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NASA NTRS1990

IRAS observations of a large circumstellar dust shell around W Hydrae

IRAS observations at 60 and 100 microns reveal a large 30-40-arcmin (about 1-pc) diameter dust shell centered on the oxygen-rich red giant W Hya. Except for SNRs, this is the largest mass-loss envelope, in apparent diameter, known around any evolved star, including PN. W Hya's radiation field, stronger than the interstellar radiation field in the outer envelope, is sufficient to heat dust grains with IR emissivity proportional to lambda exp -1.2 to temperatures of about 40 K implied by the ratio of intensities at 60 and 100 microns.

Hawkins, G. W.↗
NASA NTRS1990

Image reconstruction of IRAS survey scans

The IRAS survey data can be used successfully to produce images of extended objects. The major difficulties, viz. non-uniform sampling, different response functions for each detector, and varying signal-to-noise levels for each detector for each scan, were resolved. The results of three different image construction techniques are compared: co-addition, constrained least squares, and maximum entropy. The maximum entropy result is superior. An image of the galaxy M51 with an average spatial resolution of 45 arc seconds is presented, using 60 micron survey data. This exceeds the telescope diffraction limit of 1 minute of arc, at this wavelength. Data fusion is a proposed method for combining data from different instruments, with different spacial resolutions, at different wavelengths. Data estimates of the physical parameters, temperature, density and composition, can be made from the data without prior image (re-)construction. An increase in the accuracy of these parameters is expected as the result of this more systematic approach.

Bontekoe, Tj. Romke↗
NASA NTRS1990

Image construction from the IRAS survey and data fusion

The IRAS survey data can be used successfully to produce images of extended objects. The major difficulty, viz. non-uniform sampling, different response functions for each detector, and varying signal-to-noise levels for each detector for each scan, were resolved. The results of three different image construction techniques are compared: co-addition, constrained least squares, and maximum entropy. The maximum entropy result is superior. An image of the galaxy M51 with an average spatial resolution of 45 arc seconds, is presented using 60 micron survey data. This exceeds the telescope diffraction limit of 1 minute of arc, at this wavelength. Data fusion is a proposed method for combining data from different instruments, with different spatial resolutions, at different wavelengths. Direct estimates of the physical parameters, temperature, density and composition, can be made from the data without prior images (re-)construction. An increase in the accuracy of these parameters is expected as the result of this more systematic approach.

Bontekoe, Tj. R.↗
NASA NTRS1990

Peering beyond IRAS: The 100 to 350 micron dust emission from galaxies

Several arguments can be made to study the continuum emission from dust in galaxies at wavelengths between the cutoff of the Infrared Astronomy Satellite (IRAS) survey (about 100 microns) and the shortest wavelength that is commonly accessible from the ground (about 350 microns). Some theoretical work (see the summary by Cox and Mezger 1989) indicates that there are very cool (T sub d less than or equal to 25 K) components to the dust emission that emit primarily at wavelengths between 100 and 250 microns. In fact, a significant fraction of the total luminosity, representing a large fraction of the dust mass in some types of galaxies, is emitted at long far-infrared wavelengths. In such cases, the cool dust must play a major role in regulation of the energy balance of the Interstellar Medium (ISM) and in shielding the cores of neutral clouds.

Thronson, Harley A., Jr.↗
NASA NTRS1990

High spatial-resolution IRAS images of M51

High spatial-resolution (approx. equal to 30 seconds) images of M51 in the four Infrared Astronomy Satellite (IRAS) bands (12, 25, 60, and 100 microns) have been obtained. The spatial variation in flux in all four bands is coincident with the spiral features seen in H alpha and 6 cm with a few exceptions. In the nuclear region (4 minutes) the position of the peak of maximum intensity shifts in relation to the visual nucleus: it is coincident with the nucleus at 12 microns, shifts approximately 45 seconds to south-southwest, and is 45 seconds northwest of the nucleus at 60 and 100 microns.

Canterna, R.↗
NASA NTRS1989

Dust around Mira variables: An analysis of IRAS LRS spectra

The spatial extent and spectral appearance of the thin dust shell around Mira variables is determined largely by the dust absorptivity, Q(sub abs)(lambda), and the dust condensation temperature T(sub cond). Both Q(sub abs)(lambda) and T(sub cond) are extracted from IRAS low-resolution spectra (LRS) spectra. In order to do this, the assumption that the ratio of total power in the 10 micron feature to that in the 20 micron feature should be equal to that measured in other amorphous silicates (e.g., synthesized amorphous Mg2SiO4). It was found that T(sub cond) decreases with decreasing strength of the 10 micron feature, from T(sub cond) = 1000 K to 500 K (estimated error 20 percent). A value for the near-infrared dust absorptivity could not be determined. Although this parameter strongly affects the condensation radius, it hardly affects the shape of the LRS spectrum (as long as the optically thin approximation is valid), because it scales the spatial distribution of the dust. Information on the magnitude of the near-infrared dust absorptivity may be deduced from the unique carbon star BM Gem. This star has a LRS spectrum with silicate features indication an inner dust shell temperature of at least 1000 K. However, on the basis of observations in the 1920s-30s one may infer an inner dust shell radius of at least 6x10(exp 12)m. To have this high temperature at such a large distance, the near-infrared absorptivity of the dust must be high.

Slijkhuis, S.↗
NASA NTRS1990

Galactic survey of interstellar abundances and selective extinction with IUE and IRAS

A progress report on the results of a survey of UV selective extinction curves using the International Ultraviolet Explorer (IUE) satellite is presented. From IUE high resolution spectra, UV selective extinction curves are derived. These are fitted to six parameters characterizing the grey extinction, the 2175 angstrom bump, and the far ultraviolet curvature. These parameters are compared to the results of previous surveys of interstellar metals and infrared Cirrus intensities. With IUE high resolution spectra, equivalent widths of 18 ultraviolet resonance transitions toward 273 early type stars are measured and column densities are derived. Using the Infrared Astronomy Satellite (IRAS) all sky survey, infrared Cirrus intensities at 12, 25, 60, and 100 microns are measured.

Vansteenberg, Michael E.↗
NASA NTRS1990

Workstation-based preprocessing of IRAS sky-flux images

Computer algorithms to remove two types of degradations in IRAS sky-flux images, namely slowly varying background illumination (strongly effected by the presence of zodiacal light) and periodic stripes, are presented. This paper discusses both algorithms in detail and shows results of its use on various sky-flux images. Focus of the work was on the implementation within a workstation environment, and its value as a preprocessing tool for researchers. Speed of the process, usability of the programs, and correctness of the results were the main goals in developing these tools.

Domik, Gitta↗
NASA NTRS1991

On the possible bipolar nature of 21 micron IRAS sources

The discovery of another IRAS source (22574 + 6609) showing the unidentified 21-micron emission feature is reported. Its overall energy distribution is similar to the well-known edge-on bipolar nebulae AFGL 2688 and AFGL 618. Ground-based optical and infrared observations of this object and two other 21-micron sources show that while all three have very similar infrared properties, they differ greatly in the visual region. All three of these 21-micron sources are intrinsically similar bipolar nebulae, viewed at different orientations.

Hrivnak, Bruce J.↗
NASA NTRS1991

IRAS low-resolution spectral observations of the 10 and 18 micron silicate emission feature

The IRAS LRS Atlas contains 8-23 micron spectra of 1816 stars with silicate emission features. In order to study the shapes of the silicate dust features, the 117 stars whose emission features appear to be optically thin and which have the best signal-to-noise ratio at the longest wavelengths were analyzed. Simple spherical dust shell models were calculated in both the optically thin and the slightly optical thick approximations. From the comparison of the predicted spectra of the dust and stellar continuum, the emissivity function Kappa(lambda) was derived. In the different stars, the shape of the 10-micron feature is either narrow or broad and it is peaked either at about 9.7 microns or at 10 microns. Either particle size effects (particles above 0.75 microns in radius) or optical depth effects (central optical depth about 1) could broaden the 10-micron feature. Chemical composition differences no doubt are also important, particularly as regards the position of the peak of the 10-micron feature. The stars with the peak at 10 microns are more closely confined to the Galactic plane than the stars with the peak at 9.7 microns. The shape of the 18-micron feature is essentially the same in all stars, and can be used to extend the interstellar extinction curve past 13 to 22 microns.

Simpson, Janet P.↗
NASA NTRS1991

The carbon-rich proto-planetary nebula IRAS 22272 + 5435

Ground-based photometry and spectroscopy is presented for IRAS 22272 + 5435, a 9th mag star with a large infrared excess. The flux distribution is 'double-peaked', with a visible and near-infrared component due to the reddened photosphere and a far-infrared component presumably due to a detached dust shell. About equal amounts of energy are detected from each. The spectrum is that of a peculiar G supergiant, Gp Ia. In addition, strong molecular bands of C3 and C2 are observed. The flux distribution is modeled and, together with published molecular-line radio observations, allows the determination of basic parameters of the central star and the dust shell. The source has the characteristics one would expect of a proto-planetary nebula, an object in transition from the asymptotic giant branch to the planetary nebula phase.

Hrivnak, Bruce J.↗
NASA NTRS1990

IRAS low resolution spectra of 26 symbiotic stars

Data related to the spectral scans for 26 symbiotic stars are described which were extracted from the IRAS low resolution database. Data from the 8-15- and 15-23-micron bands are merged in a program that scales the longer wavelength and produces a weighted average of the spectral scans for each source. The survey shows that active dust producers can probably be isolated and some theories related to the presence of dust emission features are discussed in terms of source variability for measurements made with low resolution spectra.

Stencel, Robert E.↗
NASA NTRS1991

Distances to diffuse interstellar clouds from IRAS measurements and observations of optical absorption lines

Distances to diffuse interstellar clouds were determined from the amount of ultraviolet radiation that penetrates into the cloud and that arises from a nearby B star. The environment around 67 Ophiuchi, 20 Aquilae, kappa Aquilae, and 9 Cephei were studied. The intensities at 60 and 100 micron, as measured by IRAS, were used to derive dust temperatures for the clouds. Enhanced dust temperatures would indicate an influence of the star's radiation field on a cloud. Observation of molecular absorption were compared to the results of simplified chemical models in order to search for enhanced photodissociation that is caused by the star. Enhanced dust temperatures were observed for clouds in the vicinities of 20 Aql, kappa Agl, and 9 Cep. The range of the star's influence was found to be typically 1-5 pc. On the other hand, chemical analyses of the molecular data, which pertain to foreground gas, did not reveal the presence of enhanced dissociative flux from the sample of stars. Thus, upper limits were derived for the distances from the sun to the foreground material.

Federman, S. R.↗
NASA NTRS1991

Image construction from the IRAS survey

IRAS survey data can be used successfully to produce images of extended objects. The major difficulties, i.e., nonuniform sampling, different response functions for each detector, and varying signal-to-noise levels for each detector for each scan, have been resolved. The results of three different image construction techniques are compared: coaddition, constrained least-squares, and maximum entropy. The maximum entropy result is superior. An image of the galaxy M51 with an average spatial resolution of 45 arcsecods, using 60 microns survey data is presented. This exceeds the telescope diffraction limit of about 1 minute of arc at this wavelength.

Bontekoe, T. R.↗
NASA NTRS1991

IRAS software analysis library

The goal of this project was to collect 'research software' written in Interactive Data Language (IDL) to support analysis of data from the Infrared Astronomical Satellite (IRAS) and make it available to the larger community. 'Research Software' describes software created by researchers and staff for a specific research goal, but lacks sufficient documentation, easy to use interfaces, and rigorous debugging. Additionally, most of the IDL/IRAS code available needed to be ported to a (largely) hardware independent new version of IDL.

Domik, Gitta↗
NASA NTRS1991

Radio properties of extragalactic IRAS sources

The present study identifies extragalactic sources from the IRAS Faint Source Catalog by position coincidence with radio sources stronger than 25 mJy and lying north of +5 deg on the Green Bank 4.85 GHz sky maps. Published VLA maps, new 4.86 GHz VLA maps made with 15-arcsec resolution, and accurate optical positions are used to confirm 122 of these candidate identifications. Normal and starburst spiral galaxies were found to comprise about 97 percent of the FIR flux-limited sample. Radio-loud 'monsters' with q less than 2.25 dominate the radio emission from about 2 percent of the FIR source sample, and radio-quiet monsters are responsible for the FIR emission from less than about 1 percent of the FIR sample. All of the radio-identified sources are optically identified, mostly with relatively bright nearby galaxies. No evidence was found for any new populations of high-redshift FIR sources, nonthermal sources with steep FIR/optical spectra, or dust-shrouded sources visible only at FIR and radio wavelengths.

Condon, J. J.↗
NASA NTRS1991

Near-infrared observations of the z about 2.3 IRAS source FSC 10214 + 4724

Near-infrared imaging and spectroscopy of the extremely luminous IRAS source FSC 10214 + 4724 have been obtained using the Cassegrain infrared camera on the 200-inch Hale Telescope. A low-resolution spectrum in the 2.0-2.4 micron atmospheric window shows a very strong H-alpha line at the optically determined redshift z = 2.286. The observed rest-frame equivalent width of H-alpha is 0.07 +/-0.02 microns, consistent with the largest values found in quasars. The images show an unresolved source, while the near-infrared colors are somewhat redder than the mean colors of quasars observed at the same redshift. The reddening inferred is about 1.5 mag, with an upper limit of about 3.0 mag. If FSC 10214 + 4724 is a quasar, the reddening-corrected bolometric luminosity is approximately equal to the observed infrared luminosity.

Soifer, B. T.↗
NASA NTRS1991

Constraints on cosmological models from the IRAS QDOT survey results

The results of the recent IRAS survey are used to explore constraints on the nature of the Galaxy density field. Realistic simulations of the survey made from N-body simulations of the n = -1 power-law power spectrum and the Omega = 1 CDM models show that the power-law model is basically consistent with all QDOT variance results, while the CDM model has a smaller variance than the observation at the 20/h Mpc Gaussian smoothing scale. It is concluded that the QDOT results are best explained if the power spectrum of the Galaxy density field has a slope of about -1 near a wavelength of about 100/h Mpc, while the CDM model has a slope of about 0 at that scale. The possibility of marginal non-Gaussian behavior at the 20/h Mpc scale reported by Saunders et al. (1990) is examined for gravitational instability models with Gaussian initial conditions. It is argued that the observed (delta exp 3) versus (delta exp 2) in redshift space is not what these models would predict.

Park, Changbom↗