Engineering topics

Stecher, T. P.

Publications and source records attributed to Stecher, T. P..

At least 37 records · Page 2

NASA NTRS1982

The scattering phase function of interstellar grains - The case of the reflection nebula NGC 7023

IUE observations of the reflection nebula NGC 7023 and the illuminating star, HD 200775, in the spectral range from 1300 to 3100 A are combined with ground-based measurements of the nebular brightness distribution at 3500, 4100, 4700, and 5500 A to determine the scattering properties of the nebular dust grains. Total nebular fluxes are derived from existing data obtained by satellite and ground-based observations. The relevance of far-IR fluxes from the nebula is considered, and models are derived that are applicable to NGC 7023 both with respect to the ratio of nebular to stellar fluxes and with respect to the nebular surface brightness distribution. It is shown that the average grain albedo in the UV is about 0.54 and that the albedo increases to a level of about 0.6 at 1400 A after reaching a minimum of approximately 0.4 near 2200 A. The results suggest that isotropically scattering particles of high albedo make a significant contribution to interstellar scattering in the far-UV.

Witt, A. N.↗

NASA NTRS1982

Ultraviolet images of M101 - Observations of dust and inferences on the metallicity

Since extreme Population I dominates three rocketborne telescope UV images of the Sc I galaxy NGC 5457, the delineation of the spiral structure is sharper than that of ground photographs. The UV pictures, in which the H II regions appear as reflection nebulae and illumine the nearby dust, reveal faint arms which, although correlated with the 21 cm H I radiograph, are only marginally detectable on deep visual photographs. The surface brightness of the arms is explainable in terms of UV light scattering off dust from hot stars and bright H II regions, in which case the dust would have extinction properties and a dust-to-gas ratio similar to that in the solar neighborhood and would in addition suggest that the NGC 5457 outer arm metal content is similar to that of the galaxy.

Stecher, T. P.↗

NASA NTRS1982

High-resolution photographs in the rocket ultraviolet of the Orion Nebula

High-resolution photographs of the Orion Nebula have been obtained in UV bands centered near 1400, 1820, 2240 and 2620 by a 31-cm telescope mounted on an Astrobee sounding rocket. Reduction of the data by image processing, photometric calibration and error analysis steps was used to derive a matrix of values of specific intensity in absolute units. Data indicate that the Orion Nebula is primarily a reflection nebula in the UV, so that morphology is determined by the distribution of dust with respect to the illuminating stars. Ultraviolet color gradients for each color relative to the 1820-A image indicate reddening towards the outer parts of the nebula, as well as fairly symmetric dust scattering in the Theta 1 and Theta 2 Ori region. The color gradients also imply a monotonic change in scattering phase function asymmetry between 2620 and 1400 A. Total nebular flux in each bandpass is observed to be about 2.5 times the stellar flux from the Trapezium, indicating a high scattering efficiency.

Bohlin, R. C.↗

NASA NTRS1982

The ultraviolet spectrum of the Crab Nebula

Ultraviolet spectroscopy of the Crab Nebula done by the International Ultraviolet Explorer satellite is described, and an estimate of the carbon abundance is made, noting data reduction to remove spectral defects caused by radiation hits. The important C IV 1549, He II 1640, and semiforbidden C III 1908 emission line intensities were measured and upper limits placed on other ultraviolet features for the brightest filamentary region in the Nebula. The emission lines imply an average ionic abundance ratio n(C+2)/n(O+2) in the range from 0.4 to 1.5 in the observed gaseous condensations. The elemental abundance ratio of carbon to oxygen is probably in the same range. Analysis shows that there is no perceptible excess of carbon due to presupernova nucleosynthesis in the observed region. The large helium abundance, small carbon and oxygen abundances, and presence of a neutron star in the Crab Nebula suggest that the presupernova star had a mass close to eight solar masses.

Davidson, K.↗

NASA NTRS1982

Ultraviolet spectroscopy of planetary nebulae in the Magellanic Clouds

Ultraviolet spectra of the high excitation planetary nebulas LMC P40, SMC N2, and SMC N5 are presented in an investigation of their chemical composition in an effort to make reliable estimates of the carbon abundance in extragalactic planetary nebulas. The carbon abundance is found to be almost 40 times more abundant in the SMC planetaries than in the SMC interstellar medium, and is about 6 times more abundant in the LMC planetary than in the LMC interstellar medium. The net result of carbon synthesis and convective dredgeup in the progenitors of planetary nebulas as reflected in the nebular carbon abundance is roughly the same in the Galaxy, the LMC, and the SMC.

Maran, S. P.↗

NASA NTRS1982

IUE observations of planetary nebulae and their central stars in the Magellanic Clouds

The planetary nebulae LMC P40, SMC N2, and SMC N5 and their central stars were observed with IUE. The C abundances in the nebulae, compared with those in galactic planetaries, indicate that convective dredgeup of locally nucleosynthesized C has occurred. The progenitors of the nebulae were C stars at the theoretical upper luminosity threshold, thus such stars do occur as predicted, although none so bright have been found in the Clouds. The central stars of the nebulae have masses approximately 1 solar mass, luminosities approximately 40,000 solar luminosity, and radii approximately 0.7 solar radius; they have probably not yet reached their maximum luminosities. With M(subv) 19.1-19.8, they may be the visually faintest stars yet observed by UV spectroscopy. Clearly, it is not true that planetary nebulae nuclei all have masses M = (0.6 + or - 0.1) solar mass.

Maran, S. P.↗

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 NTRS1981

Discovery of the molecular hydrogen ion (h2(+)) in the planetary nebulae

Low-dispersion spectra of fifteen planetaries and hot subdwarfs were obtained with the short wavelength prime camera on IUE and continuous flux distributions corrected for interstellar extinction were derived. Several planetaries, particularly the young planetaries of high surface brightness, show anomalous flux distributions. The most anomalous case is NGC 6210. These anomalies may be explained as absorption by H2+ H2(+) in the nebula.

Heap, S. R.↗

NASA NTRS1981

Preliminary report on IUE spectra of the Crab Nebula

The Crab Nebula is marginally observable with the IUE. Observations of the optically brightest filamentary regions, made with IUE in August 1979, show the C IV lambda 1549, He II lambda 1640, and C III lambda 1909 emission lines. The intensities of these lines were compared with the visual wavelength data. It appears that carbon is not overabundant in the Crab; carbon/oxygen is approximately normal and oxygen is slightly scarcer than normal as a fraction of the total mass.

Davidson, K.↗

NASA NTRS1981

Astronomical imaging investigations for Shuttle and Spacelab missions

General objectives of space astronomical imagery are briefly examined, taking into account the improved angular resolution achievable in space observations, the utilization of an accessibility of the far ultraviolet spectral range for an easier detection and more accurate measurement of very hot stars, the determination of the structure and evolution of galaxies, and the determination of the cosmic distance scale. Attention is given to the Ultraviolet Imaging Telescope, the Spacelab Wide Angle Telescope, and the Starlab UV/optical telescope facility. It is pointed out that Starlab is envisioned as a multi-instrument, multi-purpose astronomical facility based on a near-diffraction-limited Ritchey-Chretien telescope of about 1 meter aperture and f/15 focal ratio. These resolutions are a factor of 10 better than typically achieved with ground-based telescopes.

Carruthers, G. R.↗

NASA NTRS1980

Production of carbon, sulfur, and CS in Comet West

Results from ultraviolet spectrographic observations of Comet West (1975n) in the 1620-3960 A wavelength range are presented. Objective grating imagery of the comet reveals for the first time emission from sulfur atoms and CS molecules as well as previously observed lines or bands in carbon atoms and CO(+), CO2(+) and OH molecules. Some evidence exists for emission in the C2 Mulliken bands and in the CO Cameron bands. On the basis of the observations, production rates of C (ground state), C(1D), S, and CS are inferred to be 3.8 x 10 to the 29th per sec, 4.4 x 10 to the 29th per sec, 1.7 x 10 to the 28th per sec, and greater than approximately 3.1 x 10 to the 27th per sec, respectively.

Smith, A. M.↗

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

Supernovae effects on the terrestrial atmosphere

The first effects of a nearby (10 parsec) supernova on the earth's atmosphere will be caused by ultraviolet radiation dissociating molecular oxygen. The event will be of about one month's duration. Several months later nuclear gamma radiation may arrive, causing a decrease in atmospheric ozone. Cosmic radiation from the supernova remnant will not intercept the earth for at least 1000 years at which time ozone will be seriously depleted. Supernova ultraviolet radiation increases column ozone and atomic oxygen. Atmospheric thermal structure is modified with a large temperature increase in the mesosphere and lower thermosphere and a decrease at higher altitudes caused by enhanced heat loss due to atomic oxygen radiation and conduction.

Aikin, A. C.↗

NASA NTRS1978

Ionization of interstellar H2 clouds by supernovae

Two destruction mechanisms for molecular hydrogen in the H I regions of the interstellar medium were given by Stecher and Williams (1967). The first is dominant in weak radiation fields and involves photon excitation (at about 1000 A) to the first excited electronic state with a subsequent decay into the vibrational continuum of the ground state. The second destruction mechanism is dominant in the strong photon fields that exist when a supernova explosion occurs. It involves photoionization (at about 1000 A) from metastable high vibrational levels produced by photon excitation (in the same wavelength range) to an excited electronic state and subsequent decay to bound vibrational levels. It is shown that ionization of molecular hydrogen can occur at considerable distances from such supernova events while the atomic hydrogen remains neutral. The effect should be observable in the dispersion measures of new pulsars.

Stecher, T. P.↗

NASA NTRS1978

An interstellar cloud density from Copernicus observations of CO in the spectrum of Zeta Ophiuchi

Interstellar CO absorption bands in Copernicus spectra of Zeta Oph have been studied. Absorption profiles, computed under the assumption that excitation is due to collisions with H2 molecules and interaction with the 3-K background radiation field, were fitted to the reduced data of nine bands. When a gas kinetic temperature of 56 K is assumed, the best-fit condition implies a hydrogen-nucleus density of 120 per cu cm, a CO column density of 1.2 by 10 to the 15th power per sq cm, and a radial-velocity dispersion of 0.9 km/s. The relevance of these results to existing ideas concerning the Zeta Oph interstellar clouds is discussed. It is suggested that the strongest interstellar component is not circumstellar in origin but is instead part of a supernova remnant. Simple calculations are made to establish the plausibility of the supernova-remnant identification. This suggestion is also supported by Heiles's (1976) 21-cm pictures.

Smith, A. M.↗

NASA NTRS1978

The rocket-ultraviolet spectrum and models of the planetary nebula NGC 7662

Ultraviolet spectral analysis of NGC 7662 has revealed a Balmer line distribution indicating a C/O ratio of unity. This abundance of carbon is characteristic of mixing between the central star and the outer envelope of the planetary nebula. A continuum spectrum is observed from the star and the nebula, radiating its energy via the two photon process through neutral hydrogen. Spectral lines were fitted to apply to the predictions of three models of the nebular structure. UV observations were compared with visible, IR, and radio observations.

Bohlin, R. C.↗

NASA NTRS1975

The rocket-ultraviolet spectrum of the planetary nebula NGC 7027

An ultraviolet spectrum of NGC 7027, obtained with a rocket-borne telescope, is analyzed. Absolute values are presented for the observed fluxes, and upper limits are given for the strongest predicted lines that were not observed. The results are corrected for interstellar extinction using the observed and calculated line ratios between H-beta and the hydrogenic recombination line of He II at 1640 A. The corrected C IV resonance line at 1549 A is found to be in good agreement with the intensity calculated from models, but the intercombination line of C III at 1909 A is found to be too bright by a factor of 10. This discrepancy is reduced to a factor of 4 by taking dielectric recombination into account and is eliminated by using the solar carbon abundance, which implies attenuation in the C IV line. It is shown that no appreciable number of absorbing grains can exist in the C IV-producing region of the nebula since the optical depth is of the order of 10,000 at the line center.

Bohlin, R. C.↗