J. D. Bregman

Anatomy of the photodissociation region in the orion bar.

Much of the interstellar gas resides in photodissociation regions whose chemistry and energy balance is controlled by the flux of far-ultraviolet radiation upon them. These photons can ionize and dissociate molecules and heat the gas through the photoelectric effect working on dust grains. These regions have been extensively modeled theoretically, but detailed observational studies are few. Mapping of the prominent Orion Bar photodissociation region at wavelengths corresponding to the carbon-hydrogen stretching mode of polycyclic aromatic hydrocarbons, the 1-0 S(1) line of molecular hydrogen, and the J = 1-0 rotational line of carbon monoxide allows the penetration of the far-ultraviolet radiation into the cloud to be traced. The results strongly support the theoretical models and show conclusively that the incident far-ultraviolet radiation field, not shocks as has sometimes been proposed, is responsible for the emission in the Orion Bar.

Ice and minerals on Callisto: A reassessment of the reflectance spectra

Abstract Laboratory spectral reflectance measurements, including the 3 μm H2O fundamental region, of particulate mixtures of both hydrated silicates and palagonite with water ice are presented. In all cases, the reflectances of these mixtures, at and beyond 3 μm, are dominated by absorptions due to water ice until ≥50 weight percent of the non-ice component is present. The laboratory measurements are compared to two previously unpublished reflectance spectra of Callisto. This comparison provides direct support for previous suggestions that the measured reflectance of Callisto contains a significant spectral contribution from a non-ice component, especially at and beyond 3 μm. Theoretical modeling, using Hapkes (1981, J. Geophys. Res. 86, 3039–3054; 1986, Icarus, 67, 264–280) equations, is used to more thoroughly analyze the telescopic data. The modeling considers homogeneous (intimate) and heterogeneous (areal) mixtures of both three components (ice, serpentine, and magnetite) and two components (ice and serpentine or magnetite) as a function of the relative particle size and abundance of the individual components. Comparison of the resultant calculations to the measured reflectance of Callisto imply that (1) three component mixtures of ice-magnetite-serpentine provide a better comparison to the telescopic data than any two component mixtures of ice and serpentine or magnetite; (2) the ice-magnetite-serpentine intimate mixtures which provide the best comparisons to the telescopic spectra indicate that the water ice contributes more to the spectrum of the leading hemisphere than the trailing hemisphere of Callisto; (3) all ice-magnetite mixtures fail to provide the spectral shape exhibited by the telescopic spectra of Callisto at and beyond 3 μm, indicating that the non-ice component possesses its own characteristic spectral features in this region.

The infrared spectrum of the Galactic center and the composition of interstellar dust.

We have obtained 5-8 micrometers spectra of the Galactic center from the Kuiper Airborne Observatory at resolving powers of approximately 50, approximately 150, and approximately 300. These spectra show absorption features at 5.5, 5.8, 6.1, and 6.8 micrometers. Together with previously observed features in the 3 micrometers region, these features are compared with laboratory spectra of candidate materials. The 3.0 and 6.1 micrometers features are due to the OH stretching and bending variations of H2O and are well fitted by water of hydration in silicates (e.g., talc). The 3.0 micrometer band is equally well fitted by ice mixtures containing 30% H2O, but such mixtures do not provide a good fit to the observed 6.1 micrometer band. The 3.4 and 6.8 micrometers features are identified with the CH stretching and deformation modes in CH2 and CH3 groups in saturated aliphatic hydrocarbons. The 6.1 micrometer band shows a short wavelength shoulder centered on 5.8 micrometer, attributed to carbonyl (C double bond O) groups in this interstellar hydrocarbon dust component. Finally, the narrow 5.5 micrometer feature is also attributed to carbonyl groups, but in the form of metal carbonyls [e.g., Fe(CO)4]. We have derived column densities and abundances along the line of sight toward the Galactic center for the various identified dust components. This analysis shows that hydrocarbon grains contain only 0.08 of the elemental abundance of C and contribute only a relatively minor fraction (0.1) of the total dust volume. Most of the interstellar dust volume is made up of silicates (approximately 0.6). Small graphite grains, responsible for the 2200 angstroms bump, account for 0.07 of the total dust volume. The remaining one-quarter of the interstellar dust volume consists of a material(s) without strong IR absorption features. Likely candidates include large graphite grains, diamonds, or amorphous carbon grains, which all have weak or no IR active modes. Finally, various models for the origin of the hydrocarbon dust component of the interstellar dust are discussed. All of them face some problems in explaining the observations, in particular, the absence of the spectroscopic signature of hydrocrbon grains in sources associated with molecular clouds.

Direct Spectroscopic Evidence for Ionized Polycyclic Aromatic Hydrocarbons in the Interstellar Medium

Long-slit 8-13 micrometers spectroscopy of the nebula around NGC 1333 SVS 3 reveals spatial variations in the strength and shape of emission features that are probably produced by polycyclic aromatic hydrocarbons (PAHs). Close to SVS 3, the 11.2 micrometers feature develops an excess at approximately 10.8-11.0 micrometers and a feature appears at approximately 10 micrometers. These features disappear with increasing distance from the central source, and they show striking similarities to recent laboratory data of PAH cations, providing the first identification of emission features arising specifically from ionized PAHs in the interstellar medium.

Io: An Intense Brightening Near 5 Micrometers

Spectrophotometric observations of the jovian satellite Io on 20 and 21 February 1978 (Universal Time) were made from 1.2 to 5.4 micrometers. Ios brightness at 4.7 to 5.4 micrometers was found to be three to five times greater at an orbital phase angle of 68� than at orbital phase angles of 23� (5.5 hours before the brightening) and 240� (20 hours after the brightening). Since the 5-micrometer albedo of Io is near unity under ordinary conditions, the observed transient phenomenon must have been the result of an emission mechanism. Although several such mechanisms were examined, the actual choice is not clear.

Sub-Astronomical Unit Structure of the Near-Infrared Emission from AB Aurigae

We present near-infrared, long-baseline, interferometric observations of AB Aurigae, a well-known Herbig Ae/Be star, obtained with the Infrared Optical Telescope Array. The near-infrared emission from this source has been spatially resolved for the first time, and the observations indicate a characteristic diameter of 5 mas for the emission, corresponding to 0.7 AU at the distance of AB Aur. The data appear to be most consistent with models in which the circumstellar material around AB Aurigae lies in a flattened structure with a central large hole or optically thin region of radius about 0.3 AU.

The 2.5–5.0 μm spectra of Io: Evidence for H2S and H2O frozen in SO2

Infrared spectra of Io in the region 2.5-5.0 micrometers, including new observational data, are analyzed using detailed laboratory studies of plausible surface ices. Besides the absorption bands attributable to sulfur dioxide frosts, four infrared spectral features of Io are shown to be unidentified. These unidentified features show spatial and temporal band strength variations. One pair is centered around 3.9 micrometers (3.85 and 3.91 micrometers) and the second pair is centered around 3.0 micrometers (2.97 and 3.15 micrometers). These absorptions fall close to the fundamental stretching modes in H2S and H2O, respectively. The infrared absorption spectra of an extensive set of laboratory ices ranging from pure materials, to binary mixtures of H2S and H2O (either mixed at different concentrations or layered), to H2O:H2S:SO2 mixtures are discussed. The effects of ultraviolet irradiation (120 and 160 nm) and temperature variation (from 9 to 130 K) on the infrared spectra of the ices are examined. This comparative study of Io reflectance spectra with the laboratory mixed ice transmission data shows the following: (1) Ios surface most likely contains H2S and H2O mixed with SO2. The 3.85- and 3.91-micrometers bands in the Io spectra can be accounted for by the absorption of the S-H stretching vibration (nu 1) in H2S clusters and isolated molecules in an SO2-dominated ice. The weak 2.97- and 3.15-micrometers bands which vary spatially and temporally in the Io spectra coincide with the nu 3 and nu 1 O-H stretching vibrations of clusters of H2O molecules complexed, through hydrogen bonding and charge transfer interactions, with SO2. (2) The observations are well matched qualitatively by the transmission spectra of SO2 ices containing about 3% H2S and 0.1% H2O which have been formed by the condensation of a mixture of the gases onto a 100 K surface. (3) No new features are produced in the region 2.5 to 5.0 micrometers in the spectrum of these ices under prolonged ultraviolet irradiation or temperature variation up to 120 K. (4) Comparison of the Io spectra to transmission spectra of both mixed molecular ices and layered ices indicates that only the former can explain the shifts and splitting of the absorption bands seen in the Io spectrum and additionally can account for the fact that solid H2S is observed in the surface material of Io at temperature and pressure conditions above the sublimation point of pure H2S.

Emission features in the 4-13 micron spectra of the reflection nebulae NGC 7023 and NGC 2023

Spectroscopy from 4 to 13 microns of the visual reflection nebulae NGC 7023 and NGC 2023 has been obtained. These data, together with previous work from 1 to 4 microns, show that the spectra of these sources consist of a relatively flat continuum from 1 to 13 microns and six emission features at 3.3, 3.4, 6.2, 7.7, 8.6, and 11.3 microns. The observations rule out equilibrium thermal emission for the features and continuum in reflection nebulae, and point toward a nonequilibrium emission mechanism such as thermal emission from thermally fluctuating small grains or fluorescence from large molecules. The similarity of the emission feature spectra in reflection nebulae to those in other sources suggests a universal emission mechanism, thus implying nonequilibrium emission mechanisms in other sources.

Absorption features in the 5-8 micron spectra of protostars

High signal-to-noise ratio 5-8 ..mu..m spectra of four sources embedded in molecular clouds are presented. All four sources show evidence for the presence of absorption features. The shape of these features changes, however, dramatically from source to source. They range from two relatively narrow bands at 6.0 and 6.8 ..mu..m in W33A to a broad, shallow feature, which extends from about 5.2 to 7.8 ..mu..m and shows some structure, in Mon R2-IRS2, BN, and NGC 2264.

Observation of the nu-squared band of PH3 in the atmosphere of Saturn

Observation of the