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At least 19 records
All sourcesNASA NTRSDOE OSTI
NASA NTRS1990

Distances of Galactic WC stars from emission-line fluxes and a quantification of the WC classification

The extent to which later-type WC stars in the Galaxy show constant flux in the carbon lines is assessed. It is found the the WC stars are an extremely 'well-behaved' sequence in terms of quantitative spectroscopic properties. The two classification parameters are closely correlated with a scatter of the order of uncertainty. There appear to be natural breaks in the sequence between subclasses, except between WC5 and 6. The classification is quantified using the C IV 5808 A/C III 5696 A ratio as the principal defining parameter for WC7-9 stars and the C III 5696 A/O V 5590 A ratio as the principal defining parameter for WC4-6 stars. Preliminary calibration of the line fluxes indicate that the flux of C IV 5808 A in Galactic WC stars is constant at F(0) 5808 = -log F(0)(5808) = 8.1 at 1 kpc for subclasses WC5-7. The intrinsic line flux ratio C(0) = log f(0)(5808/4650) is a smooth function of subclass number, decreasing from -0.22 dex for WC4 stars to -0.56 dex for WC9 stars.

Smith, Lindsey F.↗
NASA NTRS1986

Spectroscopic studies of Wolf-Rayet stars. III - The WC subclass

Wolf-Rayet (W-R) stars, which are the descendants of massive O-type stars, can be subdivided into three groups depending on their spectral appearance. These groups include the nitrogen class (WN), the carbon class (WC), and the oxygen class (WO). The present paper is concerned with the WC stars. The assignment of WC subtypes has been based on visual inspections of photographic plates. One of the aims of this study is related to the quantification of the visual estimates. The measured ratios of equivalent widths and the FWHM of the 4650 A line for Galactic and LMC stars are presented, and the reclassification of some stars is proposed on this basis. In particular, it is shown that the majority of the LMC WC stars should logically be classified WC4 instead of WC5. Comments on individual stars are provided, and terminal velocities are discussed. It is attempted to give a complete overview of the most important spectroscopic features of the WC stars in the optical region.

Torres, A. V.↗
NASA NTRS1973

An interpretation of the WC stars.

The difference in separation that is characteristic of WN and WC binaries is shown to provide an explanation for the observation that spectra of WC stars are apparently unaffected by the presence of a companion, whereas the spectra of WN stars in binary systems appear to be significantly different from those of single stars with a comparable degree of excitation. WC binaries are sufficiently widely separated for the WC atmosphere to remain unperturbed by the presence of the companion, while the companions of WN stars are closer and gravitational perturbation, reflection effects, and limitations of the atmosphere's extent by the Roche lobe may each have an effect.

Smith, L. F.↗
NASA NTRS1980

Effect of W and WC on the oxidation resistance of yttria-doped silicon nitride

The effect of W and WC contamination on the oxidation and cracking in air of sintered Si3N4 - 8 w/o Y2O3 ceramics at 500, 750, and 1350 C is examined. A mixture of Si3N4 - 8Y2O3, milled with alumina balls, was divided into four portions. Three portions were doped with 2 w/o WC W, and 4 w/o W respectively, in order to simulate contamination during milling. The fourth portion was undoped and used on a control. The addition of W or WC did not affect the phase relationships in the system, as all bars with or without additions contained melilite as the major Si-Y-O-N phase after sintering. At 750 C, instability (rapid oxidation and cracking) of W-doped bars appears to have occurred as a result of oxidation of the tungsten containing melilite phase. No intermediate temperature instability was observed in bars containing 2 w/o WC or in bars with no additive. Specimens exposed at 1350 C had good oxidation resistance due to the formation of a protective siliceous oxide layer. A specimen containing 4 w/o W which was preoxidized at 1350 C had improved oxidation resistance at 750 C. The tendency towards oxidation and cracking of Si3N4 - 8 Y2O3 at 750 C is concluded to be related to tungsten content of the sintered bars.

Schuon, S.↗
NASA NTRS1988

Tensile and creep rupture behavior of P/M processed Nb-base alloy, WC-3009

Due to its high strength at temperatures up to 1600 K, fabrication of niobium base alloy WC-3009 (Nb30Hf9W) by traditional methods is difficult. Powder metallurgy (P/M) processing offers an attractive fabrication alternative for this high strength alloy. Spherical powders of WC-3009 produced by electron beam atomizing (EBA) process were successfully consolidated into a one inch diameter rod by vacuum hot pressing and swaging techniques. Tensile strength of the fully dense P/M material at 300-1590 K were similar to the arc-melted material. Creep rupture tests in vacuum indicated that WC-3009 exhibits a class 1 solid solution (glide controlled) creep behavior in the 1480 to 1590 K temperature range and stress range of 14 to 70 MPa. The creep behavior was correlated with temperature and stress using a power law relationship. The calculated stress exponent n, was about 3.2 and the apparent activation energy, Q, was about 270 kJ/mol. The large creep ductility exhibited by WC-3009 was attributed to its high strain rate sensitivity.

Hebsur, Mohan G.↗
NASA NTRS1974

Free energies of formation of WC and WzC and the thermodynamic properties of carbon in solid tungsten

The activity of carbon in the two-phase regions - W + WC and W + W2C was obtained from the carbon content of iron rods equilibrated with mixtures of metal plus carbide powders. From this activity data the standard free energies of formation of WC and W2C were calculated. The temperature of the invariant reaction W2C = W + WC was fixed at 1570 + or - 5K. Using available solubility data for C in solid W, the partial molar free energy of C in the dilute solid solution was also calculated. The heat of solution of C in W, and the excess entropy for the interstitial solid solution, were computed, assuming that the carbon atoms reside in the octahedral interstices of bcc W.

Gupta, D. K.↗
NASA NTRS1980

Effect of W and WC on the oxidation resistance of yttria-doped silicon nitride

The effect of tungsten and tungsten carbide contamination on the oxidation and cracking in air of yttria-doped silicon nitride ceramics is investigated. Silicon nitride powder containing 8 wt % Y2O3 was doped with 2 wt % W, 4 wt % W, 2 wt % WC or left undoped, and sintered in order to simulate contamination during milling, and specimens were exposed in air to 500, 750 and 1350 C for various lengths of time. Scanning electron and optical microscopy and X-ray diffraction of the specimens in the as-sintered state reveals that the addition of W or WC does not affect the phase relationships in the system, composed of alpha and beta Si3N4, melilite and an amorphous phase. Catastrophic oxidation is observed at 750 C in specimens containing 2 and 4 wt % W, accompanied by the disappearance of alpha Si3N4 and melilite from the structure. At 1350 C, the formation of a protective glassy oxide layer was observed on all specimens without catastrophic oxidation, and it is found that pre-oxidation at 1350 C also improved the oxidation resistance at 750 C of bars doped with 4 wt % W. It is suggested that tungsten contamination from WC grinding balls may be the major cause of the intermediate-temperature cracking and instability frequently observed in Si3N4-8Y2O3.

Schuon, S.↗
NASA NTRS1988

Carbon and helium abundances in Wolf-Rayet WC stars determined from optical recombination lines

Carbon to helium ratios and ionization fractions are derived for 74 WC stars in the Galaxy and the LMC from optically thin recombination lines at visual wavelengths. The range of C/He ratios is 0.13 to 0.79 by number, similar to the ratios calculated by Nugis (1975) and about 20 times greater than the ratios of Smith and Willis (1982). These ratios also agree with the abundances determined in the most recent evolutionary models of massive stars, which include mass loss, convective dredge-up, and new nuclear reaction rates. The range of values found for the C/He ratios implies that the convective cores of WC stars do not occupy a very large mass fraction. The C/He ratio differs within a subtype by a factor of three or less, but the mean tends to increase toward earlier subtypes, although the standard deviation from the mean is large enough to allow the interpretation that the C/He ratio is constant for all WC subtypes.

Torres, Ana V.↗
NASA NTRS1987

Decision support for workload assessment - Introducing WC FIELDE

Currently there is a great demand for mental workload evaluation in the course of system design and modification. In light of this demand, a microprocessor-based decision support system has been created called WC FIELDE: Workload Consultant for FIELD Evaluation. The system helps the user select workload measures appropriate to his or her application from the large pool of currently available techniques. Both novices and those with some workload experience may benefit from using WC FIELDE, since the system's operation is entirely transparent and all rules involved in the decision process are available for the user to examine. WC FIELDE recommends several assessment methodologies in decreasing order of appropriateness, and provides additional information on each measure at the end of the program in the form of text files.

Casper, Patricia A.↗
NASA NTRS1999

Stellar C III Emissions as a New Classification Parameter for (WC) Central Stars

We report detection of stellar C III lambda 1909 emission in International Ultraviolet Explorer (IUE) echelle spectra of early-type [WC] planetary nebula central stars (CSPNs). Additionally, stellar C III emission at lambda 2297 is observed in early- and late-type [WC) CSPNS. Inclusion of these C III features for abundance determinations may resolve a conflict of underabundance of C/O for early type [WC2] - [WC4] CSPNS. A linear dependence on stellar C III lambda 2297 equivalent widths can be used to indicate a new classification of type [WCUV] central stars.

Feibelman, W. A.↗
NASA NTRS1988

C/He abundances in WC stars

Recombination theory has been used to determine the C/He abundances in 17 southern WC stars from spectra in the H and K bands. An LTE treatment of C II provides C/He values in the 0.04-0.3 range, by number, and maximum allowance for possible non-LTE effects yields values in the 0.1-0.7 range, by number. C/He is shown to decrease from WC4 to WC7. The ionization balance is found to be lower than in previous reports. He(+)/He(2+) values are between 2.6 and 3.0 for WC5-8 stars, increasing to 7 for WC9 stars. Continuum slopes and corrections for the emission line contributions to the H and K magnitudes are evaluated.

Smith, Lindsey F.↗
NASA NTRS1989

A case study of a WC planetary nebula nucleus - Henize 2-99

The WC spectrum of the nucleus of the planetary nebula Henize 2-99. Fluxes are given, emission lines are identified, and blends are described. The most powerful emissions are C III, C II, and C IV. O, H, and Si lines are also observed, but little stellar nitrogen is found except for N V. The interstellar extinction is estimated at c = 0.9 + or - 0.2. Blackbody, Zanstra, and Stoy temperatures are found to be 70,000 K, 26,000 K, and 27,000 K, respectively. It is shown that He 2-99 is binebulous with a prominent enhancement in H-alpha along the minor axis. The structural characteristics of He 2-99 are very similar to BD +30 deg 3639 and NGC 40.

Kaler, James B.↗
NASA NTRS1989

Mid-infrared spectra of WC 9 stars - The composition of circumstellar and interstellar dust

Broad emission features at about 7.7 microns and absorption features at about 6 and 10 microns observed from the WC 9 stars AFGL 2104 and Ve 2-45 are analyzed. The 7.7-micron feature is shown to resemble the well-known PAH band attributed to CC stretching modes in carbonaceous materials. The results suggest a low H abundance in the outflow of these objects, and that, given a strong UV source, all C-rich dust-enshrouded objects will show (some of) the IR emission bands. It is noted that at least 6 percent of the C and 4 percent of the O are locked up in C=O groups in interstellar silicates.

Cohen, M.↗
NASA NTRS1993

C III spectra in WC Wolf-Rayet stars - Does collisional excitation dominate?

A direct comparison of the spectra emitted by an improved collisionally excited C III atomic model, with observations of C III spectra in Wolf-Rayet WC stars, shows agreement for UV, visible, and near-infrared lines including lines usually considered to be recombination lines. The agreement implies high-density and temperature source conditions corresponding to log (Ne Te) is greater than 16 as a lower limit, whereas most current modeling assumes log (Ne Te) is less than 15.5. This raises questions concerning the photoionization/recombination assumptions on which most WR modeling is based. Recent models are discussed from this point of view.

Kastner, S. O.↗
NASA NTRS1995

An IRAS-based search for new Dusty Late-Type WC Wolf-Rayet stars

I have examined all Infrared Astronomical Satellite (IRAS) data relevant to the 173 Galactic Wolf-Rayet (W-R) stars in an updated catalog, including the 13 stars newly discovered by Shara and coworkers. Using the W-R coordinates in these lists, I have examined the IRAS Point Source Catalog (PSC), the Faint Source Catalog, and the Faint Source Reject Catalog, and have generated one-dimensional spatial profiles ('ADDSCANs') and two-dimensional full-resolution images ('FRESCOs'). The goal was to assemble the best set of observed IRAS color indices for different W-R types, in particular for known dusty late-type WC Wolf-Rayet (WCL) objects. I have also unsuccessfully sought differences in IRAS colors and absolute magnitudes between single and binary W-R stars. The color indices for the entire ensemble of W-R stars define zones in the IRAS color-color plane. By searching the PSC for otherwise unassociated sources that satisfy these colors, I have identified potential new W-R candidates, perhaps too faint to have been recognized in previous optical searches. I have extracted these candidates' IRAS low-resolution spectrometer (LRS) data and compared the spectra with the highly characteristic LRS shape for known dusty WCL stars. The 13 surviving candidates must now be examined by optical spectroscopy. This work represents a much more rigorous and exhaustive version of the LRS study that identified IRAS 17380 - 3031 (WR98a) as the first new W-R (WC9) star discovered by IRAS. This search should have detected dusty WCL stars to a distance of 7.0 kpc from the Sun, for the absolute value of l greater than 30 deg, and to 2.9 kpc even in the innermost Galaxy. For free-free-dominated W-R stars the corresponding distances are 2.5 and 1.0 kpc, respectively.

Cohen, Martin↗
NASA NTRS1995

An IRAS-Based Search for New Dusty Late-Type WC Wolf-Rayet Stars

I have examined all Infrared Astronomical Satellite (IRAS) data relevant to the 173 Galactic Wolf-Rayet (W-R) stars in an updated catalog, including the 13 stars newly discovered by Shara and coworkers. Using the W-R coordinates in these lists, I have examined the IRAS Point Source Catalog (PSC), the Faint Source Catalog, and the Faint Source Reject Catalog, and have generated one-dimensional spatial profiles, 'ADDSCANs', and two-dimensional full-resolution images, 'FRESCOS'. The goal was to assemble the best set of observed IRAS color indices for different W-R types, in particular for known dusty late-type WC Wolf-Rayet (WCL) objects. I have also unsuccessfully sought differences in IRAS colors and absolute magnitudes between single and binary W-R stars. The color indices for the entire ensemble of W-R stars define zones in the IRAS color-color ([12] - [25], [25] - [60])-plane. By searching the PSC for otherwise unassociated sources that satisfy these colors, I have identified potential new W-R candidates, perhaps too faint to have been recognized in previous optical searches. I have extracted these candidates' IRAS low-resolution spectrometer (LRS) data and compared the spectra with the highly characteristic LRS shape for known dusty WCL stars. The 13 surviving candidates must now be ex amined by optical spectroscopy. This work represents a much more rigorous and exhaustive version of the LRS study that identified IRAS 17380 - 3031 (WR98a) as the first new W-R (WC9) star discovered by IPAS. This search should have detected dusty WCL stars to a distance of 7.0 kpc from the Sun, for l is greater than 30 degrees, and to 2.9 kpc even in the innermost galaxy. For free-free-dominated W-R stars the corresponding distances are 2.5 and 1.0 kpc, respectively.

Cohen, Martin↗