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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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NASA NTRS1988

Why is Pluto bright? Implications of the albedo and lightcurve behavior of Pluto

Several mechanisms accounting for Pluto's high geometric albedo in the presence of CH4 and various radiations are investigated, in light of the hypothesis that the planet must replenish its surface with clean volatiles on a time-scale of less than 20,000 years. Pluto's high albedo is explained by a CH4 snow cover created by orbital atmospheric sublimation and freeze-out cycles. As a result of the 250-year atmospheric regeneration time-scale, large surface CH4 crystals will be deprived of sufficient time for growth; it is predicted that the albedo decline will reverse following a 7-17 year phase lag after perihelion.

Stern, S. Alan↗

NASA NTRS1989

Piezoresistive silicon pressure sensors in cryogenic environment

This paper presents data on low-temperature measurements of silicon pressure sensors. It was found that both the piezoresistance coefficients and the charge-carrier mobility increase with decreasing temperature. For lightly doped semiconductor materials, the density of free charge carriers decreases with temperature and can freeze out eventually. However, the effect of carrier freeze-out can be minimized by increasing the impurity content to higher levels, at which the temperature dependency of piezoresistance coefficients is reduced. An impurity density of 1 x 10 to the 19th/cu cm was found to be optimal for cryogenic applications of pressure sensor dies.

Kahng, Seun K.↗

NASA NTRS1992

Frozen start-up behavior of low-temperature heat pipes

Start-up and subsequent operation of a low-temperature heat pipe requires the liquid phase of the operating fluid to be continuously pumped back to the evaporator by the capillary action of the wick. If the pipe has been in an environment where ambient temperatures are below the freezing point of the working fluid prior to start-up, the frozen fluid in the condenser and adiabatic region scan prevent initial flow to the evaporator, causing dryout of the evaporator before all of the working fluid is in the liquid phase. This paper examines the time-dependent wall and vapor temperature profiles along the axial length of a low-temperature heat pipe during start-up from the frozen state, and freeze-out during a normal operation by applying a subfreezing temperature fluid through the condenser. In addition, the experimental transient frozen start-up wall temperature profile is compared with a two-dimensional numerical phase-change model. A successful start-up method using a pulsed power input is presented.

Faghri, Amir↗

NASA NTRS1992

The alpha-process and the r-process

The paper explores the alpha-rich freeze-out for values of neutron excesses larger than previously treated, and reports the discovery of nuclear systematics that, for neutron excesses greater than about 0.05, allow the creation of heavier elements all the way up to A of about 100, even when most of the ejecta is in the form of heavy elements. It is found that a large part of the nuclear flow in the r-process may be carried by charged particle reactions up to about A of about 100. It is speculated that the site of these processes is the high-entropy 'wind' of a young neutron star in which neutrino energy deposition is driving mass loss. The pass of r-process ejected per supernova is expected to be about 0.0001 solar mass per Type II or Ib supernova, but this is sensitive to details of the presupernova structure, the explosion mechanism, and the amount of material that falls back onto the neutron star when the reverse shock arrives there.

Woosley, S. E.↗

NASA NTRS1993

Airborne observations of the 1992 Arctic winter stratosphere by FTIR solar absorption spectroscopy

The JPL MkIV interferometer, a Fourier Transform Infra-Red (FTIR) spectrometer designed specifically for atmospheric remote sensing, made measurements of the composition of the Arctic stratosphere in January, February and March 1992. These measurements were made from the NASA DC-8 aircraft as part of the AASE2 campaign. The data reveal that despite 5 to 6 km of subsidence inside the vortex, which more than doubled the vertically integrated column amounts (burdens) of HF and HNO3 with respect to outside the vortex, considerable losses of NO2, HCl and ClNO3 were evident by mid-January. Temporary freeze-out of HNO3 was observed only on one occasion, Jan. 19, and was accompanied by substantial reductions in HCl and ClNO3. During February and March, ClNO3 and NO2 amounts increased dramatically. HCl also recovered but at a much slower rate, so that by March ClNO3 was the major reservoir of inorganic chlorine, at times exceeding HCl by a factor 2.

Toon, G. C.↗

NASA NTRS2003

The Chemistry and Excitation of Water in Molecular Clouds

We model the chemistry and thermal balance of opaque molecular clouds exposed to an external flux of ultraviolet photons. We include the processes of gas phase and grain surface chemical reactions; in particular we examine closely the freezing of atoms and molecules onto grain surfaces and the desorption of molecules from grain surfaces as a function of depth into a molecular cloud. We find that on the surface of a molecular cloud the gas phase water abundances are low because of photodissociation, and the grain phase water (ice) abundance is low because of photodesorption of water from the grain surfaces. Deeper into the cloud, at A(sub v) less than or approximately 2-8 depending on the strength of the external ultraviolet flux, the gas phase water abundance increases with depth as the photodissociation rates decline due to dust attenuation of the ultraviolet field. However, beyond A(sub v) less than or approximately 2-8 the gas phase water abundance declines because the water freezes as water ice on the grains, and photodesorption is no longer effective in clearing the ice. A peak water abundance of about 10(exp -6) to 10(exp -7) occurs at about A(sub v) approximately 2-8, relatively independent of the gas density and the ultraviolet field. We show that such a model matches very closely the observations of the Submillimeter Wave Astronomical Satellite (SWAS), a NASA Small Explorer Mission. The model elucidates several mechanisms that have been recently invoked to understand gas phase chemistry in clouds, including-the freeze-out of molecules onto grain surface, the desorption of these molecules from the surfaces, and the abundance gradients of molecules as functions of depth into molecular clouds.

Hollenbach, David↗

NASA NTRS2004

South Polar Ar Enhancement as a Tracer for Southern Winter Horizontal Meridional Mixing

Measurements made by the Gamma Ray Spectrometer (GRS) on Mars Odyssey during 2002 and 2003 show an obvious increase in the gamma flux of 1294 keV gamma rays resulting from the decay of (41)Ar. (41)Ar is made by the capture of thermal neutrons by atmospheric (40)Ar. The increase measured above the southern polar region has permitted calculation of the increase in mixing ratio of Ar from L(sub s) 8 to 100 between latitudes 75 S and 90 S. The peak in Ar enhancement occurs about 200 Earth days after CO2 freeze-out has begun, indicating that up to this time equatorward meridional mixing is rapid enough to move enhanced Ar from the polar regions northward. Although the CO2 frost depth continues to increase from L(sub s) 110 deg to 190 deg, the Ar enhancement steadily decreases to its baseline value reached at about L(sub s) 200 deg. Our data permit an estimate of the horizontal eddy mixing coefficient useful for constraining equatorward meridional mixing during southern winter and a characteristic mixing time for the polar southern winter atmosphere. Also, using the drop in excess Ar measured by the GRS from L(sub s) 110 deg to 200 deg, we estimate an eddy coefficient appropriate for meridional mixing of the entire Ar excess back to the baseline value. The horizontal eddy mixing coefficients are derived using Ar as a tracer much as the vertical eddy mixing coefficient for the Earth's troposphere is derived using CH4 as a minor constituent tracer. The estimation of meridional mixing for high latitudes at Mars is important for constraining parameters used in atmospheric modeling and predicting seasonal and daily behavior. The calculations are order of magnitude estimates that should improve as the data set becomes more robust and improves our models.

Sprague, A. L.↗

NASA NTRS2010

Isotopic Fractionation in Primitive Material: Quantifying the Contribution of Interstellar Chemistry

Anomalously fractionated isotopic material is found in many primitive Solar System objects, such as meteorites and comets. It is thought, in some cases, to trace interstellar matter that was incorporated into the Solar Nebula without undergoing significant processing. We will present the results of models of the nitrogen, oxygen, and carbon fractionation chemistry in dense molecular clouds, particularly in cores where substantial freeze-out of molecules on to dust has occurred. The range of fractionation ratios expected in different interstellar molecules will be discussed and compared to the ratios measured in molecular clouds, comets and meteoritic material. These models make several predictions that can be tested in the near future by molecular line observations, particularly with ALMA.

Charnley, Steven↗

NASA NTRS2010

Interstellar Isotopes: Prospects with ALMA

Cold molecular clouds are natural environments for the enrichment of interstellar molecules in the heavy isotopes of H, C, N and O. Anomalously fractionated isotopic material is found in many primitive Solar System objects, such as meteorites and comets, that may trace interstellar matter that was incorporated into the Solar Nebula without undergoing significant processing. Models of the fractionation chemistry of H, C, N and O in dense molecular clouds, particularly in cores where substantial freeze-out of molecules on to dust has occurred, make several predictions that can be tested in the near future by molecular line observations. The range of fractionation ratios expected in different interstellar molecules will be discussed and the capabilities of ALMA for testing these models (e.g. in observing doubly-substituted isotopologues) will be outlined.

Charnley Steven B.↗

NASA NTRS2010

Observations of Molecular Isotope Fractionation in Prestellar Cores

Anomalously fractionated isotopic material is found in many primitive Solar System objects, such as meteorites and comets. It has been suggested that these extreme isotope ratios, are tracers of interstellar chemistry. We will present observations of the nitrogen and carbon fractionation chemistry in dense molecular clouds, particularly in cores where sUbstantial freeze-out of molecules, namely CO, onto dust has occurred. Recent models have suggested that non-depleted species, carbon and nitrogen-rich, may undergo isotopic enhancements in these conditions. The fractionation ratios measured in different interstellar molecules will be discussed and compared to the ratios determined in molecular clouds, comets, and meteoritic material.

Milam, Stefanie N.↗

NASA NTRS2011

A Chandra X-Ray Survey of Ejecta in the Cassiopeia A Supernova Remnant

We present a survey of the X-ray emitting ejecta in the Cassiopeia A supernova remnant based on an extensive analysis of over 6000 spectral regions extracted on 2.5-10" angular scales using the Chandra 1 Ms observation. We interpret these results in the context of hydrodynamical models for the evolution of the remnant. The distributions of fitted temperature and ionization age are highly peaked and suggest that the ejecta were subjected to multiple secondary shocks. Based on the fitted emission measure and element abundances, and an estimate of the emitting volume, we derive masses for the X-ray emitting ejecta as well as showing the distribution of the mass of various elements over the remnant. The total shocked Fe mass appears to be roughly 0.14 Solar Mass, which accounts for nearly all of the mass expected in Fe ejecta. We find two populations of Fe ejecta, that associated with normal Si-burning and that associated with alpha-rich freeze-out, with a mass ratio of approximately 2:1. Surprisingly, essentially all of this Fe (both components) is well outside the central regions of the SNR, presumably having been ejected by hydrodynamic instabilities during the explosion. We discuss this, and its implications for the neutron star kick.

Hwang, Una↗

NASA NTRS2011

Models of Isotopic Fractionation in Prestellar Cores

Anomalously fractionated isotopic material is found in many primitive Solar System objects, such as meteorites and comets. It is thought, in some cases, to trace interstellar matter that was incorporated into the Solar Nebula without undergoing significant processing. We will present the results of models of the nitrogen, oxygen, and carbon fractionation chemistry in dense molecular clouds, particularly in cores where substantial freeze-out of molecules on to dust has occurred. The range of fractionation ratios expected in different interstellar molecules will be discussed and compared to the ratios measured in molecular clouds, comets and meteoritic material. These studies make several predictions that can be tested in the near future by high-resolution molecular line observations with ALMA.

Charnley, Steven B.↗

NASA NTRS2011

Isotope Fractionation in the Interstellar Medium

Charnley, Steven↗

NASA NTRS2012

Gas-Grain Models for Interstellar Anion Chemistry

Long-chain hydrocarbon anions C(sub n) H(-) (n = 4, 6, 8) have recently been found to be abundant in a variety of interstellar clouds. In order to explain their large abundances in the denser (prestellar/protostellar) environments, new chemical models are constructed that include gas-grain interactions. Models including accretion of gas-phase species onto dust grains and cosmic-ray-induced desorption of atoms are able to reproduce the observed anion-to-neutral ratios, as well as the absolute abundances of anionic and neutral carbon chains, with a reasonable degree of accuracy. Due to their destructive effects, the depletion of oxygen atoms onto dust results in substantially greater polyyne and anion abundances in high-density gas (with n(sub H2) approx > / cubic cm). The large abundances of carbon-chain-bearing species observed in the envelopes of protostars such as L1527 can thus be explained without the need for warm carbon-chain chemistry. The C6H(-) anion-to-neutral ratio is found to be most sensitive to the atomic O and H abundances and the electron density. Therefore, as a core evolves, falling atomic abundances and rising electron densities are found to result in increasing anion-to-neutral ratios. Inclusion of cosmic-ray desorption of atoms in high-density models delays freeze-out, which results in a more temporally stable anion-to-neutral ratio, in better agreement with observations. Our models include reactions between oxygen atoms and carbon-chain anions to produce carbon-chain-oxide species C6O, C7O, HC6O, and HC7O, the abundances of which depend on the assumed branching ratios for associative electron detachment

Cordiner, M. A.↗

NASA NTRS2012

Models of Isotopic Fractionation in Prestellar Cores

Anomalously fractionated isotopic material is found in many primitive Solar System objects, such as meteorites and comets. It is thought, in some cases, to trace interstellar matter that was incorporated into the Solar Nebula without undergoing significant processing. We will present the results of models of the nitrogen, oxygen, and carbon fractionation chemistry in dense molecular clouds, particularly in cores where substantial freeze-out of molecules on to dust has occurred. The range of fractionation ratios expected in different interstellar molecules will be discussed and compared to the ratios measured in molecular clouds, comets and meteoritic material. These studies make several predictions that can be tested in the near future by high-resolution molecular line observations with ALMA.

Charnley, Steven B.↗

NASA NTRS2016

Update on the NASA Glenn PSL Ice Crystal Cloud Characterization (2016)

NASA Glenn's Propulsion Systems Laboratory (PSL) is an altitude engine research test facility capable of producing ice-crystal and supercooled liquid clouds. The cloud characterization parameter space is fairly large and complex, but the phase of the cloud seems primarily governed by wet bulb temperature. The presentation will discuss some of the issues uncovered through four cloud characterization efforts to date, as well as some of instrumentation that has been used to characterize cloud parameters including cloud uniformity, bulk total water content, median volumetric diameter and max-diameter, percent freeze-out, relative humidity, and an update on the NASA Glenn PSL Ice Crystal Cloud Characterization (2016).

Calibration↗

NASA NTRS2017

The Origin of the Iron-Rich Knot in Tycho's Supernova Remnant

X-ray observations of supernova remnants (SNRs) allow us to investigate the chemical inhomogeneity of ejecta, offering unique insight into the nucleosynthesis in supernova explosions. Here we present detailed imaging and spectroscopic studies of the Fe knot located along the eastern rim of the Type Ia SNR Tycho ( SN 1572) using Suzaku and Chandra long-exposure data. Surprisingly, the Suzaku spectrum of this knot shows no emission from Cr, Mn, or Ni, which is unusual for the Fe-rich regions in this SNR. Within the framework of the canonical delayed-detonation models for SN Ia, the observed mass ratios M(sub Cr)/M(sub Fe) is less than 0.023, M(sub Mn)/M(sub Fe) is less than 0.012, and M(sub Ni)/M(sub Fe) is less than 0.029 (at 90% confidence) can only be achieved for a peak temperature of (5.3 - 5.7) x 10(exp. 9) K and a neutron excess of approximately less than 2.0 x 10(exp. -3). These constraints rule out the deep, dense core of a Chandrasekhar-mass white dwarf as the origin of the Fe knot and favor either incomplete Si burning or an Alpha-rich freeze-out regime, probably close to the boundary. An explosive He burning regime is a possible alternative, although this hypothesis is in conflict with the main properties of this SNR.

ISM supernova remnants↗

NASA NTRS2017

Very Large Array Ammonia Observations of the HH 111/HH 121 Protostellar System: A Detection of a New Source with a Peculiar Chemistry

We present the results of Very Large Array NH3 (J, K) = (1, 1) and (2, 2) observations of the HH 111/HH 121 protostellar system. HH 111, with a spectacular collimated optical jet, is one of the most well-known Herbig-Haro objects. We report the detection of a new source, NH3-S, in the vicinity of HH 111/HH 121 (approximately 0.03 parsecs from the HH 111 jet source) in two epochs of the ammonia observations. This constitutes the first detection of this source, in a region that has been thoroughly covered previously by both continuum and spectral line interferometric observations. We study the kinematic and physical properties of HH 111 and the newly discovered NH3-S. We also use HCO plus and HCN (J=4-3) data obtained with the James Clerk Maxwell Telescope and archival Atacama Large Millimeter/submillimeter Array (sup 13) CO, (sup 12) CO, and C (sup 18) O (J=2-1), N2D plus (J=3-2), and (sup 13) CS (J=5-4) data to gain insight into the nature of NH3-S. The chemical structure of NH3-S shows evidence for "selective freeze-out," an inherent characteristic of dense cold cores. The inner part of NH3-S shows subsonic nonthermal velocity dispersions indicating a "coherent core," while they increase in the direction of the jets. Archival near- to far-infrared data show no indication of any embedded source in NH3-S. The properties of NH3-S and its location in the infrared dark cloud suggest that it is a starless core located in a turbulent medium, with the turbulence induced by Herbig-Haro jets and associated outflows. More data are needed to fully understand the physical and chemical properties of NH3-S and if/how its evolution is affected by nearby jets.

James Clerk Maxwell Telescop↗