Engineering topics

Dufour, R. J.

Publications and source records attributed to Dufour, R. J..

At least 19 records

NASA NTRS2002

Temperature Variations from HST Imagery and Spectroscopy of NGC 7009

We present new HST/WFPC2 imagery and STIS long-slit spectroscopy of the planetary nebula NGC 7009. The primary goal was to obtain high spatial resolution of the intrinsic line ratio [O III] 4364/5008 and thereby evaluate the electron temperature (Te) and the mean-square Te variation (t(sup 2, sub A)) across the nebula. The observations here do not address Te fluctuation along the line of sight. The WFPC2 Te map is rather uniform; almost all values are between 9000 - 11,000 K, with the higher Te's closely coinciding with the inner He(++)-zone. The results indicate very small values - certainly less than 0.01 - for t(sup 2, sub A) throughout. Our STIS data allow an even more direct determination of Te and t(sup 2, sub A), albeit for a much smaller area than with WFPC2. We present results from binning the data along the slit into tiles that are 0.5 in square (matching the slit width). The average [O III] temperature using 45 tiles (excluding the central star and STIS fiducial bars) is 10,146 K; t(sup 2, sub A) is 0.0036. Although we have measured t(sup 2, sub A) in only 2-dimensions, we conclude that temperature fluctuations alone are unlikely to explain for NGC 7009 the large discrepancy between heavy element abundances inferred from emission lines that are collisionally excited compared with those that are due to recombination lines.

Rubin, R. H.↗

NASA NTRS1995

The evolution of C/O in dwarf galaxies from Hubble Space Telescope FOS observations

We present UV observations of seven H II regions in low-luminosity dwarf irregular galaxies and the Magellanic Clouds obtained with the Faint Object Spectrograph (FOS) on the Hubble Space Telescope (HST) in order to measure the C/O abundance ratio in the interstellar medium (ISM) of those galaxies. We measure both O III 1666 A and C III 1909 A in our spectra, enabling us to determine C(+2)/O(+2) with relatively small uncertainties. The results from our HST observations show a continuous increase in C/O with increasing O/H, consistent with a power law having an index of 0.43 +/- 0.09 over the range -4.7 to -3.6 in log (O/H). One possible interpretation of this trend is that the most metal-poor galaxies are the youngest and dominated by the products of early enrichment by massive stars, while more metal-rich galaxies show increasing, delayed contributions of carbon from intermediate-mass stars. Our results also suggest that it may not be appropiate to combine abundances in irregular galaxies with those in spiral galaxies to study the evolution of chemical abundances. Our measured C/O ratios in the most metal-poor galaxies are consistent with predictions of nucleosynthesis from massive stars for Weaver & Woosley's best estimate for the 12C(alpha, gamma) 16O nuclear reaction rate, assuming negligible contanmination from carbon produced in intermediate-mass stars in these galaxies. We detect a weak N III 1750 A multiplet in SMC N88A and obtain interesting upper limits for two other objects. Our 2 sigma uppr limits on the 1750 A feature indicate that the N(+2)/O(+2) ratios in these objects are not significantly larger than the N(+)/O(+) ratios measured from optical spectra. This behavior is consistent with predictions of photionization models, although better detections of N III are needed to confirm the results.

Garnett, D. R.↗

NASA NTRS1992

The H II regions of the galaxy M101

The physical properties and composition of the H II region population of the spiral galaxy M101 (NGC 5457) were examined using spectrophotometrically calibrated CCD imagery of two overlapping 16-arcmin fields in the M101, in which individual H II regions were identified and mapped using an automated algorithm. The characteristics obtained for the H II region population of M101 include the H II region luminosity function and radial variations in extinction; the forbidden O III line/H-beta, O/H, and forbidden S II line/H-alpha ratios; the ionization parameter U; and the numbers of ionizing photons. Global variations of physical properties of the H II regions in M101 are compared to model expectations derived for other galaxies. The evident star formation in the M101 galaxy is discussed.

Scowen, P. A.↗

NASA NTRS1992

The origin of broad emission lines in the extragalactic giant H II region NGC 2363

High signal-to-noise long-slit spectra have been obtained of the giant H II region NGC 2363 located in the dwarf SBm galaxy NGC 2366. A discovery of low-intensity broad spectral components (FWHM is approximately equal to 40 A or 2400 km/s) in the bright nebular lines H-alpha, H-beta, and forbidden O III is reported. The broad spectral components are detected over a large spatial extent (not less than 500 pc) centered on the nebula. Several mechanisms for broadening nebular lines are explored: stellar winds, Thomson scattering by hot gas, supernova remnants, and superbubble blowout. All mechanisms have problems. Superbubble blowout, which is the only known mechanism capable of accelerating interstellar gas over such a volume of space, does not appear consistent with the physical properties of the H II region NGC 2363 or with the nature of the host galaxy. It is concluded that the broad nebular lines are probably due to very high velocity gas whose origin is, at present, unknown.

Roy, Jean-Rene↗

NASA NTRS1987

A five-level program for ions of astrophysical interest

An interactive FORTRAN program is presented which determines the physical conditions in a nebula given an appropriate emission-line intensity ratio, or the line emissivities given a density and temperature. A method for estimating ionic abundances based on the H-beta emissivity is also included. The relevant underlying physics is reviewed and a detailed description of the operation of the program given. Uncertainties in the interpretation of the results are discussed briefly.

De Robertis, M. M.↗

NASA NTRS1986

Ultraviolet and visual wavelength spectroscopy of gas around ETA Carinae

Observational results of Eta Carinae are reported, especially spectroscopy of the outer 'S condensation' supplemented by data on the homunculus and its core. Theoretical calculations of atmosphere/wind models and of the shock-heated S condensation are needed for a proper analysis of the data, but some simplified results are discussed. The helium abundance at the surface of Eta Car appears to be roughly 0.4, and most of the CNO is nitrogen. There does not appear to be any reason, at present, to alter the often quoted temperature estimate of 30,000 K for the radiating surface (which may or may not be the surface of the star itself). The presently observed mass loss rate is probably less than 10 exp -2.4 solar masses/yr if the outflow is not strongly direction-dependent. Finally, a largely forgotten but highly relevant historical conjecture concerning Eta Car is mentioned.

Davidson, K.↗

NASA NTRS1984

The unique planetary nebula NGC 2818

The planetary nebula NGC 2818, or PK 261 + 8 deg 1, is of special interest due to its apparent association with a Population I open cluster (of the same designation). The results of new observations of the nebula are presented, including interference-filter imagery, photoelectric photometry, and ground-based and IUE spectroscopy. Analysis of its composition based on new IUE and ground-based spectroscopy and model calculations of the very similar planetary nebula NGC 2440 suggests that NGC 2818 is a prototype of the He-rich and N-rich Type I group. Compared to the composition of H II regions in the solar neighborhood, He is 60 percent overabundant, C is 0.3 dex lower, N is 1.0 dex higher, and O, Ne, Si, Cl, and Ar are essentially at comparable values. Analysis of kinematical and morphological data suggests that the nebula is probably associated with the star cluster (revised distance = 3.5 kpc). If so, then it is of exceptional size (radius approximately 1.1 pc), mass (greater than or equal to 0.6 solar mass), expansion velocity (52 + or - 3 km/s), and age (possibly as old as 22,000 yr). Some implications of the results regarding the masses of planetary-nebula progenitors, the nucleosynthesis processes in stars of intermediate mass, and the formation of planetary nebulae are also discussed.

Dufour, R. J.↗

NASA NTRS1984

Imagery and spectroscopy of supernova remnants and H-2 regions

Research activities relating to supernova remnants were summarized. The topics reviewed include: progenitor stars of supernova remnants, UV/optical/radio/X-ray imagery of selected regions in the Cygnus Loop, UV/optical spectroscopy of the Cygnus Loop spur, and extragalactic supernova remnant spectra.

Dufour, R. J.↗

NASA NTRS1983

Digital analysis of narrow-band imagery of the Cygnus Loop

Calibrated surface brightness and spectral line ratio maps which involve emission from six different ionic species are derived from the digital analysis of narrow passband direct imagery of a field in the southeast part of the Cygnus Loop SNR, and the spatial structure is categorized on the basis of the presence or absence of features in images isolating different emission lines. While features present in all emission lines are found to have spectral behavior consistent with steady flow shock model calculations for a range of v(s), the features present only in such 'hot' lines as the O III forbidden line 5007 A are found to account for relative intensities of that line greater than can be matched by steady flow model calculations.

Parker, R. A. R.↗

NASA NTRS1983

The probable association of SN 1957d in NGC 5236 (M83) with an unresolved radio source

Astrometric measurements of the position of SN 1957d in the spiral galaxy NGC 5236 using archival plates indicate that it is within 2 arcsec of the position of a point radio source of approximately Cas A strength on a VLA map made by Cowan and Branch (1982). This coincidence strongly suggests that the radio source originated from the supernova. If further investigation shows this point source to indeed be a supernova remnant, then this would be the first reported detection of an intermediate-age supernova remnant.

Pennington, R. L.↗

NASA NTRS1982

Velocity dispersions of knots in the Cygnus Loop and IC 443

Very high resolution spectroscopy of optical emission lines indicates that the velocity dispersions of knots in Cygnus Loop and IC 443 filaments result primarily from turbulence within the emitting regions. Line-of-sight velocity dispersions (half-widths at half-maximum) for the observed knots are on the order of 10-30 km/s, including associated thermal velocity dispersions of roughly 15 km/s for hydrogen, 6 km/s for oxygen, and 4 km/s for nitrogen. The knots themselves move randomly relative to each other with speeds of 10-30 km/s. Occasionally, diffuse components of the filaments are observed. These diffuse components have velocity dispersions between 60 and 85 km/s, but the relative contributions of turbulent and thermal motion to the dispersions are unkown.

Shull, P., Jr.↗

NASA NTRS1982

The carbon abundance in the Magellanic Clouds from IUE observations of H II regions

Results are given of observations of the UV spectra of several H II regions in the Magellanic Clouds, with emphasis on obtaining the carbon abundance from the UV lines of C II, C III, and C IV. The observations and results are described, and theoretical model calculations are presented of the H II regions and the relative abundances of H, He, C, N, O, S, Cl, and Ar computed from the UV and visual spectra. In addition, the abundance results for the Magellanic Clouds are compared with the sun, the Orion Nebula, and galactic planetary nebulae. In comparison with solar abundances, carbon is found to be depleted relative to hydrogen in the Magellanic Clouds by an amount larger than that of nitrogen and oxygen in the SMC and by an amount between that of nitrogen and oxygen in the LMC. It is noted that the results are consistent with a scenario that nitrogen and oxygen have greater primary yields in massive stars than carbon, which may be produced predominantly in less massive stars (such that C/N and C/O in galaxies are initially very low and subsequently increase with evolution). Subsequently, the C/N ratio may decrease as secondary production of nitrogen in less massive stars becomes significant.

Dufour, R. J.↗

NASA NTRS1982

CNO abundances in H II regions of the Magellanic clouds and the galaxy with implications regarding the nucleosynthesis of the CNO element group

Final abundance results of IUE observations of the UV spectra of three H II regions in the Small Magellanic Cloud and four H II regions in the Large Magellanic Cloud are presented. Calculated yields of carbon and oxygen derived are y(C)=.00063 and y(O)=.0016. The nucleosynthetic origin of nitrogen was evaluated as being predominantly a secondary element produced from carbon as its seed. Plotting log N/C versus log C/H yielded the rather unexpected result that log N/C decreases with lo C/H over the SMC-LMC-Orion range. The cause of this relationship is discussed.

Dufour, R. J.↗

NASA NTRS1981

Optical studies of the regions of the gamma-ray bursts of 19 November, 1978 and 5 March, 1979

Optical studies were made of two well-located gamma-ray burst regions in the Southern Hemisphere using the 1.5 m telescope at the Cerro Tololo Inter-American Observatory in Chile. Multi-filter image tube plates and UBV photometry surveys were made of both regions. The field of the 19 November, 1978 gamma-ray burst shows no early star or star with unusual colors out to several diameters of the nominal error box. The region of the 5 March, 1979 gamma-ray burst, which includes N49, contains several blue giants in the LMC, two of which deserve further study.

Fishman, G. J.↗

NASA NTRS1981

The carbon abundance in two h 2 regions of the small Megallanic Cloud

Observations of the ultraviolet spectra of two locations in the H II region NGC 346 and of the entire H II region IC 1644 in the Small Magellanic Cloud (SMC) were made using the International Ultraviolet Explorer (IUE) satellite. The abundance of carbon in the nebulae was derived using theoretical model analysis combined with ground-based spectrophotometry of other emission lines. The abundance of C relative to H in the SMC was found to be lower by -0.9 dex compared with the Sun and lower by -0.8 dex compared with the Orion Nebula. This C deficiency is similar to that of O, Ne, S, and Ar in the SMC, but not as great as found for N. The sites and history of C nucleosynthesis in galaxies is similar to that of O, Ne, S, and Ar, in contrast to that of N, which appears to be more complex, perhaps because of a mixture of secondary primary sources or a significant contribution from intermediate-mass long-lived stars.

Dufour, R. J.↗

NASA NTRS1979

Picture processing analysis of the optical structure of NGC 5128 /Centaurus A/

Results are presented for a detailed study of the peculiar elliptical galaxy NGC 5128 (Cen A), based on computer video analysis of several photographic plates of exceptional quality reduced to the standard UBV system. The picture-processing results and the measured properties of the elliptical and gaseous-disk components of NGC 5128 are examined, along with the distribution, spectral characteristics, and chemical composition of the H II regions in the disk. The data show that NGC 5128 consists of a giant E2 galaxy containing a significant amount of gas and dust situated predominantly in an equatorial disk where vigorous star formation is occurring. Reasons why NGC 5128 is so different from giant ellipticals in clusters are considered.

Dufour, R. J.↗

NASA NTRS1977

The chemical composition of three planetary nebulae in the Magellanic Clouds

The paper studies the detailed spectral characteristics of the planetary nebulae N97 and N153 in the Large Magellanic Cloud (LMC), the planetary nebula N67 in the Small Magellanic Cloud (SMC), and the small H II regions N9, N61, and N81 in the SMC. Electron temperatures and densities for each nebula are derived from emission-line strengths determined by photographic spectrophotometry, and relative abundances are estimated for H, He, N, O, Ne, Ar, and S. The results show that: (1) N67 has a 60% overabundance in He/H while N97 and N153 have approximately normal He/H values; (2) all three planetaries have N/H values comparable to those of galactic planetaries but substantially higher than found in the H II regions of their respective Cloud; (3) the O/H values in the planetaries are similar to or lower than those in the H II regions of the respective Cloud; (4) the O/Ne, O/S, and O/Ar ratios in the two LMC planetaries are comparable to those in galactic planetaries; (5) the compositions of the small H II regions in the SMC are nearly identical to those of previously studied large H II regions in the same Cloud; and (6) the He/H ratio of the interstellar gas in the SMC is about 25% less than that in the Galaxy. It is concluded that most of the N abundance in both Clouds arose from nucleosynthesis sources other than planetary nebulae.

Dufour, R. J.↗

NASA NTRS1976

Emission nebulae in the Magellanic Clouds. I - The H II regions

A review of optical and radio studies of the H II regions in the Magellanic Clouds is presented. Recent studies are emphasized, particularly optical studies of the kinematics, spectra, and chemical composition. The implications of the studies for the understanding of the evolution of stars and galaxies in general, and the Magellanic Clouds in particular, are noted. Suggestions for future work are also given.

Dufour, R. J.↗