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At least 379 records · Page 21

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NASA NTRS2012

Measurements of Thermal Effects on Acoustic Screech in a Choked Circular Jet Emanating from a Sharp-Edged Orifice

Experiments are performed in a 24.4 mm diameter choked circular hot and cold jets issuing from a sharp-edged orifice at a fully expanded jet Mach number of 1.85. The stagnation temperature of the hot and the cold jets are 319 K and 299 K respectively. The results suggest that temperature effects on the screech amplitude and frequency are manifested for the fundamental, with a reduced amplitude and increased frequency for hot jet relative to the cold jet. Temperature effects on the second harmonic are also observed.

Kandula, Max↗

NASA NTRS1988

Evidence for a common high-temperature superconducting effect in La(1.85)Sr(0.15)CuO4 and YBa2Cu3O7

The paper reports the positron-annihilation lifetime and Doppler-broadening energy spectra in La(1.85)Sr(0.15)CuO4 superconductors for T = 10-297 K. A strong temperature dependence in positron lifetime and momentum distribution parameters is observed for positron-electron annihilation data while the sample is in the superconducting state, but not while it is in the normal state. Experimental evidence is presented that the superconducting mechanisms in La(1.85)Sr(0.15)CuO4 and YBa2Cu3O7 are similar and it is suggested that this is a consequence of a common physics involving the delicate balance between localized- and itinerant-electron behavior.

Jean, Y. C.↗

NASA NTRS1985

Stellar Winda: Observational Evidence for a Hot-cool Star Connection

Stellar wind data was collected for a total of 272 stars representing all spectral types including Wolf-Rayet stars. Two significant correlations are found relating the wind luminosity (L sub w = 1/2 MV sub infinity squared) to the bolometric luminosity and the terminal velocity of the stellar wind to the stellar effective temperature. Least-squared fits to the data suggest that the wind luminosity is approximately the bolometric luminosity squared and the terminal velocity is approximately the effective temperature to the 1.8 power. The surprising result is that all spectral types throughout the HR diagram are represented in these correlations.

Waldron, W. L.↗

NASA NTRSDate not supplied

Electron Bombardment on Dione: Surface Compositional Effects and Temperature Anomalies

Saturn’s icy moons are composed primarily of water ice with minor other “contaminants,” such as CO2 ice, and a dark component thought to be organics, hematite and/or metallic Fe. The space weathering process of electron bombardment is expected to be particularly important on the surfaces of Saturn’s inner moons (Mimas, Tethys, Dione, and Rhea), as they orbit within Saturn’s inner magnetosphere. Terrains exhibiting thermal anomalies (i.e., colder temperatures in the day and warmer temperatures at night than surrounding areas) correspond to regions of high energy electron bombardment. Energetic electrons impact the surfaces, sintering ice grains together, and this process becomes more effective for increasing particle energies. Solar UV radiation, cosmic rays, dust in-fall, and cold plasma particles trapped in Saturn’s magnetic field also play an important role in altering the nature and the structure of the native surface ices by the implantation of contaminants, ionization, sputtering, and dissociation of water ice molecules. Additionally, CO2 could be sourced from irradiation of dark organic material. Many of these surface alterations are observable in Cassini’s Visible and Infrared Mapping Spectrometer (VIMS) spectra. We used similar methodologies as have been employed in previous works to derive surface temperatures from Cassini’s Composite Infrared Spectrometer (CIRS), which helped to isolate areas where space weathering due to electron bombardment is more predominant. The subtle changes in VIMS spectra were investigated using machine learning techniques. We present here our results for Dione.

C L Young↗

NASA NTRS1985

Bolometric albedos of Titan, Uranus, and Neptune

The energy budgets of Titan, Uranus, and Neptune are reevaluated using new observational data on energy input as well as unpublished data on energy output. The bolometric geometric albedo of each object was determined, and preliminary determinations of the phase functions were used to compute the Bond albedos and effective temperatures. The values for the latter are 83 + or - 2 K for Titan, 57 + or - 2 K for Uranus, and 47 + or - 2 K for Neptune. The effective temperature of Titan is greater than the observed brightness temperatures in the thermal infrared region of the spectrum, indicating that the emissivity is less than unity for this part of the spectrum. An internal luminosity of (3.9 + or 1.1) x 10 to the 15th W is found for Neptune, and an upper limit of (0.6 + or - 1.4) x 10 to the 15th W is found for Uranus.

Neff, J. S.↗

NASA NTRSDate not supplied

Optimizing Surfactant Templating of Yttria-stabilized Zirconia Aerogels for High-temperature Applications: Effect of Anionic & Nonionic Surfactant

Aerogels are useful thermal insulators due to low density, low thermal conductivity, and a tortuous path for solid conduction. However, the formulation must be optimized to increase thermal stability as drastic densification and surface area decrease is often exhibited after high-temperature exposure. Current research investigates yttria-stabilized zirconia (YSZ) aerogels for expected use between 600°C and 1000°C. We anticipate that new sol-gel chemistries will yield YSZ aerogels with retained mesoporous structure, increased surface area, and minimized shrinkage upon high-temperature exposure. This work investigates the anionic surfactant, sodium dodecyl sulfate (SDS), and the nonionic surfactant, Pluronic® P-123, as templating agents to prevent the collapse of the aerogel pore structure upon gelation and drying. To determine the effect of surfactant, characterization techniques such as nitrogen physisorption, x-ray diffraction, and scanning electron microscopy are used. By optimizing surfactant concentration, the thermal stability of YSZ aerogels can be enhanced, producing more efficient thermal insulators.

Rebecca C. Walker↗

NASA NTRS1987

Possible temperature variation effects on the interpretation of spatially resolved reflectance observations of asteroid surfaces

In virtue of the extent to which the visible and near-IR spectral reflectance of minerals that may be encountered on asteroid surfaces is a function of temperature, surface temperature distributions must be taken into account in the mineralogical evaluation of spectral reflectance data obtained from a spatially resolved asteroid surface. Possible errors of this type are presently illustrated for the case of a silicate mineral assemblage measured at the various temperatures that may be encountered over an asteroid surface, in order to allow optimal mineralogical data extraction.

Roush, Ted L.↗

NASA NTRS2021

Residual Temperature Bias Effects in Stratospheric Species Distributions from LIMS

The Nimbus 7 Limb Infrared Monitor of theStratosphere (LIMS) instrument operated from 25 Octo-ber 1978 through 28 May 1979. Its version 6 (V6) profileswere processed and archived in 2002. We present severaldiagnostic examples of the quality of the V6 stratospheric species distributions based on their level 3 zonal Fourier co-efficient products. In particular, we show that there are smalldifferences in the ascending (A) minus descending (D) or-bital temperature–pressure orT(p) profiles (theirA−Dval-ues) that affect (A−D) species values. SystematicA−Dbi-ases inT(p) can arise from small radiance biases and/or fromviewing anomalies along orbits. There can also be (A−D)differences inT(p) due to not resolving and correcting forall of the atmospheric temperature gradient along LIMS tan-gent view-paths. An error inT(p) affects species retrievalsthrough (1) the Planck blackbody function in forward calcu-lations of limb radiance that are part of the iterative retrievalalgorithm of LIMS, and (2) the registration of the measuredLIMS species radiance profiles in pressure altitude, mainlyfor the lower stratosphere. There are clearA−Ddifferencesfor ozone, H2O, and HNO3but not for NO2. Percentage differences are larger in the lower stratosphere for ozone andH2O because those species are optically thick. We evaluateV6 ozone profile biases in the upper stratosphere with the aid of comparisons against a monthly climatology of UV–ozone soundings from rocketsondes. We also provide resultsof time series analyses of V6 ozone, H2O, and potential vor-ticity for the middle stratosphere to show that their average(A+D) V6 level 3 products provide a clear picture of the evolution of those tracers during Northern Hemisphere win-ter. We recommend that researchers use the average V6 level3 product for their science studies of stratospheric ozone andH2O, while keeping in mind that there are uncorrected non-local thermodynamic equilibrium effects in daytime ozone inthe lower mesosphere and in daytime H2O in the uppermoststratosphere. We also point out that the present-day Soundingof the Atmosphere using Broadband Emission Radiometry(SABER) experiment provides measurements and retrievalsof temperature and ozone that are nearly free of anomalousdiurnal variations and of effects from gradients at low and middle latitudes.

Ellis Remsberg↗

NASA NTRS1987

Evolutionary sequences for horizontal branch stars

A new grid of canonical evolutionary horizontal branch (HB) sequences is presented. Sequences are computed for each combination of the following helium and heavy-element abundances, respectively: Y(main sequence) = 0.20, 0.25, 0.30, and Z = 0.0001, 0.001, and 0.01. The results show that the bifurcation point at which the HB morphology changes from redward-evolving tracks to tracks with blueward loops shifts to higher effective temperatures with increasing helium abundance or metallicity. The sequences can be used to study in more detail how a number of HB properties such as the HB lifetime, the effective temperature at the bifurcation point in the track morphology, the luminosity dropoff of the blue HB, and the luminosity width of the red HB depend on the composition.

Sweigart, Allen V.↗

NASA NTRS1991

A critical analysis of the ultraviolet temperature scale of the helium-dominated DB and DBV white dwarfs

The temperatures of the hottest of the DB and of some of the variable DBV white dwarfs have been reanalyzed, using a new helium-rich model atmosphere grid and archival IUE data. The grid covers variations in the effective temperature, surface gravity, and trace hydrogen abundance. Significantly lower temperatures than those found by earlier investigations are found. This has influence on the issue of which mixing-length theory is the best one for treatment of convection in DB white dwarfs. The prototype of the DBV stars, GD 358, is given particular attention in view of the recently reported HE II 1640-A line. The sensitivity of the new results to input physics is analyzed. It is found that uncertainties in Stark broadening parameters, convective efficiency, and equation of state and the assumptions of a fixed gravity (log g = 8) and a negligible abundance of hydrogen can at most give an error in the effective temperature of 400 K for a typical DB star. The uncertainty on the absolute calibration of the IUE cameras and the effect of small interstellar reddening dominate the error analysis.

Thejll, Peter↗

NASA NTRSDate not supplied

Electron Bombardment on Dione: Surface Compositional Effects and Temperature Anomalies

Saturn’s icy moons are composed primarily of water ice with minor other “contaminants,” such as CO2 ice, and a dark component thought to be organics, hematite and/or metallic Fe [1]. The space weathering process of electron bombardment is expected to be particularly important on the surfaces of Saturn’s inner moons (Mimas, Tethys, Dione, and Rhea), as they orbit within Saturn’s inner magnetosphere. Terrains exhibiting thermal anomalies (i.e., colder temperatures in the day and warmer temperatures at night than surrounding areas) correspond to regions of high energy electron bombardment [2; 3; 4; 5]. Energetic electrons impact the surfaces, sintering ice grains together, and this process becomes more effective for increasing particle energies [6]. Solar UV radiation, cosmic rays, dust in-fall, and cold plasma particles trapped in Saturn’s magnetic field also play an important role in altering the nature and the structure of the native surface ices by the implantation of contaminants, ionization, sputtering, and dissociation of water ice molecules [7]. Additionally, CO2 could be sourced from irradiation of dark organic material [8]. Many of these surface alterations are observable in Cassini’s Visible and Infrared Mapping Spectrometer (VIMS) spectra [9]. We used similar methodologies as have been employed in previous works [e.g., 10] to derive surface temperatures from Cassini’s Composite Infrared Spectrometer (CIRS), which helped to isolate areas where space weathering due to electron bombardment is more predominant. The subtle changes in VIMS spectra were investigated using machine learning techniques. We present here our results for Dione.

Cindy Lee Young↗

NASA NTRSDate not supplied

Electron Bombardment on Dione: Surface Compositional Effects and Temperature Abnormalities

C L Young↗

DOE OSTI2025

Effects of Temperature Fluctuations on Surface Mobility of Atomic Steps and Oxidation Dynamics in High-Temperature Alloys

In contrast to the traditional perspective that thermal fluctuations are insignificant in surface dynamics, here we report their influence on surface reaction dynamics. Using real-time low-energy electron microscopy imaging of NiAl(100) under both vacuum and O 2 atmospheres, we demonstrate that transient temperature variations substantially alter the direction of atom diffusion between the surface and bulk, leading to markedly different oxidation outcomes. During heating, substantial outward diffusion of atoms from the bulk to the surface results in step growth. Conversely, cooling induces considerable inward diffusion of adatoms, producing a distinct oxide morphology. In both scenarios, initially formed oxide islands impede local atomic step mobility, thereby increasing step length due to mass transfer between the surface and bulk, with atomic steps acting as adatom sinks during heating and sources during cooling. Furthermore, we show that this pinning effect on atomic step mobility can be mitigated by applying persistent temperature fluctuations. As a result, understanding these nuances is vital for accurately predicting and dynamically manipulating the performance of active materials in various chemical processes under transient thermal conditions.

36 MATERIALS SCIENCE↗

NASA NTRS1984

Similar photospheres, different mantles - A study of four O stars

The present investigation is concerned with a comparison of the spectra of four O stars (10 Lac, S Mon, HD 46223, and Zeta Pup) which have similar effective temperatures, taking into account an attempt to find the reason for differences regarding the spectral type of the stars. The photospheres of the stars are examined, taking into account the effective temperatures, radii, speed of rotation, and masses. Information is presented about the equivalent widths and profiles of strong lines in the visible and ultraviolet spectra of each star. It is found that the four stars have similar photospheres but different mantles. The stars are assigned different spectral types because the empirically selected lines used for determining spectral type in class O are sensitive primarily to conditions in the mantle of the star and not to conditions in the photosphere. Questions concerning the reasons for the differences regarding the mantles are also explored.

Underhill, A. B.↗

NASA NTRS1991

Observational tests of convective core overshooting in stars of intermediate to high mass in the Galaxy

This study presents the results of 14 tests for the presence of convective overshooting in large convecting stellar cores for stars with masses of 4-17 solar masses which are members of detached close binary systems and of open clusters in the Galaxy. A large body of theoretical and observational data is scrutinized and subjected to averaging in order to minimize accidental and systematic errors. A conservative upper limit of d/HP less than 0.4 is found from at least four tests, as well as a tighter upper limit of d/HP less than 0.2 from one good test that is subject to only mild restrictions and is based on the maximum observed effective temperature of evolved blue supergiants. It is concluded that any current uncertainty about the distance scale for these stars is unimportant in conducting the present tests for convective core overshooting. The correct effective temperature scale for the B0.5-B2 stars is almost certainly close to one of the proposed hot scales.

Stothers, Richard B.↗

NASA NTRS1976

Pioneer 10 and 11 observations and the dynamics of Jupiter's atmosphere

Three results of the Pioneer 10 and 11 infrared radiometer experiment are examined. The first result is that the effective temperature is the same at the poles and equator, in spite of the excess solar heat deposited at the equator. This is consistent with theories of convection which indicate that an extremely small equator-to-pole temperature difference could suppress the internal heat flux at the equator relative to the pole by an amount sufficient to balance the difference in solar energy deposition. The second result is that the effective temperature of belts is 3 to 4 K greater than that of zones. This is almost exactly accounted for by the lower albedo of the belts, and is consistent with a model where the internal heat flux is the same under belts and zones, and the horizontal atmospheric heat flux is zero. The third results is the clearly visible instability along the equatorward edges of zones in the Northern Hemisphere.

Ingersoll, A. P.↗

NASA NTRS1990

IRAS low-resolution spectral observations of H II regions

Abundances of neon and sulfur for 95 H II regions whose spectra were measured by the IRAS Low Resolution Spectrometer (LRS) are determined; and theoretical implications of the neon ionization level and the sulfur-to-neon abundance ratio are discussed. The abundances show a distinct gradient of Ne/H with respect to Galactocentric radius RG, and a smaller gradient for S/H. The neon ionization is high only for sources with both low neon abundance and high luminosity. It is concluded that, due to the fact that the luminosities observed are so high compared to those predicted for the zero-age main sequence star, the most luminous H II regions are probably excited by several stars or a cluster of stars. The observed ionization of neon shows that the effective temperatures of the exciting stars cannot be more than 40,000 K, with an alternative explanation that 04 and 03 stars have Lyman continuum spectra cooler than the 45,000 - 50,000 K effective temperatures that are found for their visible and UV spectra.

Simpson, J. P.↗

NASA NTRS1992

Determination of nitrogen to carbon abundance ratios from transition layer emission lines

We have finished studying the nitrogen to carbon abundance ratios for stars with different effective temperatures T(sub eff) and luminosities using transition layer emission lines and using spectra available in the IUE archives. The N/C abundance ratio determinations using transition layer emission lines are as accurate as the photospheric abundance determinations as found by comparison of results obtained by both methods for the same stars. Our measurements confirm photospheric abundance determinations in regions of the HR diagram where they can be obtained. Our studies have extended the temperature range to higher temperatures. They have shown the exact positions in the HR diagram where the mixing due to the outer convection zones reaches deep enough to bring nuclear processed material to the surface. This occurs at effective temperatures which are higher by delta log T(sub eff) approximately 0.04 or roughly 400 K than expected theoretically. Since the depth of the convection zone increases rapidly with decreasing T(sub eff) this may indicate considerable overshoot beyond the lower boundary of the convection zone. Our N/C abundance ratio determinations from transition layer emission lines have confirmed that the actual enrichment observed for some cool giants is larger than expected theoretically, again indicating a larger degree of mixing in several stars either from below or from above. For the supergiants it probably indicates overshoot above the convective core in the progenitor main sequence stars. For the more massive giants this may also be the case, though we did not find a correlation between delta log N/C and the absolute magnitudes, but these are rather uncertain. As byproducts of these studies we also found anomalies in Si/C and N/C abundance ratios for F giants which can be understood as the relict of surface abundance changes for their main sequence progenitors due to diffusion. This anomaly disappears for G giants, for which the depths of the convection zones are apparently deep enough to wipe out these element separations (Bohm-Vitense 1992).

Boehm-Vitense, Erika↗