grains extending to [?]4 AU and an outer disk of small silicate grains extending out to [?]1200 AU. Our model is consistent with an inner "birth" disk of continually colliding parent bodies producing an extended envelope of ejected small grains. This picture resembles the disks around Vega, AU Microscopii, and b Pictoris, supporting the idea that 51 Ophiuchius may be a b Pictoris analog.

We present spectra of a sample of Herbig Ae and Be (HAeBe) stars obtained with the Infrared Spectrograph on Spitzer. All but one of the Herbig stars show emission from PAHs, and seven of the spectra show PAH emission, but no silicate emission at 10 mm. The central wavelengths of the 6.2, 7.7-8.2, and 11.3 mm emission features decrease with stellar temperature, indicating that the PAHs are less photoprocessed in cooler radiation fields. The apparent low level of photoprocessing in HAeBe stars, relative to other PAH emission sources, implies that the PAHs are newly exposed to the UV-optical radiation fields from their host stars. HAeBe stars show a variety of PAH emission intensities and ionization fractions but a narrow range of PAH spectral classifications based on positions of major PAH feature centers. This may indicate that, regardless of their locations relative to the stars, the PAH molecules are altered by the same physical processes in the protoplanetary disks of intermediate-mass stars. Analysis of the mid-IR SEDs indicates that our sample likely includes both radially flared and more flattened/settled disk systems, but we do not see the expected correlation of overall PAH emission with disk geometry. We suggest that the strength of PAH emission from HAeBe stars may depend not only on the degree of radial flaring but also on the abundance of PAHs in illuminated regions of the disks and possibly on the vertical structure of the inner disk as well.

The central regions of many interacting and early-type spiral galaxies are actively forming stars. This process affects the physical and chemical properties of the local interstellar medium, as well as the evolution of the galaxies. We observed near-infrared H2 emission lines: v = 1-0 S(1), 3-2 S(3), 1-0 S(0), and 2-1 S(1) from the central ~1 kpc regions of the archetypical starburst galaxies M82 and NGC 253 and the less dramatic but still vigorously star-forming galaxies NGC 6946 and IC 342. Like the far-infrared continuum luminosity, the near-infrared H2 emission luminosity can directly trace the amount of star formation activity because the H2 emission lines arise from the interaction between hot and young stars and nearby neutral clouds. The observed H2 line ratios show that both thermal excitation and nonthermal excitation are responsible for the emission lines but that the great majority of the near-infrared H2 line emission in these galaxies arises from energy states excited by ultraviolet fluorescence. The derived physical conditions, e.g., far-ultraviolet radiation field and gas density, from [C II] and [O I] lines and far-infrared continuum observations when used as inputs to photodissociation models also explain the luminosity of the observed H2 1-0 S(1) line. The ratio of the H2 1-0 S(1) line to far-IR continuum luminosity is remarkably constant over a broad range of galaxy luminosities: Limg1.gif/LFIR [?] 10-5, in normal late-type galaxies (including the Galactic center), in nearby starburst galaxies, and in luminous IR galaxies (LIRGs: LFIR > 1011 L). Examining this constant ratio in the context of photodissociation region models, we conclude that it implies that the strength of the incident UV field on typical molecular clouds follows the gas density at the cloud surface.

We have fabricated large, coarsely ruled, echelle patterns on silicon wafers by using photolithography and chemical-etching techniques. The grating patterns consist of 142-µm-wide, V-shaped grooves with an opening angle of 70.6°, blazed at 54.7°. We present a detailed description of our grating-fabrication techniques and the results of extensive testing. We have measured peak diffraction efficiencies of 70% at λ = 632.8 nm and conclude that the gratings produced by our method are of sufficient quality for use in high-resolution spectrographs in the visible and near IR (λ ≃ 500–5000 nm).