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Witt, A. N.

Publications and source records attributed to Witt, A. N..

35 records · Page 2
All sourcesNASA NTRSDOE OSTI
NASA NTRS1982

IUE observations of reflection nebulae

Low-resolution IUE spectra in the SWP and LWR ranges were obtained of several reflection nebulae in the vicinity of their respective illuminating stars. Data for NGC 7023 (HD200775), NGC 1435 (Merope, 23 Tau), NGC 1432 (Maia, 20 Tau) and the Electra (17 Tau) nebula were found to display significant differences in the shape of their normalized nebular spectra. If the dust in these nebulae is similar from one object to the next, these differences can be most readily explained by differences in the line-of-sight dust distribution, coupled with a wavelength-dependent phase function, changing from a strongly forward throwing form (g = 0.6-0.7) in the visible to a more nearly isotropic shape (g approximately 0.25) at 1400A.

Witt, A. N.↗
NASA NTRS1981

The ultraviolet interstellar extinction curve in the Pleiades

The wavelength dependence of ultraviolet extinction in the Pleiades dust clouds has been determined from IUE observations of HD 23512, the brightest heavily reddened member of the Pleiades cluster. There is evidence for an anomalously weak absorption bump at 2200 A, followed by an extinction rise in the far ultraviolet with an essentially normal slope. A relatively weak absorption band at 2200 A and a weak diffuse absorption band at 4430 A seem to be common characteristics of dust present in dense clouds. Evidence is presented which suggests that the extinction characteristics found for HD 23512 are typical for a class of extinction curves observed in several cases in the Galaxy and in the LMC.

Witt, A. N.↗
NASA NTRS1980

Long-slit spectroscopy in the rocket ultraviolet of the Orion Nebula

The study deals with the extensive nebulosity in the Trapezium region of Orion. An entrance slit was placed at the focal plane of rocketborne telescope, creating a long-slit spectrograph with a wavelength resolution of 22 A FWHM and a spatial resolution of 14 in. along the slit. Data reduction is outlined, the structure of the Orion Nebula is discussed, and radiative transfer considerations are presented.

Bohlin, R. C.↗
NASA NTRS1980

The HD 200775/NGC 7023 complex - A question of reddening

The separation of the observed reddening of HD 200775 into intrinsic and interstellar components is rediscussed in the light of new surface-brightness data for NGC 7023, the reflection nebula surrounding HD 200775. Appropriate correction for the nebular contribution to reported ultraviolet flux measurements of HD 200775 leads to new values due to dust reddening: E(B-V) sub D = 0.44, and due to intrinsic reddening: E(B-V) sub I = 0.13-0.26. The newly derived extinction curve for HD 200775 is characterized by an abnormally weak interstellar 2200 A absorption feature, and the deduced nebular brightness in the far-ultraviolet is consistent either with dust in NGC 7023 having a relatively high albedo (a approximately equal to 0.6) and a nearly isotropic phase function (g approximately equal to 0.2), or with low albedo dust (a approximately equal to 0.3) with a forward-throwing phase function (g approximately equal to 0.6).

Witt, A. N.↗
NASA NTRS1980

A surface brightness study of the reflection nebula NGC 7023

New photoelectric surface brightness measurements of NGC 7023 at 3500, 4100, 4700, and 5500 A in 38 positions are reported. The systematic behavior in the changes of surface brightness and color with position has been examined, and conclusions are drawn concerning the nebular density, the geometry of the star's location, and the variation of the dust scattering properties with wavelength. Indications are found for the existence of a small H II region surrounding the illuminating star HD 200775.

Witt, A. N.↗
NASA NTRS1978

Ultraviolet photometry from the orbiting astronomical observatory. XXX - The Orion reflection nebulosity

Surface-brightness measurements are presented that cover the region of Orion in nine intermediate-width bandpasses ranging from 4250 to 1550 A. The existence of an extended ultraviolet reflection nebulosity in this area is confirmed, and the characteristics of its spectrum and spatial distribution are derived. The observations are consistent with a model in which the dense molecular cloud complex in Orion is illuminated by the foreground Orion aggregate of early-type stars. The interpretation is complicated by the fact that foreground dust may contribute to the observed scattered light. The scattering particles in the cloud appear to exhibit a wavelength-dependent albedo similar to that found for interstellar grains in general, with a strong indication that the phase function changes to a less forward-scattering form in the ultraviolet.

Witt, A. N.↗
NASA NTRS1977

Multiple scattering in reflection nebulae. I - A Monte Carlo approach. II - Uniform plane-parallel nebulae with foreground stars. III - Nebulae with embedded illuminating stars

A method for calculating the surface brightness distribution on a plane-parallel reflection nebula of uniform density illuminated by a star located either in front of, behind, or arbitrarily inside the scattering medium is proposed. The Monte Carlo technique is used to find solutions to the radiative transfer problem. The scattering properties of the nebular particles are parameterized by the albedo for single scattering and a three-parameter analytic phase function. Calculations are then presented for the surface brightness distribution across the face of such nebulae with (1) a foreground star and (2) immersed stars. The calculations include the full effects of multiple scattering, are independent of a particular assumed grain material or size distribution, and are applicable to any wavelength region for which observations can be obtained.

Witt, A. N.↗
NASA NTRS1977

Multiple scattering in reflection nebulae. IV - The multiplicity of scattering

The characteristics of higher order scattering in plane-parallel reflection nebulae of finite optical thickness have been examined. Measured both by the mean number of scatterings and the effective multiplicity, the importance of multiple scattering in such systems varies strongly with position as well as direction, and the effects cannot be taken into account by simple corrections only. It is found that the amount of higher order scattering is relatively insensitive to the shape of the phase function over a wide range of asymmetries, while the dependence of multiplicity on particle albedo, optical thickness of the nebula, and other geometrical factors is as expected. Pronounced asymmetries in the surface brightness distribution of strongly tilted plane-parallel nebulae are explained by different radial variations of the effective multiplicity along opposite radii on the projected nebular face.

Witt, A. N.↗
NASA NTRS1975

Search for continuous fluorescence in reflection nebulae

Photometric and spectrophotometric observations have been made of the reflection nebulae NGC 1435, NGC 2068, NGC 7023, and IC 1287 in an attempt to detect continuous fluorescence by dust grains. Several effects of importance for observations of such faint objects are discussed, including instrumental light scattering, a photographic effect, and a time-delay effect which can occur if the illuminating star is a spectrum variable. It is found that continuous fluorescence by interstellar grains is not likely to exist and that it cannot account for more than 10% of the total surface brightness of these reflection nebulae. No evidence of diffuse interstellar features is found in the spectra of these nebulae.

Rush, W. F.↗
NASA NTRS1974

Monte Carlo calculations of the surface brightness profiles of spherical dark nebulae

Theoretical surface brightness profiles of spherical dark nebulae of both uniform density and a density proportional to 1/r squared are being discussed. The albedo and asymmetry factor of the dust grains and the optical radius measured from the cloud center were the variable parameters in this study, which further assumed an isotropically incident radiation field. The occurrence of dark cores in the surface brightness profiles is examined, and their frequent existence in galactic dark nebulae is interpreted as resulting from a strongly forward scattering phase function and a high optical depth of the associated objects. The shape and depth of the profiles of the dark cores is closely related to the internal density distribution of the clouds, and it is suggested that detailed observations of the surface brightness profiles of dark nebulae will reveal important information on the type of their internal density distribution and, thus, their evolutionary status.

Witt, A. N.↗
NASA NTRS1973

A search for continuous fluorescence in reflection nebulae

Photometric and spectrophotometric observations were made of the reflection nebulae NGC1435, NGC2068, NGC7023, and IC1287 in an attempt to detect continuous fluorescence by dust grains. Several effects of importance for observations of such faint objects are discussed, including instrumental light scattering, a photographic effect, and a time delay effect which can occur if the illuminating star is a spectrum variable. It is found that continuous fluorescence by interstellar grains is not likely to exist and that it cannot account for more than 10 percent of the total surface brightness of these reflection nebulae. No evidence of diffuse interstellar features is found in the spectra of these nebulae.

Rush, W. F.↗
NASA NTRS1972

Diffuse galactic light in the 1500-4200 angstrom region

Diffuse galactic light has been observed with the four stellar photometers of the OAO-2 in 29 of Kapteyn's selected areas. The data can be understood in terms of a wavelength dependent albedo of the interstellar grains with a pronounced minimum around 2200 A with a rapid increase towards unity at wavelengths below 2000 A.

Witt, A. N.↗