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NASA NTRS2011

Observational Evidence of How Magnetofluid Turbulence in the Solar Wind Dissipates

The solar wind appears to be a fully developed turbulent magnetofluid. As this magnetofluid expands into the heliosphere, it cools significantly less rapidly than would be expected of an adiabatically expanding gas. The evolution of the temperature with distance is roughly what would be expected if the turbulence dissipated by heating the thermal plasma. Several physical mechanisms have been proposed, including resonance absorption of waves and Landau damping. Recently, high-time resolution magnetic field data from the four Cluster spacecraft have illustrated damping of the fluctuations out to the electron inertial scale. Use of the wave telescope/k-filtering technique during two intervals of busrt mode data suggests that dissipation of the fluctuations is due to Landau damping, first on protons, then on electrons.

Goldstein, Melvyn L.↗

NASA NTRS2013

Effect of Residual Noncondensables on Pressurization and Pressure Control of a Zero-Boil-Off Tank in Microgravity

The Zero-Boil-Off Tank (ZBOT) Experiment is a small-scale experiment that uses a transparent ventless Dewar and a transparent simulant phase-change fluid to study sealed tank pressurization and pressure control with applications to on-surface and in-orbit storage of propellant cryogens. The experiment will be carried out under microgravity conditions aboard the International Space Station in the 2014 timeframe. This paper presents preliminary results from ZBOT's ground-based research that focuses on the effects of residual noncondensable gases in the ullage on both pressurization and pressure reduction trends in the sealed Dewar. Tank pressurization is accomplished through heating of the test cell wall in the wetted and un-wetted regions simultaneously or separately. Pressure control is established through mixing and destratification of the bulk liquid using a temperature controlled forced jet flow with different degrees of liquid jet subcooling. A Two-Dimensional axisymmetric two-phase CFD model for tank pressurization and pressure control is also presented. Numerical prediction of the model are compared to experimental 1g results to both validate the model and also indicate the effect of the noncondensable gas on evolution of pressure and temperature distributions in the ullage during pressurization and pressure control. Microgravity simulations case studies are also performed using the validated model to underscore and delineate the profound effect of the noncondensables on condensation rates and interfacial temperature distributions with serious implications for tank pressure control in reduced gravity.

Marangoni and Natural Convection↗

NASA NTRS2013

Characterization of the Gaseous Companion k Andromedae B* New Keck and LBTI High-contrast Observations

Context. We previously reported the direct detection of a low mass companion at a projected separation of 55+/-2 astronomical units around the B9 type star kappa Andromedae. The properties of the system (mass ratio, separation) make it a benchmark for the understanding of the formation and evolution of gas giant planets and brown dwarfs on wide-orbits. Aims. We present new angular differential imaging (ADI) images of the system at 2.146 (K(sub s)), 3.776 (L'), 4.052 (NB 4.05) and 4.78 micrometers (M') obtained with Keck/NIRC2 and LBTI/LMIRCam, as well as more accurate near-infrared photometry of the star with the MIMIR instrument. We aim to determine the near-infrared spectral energy distribution (SED) of the companion and use it to characterize the object. Methods. We used analysis methods adapted to ADI to extract the companion flux. We compared the photometry of the object to reference young/old objects and to a set of seven PHOENIX-based atmospheric models of cool objects accounting for the formation of dust. We used evolutionary models to derive mass estimates considering a wide range of plausible initial conditions. Finally, we used dedicated formation models to discuss the possible origin of the companion. Results. We derive a more accurate J = 15.86 +/- 0.21, H = 14.95 +/- 0.13, K(sub s) = 14.32 +/- 0.09 mag for kappa And b. We redetect the companion in all our high contrast observations. We confirm previous contrasts obtained at K(sub s) and L' band. We derive NB 4.05 = 13.0 +/- 0.2 and M' = 13.3 +/- 0.3 mag and estimate Log(base 10)(L/solar luminosity) = −3.76 +/- 0.06. Atmospheric models yield T(sub eff) = 1900(+100/−200) K. They do not set constrains on the surface gravity. "Hot-start" evolutionary models predict masses of 14(+25/−2) Jupiter mass based on the luminosity and temperature estimates, and considering a conservative age range for the system (30(+120/−10) million years). "warm-start" evolutionary tracks constrain the mass to M greater than or equal to 11 Jupiter mass. Conclusions. The mass of kappa Andromedae b mostly falls in the brown-dwarf regime, due to remaining uncertainties in age and mass-luminosity models. According to the formation models, disk instability in a primordial disk could account for the position and a wide range of plausible masses of kappa and b.

spectral energy distribution (SED)↗

NASA NTRS2017

Zero-Boil-Off Tank (ZBOT) Experiment: Ground-Based Validation of Self-Pressurization and Pressure Control Two-Phase CFD Model

Integral to all phases of NASA's projected space and planetary expeditions is affordable and reliable cryogenic fluid storage for use in propellant or life support systems. Cryogen vaporization due to heat leaks into the tank from its surroundings and support structure can cause self-pressurization relieved through venting. This has led to a desire to develop innovative pressure control designs based on mixing of the bulk tank fluid together with some form of active or passive cooling to allow storage of the cryogenic fluid with zero or reduced boil-off. The Zero-Boil-Off Tank (ZBOT) Experiments are a series of small scale tank pressurization and pressure control experiments aboard the International Space Station (ISS) that use a transparent volatile simulant fluid in a transparent sealed tank to delineate various fundamental fluid flow, heat and mass transport, and phase change phenomena that control storage tank pressurization and pressure control in microgravity. The hardware for ZBOT-1 flew to ISS on the OA-7 flight in April 2017 and operations are planned to begin in September 2017, encompassing more than 90 tests. This paper presents preliminary results from ZBOT's ground-based research delineating both pressurization and pressure reduction trends in the sealed test tank. Tank self-pressurization tests are conducted under three modes: VJ heating, strip heating and simultaneous VJ and strip heating in attempt to simulate heat leaks from the environment, the support structure and both. The jet mixing pressure control studies are performed either from an elevated uniform temperature condition or from thermally stratified conditions following a self-pressurization run. Jet flow rates are varied from 2-25 cm/s spanning a range of jet Re number in laminar, transitional, and turbulent regimes and a range of Weber numbers covering no ullage penetration, partial penetration and complete ullage penetration and break-up (only in microgravity). Numerical prediction of a two-phase CFD model are compared to experimental 1g results to both validate the model and also indicate the effect of the residual non-condensable gas on evolution of pressure and temperature distributions in the tank during pressurization and pressure control.

microgravity evaporation condensatio↗

NASA NTRS1984

Spiral structure and star formation. I - Formation mechanisms and mean free paths

The wide variety of optical appearances observed in spiral galaxies has encouraged the growth of two theoretical approaches to explain the spiral patterns exhibited by such young objects as OB associations, giant H II regions, and dark dust lanes. These approaches are related to the density wave theory of spiral structure and 'stochastic, self-propagating star formation'. Levinson and Roberts (1981) tried to reconcile these two theoretical approaches, and considered a disk filled with discrete gas clouds. The present investigation is concerned with refinements and extensions of the model of Levinson and Roberts. Attention is given to gravitational forces and dynamical propagation, cloud-cloud collisions, supernova interactions, computational models, a theory concerning the interactions of the gas cloud and stellar association systems, the time evolution of the gas cloud-stellar association systems, and aspects of collisionally triggered star formation.

Roberts, W. W., Jr.↗

NASA NTRS2013

ALMA Observations of the Molecular Gas in the Debris Disk of the 30 Myr Old Star HD 21997

The 30 Myr old A3-type star HD 21997 is one of the two known debris dust disks having a measurable amount of cold molecular gas. With the goal of understanding the physical state, origin, and evolution of the gas in young debris disks, we obtained CO line observations with the Atacama Large Millimeter/submillimeter Array (ALMA). Here, we report on the detection of (12)CO and (13)CO in the J = 2-1 and J = 3-2 transitions and C(18)O in the J = 2-1 line. The gas exhibits a Keplerian velocity curve, one of the few direct measurements of Keplerian rotation in young debris disks. The measured CO brightness distribution could be reproduced by a simple star+disk system, whose parameters are r(sub in) < 26 AU, r(sub out) = 138 +/- 20 AU, Stellar M = 1.8 +0.5/−0.2 Solar M, and i = 32. Deg. 6 +/- 3 deg..1. The total CO mass, as calculated from the optically thin C(18)O line, is about (4-8) ×10(exp −2 ) Solar M, while the CO line ratios suggest a radiation temperature on the order of 6-9 K. Comparing our results with those obtained for the dust component of the HD 21997 disk from ALMA continuum observations by Moor et al., we conclude that comparable amounts of CO gas and dust are present in the disk. Interestingly, the gas and dust in the HD 21997 system are not colocated, indicating a dust-free inner gas disk within 55 AU of the star. We explore two possible scenarios for the origin of the gas. A secondary origin, which involves gas production from colliding or active planetesimals, would require unreasonably high gas production rates and would not explain why the gas and dust are not colocated. We propose that HD 21997 is a hybrid system where secondary debris dust and primordial gas coexist. HD 21997, whose age exceeds both the model predictions for disk clearing and the ages of the oldest T Tauri-like or transitional gas disks in the literature, may be a key object linking the primordial and the debris phases of disk evolution.

molecular gas↗

NASA NTRS1989

Thermal evolution of Titan's atmosphere

The present model of Titan's atmosphere/surface evolution assumes the presence of a kilometer-deep ethane-methane ocean at the surface that serves as a tropospheric thermal balance-determining atmospheric-gas reservoir. The evolution of the atmospheric thermal structure is driven by gas-abundance changes occurring as stratospheric photolysis irreversibly converts methane into higher hydrocarbons, whereby the ocean becomes ethane-enriched and methane-depleted. A family of evolution models is presented whose various members correspond to different present-day ocean compositions, various atmospheric hydrogen abundances, and different schemes for frequency-averaging the gas opacity. A slow evolution over several billion years is envisioned.

Lunine, Jonathan I.↗

NASA NTRS1953

Relative Importance of Various Sources of Defect-Producing Hydrogen Introduced into Steel During Application of Vitreous Coatings

When porcelain enamels or vitreous-type ceramic coatings are applied to ferrous metals, there is believed to be an evolution of hydrogen gas both during and after the firing operation. At elevated temperatures rapid evolution may result in blistering while if hydrogen becomes trapped in the steel during the rapid cooling following the firing operation gas pressures may be generated at the coating-metal interface and flakes of the coating literally blown off the metal. To determine experimentally the relative importance of the principal sources of the hydrogen causing the defects, a procedure was devised in which heavy hydrogen (deuterium) was substituted in turn for regular hydrogen in each of five possible hydrogen-producing operations in the coating process. The findings of the study were as follows: (1) the principal source of the defect-producing hydrogen was the dissolved water present in the enamel frit that was incorporated into the coating. (2) the acid pickling, the milling water, the chemically combined water in the clay, and the quenching water were all minor sources of defect-producing hydrogen under the test conditions used. Confirming experiments showed that fishscaling could be eliminated by using a water-free coating.

Moore, Dwight G↗

NASA NTRS1979

Gaseous halos - Implications for cluster, galaxy, and radio-source evolution

Substantial amounts of enriched gas produced during the early stages of galactic evolution can be stably contained within massive extended halos and released into the intracluster medium by galaxy collisions at a relatively late epoch. If most of the intracluster gas in rich clusters originates in this manner, the onset of ram-pressure stripping can be delayed until z is no more than about 0.5. Prior to this, such clusters contain many spiral and gas-rich elliptical galaxies, thereby accounting for the unusually large proportion of blue galaxies found in distant clusters. Implications are drawn for X-ray observations of rich clusters and for the morphology of extended extragalactic radio sources.

Norman, C.↗

NASA NTRS2011

Molecular Gas in Young Debris Disks

Gas-rich primordial disks and tenuous gas-poor debris disks are usually considered as two distinct evolutionary phases of the circumstellar matter. Interestingly, the debris disk around the young main-sequence star 49 Ceti possesses a substantial amount of molecular gas and possibly represents the missing link between the two phases. Motivated to understand the evolution of the gas component in circumstellar disks via finding more 49 Ceti-like systems, we carried out a CO J = 3-2 survey with the Atacama Pathfinder EXperiment, targeting 20 infrared-luminous debris disks. These systems fill the gap between primordial and old tenuous debris disks in terms of fractional luminosity. Here we report on the discovery of a second 49 Ceti-like disk around the 30 Myr old A3-type star HD21997, a member of the Columba Association. This system was also detected in the CO(2-1) transition, and the reliable age determination makes it an even clearer example of an old gas-bearing disk than 49 Ceti. While the fractional luminosities of HD21997 and 49 Ceti are not particularly high, these objects seem to harbor the most extended disks within our sample. The double-peaked profiles of HD21997 were reproduced by a Keplerian disk model combined with the LIME radiative transfer code. Based on their similarities, 49 Ceti and HD21997 may be the first representatives of a so far undefined new class of relatively old > or approx.8 Myr), gaseous dust disks. From our results, neither primordia1 origin nor steady secondary production from icy planetesima1s can unequivocally explain the presence of CO gas in the disk ofHD21997.

Moor, A.↗

NASA NTRS1981

The stochastic evolution of asteroidal regoliths and the origin of brecciated and gas-rich meteorites

A model is constructed which views regolith evolution on asteroids as a stochastic process. Average values are shown to be poor descriptors of regolith depth. The utility of the average depth is not significantly increased by avoiding large craters or thick ejecta deposits, a procedure adopted in previous regolith studies. The statistical uncertainty associated with regolith depth severely limits the power of regolith models in predicting parent-body size for brecciated meteorites. A Monte Carlo algorithm was used to simulate the random walks and corresponding charged-particle irradiation histories of grains in regoliths. On rocky asteroids, only about 20 percent of the grains was exposed to solar cosmic ray ions. Results based on present-day conditions in the asteroid belt agree well with irradiation features observed in gas-rich meteorites. An origin during epochs of early solar system evolution is not required.

Housen, K. R.↗

NASA NTRS1981

A search for neutral hydrogen in D and cD galaxies

Recent X-ray spectroscopic results suggest that 2-3 solar masses/yr of cooling gas may be falling into M87 and as much as 300 solar masses/yr into giant galaxies in rich clusters. The eventual fate of the gas in the radiatively accreting flow is an open question. In an attempt to set some bounds on the evolution of the gas, a sensitive search was performed for H I in a sample of eight D and cD galaxies in rich and poor clusters. Null detections in all cases set upper limits of (4-20) x 10 to the 8th solar masses of H I in the giant galaxies. Possible reasons for these nondetections and alternative evolutionary fates for the accreting gas are discussed.

Burns, J. O.↗

NASA NTRS2013

GBT Detection of Polarization-Dependent HI Absorption and HI Outflows in Local ULIRGs and Quasars

We present the results of a 21-cm HI survey of 27 local massive gas-rich late-stage mergers and merger remnants with the Green Bank Telescope (GBT). These remnants were selected from the Quasar/ULIRG Evolution Study (QUEST) sample of ultraluminous infrared galaxies (ULIRGs; L(sub 8 - 1000 micron) > 10(exp 12) solar L) and quasars; our targets are all bolometrically dominated by active galactic nuclei (AGN) and sample the later phases of the proposed ULIRG-to-quasar evolutionary sequence. We find the prevalence of HI absorption (emission) to be 100% (29%) in ULIRGs with HI detections, 100% (88%) in FIR-strong quasars, and 63% (100%) in FIR-weak quasars. The absorption features are associated with powerful neutral outflows that change from being mainly driven by star formation in ULIRGs to being driven by the AGN in the quasars. These outflows have velocities that exceed 1500 km/s in some cases. Unexpectedly, we find polarization-dependent HI absorption in 57% of our spectra (88% and 63% of the FIR-strong and FIR-weak quasars, respectively). We attribute this result to absorption of polarized continuum emission from these sources by foreground HI clouds. About 60% of the quasars displaying polarized spectra are radio-loud, far higher than the approx 10% observed in the general AGN population. This discrepancy suggests that radio jets play an important role in shaping the environments in these galaxies. These systems may represent a transition phase in the evolution of gas-rich mergers into "mature" radio galaxies.

Teng, Stacy H.↗

NASA NTRS1991

Effects of high pressure nitrogen on the thermal stability of SiC fibers

Polymer-derived SiC fibers were exposed to nitrogen gas pressures of 7 and 50 atm at temperatures up to 1800 C. The fiber weight loss, chemical composition, and tensile strength were then measured at room temperature in order to understand the effects of nitrogen exposure on fiber stability. High pressure nitrogen treatments limited weight loss to 3 percent or less for temperatures up to 1800 C. The bulk Si-C-O chemical composition of the fiber remained relatively constant up to 1800 C with only a slight increase in nitrogen content after treatment at 50 atm; however, fiber strength retention was significantly improved. To further understand the effects of the nitrogen atmosphere on the fiber stability, the results of previous high pressure argon treatments were compared to those of the high pressure nitrogen treatments. High pressure inert gas can temporarily maintain fiber strength by physically inhibiting the evolution of gaseous species which result from internal reactions. In addition to this physical effect, it would appear that high pressure nitrogen further improved fiber temperature capability by chemically reacting with the fiber surface, thereby reducing the rate of gas evolution. Subsequent low pressure argon treatments following the initial nitrogen treatments resulted in stronger fibers than after argon treatment alone, further supporting the chemical reaction mechanism and its beneficial effects on fiber strength.

Jaskowiak, Martha H.↗

NASA NTRS2008

Large-Scale Structure of the Molecular Gas in Taurus Revealed by High Spatial Dynamic Range Spectral Line Mapping

Viewgraph topics include: optical image of Taurus; dust extinction in IR has provided a new tool for probing cloud morphology; observations of the gas can contribute critical information on gas temperature, gas column density and distribution, mass, and kinematics; the Taurus molecular cloud complex; average spectra in each mask region; mas 2 data; dealing with mask 1 data; behavior of mask 1 pixels; distribution of CO column densities; conversion to H2 column density; variable CO/H2 ratio with values much less than 10(exp -4) at low N indicated by UV results; histogram of N(H2) distribution; H2 column density distribution in Taurus; cumulative distribution of mass and area; lower CO fractional abundance in mask 0 and 1 regions greatly increases mass determined in the analysis; masses determined with variable X(CO) and including diffuse regions agrees well with the found from L(CO); distribution of young stars as a function of molecular column density; star formation efficiency; star formation rate and gas depletion; and enlarged images of some of the regions with numerous young stars. Additional slides examine the origin of the Taurus molecular cloud, evolution from HI gas, kinematics as a clue to its origin, and its relationship to star formation.

molecular clouds↗

NASA NTRS1988

The evolution of cooling flows. II - Galaxies and galaxy formation

Self-similar solutions are presented for the idealized case of a spherical galaxy with a singular isothermal gravitational potential where the ISM is built up from mass loss by stars or by accretion of external gas. For the former case, where a galaxy initially devoid of gas obtains gas from stellar evolution, steady flow only is obtained inside the cooling radius. Several possibilities occur and are discussed for flow outside the cooling radius. For the case involving infall of external gas, the nature of the flow depends on the initial density of the infalling gas. At high density, only a cold, unshocked flow is possible. At low density, the flow is accelerated inward, is heated in a shock front, approaches a hydrostatic state, passes through the cooling radius and enters a steady subsonic cooling flow, and finally passes through a sonic point. The effects of radiative cooling are discussed.

Chevalier, Roger A.↗

NASA NTRS1988

Star formation via the phase transition of an adiabatic gas

An analytic model based on a second-order accurate global virial analysis is used to describe two stable equilibrium states for rotating gas clouds: a pressure-supported, diffuse state and a rotationally supported, compact state. Diffuse and compact equilibrium sequences are identified along which slow, secular evolution of a gas cloud can occur. A virial analysis provides analytic expressions for the limiting masses of gas clouds at points of dynamical instability. The minimum mass below which external triggers of any amplitude cannot effectively induce star formation is identified for adiabatic gas clouds. It is suggested that fluctuations in the ISM of some galaxies may actually cause star formation to proceed primarily via a phase transition, rather than via the classical Jeans instability.

Tohline, Joel E.↗

NASA NTRS1977

Thermochemical characterization of polymers for improved fire safety

Apparatus has been constructed for studying the thermal decomposition of polymers as a function of temperature. Such data is needed to evaluate the toxic threat presented by polymeric materials under fire conditions such as the smoldering fire of the type that occurs in closed areas such as coat closets in which anaerobic decomposition of polymers occurs. The apparatus allows the products of thermal decomposition to be collected and analyzed by infrared spectrometry and mass spectrometry. Data obtained from dog hair, an aromatic polyamide, polyphenylene sulfide, and polybenzimidazole are presented. It was found that significant amounts of toxic gas were evolved from dog hair at temperatures as low as 250 C, while temperatures in excess of 500 C were necessary in order for the evolution of toxic gas from the aromatic polymers to become significant.

Lerner, N. R.↗