John A. Hackwell

The Silicates in the Disk of beta Pictoris

We have obtained intermediate-resolution (R≃50) infrared (2.6-13.5 μm) spectra of the particles in the circumstellar disk of β Pic. The silicate dust feature near 10 μm is broader and contains more structure than interstellar and most circumstellar emission features. The silicate feature in β Pic is remarkably similar to those in comets Halley, Bradfield 1987s, and Levy 1990 XX which have emission features characteristic of crystalline silicates. This result supports the inference based on IRAS results that cometary bodies resupply the grains in the β Pic disk. Detailed models of the dust disk and grains are used to derive plausible disk temperature and density gradients

First use of an airborne thermal infrared hyperspectral scanner for compositional mapping

In May 1999, the airborne thermal infrared hyperspectral imaging system, Spatially Enhanced Broadband Array Spectrograph System (SEBASS), was flown over Mormon Mesa, NV, to provide the first test of such a system for geological mapping. Several types of carbonate deposits were identified using the 11.25-μm band. However, massive calcrete outcrops exhibited weak spectral contrast, which was confirmed by field and laboratory measurements. Because the weathered calcrete surface appeared relatively smooth in hand specimen, this weak spectral contrast was unexpected. Here we show that microscopic roughness not readily apparent to the eye has introduced both a cavity effect and volume scattering to reduce spectral contrast. The macroroughness of crevices and cobbles may also have a significant cavity effect. The diminished spectral contrast is important because it places higher signal-to-noise ratio (SNR) requirements for spectroscopic detection and identification. This effect should be factored into instrumentation planning and interpretations, especially interpretations without benefit of ground truth. SEBASS had the required high SNR and spectral resolution to allow us to demonstrate for the first time the ability of an airborne hyperspectral thermal infrared scanner to detect and identify spectrally subtle materials.

Infrared colors and the diffuse interstellar bands

Broad-band infrared photometric measurements have been gathered for 105 stars which exhibit diffuse interstellar bands in their spectra. All normal stars obey a single reddening law, and a value of R equal to 3.08 + or - 0.15 is derived. This value is consistent with other recent determinations of R. The diffuse band indicators, the central depth of the 4430-A feature and the equivalent widths of the 5780-A and 6284-A features, show as large a scatter with the infrared color excesses as they do with E(B - V). No single-valued relation between the color excesses and the diffuse band strengths appears to exist. This casts doubt on whether dust grains which produce the visual and infrared extinctions are the carriers for the diffuse interstellar features.

Infrared Spectroscopy of Ultracompact H II Regions

We present intermediate-resolution (λ/Δλ ≈ 60) spectra of 21 ultracompact H II regions in the spectral range from 3 to 13 μm. The 9.7 μm silicate feature is seen in absorption, and the 12.8 μm [Ne II] fine structure line is seen in emission toward most of the observed nebulae. The strengths of both features vary enormously from nebula to nebula, suggesting large variations in the column densities of both Ne II and silicates toward these objects. Near-IR features attributed to polycyclic aromatic hydrocarbons (PAHs) are detected in six of the sources. Spherically symmetric dust shell models were calculated to obtain the best fits to those nebulae for which distances are known, and spectral energy distributions are available in the range of 1 mm to 1 μm. The models are used to infer properties of the dust cocoon such as the distribution of density and temperature with radius, shell thickness, outer shell radius, and dust abundances. Our results are consistent with previous models that predict large dust cocoons with central cavities, sharp temperature gradients, and approximately constant density in the outer regions.

Compact prism spectrograph suitable for broadband spectral surveys with array detectors

We describe a novel design for a low-resolution spectrograph that is capable of covering the entire 2.9-13.5 μm region, without scanning, at a resolving power of 20-100. The spectrograph uses two unique curved prisms to disperse radiation onto two 58-element arrays of infrared detectors. It has no moving parts. This spectrograph is an effective replacement for scanning circular variable filter (CVF) spectrometers or for Michelson Interferometers working at low resolution. Because all of the detectors in the spectrograph view an object through the same aperture, time-varying sources introduce no ambiguities into the spectra. The use of BIB detector arrays gives the instrument high sensitivity. We describe a ground-based version of the spectrograph that is currently under construction.

The evolution of the dust shell of Nova Serpentis 1978

Photometric measurements of Nova Serpentis 1978 from V to 19.5 microns covering the first 240 days of the novas development are reported. A free-free expansion phase was followed by the condensation of a dust shell that became optically thick at both visible and infrared wavelengths. Carbon grains grew to radii of approximately 0.3 microns during the condensation period. The grains appeared to sputter or evaporate for a period of approximately 100 days following infrared maximum. It is found that the condensed carbon comprised 2% of the ejected mass, which implies that carbon abundance was enhanced in the nova shell. The angular expansion rate of the shell yields a distance of approximately 5 kpc and a luminosity of approximately 3000 solar luminosities for Nova Serpentis 1978.

Low-resolution array spectrograph for the 2.9- to 13.5-um spectral region

This paper describes the optical system and the electronics of a newly developed low-resolution IR spectrograph, designed for ground-based and airborne observations. The spectrograph covers the entire 2.9- to 13.5-micron spectral region simultaneously, without scanning, at a nominal resolving power of 50 and a minimum resolving power of 20. The new spectrograph equals in spectral coverage to circular variable filter spectrometers that contain three filter segments. Because all of the detectors view the source through the same aperture, telescope tracking errors do not result in spectral ambiguities such as those that can arise in scanning spectrometers.

8- to 13 μm spectroscopy of comet Levy 1990 XX

Abstract Comet Levy 1990 XX was observed from Aug. 12 to Aug. 15, 1990, UT before perihelion with the 3-m Infrared Telescope Facility (IRTF) and the Aerospace Broadband Array Spectrograph System (BASS) using a 3.7-arcsec-diameter aperture. The spectra showed strong, structured silicate emission with peaks near 9.8 and 11.2 μm that are similar to those displayed by Comets P/Halley and Bradfield 1987 XXIX (1987s). The continuum was well fit by a graybody whose temperature is 262–277 K, significantly above the radiative equilibrium blackbody temperature of 223 K for the comets heliocentric distance of 1.54 AU. This temperature excess requires the silicate grains to be small, less than or about 1 μm radius.

The optically thin dust shell of Nova Cygni 1978

Nova Cygni 1978 was monitored photometrically from V to 19.5 microns for 120 days after the eruption. Following the initial expansion of the hot gas shell, an optically thin dust shell formed and reached a maximum visual optical depth of about 0.1 by day 60. No visible transition phase of the type observed in the very dusty DQ Herculis novae occurred in Nova Cygni 1978. It is argued that dust grain growth was inhibited because of the low mass of condensable atoms in the shell. Although the dust shell in Nova Cygni 1978 was optically thin at visual and infrared wavelengths, the grains grew to a radius of 0.3 micron which is comparable to the radius of the grains which grow in the optically thick shells of the DQ Herculis type novae.

An infrared study of Orion Molecular Cloud-2 (OMC-2)

This paper reports 1.2-23 micron photometry for 11 discrete sources in Orion Molecular Cloud-2 (OMC-2). These data, combined with H and K photometric and K polarimetric images, are used to model the cluster sources. Most appear to be young stars of roughly solar mass. Some have circumstellar dust reradiation or reflection nebulosity. A model based on single scattering of light from an exciting star explains some features of the IRS 1 nebula, the largest reflection nebula in OMC-2. However, the red colors and high surface brightness of the IRS 1 nebula require a cool excitation source that is more luminous than far-infrared observations would indicate. 34 refs.