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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 667 records · Page 37
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NASA NTRS1995

Magnetic Compton-induced pair cascade model for gamma-ray pulsars

Electrons accelerated to relativistic energies in pulsar magnetospheres will Compton scatter surface thermal emission and nonthermal optical, UV, and soft X-ray emission to gamma-ray energies, thereby initiating a pair cascade through synchrotron radiation and magnetic pair production. This process is proposed as the origin of the high-energy radiation that has been detected from six isolated pulsars. We construct an analytic model of magnetic Compton scattering near the polar cap of isolated pulsar magnetospheres and present approximate analytic derivations for scattered spectra, electron energy-loss rates, and photon luminosities. A Monte Carlo simulation is used to model the pair cascade induced by relativistic electrons scattering photons through the cyclotron resonance. For simplicity, the primary electrons are assumed to be monoenergetic and the nonresonant emission is omitted. Assuming that the angle phi(sub B) between the magnetic and spin axes is approximately equal to the polar-cap angle theta(sub pc), this model can produce both double-peaked and broad single-peaked pulse profiles and account for the trend of harder gamma-ray spectra observed from older pulsars.

Sturner, Steven J.↗
NASA NTRS1995

The stellar wind of an O8.5 I(f) star in M 31

We rediscuss the UV spectrum of OB 78#231, an O8.5 I(f) star in the Andromeda galaxy M 31, which has been obtained with the Faint Object Spectrograph on the Hubble Space Telescope by Hutchings et al. (1992). The spectrum has been re-extracted with better knowledge of background, calibration, and scattered light. The empirical analysis of the stellar wind lines results in a terminal velocity and mass loss rate similar to those typically found in comparable galactic objects. Furthermore, a comparison with an FOS spectrum of an O7 supergiant in the Small Magellanic Cloud and IUE spectra of galactic objects implies a metallicity close to galactic counterparts. These results are confirmed quantitatively by spectrum synthesis calculations using a theoretical description of O-star winds.

Haser, S. M.↗
NASA NTRS1995

Multiple rings around Wolf-Rayet evolution

We present optical narrow-band imaging of multiple rings existing around galactic Wolf-Rayet (WR) stars. The existence of multiple rings of material around Wolf-Rayet stars clearly illustrates the various phases of evolution that massive stars go through. The objects presented here show evidence of a three stage evolution. O stars produce an outer ring with the cavity being partially filled by ejecta from a red supergiant of luminous blue variable phase. A wind from the Wolf-Rayet star then passes into the ejecta materials. A simple model is presented for this three stage evolution. Using observations of the size and dynamics of the rings allows estimates of time scales for each stage of the massive star evolution. These are consistent with recent theoretical evolutionary models. Mass estimates for the ejecta, from the model presented, are consistent with previous ring nebula mass estimates from IRAS data, showing a number of ring nebulae to have large masses, most of which must in be in the form of neutral material. Finally, we illustrate how further observations will allow the determination of many of the parameters of the evolution of massive stars such as total mass loss, average mass loss rates, stellar abundances, and total time spent in each evolutionary phase.

Marston, A. P.↗
NASA NTRS1995

Winds and accretion in delta Sagittae

The ten-year binary delta Sge (M2 Ib-II+B9.5 V) is a zeta Aur binary containing an abnormally cool component. Combining our analysis of the system as a visual binary with Batten's radial-velocity solution leads to the following properties: i = 40 deg, a = 51 mas = 8.83 A.U. = 1893 solar radius, hence d = 173 pc; M(sub B) = 2.9 solar mass and M(sub M) = 3.8 solar mass; and R(sub B) = 2.6 solar radius and R(sub M) = 152 solar radius. This interpretation of the orbit places the M supergiant on the asymptotic giant branch. We have collected ultraviolet spectra throughout the star's 1980-90 orbit, concentrated around the conjuction of 1990. The wind of the M giant appears in these as narrow shell lines of singly ionized metals, chiefly Fe II, with P-Cyg profiles at many phases, which show the slow variation in strength expected for the orbit but no pronounced atmospheric eclipse. The terminal velocity of the wind is 16-18 km/s, and its excitation temperature is approximately 10,000 K. Most of the broadening of the wind lines is caused by differential expansion of the atmosphere, with (unmeasurably) low turbulent velocities. Nontheless, the mass loss rate (1.1 +/- 0.4 X 10 (exp -8) solar mas/yr) is almost the same as found previously by Reimers and Schroder for very different assumptions about the velocity structure. Also seen in the spectrum throughout the orbit are the effects of a variable, high-speed wind as well as evidence for accretion onto the B9.5 star. This high-speed wind absorbs in species of all ionization stages observed, e. g., C II, Mg II, Al III, SI IV, C IV, and has a terminaal velocity in the range 200-450 km/s. We presume this wind originates at the B dwarf, not the M supergiant, and speculate that it comes from an accretion disk, as suggested by recent models of magnetically moderated accretion. Evidence for accretion is redshifted absorption in the same transitions formed in the high-speed wind, as well as broad emission lines of singly ionized metals. This emission seems to be scattered out of the continuum of the B star. Finally, we discuss Auger ionization by accretion luminosity as the cause of the high ionization in these high-speed flows as well as the source of the extended circumstellar clouds of Si(3+) and C(3+) ions in zeta Aur binaries.

Eaton, Joel A.↗
NASA NTRS1995

Fine structure line emission from supergiants

We have detected (O I) 63 micron and (Si II) 35 micron emission from the oxygen-rich, M supergiants alpha Orionis (Betelgeuse), alpha Scorpii (Antares), and alpha Herculis (Rasalgethi). The measured fluxes indicate that the emission originates in dense, warm gas in the inner envelope or transition region where molecules and dust are expected to form and the acceleration of the wind occurs. Mass-loss rates are derived, evidence for time variability is presented, and results for other evolved stars are included.

Haas, Michael R.↗
NASA NTRS1996

The 1988 Arctic Survey, Diurnal Study (Sunrise and Sunset) and Peak Altitude (22 km) Flights for the In Situ Detection of ClO and BrO from the NASA ER-2 Aircraft

Two critical areas of research were addressed successfully by this research. The first involves NASA ER-2 airborne observations of ClO and BrO radical destruction of ozone within the arctic vortex. The second involves the analysis of diurnal variations in ClO, to test the production and loss rates of ClO that constitutes the test for coupling reactions between the chlorine and nitrogen systems. We discuss results from this research in order.

Anderson, James G.↗
NASA NTRS1996

Composition and evolution of the atmosphere of Venus

The contract year started by analyzing Jovian atmospheric data acquired by the Galileo Probe Mass Spectrometer (GPMS). Two Venus hydrogen projects got underway as well. The first study strives to understand how to reconcile the standard treatment of the evolution of the H2O and HDO resevoirs on Venus over 4.5 Gyr in the presence of H and D escape and injection by comets. The second study is calculating the charge exchange contribution to hydrogen loss rates, using realistic models for exospheric H, H(+), D, D(+), and ion temperature from PV data. This report includes the following papers as attachments and supporting data: 'The Galileo Probe Mass Spectrometer: Composition of Jupiter's Atmosphere'; 'Chemical Composition Measurements of the Atmosphere of Jupiter with the Galileo Probe Mass Spectrometer'; 'Ion/Neutral Escape of Hydrogen and Deuterium: Evolution of Water'; 'Hydrogen and Deuterium in the Thermosphere of Venus: Solar Cycle Variations and Escape'; and 'Solar Cycle Variations in H(+) and D(+) Densities in the Venus Ionosphere: Implications for Escape'.

Donahue, Thomas↗
NASA NTRS1996

Externally Induced Evaporation of Young Stellar Disks in Orion

In this paper we propose a model for the evaporation of disks around young low-mass stars by external sources of high energy photons. Two evaporation techniques are possible. Lyman continuum radiation can ionize hydrogen at the disk surface powering a steady thermal ionized disk-wind, or FUV radiation can heat the disk through photo-electric grain processes powering a slower thermal neutral disk-wind. Applying these two models to the evaporating objects in the Trapezium produces a satisfactory solution to both the mass-loss rate and size of the ionized envelopes.

Johnstone, D.↗
NASA NTRS1995

Radiation Pressure-Driven Magnetic Disk Winds in Broad Absorption Line Quasi-Stellar Objects

We explore a model in which QSO broad absorption lines (BALS) are formed in a radiation pressure-driven wind emerging from a magnetized accretion disk. The magnetic field threading the disk material is dragged by the flow and is compressed by the radiation pressure until it is dynamically important and strong enough to contribute to the confinement of the BAL clouds. We construct a simple self-similar model for such radiatively driven magnetized disk winds, in order to explore their properties. It is found that solutions exist for which the entire magnetized flow is confined to a thin wedge over the surface of the disk. For reasonable values of the mass-loss rate, a typical magnetic field strength such that the magnetic pressure is comparable to the inferred gas pressure in BAL clouds, and a moderate amount of internal soft X-ray absorption, we find that the opening angle of the flow is approximately 0.1 rad, in good agreement with the observed covering factor of the broad absorption line region.

DeKool, Martin↗
NASA NTRS1997

Io's Sodium Corona and Spatially Extended Cloud: A Consistent Flux Speed Distribution

For Io neutral cloud calculations, an SO2 source strength of approximately 4x10(exp 27) molecules/sec was determined by successfully matching the SO2(+) density profile near the satellite deduced from magnetometer data acquired by the Galileo spacecraft during its close flyby on December 7, 1995. The incomplete collision source velocity distribution for SO2 is the same as recently determined for the trace species atomic sodium by Smyth and Combi (1997). Estimates for the total energy loss rate (i.e. power) of O and S atoms escaping Io were also determined and imply a significant pickup current and a significant reduction in the local planetary magnetic field near Io.

Smyth, William H.↗
NASA NTRS1997

Quantitative UV spectroscopy of early O stars on the Magellanic Clouds: The determination of the stellar metallicities

Ultraviolet spectra of four O stars in the Magellanic Clouds obtained with the faint object spectrograph of the Hubble Space Telescope are analyzed with respect to their metallicity. The metal abundances are derived from the stellar parameters and the mass loss rate with a two step procedure: hydrodynamic radiation-driven wind models with metallicity as a free parameter are constructed to fit the observed wind momentum rate and thus yield a dynamical metallicity, and synthetic spectra are computed for different metal abundances and compared to the observed spectra in order to obtain a spectroscopic metallicity.

Haser, Stefan M.↗
NASA NTRS1998

The Local ISM and its Interaction with the Winds of Nearby Late-type Stars

We present new Goddard High-Resolution Spectrograph (GHRS) observations of the Ly-alpha and Mg II absorption lines seen toward the nearby stars 61 Cyg A and 40 Eri A. We use these data to measure interstellar properties along these lines of sight and to search for evidence of circumstellar hydrogen walls, which are produced by collisions between the stellar winds and the Local InterStellar Medium (LISM). We were able to model the Ly-alpha lines of both stars without hydrogen-wall absorption components, but for 61 Cyg A the fit required a stellar Ly-alpha, line profile with an improbably deep self-reversal, and for 40 Eri A the fit required a very low deuterium-to-hydrogen ratio that is inconsistent with previous GHRS measurements. Since these problems could be rectified simply by including stellar hydrogen-wall components with reasonable attributes, our preferred fits to the data include these components. We have explored several ways in which the hydrogen-wall properties measured here and in previous work can be used to study stellar winds and the LISM. We argue that the existence of a hydrogen wall around 40 Eri A and a low H I column density along that line of sight imply that either the interstellar density must decrease toward 40 Eri A or the hydrogen ionization fraction (chi) must increase. We find that hydrogen-wall temperatures are larger for stars with faster velocities through the LISM. The observed temperature-velocity relation is consistent with the predictions of hydromagnetic shock jump conditions. More precise comparison of the data and the jump conditions suggests crude upper limits for both chi and the ratio of magnetic to thermal pressure in the LISM (alpha): chi less than 0.6 and alpha less than 2. The latter upper limit corresponds to a limit on the LISM magnetic field of B less than 5 micro G. These results imply that the plasma Mach number of the interstellar wind flowing into the heliosphere is M(sub A) greater than 1.3, which indicates that the collision is supersonic and that there should therefore be a bow shock outside the heliopause in the upwind direction. Finally, we estimate stellar wind pressures (P sub wind) from the measured hydrogen-wall column densities. These estimates represent the first empirical measurements of wind properties for late-type main-sequence stars. The wind pressures appear to be correlated with stellar X-ray surface fluxes, F(x), in a manner consistent with the relation P(wind) varies as F(x)(exp -1/2), a relation that is also consistent with the variations of P(sub wind) and F(sub x) observed during the solar activity cycle. If this relation can in fact be generalized to solar-like stars, as is suggested by our data, then it is possible to estimate stellar wind properties simply by measuring stellar X-rays. One implication of this is that stellar wind pressures and mass-loss rates are then predicted to increase with time, since F(sub x) is known to decrease with stellar age.

Wood, Brian E.↗
NASA NTRS1995

A slightly more massive young Sun as an explanation for warm temperatures on early Mars

The valley network channels on the heavily cratered ancient surface of Mars suggest the presence of liquid water approx. 3.8 Gyr ago. However, the implied warm climate is difficult to explain in the context of the standard solar model, even allowing for the maximum CO2 greenhouse heating. In this paper we investigate the astronomical and planetary implications of a nonstandard solar model in which the zero-age, main-sequence Sun had a mass of 1.05 +/- 0.02 Solar Mass. The excess mass was subsequently lost in a solar wind during the first 1.2(sup -0.2)(sub +.04) Gyr of the Sun's main sequence phase. The implied mass-loss rate of 4(sup +3)(sub -2) x 10(exp -11) Solar Mass/yr, or about 10(exp 3) times that of the current Sun, may be detectable in several nearby young solar type stars.

Whitmire, Daniel P.↗
NASA NTRS1999

Ionized Outflows in 3-D Insights from Herbig-Haro Objects and Applications to Nearby AGN

HST shows that the gas distributions of these objects are complex and clump at the limit of resolution. HST spectra have lumpy emission-line profiles, indicating unresolved sub-structure. The advantages of 3D over slits on gas so distributed are: robust flux estimates of various dynamical systems projected along lines of sight, sensitivity to fainter spectral lines that are physical diagnostics (reddening-gas density, T, excitation mechanisms, abundances), and improved prospects for recovery of unobserved dimensions of phase-space. These advantages al- low more confident modeling for more profound inquiry into underlying dynamics. The main complication is the effort required to link multi- frequency datasets that optimally track the energy flow through various phases of the ISM. This tedium has limited the number of objects that have been thoroughly analyzed to the a priori most spectacular systems. For HHO'S, proper-motions constrain the ambient B-field, shock velocity, gas abundances, mass-loss rates, source duty-cycle, and tie-ins with molecular flows. If the shock speed, hence ionization fraction, is indeed small then the ionized gas is a significant part of the flow energetics. For AGN'S, nuclear beaming is a source of ionization ambiguity. Establishing the energetics of the outflow is critical to determining how the accretion disk loses its energy. CXO will provide new constraints (especially spectral) on AGN outflows, and STIS UV-spectroscopy is also constraining cloud properties (although limited by extinction). HHO's show some of the things that we will find around AGN'S. I illustrate these points with results from ground-based and HST programs being pursued with collaborators.

Cecil, Gerald↗
NASA NTRS1999

Proton Effects and Test Issues for Satellite Designers

Microelectronic and photonic systems in the natural space environment are bombarded by a variety of charged particles including electrons, trapped protons, cosmic rays, and solar particles (protons and other heavy ions). These incident particles cause both ionizing and non-ionizing effects when traversing a device, and the effects can be either transient or permanent. The vast majority of the kinetic energy of an incident proton is lost to ionization, creating the single event effects (SEES) and total ionizing dose (TID) effects. However, the small portion of energy lost in non-ionizing processes causes atoms to be removed from their lattice sites and form permanent electrically active defects in semiconductor materials. These defects, i.e., "displacement damage," can significantly degrade device performance. In general, most of the displacement damage effects in the natural space environment can be attributed to protons since they are plentiful and extremely energetic (and therefore not readily shielded against). For this reason, we consider only proton induced displacement damage in this course. (Nevertheless, we identify solar cells as an important example of a case where both electron and proton damage can be important since only very light shielding is feasible.) The interested reader is encouraged to explore the three previous NSREC and RADECS short courses which also treat displacement damage issues for satellite applications. Part A of this segment of the short course introduces the space environment, proton shielding issues, and requirements specifications for proton-rich environments. In order to exercise the displacement damage analysis tools for on-orbit performance predictions, the requirements document must provide the relevant proton spectra in addition to the usual total ionizing dose-depth curves. Ion-solid interactions and the nature of the displacement damage they generate have been studied extensively for over half a century, yet they still remain a subject of investigation. In this section, a description of the mechanisms by which displacement damage is produced will be followed by a summary of the major consequences for device performance in a space environment. Often the degradation of a device parameter can be characterized by a damage factor (measured in a laboratory using monoenergetic protons) that is simply the change in a particular electrical or optical parameter per unit proton fluence. In addition, we will describe the concept of a non-ionizing energy loss rate (NIEL) which quantifies that portion of the energy lost by an incident ion that goes into displacements. It has been calculated as a function of proton energy, and is analogous to (and has the same units as) the linear energy transfer (LET) for ionizing energy. We will discover that, to first order, the calculated NIEL describes the energy dependence of the measured device damage factors. This observation provides the basis for predicting proton induced device degradation in a space environment based on both the calculated NIEL and relatively few laboratory test measurements. The methodology of such on-orbit device performance predictions will be described, as well as the limitations. Several classes of devices for which displacement damage is a significant (if not the dominant) mode of radiation induced degradation will be presented.

Marshall, Cheryl J.↗
NASA NTRS1999

Ozone Budgets from a Global Chemistry/ Transport Model and Comparison to Observations from POLARIS

The objective of the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) field mission was to obtain data to better characterize the summertime seasonal decrease of ozone at mid to high latitudes. The decrease in ozone occurs mainly in the lower stratosphere and is expected to result from in situ chemical destruction. Instrumented balloons and aircraft were used in POLARIS, along with satellites, to measure ozone and chemical species which are involved with stratospheric ozone chemistry. In order to close the seasonal ozone budget, however, ozone transport must also be estimated. Comparison to a global chemistry and transport model (CTM) of the stratosphere indicates how well the summertime ozone loss processes are simulated and thus how well we can predict the ozone response to changing amounts of chemical source gases. Moreover, the model gives insight into the possible relative magnitude of transport contributions to the seasonal ozone decline. Initial comparison to the Goddard CTM, which uses transport winds and temperatures from meteorological data assimilation, shows a high ozone bias in the model and an attenuated summertime ozone loss cycle. Comparison of the model chemical partitioning, and ozone catalytic loss rates to those derived from measurements shows fairly close agreement both at ER-2 altitudes (20 km) and higher. This suggests that the model transport is too active in resupplying ozone to the high latitude region, although chemistry failings cannot be completely ruled out. Comparison of ozone and related species will be shown along with a full diagnosis of the model ozone budget and its possible sources of error.

Kawa, S. Randy↗
NASA NTRS1998

Pulsations, Shocks, and Mass Loss

This grant provided long-term support for my investigation of the outflows powered by young stars. Several major research results emerged during the course of this research, including: (1) The discovery of giant Herbig-Haro outflows from young stars that can extend for many parsecs from their sources. The first parsec-scale outflow to be recognized led to the realization that Herbig-Haro outflows, even those produced by low mass young stellar objects, can extend orders of magnitude farther from their sources than previously thought. Our preconceptions were to a large extent driven by the narrow fields-of-view then provided by CCD detectors. With the recent advent of large format CCDs and CCD mosaics, we have come to realize that most outflows attain parsec-scale dimensions. Even at the distance of the Orion star forming clouds, such flows can subtend a degree on the sky. Our work has led to the recognition of over two dozen giant. (2) The discovery that outflows are highly clustered. Even regions of relatively isolated star formation such as those in Taurus frequently produce multiple outflows. (3) The discovery of a new family of externally irradiated jets. During the last year of support from this grant, we made the startling discovery that there is a class of jets from young stars that are illuminated by the ionizing radiation field of nearby massive stars. The first four examples were discovered in the vicinity of the a Orionis sub-group of the Orion OB Association which is believed to be at least 2 million years old. Since the jets are photo-ionized, their densities can be reliably estimated. Most HH jets are shock excited, and are therefore notoriously difficult to characterize since their visibility and observed properties depend on the complex and highly non-linear processes associated with shocks. Furthermore, many irradiated jets are one sided rather than bipolar. Thus, irradiated jets may for the first time be used to accurately diagnose jet densities and mass loss rates, and to probe the physics of jet collimation, and may indicate that the jet production phase of certain young stars may last more than a million years. These three discoveries provide us with fundamental new insights into the star formation process, into the physical conditions inside and near star forming clouds, and into new ways to probe the physics and chemistry of such clouds.

Bally, John↗
NASA NTRS1998

Ozone Budgets from a Global Chemistry/Transport Model and Comparison to Observations from POLARIS

The objective of the Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) field mission was to obtain data to better characterize the summertime seasonal decrease of ozone at mid to high latitudes. The decrease in ozone occurs mainly in the lower stratosphere and is expected to result from in situ chemical destruction. Instrumented balloons and aircraft were used in POLARIS, along with satellites, to measure ozone and chemical species which are involved with stratospheric ozone chemistry. In order to close the seasonal ozone budget, however, ozone transport must also be estimated. Comparison to a global chemistry and transport model (CTM) of the stratosphere indicates how well the summertime ozone loss processes are simulated and thus how well we can predict the ozone response to changing amounts of chemical source gases. Moreover, the model gives insight into the possible relative magnitude of transport contributions to the seasonal ozone decline. Initial comparison to the Goddard CTM, which uses transport winds and temperatures from meteorological data assimilation, shows a high ozone bias in the model and an attenuated summertime ozone loss cycle. Comparison of the model chemical partitioning and ozone catalytic loss rates to those derived from measurements shows fairly close agreement both at ER-2 altitudes (20 km) and higher. This suggests that the model transport is too active in resupplying ozone to the high latitude region, although chemistry failings cannot be completely ruled out. Comparison of ozone and related species will be shown along with a full diagnosis of the model ozone budget and its possible sources of error.

Kawa, S. Randolph↗