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Search indexed NASA NTRS and DOE OSTI research on propulsion, heat transfer, battery materials and energy systems. Follow report and document links to the original sources.

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At least 415 records · Page 23
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NASA NTRS1986

The exciting star of HH 57

The first spectral classification for the exciting star of HH 57 is provided. The star is an F8 III and suffers about 5 mag of visual extinction, far greater than that toward the HH 57A knot, only a few arcseconds away. This difference supports the previous identification of a flattened geometry for the circumstellar dust around this star. Using IRAS photometry, a distance of 940 pc to HH 57 is determined by bolometrically matching the observed luminosity of the star to those of FU Ori and V1057 Cyg.

Cohen, M.↗
NASA NTRS1985

The intrinsic ultraviolet continua of O stars

Low-resolution IUE spectra of O and B stars in open clusters have been obtained. The main-sequence B stars are used to derive UV extinction curves for the program clusters, and these curves are used to determine the intrinsic UV to visual colors of O stars in NGC 2244, Trumpler 14, and NGC 6231. These data are analyzed in conjunction with the dereddened colors of O stars with color excesses less than 0.15 mag to obtain a set of reliable UV colors for O3-9, class V-I stars. The results can be summarized as follows: none of the observed O stars have UV colors significantly bluer than a normal B0 V star, and the late O supergiants have significantly redder UV colors than main-sequence O stars, although this luminosity distinction appears to vanish for the early O stars. Several possible explanations for these unexpected results are considered, but none of the presently available nonstandard models are sufficiently developed to be definitively tested.

Massa, D.↗
NASA NTRS1985

The abundance of atomic carbon near the ionization fronts in M17 and S140

The 492 GHz ground-state line of atomic carbon in the edge-on ionization fronts in M17 and S140 were observed. It was found that, contrary to expectation, the C I emission peaks farther into the molecular cloud from the ionization front than does the CO. In fact the peak C I abundance in M17 occurs more than 60 mag of visual extinction into the cloud from the ionization front. Calculations of the ratio of C I to CO column densities yield values of 0.1-0.2. These observations do not support chemical models which predict that neutral atomic carbon should be found only near the edges of molelcular clouds. Other models are discussed which may explain the observations.

Keene, J.↗
NASA NTRS1986

The Shane-Wirtanen counts - Systematics and two-point correlation function

Residual errors in the Selder et al. (SSGP) map which caused a break in both the correlation factor (CF) and the filamentary appearance of the Shane-Wirtanen map are examined. These errors, causing a residual rms fluctuation of 11 percent in the SSGP-corrected counts and a systematic rms offset of 8 percent in the mean count per plate, can be attributed to counting pattern and plate vignetting. Techniques for CF reconstruction in catalogs affected by plate-related systematic biases are examined, and it is concluded that accurate restoration may not be possible. Surveys designed to measure the CF at the depth of the SW counts on a scale of 2.5 deg, must have systematic errors of less than or about 0.04 mag.

De Lapparent, V.↗
NASA NTRS1986

The KPD survey for galactic plane ultraviolet-excess objects Space densities of white dwarfs and subdwarfs

A survey for ultraviolet-excess objects has been performed for over about 1000 sq deg of the Galactic plane. Photographic blue and ultraviolet plates were obtained to select ultraviolet-excess candidates, and spectroscopic observations were used to determine the nature of those objects. Over 70 white dwarfs and subdwarfs were identified from the survey plates, and these objects were analyzed statistically to determine scale heights and space densities. The sample was complete to B = 15.3 mag, with the space density of white dwarfs for Mv of less than 12.25 equal to 0.72 + or 0.25/1000 cu pc, slightly higher than that from the Palomar-Green survey. The average absolute magnitude determined for both the subdwarf O and the subdwarf B stars was considerably fainter than previous determinations, while the scale height for the subdwarfs was lower than earlier suggested values. The space density for the subdwarf O stars is about 7 x 10 to the -7th/cu pc, while the value for the subdwarf B stars is about 2 x 10 to the -6th/cu pc, both values considerably higher than previous estimates. The space densities can be used to place evolutionary constraints on these objects.

Downes, R. A.↗
NASA NTRS1986

Multiple star formation and the dynamical evolution of B335

The detection of a second high-velocity bipolar flow in the dark cloud B335 located in the diffuse envelope to the east of the core is reported. This flow has the same orientation as the first one. The visual extinction in the region between the redshifted and blueshifted lobes is less than or about 1 mag and is not coincident with a condensation. The flow is not associated with any known infrared source in the IRAS catalog. The new flow is about three times older than the first, and its energetics are somewhat smaller. The flows are aligned not only with each other but also with the long axis of the structure of B335 as outlined by CO maps, suggesting that magnetic fields have a role in guiding the flow. From the presence of multiple flows and the structure of the envelope, it is concluded that B335 is at the end of its starbearing life, rather than a young cloud beginning star formation, as previously supposed.

Langer, W. D.↗
NASA NTRS1986

Diffuse interstellar bands in low color excess stars. III

Measurements of the equivalent widths of the diffuse interstellar bands at 6284 A and 6613 A in the spectra of 23 stars with E(B-V) less than 0.1 mag are reported. The band strengths do not correlate with interstellar H2 column density. They are probably produced by molecular mantles on grains or very stable molecules such as the polycyclic aromatic hydrocarbons.

Kumar, C. K.↗
NASA NTRS1985

The identification of H2311 + 77 with HD220140, a probable RS CVn star

H2311 + 77, one of the bright soft X-ray sources in the sky originally detected with HEAO 1, has been identified in an Exosat imaging observation with a 7.7-mag late-type star, HD220140. The X-ray luminosity and the optical characteristics of this star indicate that it is probably an RS CVn binary system.

Pravdo, S. H.↗
NASA NTRS1986

Photometry of small asteroids and cometary cores

The first asteroid lightcurves have now been made with a new technique of CCD photometry. The apparent magnitude is fainter (V>17) than what can be done with the 1.52-m Catalina reflector with a photomultiplier photometer. With the CCD system, however, the lightcurve shows remarkably good repetition; finding the asteroid is, of course, no problem as the object is recognized later by its motion on the CCD. Asteroid 1985RV has a lightcurve amplitude of about 0.4 mag and its period of rotation P = 4.0 hours, on the assumption that the lightcurve has two maxima and two minima as is the case for nearly all other asteroids. The diameter is about 3 km. 1985RV is a first example of results that are being obtained on asteroids and comets with CCD in the Catalinas, Kitt Peak, and Cerro Tololo.

Gehrels, T.↗
NASA NTRS1986

Translucent molecular clouds: Theory and observations

Few suitable stars behind molecular clouds have been identified. A limited survey was performed of interstellar lines toward highly reddened stars in the southern sky, using the ESO 1.4 m CAT telescope with a Reticon detector, and the Cerro Tololo 4 m telescope equipped with a GEC charge coupled device (CCD) detector. Because of the reduced extinction at longer wavelengths, molecules were searched for with transitions in the red part of the spectrum such as C2 and CN. For some lines-of-sight for which C2 was detected, the 4300 A line of CH was also observed. Absorption lines of interstellar C2 around 8750 A were detected in the spectra of about 1/4 of the 36 observed stars. The inferred C2 column densities range between 10 to the 13th power and 10 to the 14th power sq. cm., and are up to an order of magnitude larger than those found for diffuse clouds. The observed column densities of CH correlate very well with those of C2 over this range. In contrast, the measured column densities of CN vary by orders of magnitude between the various regions, and they do not correlate with those of C2 and CH. The observed rotational population distribution of C2 also provides information about the physical conditions in the clouds. Models of translucent molecular clouds have been constructed along the lines described by van Dishoeck and Black (1986) for diffuse clouds. The models compute accurately the fractions of atomic and molecular hydrogen as functions of depth into the clouds, as well as the excitation of H2 by ultraviolet pumping. They also incorporate a detailed treatment of the photodissociation processes of the molecules (cf. van Dishoeck 1986), which play an important role in the chemistry up to depths of about 3 mag.

Vandishoeck, E. F.↗
NASA NTRS1987

Ground-based 1- to 32-microns observations of ARP 220: Evidence for a dust-embedded AGN?

New observations of the 10 and 20 micron size of the emission region in Arp 220 are presented. Also given are ground based photometry from 1 to 32 micron including measurements of the strength of the silicate feature at 10 micron. The results show that the 20 micron size of Arp is smaller than 1.5 arcsec (500 pc); comparison of IRAS and ground based observations show that IRAS 12 micron flux measured with a large arcmin beam is the same as that seen from the ground with a 3 arcsec aperture. At 10 micron a deep silicate absorption feature is seen that corresponds to a visual extinction of about 50 mag. These results suggest that a very significant portion of the 10 to the 12th power L sub 0 infrared luminosity from Arp 220 comes from a region less than or of the order of 500 pc in diameter. When these results are combined with recent measurement of a broad Brackett alpha line by DePoy and an unresolved 2.2 micron source by Neugebauer, Matthews and Scoville, a very attractive possibility for the primary luminosity source Arp 220 is a dust embedded compact Seyfert type nucleus.

Becklin, E. E.↗
NASA NTRS1987

Microwave characterization and modeling of GaAs/AlGaAs heterojunction bipolar transistors

The characterization and modeling of a microwave GaAs/AlGaAs heterojunction Bipolar Transistor (HBT) are discussed. The de-embedded scattering parameters are used to derive a small signal lumped element equivalent circuit model using EEsof's Touchstone software package. Each element in the equivalent circuit model is shown to have its origin within the device. The model shows good agreement between the measured and modeled scattering parameters over a wide range of bias currents. Further, the MAG (maximum available power gain) and the h sub 21 (current gain) calculated from the measured data and those predicted by the model are also in good agreement. Consequently, the model should also be capable of predicting the f sub max and the f sub T of other HBTs.

Simons, Rainee N.↗
NASA NTRS1986

Photometry of occultation candidate stars. II - Uranus 1985-1990 and Saturn 1986-1991

K magnitudes are given for seven Uranus-occulted stars, together with V and I magnitudes for 20 stars that will be occulted by the Uranian rings in 1986-1990 and 15 that will be occulted by the rings of Saturn in the course of 1986-1991. Occultation stars are noted to be up to 1 mag brighter in the K bandpass than could be predicted on the basis of V-I colors alone; this is explainable in terms of typical degrees of interstellar reddening.

French, L. M.↗
NASA NTRS1986

Near-ultraviolet spectroscopy of Seyfert nuclei - Reddening and Bowen fluorescence

Spectra with high signal-to-noise ratios extending down to the atmospheric ultraviolet cut-off have been obtained for 11 Seyfert nuclei with narrow emission lines. In all objects the He II lines at 3204 A and 4686 A were measured simultaneously. Their ratio gives an estimate of reddening accurate to 0.14 mag in E(B-V). The redshifts of nine of the Seyfert galaxies are sufficient to allow detection of the O III 3133 line A, produced by Bowen fluorescence of He II Ly-alpha photons. The inferred efficiency of the Bowen mechanism ranges from 15 to 50 percent. The lower values, which are considerably less than the efficiencies found in planetary nebulae, are observed in Seyfert 2 nuclei with systematically weaker emission lines from high-excitation, high-density gas. In contrast, the Seyfert 1 and high-excitation Seyfert 2 nuclei have more 'normal' Bowen efficiencies. Several possible explanations are considered.

Malkan, Matthew A.↗
NASA NTRS1986

A theoretical and observational study of the Red Giant Branch phase transition in Magellanic Cloud clusters - A progress report

Preliminary results are reported for an investigation comparing theoretical models of the sudden appearance of an extended RGB (and its effects on the spectral energy distributions of stellar populations) with data from ESO CCD observations of clusters in the LMC and SMC. Isochrones for the entire RGB are being constructed on the basis of 100 new evolutionary sequences (calculated using the evolution code of Sweigart and Gross, 1976 and 1978) to permit determination of synthetic colors and spectral energy distributions. The observations so far indicate a main sequence about 0.1 mag redder than that predicted by the present models or by the isochrones of VandenBerg and Bell (1985), and fail to show a B-V color difference at the RGB phase transition.

Buonanno, R.↗
NASA NTRS1986

Time-resolved spectrophotometry of the AM Herculis system E2003 + 225

Time-resolved, medium-resolution photometry is reported for the binary system E2003 + 225 over a complete orbital period in 1984. The object was 1.5-2 mag fainter than when viewed earlier in 1984. The fluxes, equivalent widths and full widths at FWHM for dominant lines are presented for four points in the cycle. A coincidence of emission lines and a 4860 A continuum line was observed for the faster component, which had a 500 km/sec velocity amplitude that was symmetric around the zero line. An aberrant emission line component, i.e., stationary narrow emission lines displaced about 9 A from the rest wavelengths, is modeled as Zeeman splitting of emission from material close to the primary.

Mccarthy, Patrick↗
NASA NTRS1987

The moderately interacting Algol binary RS Cephei

In agreement with previous observers, it is found that the eclipsing binary RS Cep is an Algol-type semidetached system. From IUE spectra and Lick Observatory optical scans, the spectral types of the components are determined as B9.7e V + G8 III-IV. The effective temperature of the primary star is found to be 9670 K (with an uncertainty of about 150 K). From the photometric elements, the spectral types, and the semidetached nature of the system, a satisfactory model of the system is developed, giving masses 2.4 and 0.4 solar mass, radii 2.3 and 7.6 solar radii, and separation of the components 32 solar radii. This model leads to a distance of 910 pc; the color excess is E(B - V) = 0.03 mag. Balmer emission lines are observed in the optical scans, so that the system is a Be star. An IUE spectrum obtained during the total primary eclipse reveals a number of UV emission lines, typical for interacting Algols and W Ser stars. Power emitted in these lines is about twice as high as that in TT Hya or U Cep, therefore, RS Cep is a relatively active Algol system. A broad 'bulge' of excess relative flux is observed roughly between wavelengths 210 and 270 nm in the totality spectrum, and interpreted as broad blends of emission lines of Fe II; the same Fe II lines, formed in a circumstellar shell or disk, cause additional absorption in the out-of-eclipse spectra.

Plavec, Mirek J.↗
NASA NTRS1987

The moderately interacting Algol-type eclipsing binary RY Geminorum

Ultraviolet spectra of the Algol-type semidetached system RY Geminorum, whose components can be described as A0 V and K0 IV, have been matched to the ultraviolet spectrum by Kurucz (1979) model atmospheres, and a best fit is found for T(eff) = 9150 K. Comparison with standard star spectra requires that this value be raised to 9400 K. The color excess of the system is determined to be no more than E(B-V) = 0.03 mag; the distance to the system is about 360 pc. The masses are approximately 2.36 and 0.38 solar masses, and the radii are 2.5 and 5.8 solar radii, respectively. The separation of the two centers is 26 solar radii. Evidence for a circumstellar line absorption is found in optical and ultraviolet spectrograms, and evidence is found in IUE spectra taken in partial eclipse for circumstellar emission lines of the type detected previously in the WS Serpentis stars and in several semidetected systems of the Algol type.

Plavec, Mirek J.↗