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At least 415 records · Page 23
All sourcesNASA NTRSDOE OSTI
NASA NTRS1985

Planetary searches using optical astrometric interferometers

The detectability of planets is discussed, and observations made with an astrometric interferometer (AI) at Mount Wilson are presented. Planets are detectable as a result of their periodic perturbation of the position of a central observable star. This detection requires relative astrometric accuracies of about 3 x 10 to the -4th for Jovian planets and about 3 x 10 to the -7th arcsec for terrestrial planets. The principles governing astrometric interferometry are explained, and it is conjectured that a two-color optical AI on earth should be capable of about 0.0001 arcsec rms accuracy for stars brighter than magnitude 7-10; small one-color space systems should achieve at least 0.00001 arcsec. Owing to atmospheric turbulence, two-color systems permit correction every few milliseconds for the random wavefront tilt. A 3.4-meter baseline interferometer on Mount Wilson has demonstrated arcsec rms fluctuations of about 0.02 for 1-sec samples. Space-based optical astrometric interferometry is also discussed.

Staelin, D. H.↗
NASA NTRS1985

Photochemical consequences of enhanced CO2 levels in earth's early atmosphere

Greatly enhanced atmospheric CO2 concentrations are the most likely mechanism for offsetting the effects of reduced solar luminosity early in the earth's history. CO2 levels of 80 to 600 times the present value could have maintained a mean surface temperature of 0 C to 15 C, given a 25 percent decrease in solar output. Such high CO2 levels are at least qualitatively consistent with the present understanding of the carbonate-silicate geochemical cycle. The presence of large amounts of CO2 has important implications for the composition of the earth's prebiotic atmosphere. The hydrogen budget of a high-CO2 primitive atmosphere would have been strongly influenced by rainout of H2O2 and H2CO. The reaction of H2O2 with dissolved ferrous iron in the early oceans could have been a major sink for atmospheric oxygen. The requirement that this loss of oxygen be balanced by a corresponding loss of hydrogen (by escape to space and rainout of H2CO) implies that the atmospheric H2 mixing ratio was greater than 2 x 10 to the -5th and the ground level O2 mixing ratio was below 10 to the -12th, even if other surface sources of H2 were small. These results are only weakly dependent on changes in solar UV flux, rainout rates, and vertical mixing rates in the primitive atmosphere.

Kasting, J. F.↗
NASA NTRS1986

The outer atmosphere of the carbon star TX Piscium

A high-resolution LWP IUE spectrum of the bright N-type carbon star TX Psc demonstrates that the Mg II h and k emission profiles are strongly affected by absorption from Mg II, Mn I, probably Fe I, and possibly from molecules. The indication that the absorbing matter has a column density of not less than 10 to the 20th H atoms or molecules per sq cm is consistent with absorption in a slowly expanding envelope. The integrated Mg II line flux is found to be much greater than in 1981, and the radio CO (J = 1 - 0) line from the circumstellar shell is detected. Results for a column density of not larger than 10 to the 22nd H2 molecules/sq cm, and a radial velocity close to that of the star, are in agreement with those obtained from UV data. Some dust emission from carbon grains is suggested by the far infrared flux distribution, and a mass-loss rate estimation for the star of 10 to the -6th to 10 to the -8th solar masses is obtained.

Eriksson, K.↗
NASA NTRS1986

Linewidth characteristics of Raman-shifted dye laser output at 720 and 940 nm

Raman conversion efficiency and line broadening are reported for Stokes operation at 720 and 940 nm, with hydrogen and deuterium as the Raman source, and using an Nd:YAG pumped Quanta-Ray PDL-2 dye laser. The dye laser linewidth is 0.2/cm (FWHM) with the grating alone as an intracavity element, and the conversion efficiency at 400 psi was found to be 40 and 20 percent for outputs of 720 and 940 nm, respectively. Pressure broadening coefficients of (9.2 + or - 0.9) x 10 to the -5th per cm/psi for hydrogen, and 7.7 x 10 to the -5th per cm/psi for deuterium, were obtained in good agreement with previous results. The linewidth at the first Stokes wavelength was shown to be determined by pressure broadening in the Raman medium.

Grossmann, B. E.↗
NASA NTRS1986

The determination of the global average OH concentration using a deuteroethane tracer

It is proposed to measure the decreasing global concentration of an OH reactive isotopic tracer, G sub 2 D sub 6, after its introduction into the troposphere in a manner to facilitate uniform global mixing. Analyses at the level of 2 x 10 to the -19th power fraction, corresponding to one kg uniformly distributed globally, should be possible by a combination of cryogenic absorption techniques to separate ethane from air and high sensitivity isotopic analysis of ethane by mass spectrometry. Aliquots of C sub 2 D sub 6 totaling one kg would be introduced to numerous southern and northern latitudes over a 10 day period in order to achieve a uniform global concentration within 3 to 6 months by the normal atmospheric circulation. Then samples of air of 1000 l (STP) would be collected periodically at a tropical and temperate zone location in each hemisphere and spiked with a known amount of another isotopic species of ethane, C-13 sub 2 H sub 6, at the level of 10 to the -11th power mole fraction. After separation of the ethanes from air, the absolute concentration of C sub 2 D sub 6 would be analyzed using the Argonne 100-inch radius mass spectrometer.

Stevens, Charles M.↗
NASA NTRS1986

Spectroscopic detection of Ch3Cl in the upper troposphere and lower stratosphere

Absorptions due to the nu1 band of CH3Cl have been identified for the first time in infrared solar absorption spectra of the upper troposphere and lower stratosphere. The spectral data were obtained with the ATMOS Fourier transform spectrometer on board Spacelab 3 in May 1985 during four solar occultation events near latitudes of 30 deg N, 26 deg N, 25 deg N, and 49 deg S. Volume mixing ratio profiles of CH3Cl retrieved for the altitude range 12 to 23 km at these four latitudes do not show appreciable differences. Vertical mixing ratio distributions vary from 6 x 10 to the -10th at 12 km to 3 x 10 to the -10th at 23 km with an average uncertainty of about 25 percent. The retrieved mixing ratio does not decrease with altitude as rapidly as the data obtained by in situ techniques.

Park, J. H.↗
NASA NTRS1986

Kinetics and mechanism of the IO + IO reaction

The flash photolysis-absorption technique has been used to measure the rate constant for the reaction IO + IO over the temperature range 250-373 K and pressure range 20-700 torr of N2. The rate constant for the overall reaction was found to be independent of pressure and equal to 1.73 x 10 to the -12th exp (1020 + or - 200)/T cu cm/molecule s. Branching ratios for the reaction channels forming 2I + O2, I2 + O2, and I2O2, were found to be pressure dependent, with the I2 channel accounting for less than 5 percent of the total reaction over the observed pressure range. A stable reaction product was observed between 230 and 300 nm which was formed on a long time scale compared to IO disappearance. The rate constant for the reaction I + O3 + yields to IO + O2 was measured at 298 K and found to be (9.5 + or - 1.5) x 10 to the -13th cu cm/molecule s.

Sander, S. P.↗
NASA NTRS1986

The flux of meteoroids and orbital space debris striking satellites in low earth orbit

Results are presented from an analysis of 331 craters in the 0.09-250 micron size range found in the solid aluminum thermal control louvers of the Solar Maximum Mission spacecraft, distinguishing between natural meteoroid impacts and those of orbital spacecraft debris. The time-averaged flux and size distribution of both particle types over the 10 to the -13th to 10 to the -7th g mass range are directly determined.

Laurance, M. R.↗
NASA NTRS1986

Temperature dependence of the NO3 absorption spectrum

The absorption spectrum of the gas-phase NO3 radical has been studied between 220 and 700 nm by using both flash photolysis and discharge flow reactors for the production of NO3. In the flash photolysis method, cross sections at the peak of the (0,0) band at 661.9 nm were measured relative to the cross section of ClONO2 at several different wavelengths. From the best current measurements of the ClONO2 spectrum, the NO3 cross section at 661.9 nm was determined to be (2.28 + or 0.34) x 10 to the -17th sq cm/molecule at 298 K. Measurements at 230 K indicated that the cross section increases by a factor of 1.18 at the peak of the (0,0) band. The discharge flow method was used both to obtain absolute cross sections at 661.9 nm and to obtain relative absorption spectra between 300 and 700 nm at 298 and 230 K. A value of (1.83 + or - 0.27) x 10 to the -17th sq cm/molecule was obtained for sigma NO3 at 661.9 nm at 298 K. Upper limits to the NO3 cross sections were also measured between 220 and 260 nm with the discharge flow method.

Sander, Stanley P.↗
NASA NTRS1986

Two-step excitation and blue fluorescence under continuous-wave pumping in Nd:YLF

Near-UV and blue fluorescence from the 4D3/2 and 4D5/2 manifolds in Nd:YLF has been observed at room temperature under CW pumping by a rhodamine 590 dye laser. Excitation to these manifolds is attributed to two-step excitation involving excited-state absorption from the 4F3/2 metastable level. A similar phenomenon has also been observed in Nd:YAG and Nd:glass. The effective excited-state absorption cross section is measured to be (2 + or - 1) x 10 to the -20th sq cm at 587.4 nm in the pi polarization, and the peak effective stimulated emission cross section is measured to be 5 x 10 to the -20th sq cm at 411.7 nm, also in the pi polarization. Estimated laser threshold at 411.7 nm for two-step pumping at 587.4 nm is 70 mW.

Fan, T. Y.↗
NASA NTRS1986

Mass loss from giant and supergiant stars

The 12 m telescope of the National Radio Astronomy Observatory has been used at the J = 2-1 transition of CO to increase the known list of giant and supergiant stars with observable circumstellar envelopes. The candidate objects were generally M-type giants and supergiants, chosen for their strong infrared luminosities. Of the 35 objects which were previously undetected, or only marginally detected, 10 were found to produce detectable CO emission. Physical parameters of the envelopes are derived by source modeling. Mass-loss rates vary from 10 to the -7th to 4 x 10 to the -5th solar mass/yr.

Wannier, P. G.↗
NASA NTRS1987

Charge distribution and response time for a modulation-doped extrinsic infrared detector

The electric charge distribution and response time of a modulation-doped extrinsic infrared detector are determined. First, it is demonstrated theoretically that the photoconductive layer is effectively depleted of ionized majority-impurity charges so that scattering is small and mobility is high for photogenerated carriers. Then, using parameters appropriate to an actual detector, the predicted response time is 10 to the -8th to about 10 to the -9th s, which is much faster than comparable conventional detectors. Thus, the modulation-doped detector design would be valuable for heterodyne applications.

Hadek, Victor↗
NASA NTRS1987

Shallow n(+) diffusion into InP by an open-tube diffusion technique

Very shallow n(+) layers have been obtained in InP by using gallium sulfide as a source for sulfur diffusion, and chemically vapor-deposited SiO2 as a cap. Diffusions were carried out from 585 to 725 C in an open-tube system with a nitrogen ambient. The doping profile of sulfur in InP is estimated to be of the complementary error function type with a surface concentration of 5.6 x 10 to the 18th/cc and a diffusion constant of 1.1 x 10 to the -14th sq cm/s at 670 C. Diodes made on n(+)-p junctions obtained by this diffusion technique show ideality factors close to unity and saturation current densities as low as 3.4 x 10 to the -15th A/sq cm, signifying the presence of a defect-free junction. These diffusions, with junction depths in the 400-700 A range, are ideal for solar cell applications.

Ghandhi, Sorab K.↗
NASA NTRS1987

Rocket observation of the N II 2143 A emission in an aurora

Using a rocket-mounted spectrometer with a multichannel detector, the existence of the N II 2143 A doublet in the auroral ultraviolet spectrum was confirmed. The measured intensity is in agreement with previous auroral observations and is consistent with a peak electron-impact cross section of 1-2 x 10 to the -18th sq cm. By comparing an altitude profile of the 2143 A emission to the N2(+) 3914 A emission, a deactivation rate for the N(+)(5S) state was obtained. The most likely quencher is N2, although the possibility of a reaction with O2 cannot be ruled out. For N2, the deduced value of the quenching coefficient equals 5.5 x 10 to the -10th cu cm/s.

Siskind, David E.↗
NASA NTRS1987

Energy transfer of O(D-1) atoms in collision with O(P-3) atoms

Calculations are carried out of the elastic scattering and excitation exchange cross-sections in collisions of O(D-1) and O(P-3) atoms, which determine the degree of thermalization of the O(D-1) atoms produced by dissociative recombination in the thermosphere. The effective elastic scattering and excitation exchange cross-sections are calculated to be 1.55 x 10 to the -15th and 6.25 x 10 to the -16th sq cm, respectively, at a relative collision energy of 1.0 eV. The mutual diffusion coefficient between O(D-1) and O(P-3) atoms is also presented.

Yee, Jeng-Hwa↗
NASA NTRS1987

The production of NH(A 3Pi i) through photodissociative excitation of NH3

The NH(A-X) emission produced by photodissociation of NH3 is examined using a line emission source in the 600-1340 A region. It is observed that the production of the NH(A 3Pi i) photofragment involves a spin-forbidden transition. Fluorescence spectra of NH3 are derived and analyzed. The relationship between fluorescence cross sections and fluorescence count rates and gas pressure is studied. The fluorescence cross sections of NH(A-X) are calculated as 2.1 x 10 to the -22nd, 3.1 x 10 to the -22nd, 3.0 x 10 to the -21st, 1.5 x 10 to the -20th, and 3.4 x 10 to the -20th sq cm for the primary photon wavelength of 1239, 1184, 1065, 1037, and 976 A, respectively; and the corresponding fluorescence quantum yields are 0.00002, 0.00003, 0.0012, 0.0085, and 0.015.

Wu, C. Y. Robert↗
NASA NTRS1987

A systematic search for long-term variability in a large sample of X-ray sources

A method which uses the correct measurement error distribution (Poisson) and which naturally takes into account detections as well as upper limits has been developed to search for flux variability in X-ray sources. The method is applied to 256 sources observed at least twice with the Einstein Observatory Imaging Proportional Counter. These sources have, in the very large majority of cases, an X-ray flux in the range 1 x 10 to the -13th to 6 x 10 to the -12th erg/sq cm/s (0.2-3.5 keV). Thirty-one sources show flux variations with amplitude ranging from about 20 percent to a factor of 6. Flux variations of small amplitude seem to be fairly common, but can be detected only in bright sources or in sources observed with very long exposures. Variations of a factor of 2 or more are seen in about 15 percent of the sources for which variations of such an amplitude are detectable.

Maccacaro, Tommaso↗
NASA NTRS1987

Search for X-rays from the region of the Aries-Perseus flasher

The region containing the Perseus flasher was observed for 5.4 h with the Exosat observatory. Upper limits to a point source with a steady X-ray flux were 2 x 10 to the -12th erg/sq cm s and 6 x 10 to the 12th erg/sq cm s. Upper limits to X-ray flashes of about 1 s duration were 4 x 10 to the 9th erg/sq cm s and 7 x 10 to the -10th erg/sq cm s. In view of the growing number of negative optical observations, it appears that either the optical flashes do not have a celestial origin, or that the flasher has a transient character and turns off for extended periods of time.

Lewin, Walter H. G.↗