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At least 397 records · Page 22
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NASA NTRS1980

A far-infrared study of the reflection nebula NGC 2023

Multicolor mapping of the reflection nebula NGC 2023 from 40 to 160 microns is presented. These data show the shorter wavelength emission to peak on or close to the exciting star HD 37903. The longest wavelength emission, however, peaks about 1 arcmin south of HD 37903, at a position coincident with the C II recombination line peak. The dust temperature appears to peak close to HD 37903 suggesting that it is probably the most luminous heating source for the cloud. The far-infrared data together with 10 microns photometry of HD 37903 imply a roughly uniform dust mass density within 0.1 pc of the star with no significant density increase toward the star. The results imply that the gas and dust column density increase slightly to the south of HD 37903 and that the bulk of the molecular cloud lies behind the star and the reflection nebula.

Harvey, P. M.↗
NASA NTRS1980

The young open cluster NGC 3293 and its relation to Car OB1 and the Carina Nebula complex

On the basis of visible evidence, NGC 3293 appears to be associated with a number of objects in the Carina Nebula complex. UBVRI photoelectric photometry and MK-spectral specifications are presented for a large number of stars in this cluster and its surroundings (Car OB1). Over 70 cluster members with spectral types earlier than B9 are identified. Star counts indicate that the expected number of members brighter than V = 18 is 297 plus or minus 9. The cluster appears to be situated just in front of a dense dust lane, which itself appears to be part of the dust complex emanating from the center of the Carina Nebula.

Turner, D. G.↗
NASA NTRS1981

Surface mapping of selected regions in the Orion Nebula

Low dispersion, large aperture, ultraviolet spectra of selected regions in the Orion Nebula were obtained with the International Ultraviolet Explorer (IUE) scientific instrument. Spectra obtained at 35 contiguous locations defining a mosaic within the nebula were used to generate monochromatic images of high spatial resolution at the wavelengths of the ultraviolet emission lines. Image processing techniques were utilized to generate and analyze these ultraviolet surface maps. The imagery at the three wavelengths studied shows definite differences in the spatial distribution of emission from the CII CIII and OII ions. Ways of using the imagery to determine ionization structure and C/O abundance ratios throughout the regions observed are developed, in addition to means of analyzing the extensive continuum measurements in terms of dust scattering characteristics.

Perry, P. M.↗
NASA NTRS1981

An atlas of emission line fluxes of planetary nebulae in the 1150-3200 A region

Emission line fluxes for 28 planetary nebulae are presented. The nebulae were chosen to cover a wide range of excitation classes, apparent diameters, location in the sky, and types of central stars. All objects were observed in the low dispersion mode of the IUE spectrographs, using the large entrance aperture.

Feibelman, W. A.↗
NASA NTRS1980

Upper limits on X-ray line emission from the Crab Nebula

X-ray emission lines from the Crab Nebula and the surrounding region have been searched with the Focal Plane Crystal Spectrometer (FPCS) on the Einstein Observatory. A 3 x 30 arcmin and a 6 arcmin diameter circular aperture were pointed at several positions centered on, and immediately to the north and south of, the nebula. Stringent upper limits are obtained for emission in lines of N-6 (0.43 keV), O-8 (0.65 keV), Fe-17 (0.83 keV), and others - most of which are prominent in the spectra of other supernova remnants and of the sun. The observed upper limits and standard models for the emission from a thermal plasma lead to upper limits on the quantity of hot plasma within or around the Crab. In particular, previously reported emission from a 7 arcmin diameter region surrounding the Crab is probably due to scattering of X-rays from the central source by interstellar dust in the line of sight and is certainly not due to emission from a thermal plasma.

Schattenburg, M. L.↗
NASA NTRS1980

Two-dimensional photometry of planetary nebulae

Monochromatic images of three planetary nebulae, NGC 6720, NGC 7662 and NGC 2392, were taken using narrow band interference filters centered on the forbidden emission lines of Oxygen I, II, III, Nitrogen II, Sulfur II and the permitted emission lines of Hydrogen I, Helium I and Helium II. The data was taken with a silicon intensified vidicon using the Palomar 1.5 meter telescope and was recorded digitally to an encoding level of 12 bits. The data was processed at the Image Processing Laboratory of the Jet Propulsion Laboratory. The calibrated images were used to investigate the three dimensional structure of the nebulae and to generate two-dimensional electron and temperature maps.

Kupferman, P. N.↗
NASA NTRS1980

Dust to planetesimals - Settling and coagulation in the solar nebula

The behavior of solid particles in a low-mass solar nebula during settling to the central plane and the formation of planetesimals is discussed. The gravitational instability in a dust layer and collisional accretion are examined as possible mechanisms of planetesimal formation. The shear between the gas and a dust layer is considered along with the differences in the planetesimal formation mechanisms between the inner and outer nebula. A numerical model for computing simultaneous coagulation and settling is described.

Weidenschilling, S. J.↗
NASA NTRS1981

Studies of bipolar nebulae. VII - The exciting star of OH 0739-14 /equals OH 231.8 plus 4.2/

A spectral classification for the exciting star of the bipolar reflection nebula OH 0739-14, which has also been observed to be an OH/H2O maser source, is obtained from the spectrum of the nebulosity in the range 6000-9000 A. Spectrophotometry of the visible nebula was obtained with the Cassegrain image-tube scanner on the Lick Observatory 3-m telescope at a resolution of approximately 7 A. The red spectrum is found to be distinguished by TiO absorption at the bandheads at 8206, 8303, 8432, 8442 and 8452 A and VO in the 7400 and 7900 A regions. On the basis of the depths of these bandheads and the K I 7699 A and Ca II infrared triplet, OH 0739-14 is determined to be of spectral type M9 III. The underlying star thus represents the coolest known star to occur in a bipolar system.

Cohen, M.↗
NASA NTRS1981

Infrared reflection nebulae in S 106 and NGC 7538 E

Two micron linear polarimetry of the S 106 and NGC 7538 E regions reveal the presence of infrared reflection nebulae, which may confuse the count of luminosity sources in a region of star formation. S 106 and NGC 7538/IRS 9 are considered the only luminosity sources in their respective regions, and the polarization of S 106/Source 3 is probably due to absorption by aligned grains. Moreover, the reflection nebula model for NGC 7538 E is best supported if the polarization of NGC 7538/IRS 9 itself is due to the absorption by aligned grains.

Tokunaga, A. T.↗
NASA NTRS1981

Spatial observations of the Orion Nebula in the unidentified 3.28 micron feature

The unidentified infrared emission at a wavelength of 3.28 microns has been mapped in a 5 x 5 arcmin region of the Orion Nebula at a resolution of 30 arcsec along with continuum thermal emission at 3.5 microns in order to gain information on the probable source of the unidentified feature. The 3.28-micron emission is found to be extended over much of the nebula with a distribution very different from that of the 3.5-micron continuum. The ratio of 3.28-micron to 3.5-micron flux density is observed to range from a value of 7 in the Orion ionization ridge to less than 0.15 towards the Becklin-Neugebauer object, and to generally be larger toward the edges of the emitting region than toward the Trapezium. From the observations, it is concluded that the 3.28-micron emission originates in the ionization front between the foreground ionized gas and background molecular cloud, and may be due to thermal emission from complex, less volatile organic substances in grain mantles.

Sellgren, K.↗
NASA NTRS1981

Molecular hydrogen ion /H2+/ absorption in planetary nebulae

Several IUE spectra of planetary nebulae show an absorption feature shortward of 1500 A which is believed to be due to H2(+). The nebulae are excited by stars of spectral type O3-O7 or continuum, and all but one have double-shell structure.

Feibelman, W. A.↗
NASA NTRS1981

A far-infrared emission feature in carbon-rich stars and planetary nebulae

The 16-30 micron spectra of several carbon stars and the planetary nebulae IC 418 and NGC 6572 have been obtained using the NASA C-141 Kuiper Airborne Observatory. A newly observed emission feature appears in the spectrum of IRC +10216 and several other carbon stars at wavelengths greater than 24 microns. The feature is interpreted as resulting from a solid-state resonance in the dust grains which have condensed around these stars. A similar feature appears in the spectra of IC 418 and NGC 6572, implying that the same type of dust is present. Since the dust probably condensed from a carbon-rich gas, this indicates an evolutionary link between carbon stars and these planetary nebulae. No identification for the grain material has been found, but some clues are apparent which could aid in the identification.

Forrest, W. J.↗
NASA NTRS1980

A model for nitrogen isotopic variations in the lunar regolith - Possible solar system contributions from a nearby planetary nebula

To account for the observed secular increase of the N-15/N-14 ratio in lunar samples, a model is developed which envisions an encounter between a planetary nebula and the solar system during the first few hundred million years of its history. The principal effects of the encounter for the sun's outer convective zone, and hence solar wind, due to accretion of planetary nebula material with its distinct nucleosynthetic history are delineated. Associated material contributions to planets and meteorite parent bodies are considered and contraints relevant to this possibility are discussed.

Ray, J.↗
NASA NTRS1982

High dispersion observations of selected regions in the Orion Nebula

High resolution spectral observations were made of several regions of the Orion Nebula near theta (2) Ori A using the IUE. The positions were selected using a moderate spatial resolution map from a previous low dispersion IUE survery of this section of the nebula. With the SWP and LWR cameras, 28 pectra were obtained of the bright bar, three Taylor-Munch cloudlets, and several surrounding locations. Emission lines of He, C, N, O, Mg, and Si allow a characterization of these cloudlets and of the gas in and around the bar. Small aperture observations provide radial velocity information for the ultraviolet emission of these features. These data show ionization variations from region to region and are suggestive of stellar wind interactions between the cloudlets and theta(2) Ori A.

Boeshaar, G. O.↗
NASA NTRS1982

High-resolution X-ray observations of the Orion Nebula

Observations of the Trapezium region in the Orion Nebula have been obtained with the high-resolution X-ray imaging instrument on the Einstein Observatory. At least 58 sources of the X-ray emission have been revealed, and all but two can be identified with visible stars. The strongest source is the star theta(1)C, which excites the emission nebula, and has an X-ray luminosity of 6 x 10 to the 32nd ergs/s. No strong X-ray emission has been observed from members of the infrared cluster embedded within the Orion molecular cloud.

Ku, W. H.-M.↗
NASA NTRS1981

The planetary nebula NGC 6826

Monochromatic photographs have established the NGC 6826 nebula as the third member of a group of very rare triple-shell planetaries (Feibelman, 1971, 1974). Kaler (1974) also characterized NGC 6826 as a giant halo planetary. Numerous errors and confusing statements regarding its size, structure and stratification are discussed, and the correct dimensions of the nebula are reported: the inner ring is 12.7 arcsec x 8.7 arcsec (Feibelman, 1971); the outer ring is 27 arcsec x 24 arcsec according to Curtis (1918), 25.7 arcsec x 24.4 arcsec according to Feibelman (1971), and 36 arcsec x 36 arcsec according to Coleman et al. (1975). The halo measurements range in diameter from 110 arcsec (Duncan, 1937) to 130 arcsec (Kaler, 1974) to 142 arcsec (Millikan, 1974). Values for the distance of NGC 6826 range from 0.75-1.16 kpc (Cahn and Kaler, 1971) to 2.265 kpc (Cudworth, 1974).

Feibelman, W. A.↗
NASA NTRS1982

The ionization structure of the Ring Nebula. II - Ultraviolet observations

Good agreement is found for O and N abundances, and the importance of charge-transfer reactions to the ionization of Ne is confirmed, in UV observations of emission line intensities for four positions in the Ring Nebula that have been previously studied optically. It is noted that the C(2+) abundance inferred from the optical 4267 A recombination line is as much as 10 times brighter than that measured from the UV lines, and that this discrepancy is greatest near the central star. The most attractive explanation for this phenomenon is the resonance fluorescence due to light from the central star. Both Ne/O and C/H abundances are in good agreement with previously measured average values for planetary nebulae.

Barker, T.↗
NASA NTRS1982

E/B - V/ extinction values for 24 planetary nebulae derived from IUE data

Excellent agreement was found for E(B - V) values for nine planetary nebulae observed in common by IUE and ANS, derived from a strong 2200A absorption feature. E(B - V) values for seven additional objects with moderately strong 2200A absorption were determined. A third group of eight planetary nebulae for which the 2200A absorption feature is marginal or absent had E(B - V) extinctions determined from the observed He II I(1640A)/I(4686A) ratio. A comparison with optical and radio data is included.

Feibelman, W. A.↗