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Mid-infrared (mid-IR) spectra from ~5 to 14 μm of five, nearby (< 70 Mpc) elliptical galaxies are presented that were observed with the Infrared Spectrograph on the Spitzer Space Telescope. The sample galaxies have a main stellar component that is typical for normal, passively evolving ellipticals; however, they are rich in cold gas and dust and have morphological-merger signatures from which a time order of the galaxies since the merger or accretion events can be estimated. The presented results are significant because (1) emission due to Polycyclic Aromatic Hydrocarbons (PAHs) and associated species is detected for the first time in these galaxies and (2) the detected mid-IR spectra are independently exploited as a probe of current or recent star-formation that, in this case, is assumed to be triggered by the merger. As shown in exemplary spectra of the early-age merger NGC 3656, the strength of the PAH emission is more centrally peaked in the earlier-age mergers, suggesting that the PAH data are indeed probing star-formation that is correlated with the time since the mergers and systematically depletes the centrally located gas, becoming weaker and more flatly distributed as the merger evolves.

Early-type galaxies exhibit thermal and molecular resonance emission from dust that is shed and heated through stellar mass loss as a subset of the population moves through the AGB phase of evolution. Because this emission can give direct insight into stellar evolution in addition to galactic stellar mass loss and ISM injection rates, we conducted a program to search for this signature emission with CAM on ISO. We obtained 6-15 micron imaging observations in six narrow bands for nine elliptical galaxies; every galaxy is detected in every band. For wavelengths shorter than 9 microns, the spectra are well matched by a blackbody, originating from the K and M stars that dominate the integrated light of elliptical galaxies. However, at wavelengths between 9 and 15 microns, the galaxies display excess emission relative to the stellar photospheric radiation. Additional data taken with the fine resolution circular variable filter on one source clearly shows broad emission from 9 to 15 microns, peaking around 10 microns. This result is consistent with the known, broad silicate feature at 9.7 microns, originating in the circumstellar envelopes of AGB stars. This emission is compared with studies of Galactic and LMC AGB stars to derive cumulative mass loss rates. In general, these mass loss rates agree with the expected ~0.8 solar masses per year value predicted by stellar evolutionary models. Both the photospheric and circumstellar envelope emission follow a de Vaucouleurs' R^{1/4} law, supporting the conclusion that the mid-infrared excess emission originates in the stellar component of the galaxies and acts as a tracer of AGB mass loss and mass injection into the ISM.

One of the major criticisms of identifying the infrared emission bands with polycyclic aromatic hydrocarbon (PAH) molecules has been the lack of a match between laboratory spectra of individual PAHs and the emission features. Part of the difficulty arises from the complexity of modeling the emission mechanism with an a priori unknown mixture of ionized and neutral PAHs. A direct comparison between laboratory spectra of PAHs and astronomical sources is possible for absorption spectra. However, because of poor atmospheric transmission, ground-based spectra of the PAH absorption band in the C-H stretch region are too noisy to make a detailed comparison with laboratory spectra. In this paper we show that ISO Short Wavelength Spectrometer spectra of a few protostars that show a 3.25 μm absorption band can be well matched by laboratory absorption spectra of a mixture of isolated PAHs.

Interpretation of the infrared emission bands is difficult because these bands likely arise from a mixture of ionized and neutral polycyclic aromatic hydrocarbon (PAH) molecules. Modeling the emission process is also difficult because the size distribution of the molecules, their ionization state and excitation, and the geometry of the emitting regions are generally unknown. If these molecular bands could be found in absorption, many of these factors would be eliminated, making it much easier to interpret the data. We have discovered an absorption band in Monoceros R2 IRS 3 centered at 11.25 μm that we identify with a C–H out-of-plane vibrational mode of PAH molecules. The shape and position of the band are very similar to that seen in emission in planetary nebulae, H II regions, and the interstellar medium.

Infrared spectra of two celestial objects frequently used as flux standards are calibrated against an absolute laboratory flux standard at a spectral resolving power of 100 to 200. The spectrum of the K1.5III star, alpha Boo, is measured from 3 microns to 30 microns and that of the C-type asteroid, 1 Ceres, from 5 microns to 30 microns. While these 'standard' spectra do not have the apparent precision of those based on calculated models, they do not require the assumptions involved in theoretical models of stars and asteroids. Specifically they provide a model-independent means of calibrating celestial flux in the spectral range from 12 microns to 30 microns where accurate absolute photometry is not available. The agreement found between the spectral shapes of alpha Boo and Ceres based on laboratory standards, and those based on observed ratios to alpha CMa (Sirius) and alpha Lyr (Vega), flux calibrated by theoretical modeling of these hot stars strengthens our confidence in the applicability of the stellar models as primary irradiance standards.

Hydrogen-poor planetary nebulae (PNe) Abell 58, Abell 78, and IRAS 15154-5258 were observed with the ISOPHOT low-resolution spectrometer on board the Infrared Space Observatory. Polycyclic aromatic hydrocarbon (PAH) features, commonly seen in H II regions, PNe, and in the diffuse interstellar medium, are absent from their infrared (IR) spectra. A 6.4 μm dust feature is present instead in the IR spectra of Abell 78 and IRAS 15154-5258. We identify this feature as an aromatic C—C stretch feature produced in carbonaceous grains with little or no hydrogen. The 6.4 μm feature is absent in the IR spectrum of the hydrogen-poor material in PN A58, which was ejected nearly 90 yr ago. Our observations of all three PNe are consistent with the hypothesis that the 6.4 μm C—C stretch feature is produced by small, H-free carbonaceous grains, transiently heated to high temperatures by energetic stellar photons. The 6.4 μm feature is not detected in A58 because of the presence of strong IR continuum produced by large, hot grains located close to its central star.

We describe our efforts to seek "closure" in our infrared absolute calibration scheme by comparing spectra of asteroids, absolutely calibrated through reference stars, with "Standard Thermal Models" and "Thermophysical Models" for these bodies. Our use of continuous 5-14 microns airborne spectra provides complete sampling of the rise to, and peak, of the infrared spectral energy distribution and constrains these models. Such models currently support the absolute calibration of ISO-PHOT at far-infrared wave- lengths (as far as 300 microns), and contribute to that of the Mid-Infrared Spectrometer on the "Infrared Telescope in Space" in the 6-12 microns region. The best match to our observed spectra of Ceres and Vesta is a, standard thermal model using a beaming factor of unity. We also report the presence of three emissivity features in Ceres which may complicate the traditional model extrapolation to the far-infrared from contemporaneous ground-based N-band photometry that is used to support calibration of, for example, ISO-PHOT. While identification of specific materials that cause these features is not made, we discuss families of minerals that may be responsible.

The mean ages of early-type galaxies obtained from the analysis of optical spectra, give a mean age of 8 Gyr at z = 0, with 40% being younger than 6 Gyr. Independent age determinations are possible by using infrared spectra (5-21 microns), which we have obtained with the Infrared Spectrograph on the Spitzer Observatory. This age indicator is based on the collective mass loss rate of stars, where mass loss from AGB stars produces a silicate emission feature at 9-12 microns. This feature decreases more rapidly than the shorter wavelength continuum as a stellar population ages, providing an age indicator. From observations of 30 nearby early-type galaxies, 29 show a spectral energy distribution dominated by stars and one has significant emission from the ISM and is excluded. The infrared age indicators for the 29 galaxies show them all to be old, with a mean age of about 10 Gyr and a standard deviation of only a few Gyr. This is consistent with the ages inferred from the values of M/L_B, but is inconsistent with the ages derived from the optical line indices, which can be much younger. All of these age indicators are luminosity-weighted and should be correlated, even if multiple-age components are considered. The inconsistency indicates that there is a significant problem with either the infrared and the M/L_B ages, which agree, or with the ages inferred from the optical absorption lines., Accepted for publication in ApJ

An automatic interferometer fringe tracking system has been developed, implemented, and tested at the Infrared Optical Telescope Array (IOTA) Observatory at Mount Hopkins, Arizona. The system can minimize the optical path differences (OPDs) for all three baselines of the Michelson stellar interferometer at IOTA. Based on sliding window discrete Fourier-transform (DFT) calculations that were optimized for computational efficiency and robustness to atmospheric disturbances, the algorithm has also been tested extensively on offline data. Implemented in ANSI C on the 266 MHz PowerPC processor running the VxWorks real-time operating system, the algorithm runs in approximately milliseconds per scan (including all three interferograms), using the science camera and piezo scanners to measure and correct the OPDs. The adaptive DFT-based tracking algorithm should be applicable to other systems where there is a need to detect or track a signal with an approximately constant-frequency carrier pulse. One example of such an application might be to the field of thin-film measurement by ellipsometry, using a broadband light source and a Fourier-transform spectrometer to detect the resulting fringe patterns.

An automatic fringe tracking system has been developed and implemented at the Infrared Optical Telescope Array (IOTA). In testing during May 2002, the system successfully minimized the optical path differences (OPDs) for all three baselines at IOTA. Based on sliding window discrete Fourier transform (DFT) calculations that were optimized for computational efficiency and robustness to atmospheric disturbances, the algorithm has also been tested extensively on off-line data. Implemented in ANSI C on the 266 MHz PowerPC processor running the VxWorks real-time operating system, the algorithm runs in approximately 2.0 milliseconds per scan (including all three interferograms), using the science camera and piezo scanners to measure and correct the OPDs. Preliminary analysis on an extension of this algorithm indicates a potential for predictive tracking, although at present, real-time implementation of this extension would require significantly more computational capacity.

The Astrobiology Explorer (ABE) is a MIDEX mission concept, currently under Concept Phase A study at NASA's Ames Research Center in collaboration with Ball Aerospace & Technologies, Corp., and managed by NASA's Jet Propulsion Laboratory. ABE will conduct infrared spectroscopic observations to address important problems in astrobiology, astrochemistry, and astrophysics. The core observational program would make fundamental scientific progress in understanding the distribution, identity, and evolution of ices and organic matter in dense molecular clouds, young forming stellar systems, stellar outflows, the general diffuse ISM, HII regions, Solar System bodies, and external galaxies. The ABE instrument concept includes a 0.6 m aperture Ritchey-Chretien telescope and three moderate resolution (R = 2000-3000) spectrometers together covering the 2.5-20 micron spectral region. Large format (1024 x 1024 pixel) IR detector arrays will allow each spectrometer to cover an entire octave of spectral range per exposure without any moving parts. The telescope will be cooled below 50 K by a cryogenic dewar shielded by a sunshade. The detectors will be cooled to approx. 7.5 K by a solid hydrogen cryostat. The optimum orbital configuration for achieving the scientific objectives of the ABE mission is a low background, 1 AU Earth driftaway orbit requiring a Delta II launch vehicle. This configuration provides a low thermal background and allows adequate communications bandwidth and good access to the entire sky over the approx. 1.5 year mission lifetime.

We present far-IR ISO observations of three early-type galaxies in the Virgo cluster. The data were recorded using the ISOPHOT instrument in both the P32 oversampled maps and the P39/37 sparse maps. The maps reach the limiting sensitivity of the ISOPHOT instrument at 60, 90, and 180$\mu$m. Two of the most prominent elliptical galaxies in Virgo -- NGC 4472 and NGC 4649 -- clearly show no emission at far-IR wavelengths at a level of few tens of mJy, but NGC 4636 is detected at all three wavelengths. We have computed the far-IR emission from dust for NGC4472 and NGC4636 under the assumption that dusty outflows from evolving red giant stars are continuously supplying dust to the interstellar medium and that the grains, once diffused into the interstellar medium, are sputtered away by collisions with ions. While the calculated fluxes are consistent with the observed upper limits for NGC4472, the dust emission detected in NGC4636 support the hypothesis that additional dust has been accreted in a very recent ($\lta$ few $10^8$ yrs) merger with a dusty, gas-rich galaxy., Comment: Accepted for publication in ApJL

In this paper we describe a potential new Explorer-class space mission, the AstroBiology Explorer (ABE), consisting of a relatively modest dedicated space observatory having a 50 cm aperture primary mirror which is passively cooled to T less than 65 K, resides in a low-background orbit (heliocentric orbit at 1 AU, Earth drift-away), and is equipped with a suite of three moderate order (m approx. 10) dispersive spectrographs equipped with first-order cross-dispersers in an "echellette" configuration and large format (1024xl024 pixel) near- and mid-IR detector arrays cooled by a modest amount of cryogen. Such a system would be capable of addressing outstanding problems in Astrochemistry and Astrophysics that are particularly relevant to Astrobiology and addressable via astronomical observation. The observational program of this mission would make fundamental scientific progress in each of the key areas of the cosmic history of molecular carbon, the distribution and chemistry of organic compounds in the diffuse and dense interstellar media, and the evolution of ices and organic matter in young planetary systems. ABE could make fundamental progress in all of these areas by conducting an approximately one year mission to obtain a coordinated set of infrared spectroscopic observations over the 2.5-20 micrometers spectral range at spectral resolutions of R greater than or equal to 1000 of approximately 1000 galaxies, stars, planetary nebulae, and young star planetary systems.

The third telescope project to enable phase-closure observations at the IOTA interferometer is well underway, and is anticipated to be completed later this year. For this project, we present the main technical improvements which we have already made or expect to make, including a new VxWorks control system, improved star acquisition cameras, improved siderostat and primary mirror supports, five-axis control of the telescope secondary mirrors, automated control of the long delay line, trihedral retroreflectors, three-beam combination, the PICNIC camera, and fringe packet tracking.

The first tests of an infrared fringe-tracker prototype for the IOTA interferometer (Mount-Hopkins, Arizona) were carried out during 1999. The aim of this real-time system is to minimize the optical path difference (OPD) fluctuations between the two beams such that interference fringes (obtained in J, K, or L band) can always be observed within the `scan window' given by the instrument. After an introduction of the employed technology (hardware and software), we present results obtained from star observations. Finally, we discuss the possibility of improving the fringe-tracker by prediction of the OPD, using statistical properties of its fluctuations. The improvement of the fringe-tracker already existing for the FLUOR recombiner is discussed as well.

Polycyclic aromatic hydrocarbon (PAH) emission in the Spitzer Infrared Spectrograph spectral map of the northwest photon dominated region (PDR) in NGC 7023 was analyzed exclusively using PAH spectra from the NASA Ames PAH IR Spectroscopic Database (www.astrochem.org/pahdb). The 5-15 μm spectrum at each pixel is fitted using a non-negative-least-squares fitting approach. The fits are of good quality, allowing decomposition of the PAH emission into four subclasses: size, charge, composition, and hydrogen adjacency (structure). Maps tracing PAH subclass distributions across the region paint a coherent astrophysical picture. Once past some 20 seconds of arc from HD 200775, the emission is dominated by the more stable, large, symmetric, compact PAH cations with smaller, neutral PAHs taking over along the lines-of-sight toward the more distant molecular cloud. The boundary between the PDR and the denser cloud material shows up as a distinct discontinuity in the breakdown maps. Noteworthy is the requirement for PANH cations to fit the bulk of the 6.2 and 11.0 μm features and the indication of PAH photo-dehydrogenation and fragmentation close to HD 200775. Decomposition of the spectral maps into "principal" subclass template spectra provides additional insight into the behavior of each subclass. However, the general applicability of this computationally more efficient approach is presently undetermined. This is the first time the spectra of individual PAHs are exclusively used to fit the 5-15 μm region and analyze the spatial behavior of the aromatic infrared bands, providing fundamental, new information about astronomical PAH subpopulations including their dependence on, and response to, changes in local conditions.

NASA Ames Research Center and Lick Observatory have jointly developed two prototype infrared camera for astronomical imaging. The new cameras have been built using Amber Engineering Inc. 128 X 128 SiGa and InSb focal plane arrays. We report observations of the BN-KL source complex in M42 and HD44179 which are used as a demonstration of the cameras' performance when used as narrow band spectral images. The images have a dynamic range in excess of 10 3 . The high signal to noise ratio and linearity of the images are used to demonstrate the power of spectral differencing as a tool for astronomical image enhancement at mid infrared wavelengths. The observations were made over a range of wavelengths 3.1 - 3.3 micrometers and 8.4 - 11.5 microns in and out of the silicate absorption band, and in the PAH emission bands at 3.3 micrometers and 11.3 micrometers using approximately 2% spectral resolution. Bell and Crisp, 1991, have reported spatial resolution enhancement of images for the surface of Mars by division of multispectral images in and out of the H 2 O absorption feature at 3 micrometers . We find that a similar image processing technique using normalized image differencing can be used in the 10 micrometers silicate absorption feature and PAH emission bands to improve the contrast in mid-infrared astronomical images. This technique is expected to be applicable to imaging of galactic nuclei and other astronomical objects as well as embedded stellar infrared sources.

High-resolution images in [O III] λ5007 of the hydrogen-poor knots of Abell 30 reveal comet-like structures which may be indicative of interaction with the stellar wind. In the near IR, new, higher-resolution, K-band images show an equatorial ring of hot dust that corresponds closely to optical knots 2 and 4 of Jacoby and Ford, while their polar knots 1 and 3 show no comparable IR. emission. Both the thermal IR emission and the heavy internal extinction of the central star demands an extremely dusty ejecta. Greenstein showed that the UV extinction curve is fit by amorphous carbon. Our comprehensive dust models consider both the UV extinction and the IR emission from a population of carbon grains. The thermal emission from larger grains produces the far IR emission, while the stochastic heating of very small grains to high temperatures is essential to explain the near IR flux. We are able to reproduce the shape of the near IR spectrum with an a-3.0 distribution of grain radii which extends down to a minimum grain radius of 8 A.

We report near-infrared spectroscopy of three Seyfert galaxies. Velocity resolved spectra covering low excitation-potential transitions of [Ar III] 8.991 µm, [S IV] 10.514 µm, and [Ne II] 12.813 µm were obtained using the facility mid-infrared array spectrometer (SpectroCam) of the Palomar Observatory Hale 5-m Telescope, and low-resolution spectra covering [Mg V] 5.608 µm, [Ar II] 6.985 µm, and [Ne VI] 7.642 µm were obtained using the faint-object spectrograph (HIFOGS) of the Kuiper Airborne Observatory (KAO).We find high contrast [Ar III] and [S IV] in each galaxy, and [Ne II] in NGC 1068 and NGC 4151 (12.8 µm data were not obtained on NGC 5506). The line profiles are well resolved. In NGC 1068 and NGC 4151, they are fit by singlecomponent Gaussians. However, in NGC 5506, evidence for a broad pedestal is seen in the [Ar III] and [S IV] spectra, similar to the broad Paβ component reported by Blanco et al. (1990) and Rix et al. (1990).

New 4 to 8 micron infrared spectroscopic observations of two oxygen-rich stars are presented and combined with IRAS LRS data to span the 4 to 24 micron wavelength range. In the 4 to 8 micron wavelength range, we observe a 7.15 micron (1400/cm) emission feature. The new feature at 7.15 microns is not uniquely correlated with any of the sharply defined 10, 11, 13.1, and 19.7 micron emission features that are known to be present in this class of circumstellar shells, but it does not appear to be correlated with the spectrally broad dust emission in the 10 to 20 micron spectral region. The feature has not been reported previously in any other astronomical environment. A reinterpretation of prior 4 to 8 micron spectroscopy of alpha Ori and R Cas reveals the presence of the 7.15 micron emission in alpha Ori and possibly in R Cas. The spectrally narrow 19.7 micron emission, that is distinctly diffeent than the relatively broad silicate 18 micron emmision feature in oxygen-rich dust shells, is also observed to be present in the LRS spectrum of SAO 197549. The implication of these observations is that a universal astronomical silicate does not exist in oxygen-rich circumstellar shells. This variety is analogous to that observed in interplanetary dust particles and may indicate an intimate relation between the classes of condensates.

Spectral images were obtained of the Orion Bar which sample polycyclic aromatic hydrocarbon (PAH) emission at 3.3, 8.4, and 11.3 micrometers. The images are strikingly different even though they all sample PAH emission. In particular, the 3.3 and 11.3 micrometers images sample PAH emission from C-H bonds, yet the 3.3 micrometers image contains many small bright knots while the 11.3 micrometers image is much more uniform. For comparison with a fluorescent PAH model, a data set was created from the measured intensities of 250 locations in each image. From the comparison, we conclude that: (1) the size distribution of PAHs varies within the Bar, with the bright 3.3 micrometers knots containing the largest proportion of small PAHs; (2) the points along the front of the Bar have emission cross sections characteristic of neutral PAHs while within the Bar, the emission cross sections are different, consistent with the PAHs being charged; (3) the PAHs along the front of the Bar are larger than average for the Bar; (4) emission along the back of the Bar is consistent with PAH emission in an attenuated UV radiation field; (5) there is no evidence for PAH dehydrogenation.

Spectrophotometric observations of SN 1987A from the Kuiper Airborne Observatory are presented for five epochs at 60, 260, 415, 615, and 775 days after the explosion. The low-resolution (lambda/Delta lambda = 50-100) spectra of SN 1987A are combined with data from other wavelengths to model the continuum, subtract the continuum from the spectra to determine line strengths and reveal molecular bands, separate the atomic continuum radiation from the dust continuum, and derive constraints on the grain temperatures and optical depths. A scenario for the evolution of SN 1987A and that of the ejecta from which it arises is obtained on the basis of the analysis of the continuum emission.

We present 3.3, 8.5, 10.0, 11.3, and 12.5 μm narrow-band (Δλ/λ ≃3%-10%) and K-band (2.2 μm) high spatial resolution images of IRAS 21282+5050 and a 6 cm flux density measurement. The new infrared images reveal a toroidal dust nebula, 4″.5×6″, P.A. 165°, with two prominent emission peaks aligned almost E-W and separated by 1″.7-2″.1; a remarkably similar structure to that observed in NGC 7027. From the 6 cm flux density and the dust nebula size, we derive an electron density of 3.6× 10 3 cm −3 for the central ionized region. Comparison of these images with a published CO 1-0 map suggests that the structure of this dusty, ionization-bounded planetary nebula may reflect the density inhomogeneities of the progenitor's wind

Images of HD 44179 (the Red Rectangle) obtained in the 3.3 and 11.3 micron emission bands show two different spatial distributions. The 3.3 micron band image is centrally peaked and slightly extended N-S while the 11.3 micron image shows a N-S bipolar shape with no central peak. If the 3.3 micron band image shows the intrinsic emission of the 11.3 micron band, then the data suggest absorption of the 11.3 micron emission near the center of HD 44179 by a disk with an optical depth of about one, making HD 44179 the first object in which the IR emission bands have been observed to be optically thick. Since there is no evidence of absorption of the 3.3 micron emission band by the disk, the absorption cross section of the 3.3 micron band must be substantially less than for the 11.3 micron band. Since the 3.3 and 11.3 micron bands are thought to arise from different size PAHs, the similar N-S extents of the two images implies that the ratio of small to large PAHs does not change substantially with distance from the center.

Spectral images in the 3.3 and 11.3 micron IR emission band around the star SVS 3 in NGC 1333 indicate that the two bands have their origin in different locations, rendering them useless in estimates of the sizes of the molecules emitting the bands. This is in keeping with the emission bands' generation by a mixture of variously sized polycyclic aromatic hydrocarbons (PAHs) whose smallest species have been dehydrogenated in the vicinity of SVS 3. Hot bands and overtone/combination bands may account for the 3.4-micron band, rather than molecular side-groups of the PAHs.

Multiaperture 3-4-micron spectra along with K- and L-band images of the compact planetary nebula IRAS 21282 + 5050 show a 5 arcsec - 20 arcsec diameter nebula with structure similar to many other planetary nebulae. The spectral observations and the L-band image show evidence for extended PAH emission out to a radius of 20 arcsec, while the K-band image shows a 5 arcsec diameter nebula. An observed linear increase of integrated brightness with aperture size at L band implies a 1/r exp 2 volume emissivity for a spherically symmetric model. The spectral similarity of the emission in the small and large apertures suggests fluorescent emission by the PAHs. If the observed emission is from PAHs which formed during the planetary nebulae stage of IRAs 21282 + 5050, then PAHs have been forming for not less than 3000 yr. If the PAH emission is from material produced during the earlier red giant phase, then the formation time frame was much longer. The morphological and spectral similarity of IRAS 21282 + 5050 to many other planetary nebulae suggests that this phenomenon may be widespread, and that planetary nebulae may be a significant source of interstellar PAHs.

Spectra of the Martian thermal emission in the 5.4-10.5 micron region are reported. Emission features at 7.8 and 9.7 microns are attributed to surface silicates, and an emission feature at 6.1 micron is attributed to a molecular water component of the surface material. An absorption band at 8.7 micron and a possible one at 9.8 microns is attributed to sulfate or bisulfate anions probably located at a distorted crystalline site, and an absorption band at 6.7 microns is attributed to carbonate or bicarbonate anions located in a distorted crystalline site. Spectral simulations indicate that the sulfate- and carbonate-bearing minerals are contained in the same particles of airborne dust as the dominant silicate minerals, that the dust optical depth is about 0.6 at a reference wavelength of 0.3 micron over the area of the observed spots, and that sulfates and carbonates constitute 10-15 percent and 1-3 percent by volume of the airborne dust, respectively.

We have attempted to incorporate the compositional information available from interplanetary dust particles (IDPs) and the recent Halley flybys into a coherent model to account for cometary infrared emission. We show that the entire infrared emission spectrum of Comet Halley is consistent with the presence of three general dust particle types. These dust types are (i) amorphous carbon grains, (ii) crystalline silicate grains, and (iii) crystalline silicate grains with thin carbonaceous coatings. In analogy with IDPs, the thin carbonaceous coatings on the silicate minerals may be responsible for the dark appearance of most comets. Good fits to the observational data are found for particle populations having particle size distributions similar to those inferred for Comet Halley, and silicate to amorphous carbon mass ratios of 8 to 40, values consistent with IDP and flyby data. Composite particles in which all the carbon resides in thick mantles on silicate cores fail to simultaneously match both the spectral and compositional constraints. Finally, the apparent strength of the 10-μm silicate feature is found to be particle size dependent. Consequently, as has been noted before, the absence of an obvious 10-μm feature in other comets does not imply the absence of silicates, but may instead imply the dominance of large silicate grains.

IR emission from the dust comet of Comet C-G was monitored at 1-20 microns with the 3-m NASA Infrared Telescope Facility at Mauna Kea, Hawaii during its perihelion passage from September 1982 to March 1983. The maximum dust production rate of approximately 200,000 g/sec was found to occur approximately one month after perihelion. It was also found that no obvious 1-micron silicate feature was present at 1.33 AU preperihelion, and that the ratio of dust/gas production was higher in other short-period comets observed in the infrared, particularly preperihelion. The low albedos derived for the dust grains (less than 0.05 micron) are noted to be plausible for mineral mixtures expected in cometary grains.

Advances in infrared spectrometers and the theory that PAHs are responsible for the infrared emission bands have led to a wealth of new information in recent years. High quality data have shown many weak emission bands which are diagnostic of the material producing the bands. While correlations of the strengths of the narrow bands indicate that a single material can account for all of the narrow bands, independent spatial behavior of the narrow and broad components show that they have different origins. Predictions of the behavior of the spectra based on laboratory data have been confirmed observationally, strengthening the theory that PAH molecules are the origin of the infrared emission bands.

We have investigated the continuum properties of the superluminal quasar 3C 345 with monitoring studies at radio, optical, infrared, and X-ray frequencies as well as with simultaneous multifrequency spectra extending from the radio through the X-ray bands. Variability occurs more rapidly and with greater amplitude toward shorter wavelengths in the infrared-optical region (0.4-100 μm). Radio outbursts, which appear to follow infrared-optical outbursts by about a year, occur first at the highest frequencies, as expected from optical depth effects, although the peak flux is often reached at several frequencies at once. The beginning of outbursts as defined by millimeter measurements corresponds to the appearance of the three known "superluminal" components. An increase in the X-ray flux during 1979-1980 corresponds to increased radio flux, while the infrared flux changes in the opposite sense. The multifrequency spectra show that the nearly flat radio continuum steepens at 10^(11)-10^(12) Hz and has a power-law slope of -0.91 ± 0.04 from 350 to 20 μm that steepens to -1.40 ± 0.02 at 20 μm-1200 A. A "blue bump" is detected at rest wavelengths 4000-1500 A. The X-ray emission has a flatter slope (-0.7) than the infrared-ultraviolet continuum and lies above an extrapolation of that continuum to X-ray energies. This supports the finding from variability that the X-ray emission is not simply connected to the optical emission. Although the shape of the infrared-ultraviolet continuum is generally preserved during flux variations, slope variations occur and are most common in the ultraviolet region. Most of the total power (3 x 10^(47) ergs s ^(-1)) emerges in the submillimeter-optical region but about one-sixth of the power is emitted at X-ray and radio.

A liquid helium-cooled, 24 detector grating spectrometer was developed and used for low resolution astronomical observations in the 5 to 14 micron spectral range. The instrument operated on the 91 cm Kuiper Airborne Observatory, the 3 m IRTF (Mauna Kea), the 3 m Shane telescope Observatory, the 3 m Shane telescope (Lick Observatory), and the 152 cm NASA and University of Arizona telescope. The detectors are discrete Si:Bi photoconductors with individual metal oxide semiconductor field effect transistor preamplifiers operating at 4 K. The system uses a liquid helium-cooled slit, order-sorter filter, collimator mirror, grating, and camera mirror arranged in a Czerny-Turner configuration with a cold stop added between the collimator mirror and the grating. The distances between components are chosen so that the collimator mirror images the secondary mirror of the telescope onto the cold stop, thus providing a very effective baffle. Scattered radiation is effectively reduced by using liquid helium-cooled, black baffles to divide the spectrometer into three separate compartments. The system noise-equivalent flux density, when used on the 152 cm telescope from 8 to 13 microns with a resolving power of 50, is 4.4 x 10 to the minus 17th power W/sq cm micron square root of Hz. The main applications are for measuring continuum radiation levels and solid state emission and absorption features in regions of star and planet formation.

Spectrophotometry from 5–10μm (Δλ/λ ≈ 0.02) of comet Halley was obtained from the Kuiper Airborne Observatory on 1985 December 12.1 and 1986 April 8.6 and 10.5, UT. 8–13 μm data were obtained on 17.2 December 1985 from the Nickel Telescope at Lick Observatory. The spectra show a strong broad emission band at 10 μm and a weak feature at 6.8 μm. We do not confirm the strong 7.5 μm emission feature observed by the Vega 1 spacecraft.

Infrared spectra of supernova 1987A taken in April and November 1987 are presented, showing two distinctly different stages in the evolution of the expanding gas shell. The optical and infrared spectrum in April originated from the hydrogen envelope and show weak hydrogen lines rising above a 5,000-K photospheric continuum. The November spectrum was dominated by strong emission lines from heavy elements as well as many lines from highly excited levels of hydrogen, with peak flux levels in the lines at or slightly above the level of the continuum in April. It is concluded that the inner regions of the supernova were just becoming visible in early 1988. It is expected that these regions contain heavy elements produced by advanced nuclear burning stages in the progenitor star and in the shock wave that ejected all material external to the iron core.

Airborne measurements obtained for the unidentified IR (UIR) 5-8 micron emission bands of eight planetaries, eight locations in five reflection nebulae, and seven locations in four H II regions (including the Orion Bar), are presently compared with existing and new ground-based observations of the 3.3, 8.7, and 11.3 micron bands. The good correlations found between the strengths of all pairs of bands lead to the conclusion that all seven UIR features form a 'generic spectrum', although there are significant variations in the relative strengths of the features among the sources. The fraction of total far-IR luminosity radiated by a planetary in the strongest UIR feature at 7.7 microns is strongly correlated with the nebular C/O ratio, strongly suggesting that hydrocarbons are the carriers of these features.

Structure has been observed in the 3-3.6 micron preperihelion spectrum of Comet Halley consistent with either an absorption band near 3.1 microns or emission near 3.3 microns. The results suggest that a large fraction of the water molecules lost by the comet are initially ejected in the form of small ice particles rather than in the gas phase.