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At least 1,243 records · Page 69
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NASA NTRS1996

The Lyman-Continuum Fluxes and Stellar Parameters of O and Early B-Type Stars

Using the results of the most recent stellar atmosphere models applied to a sample of hot stars, we construct calibrations of effective temperature (T(sub eff)), and gravity (log(sub g)) with a spectral type and luminosity class for Galactic 0-type and early B-type stars. From the model results we also derive an empirical relation between the bolometric correction and T(sub eff) and log g. Using a sample of stars with known distances located in OB associations in the Galaxy and the Large Magellanic Cloud, we derive a new calibration of M(sub v) with spectral class. With these new calibrations and the stellar atmosphere models of Kurucz, we calculate the physical parameters and ionizing photon luminosities in the H(0) and He(0) continua for O and early B-type stars. We find substantial differences between our values of the Lyman- continuum luminosity and those reported in the literature. We also discuss the systematic discrepancy between O-type stellar masses derived from spectroscopic models and those derived from evolutionary tracks. Most likely, the cause of this 'mass discrepancy' lies primarily in the atmospheric models, which are plane parallel and hydrostatic and therefore do not account for an extended atmosphere and the velocity fields in a stellar wind. Finally, we present a new computation of the Lyman-continuum luminosity from 429 known O stars located within 2.5 kpc of the Sun. We find the total ionizing luminosity from this population ((Q(sub 0)(sup T(sub ot))) = 7.0 x 10(exp 51) photons/s) to be 47% larger than that determined using the Lyman continuum values tabulated by Panagia.

Vacca, William D.↗
NASA NTRS1993

ROSAT X-ray Study of the Stellar Population of the Chamaeleon I Dark Cloud

Two soft X-ray images of the Chamaeleon I cloud obtained with the ROSAT PSPC are presented. Seventy reliable, and perhaps 19 additional X-ray sources are found. Up to 90% of these sources are certainly or probably identified with T Tauri stars formed in the cloud. Twenty to 35 are probably new 'weak' T Tauri (WTT) stars. We summarize a statistical study of the relationships between X-ray emission and other physical properties for a number of Chamaeleon stars. We see no difference in spatial distributions, detection rate and luminosity functions between the WTTS and CTTS.

Casanova, S.↗
NASA NTRS1998

Giant Molecular Clouds and High-Mass Star Formation in the Milky Way

We are conducting an extensive investigation of high-mass (OB) star formation within the dense cores of giant molecular clouds (GMCS) throughout the first Galactic quadrant of the Milky Way using enhanced resolution Infrared Astronomical Satellite (IRAS) images in combination with high-resolution ground-based observations in millimeter wave molecular transitions and radio continuum. As part of this investigation several resolution enhancement algorithms are applied to the IRAS data, including the HIgh RESolution (HIRES) algorithm developed at the IRAS Processing and Analysis Center (IPAC), as well as others ("pixon" image reconstruction). In addition, as part of a related study, we have completed a large survey of the CO emission in the first Galactic quadrant using the 15-element array detector (QUARRY) with the Five College Radio Astronomy Observatory (FCRAO) 14 m antenna, which provides sampling at an angular resolution of 50", comparable to that attained in the reprocessed IRAS data. Both of these data sets are compared with a sample of ultra-compact (UC) H II regions taken from a high-resolution multi-wavelength (6 and 20 cm) radio survey of the Galactic plane using the NRAO Very Large Array (VLA). Selected regions are observed in 1.3 mm continuum, which has proven to be particularly sensitive to the dust column density. Extensive observations of molecular clouds at high resolution in CO, CS and HCN are combined with the reprocessed IRAS high-resolution images to give a more complete picture of the physical conditions and kinematics of high-mass star forming GMCS. Our goals are to study in detail the morphology, structure, and rate of high-mass star formation within GMCs throughout the Galactic disk from the inner edge of the molecular ring to the outer Galaxy.

Source record↗
NASA NTRS2006

Using Microwave Limb Sounder Data to Validate Model Ice Fields

MLS IWC overall tends to be higher relative to ECMWF after considering MLS track sampling and sensitivity (cutoff application). Disagreement tends to be accentuated over Indian and Western Pacific Oceans and over tropical landmasses. Large disagreement occur at upper level at 147 hPa but small at lower levels at 215 and 316 hPa suggesting the need to investigate the strength of model large-scale circulation and physics associated with the IWC formation. Future work includes: Present MMF/ECMWF Comparisons at AGU/Baltimore Session on MMF/Cloud Resolving Modeling, GMAO/GSFC & at the WPac/Bejing/AGU in Tao's Cloud-Radiation Session. Write-up Results on ECMWF/MLS Comparisons for GRL. Continue with MLS vs ECMWF Water Vapor & Temperature comparisons - will seek more interaction with other MLS colleagues. Investigate the Development of Biases in ECMWF Forecasts - i.e. in the actual model. Work with GMAO-5 development team regarding their cloud microphysics performance. Integrate CloudSat into IWC Analyses.

cloud ice↗
NASA NTRS2009

Ocean, Land and Meteorology Studies Using Space-Based Lidar Measurements

CALIPSO's main mission objective is studying the climate impact of clouds and aerosols in the atmosphere. CALIPSO also collects information about other components of the Earth's ecosystem, such as oceans and land. This paper introduces the physics concepts and presents preliminary results for the valueadded CALIPSO Earth system science products. These include ocean surface wind speeds, column atmospheric optical depths, ocean subsurface backscatter, land surface elevations, atmospheric temperature profiles, and A-train data fusion products.

Hu,Yongxiang↗
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↗
NASA NTRS2001

An Investigation of the Influence of Urban Areas on Rainfall Using a Cloud-Mesoscale Model and the TRMM Satellite

A recent paper by Shepherd and Pierce (conditionally accepted to Journal of Applied Meteorology) used rainfall data from the Precipitation Radar on NASA's Tropical Rainfall Measuring Mission's (TRMM) satellite to identify warm season rainfall anomalies downwind of major urban areas. A convective-mesoscale model with extensive land-surface processes is employed to (a) determine if an urban heat island (UHI) thermal perturbation can induce a dynamic response to affect rainfall processes and (b) quantify the impact of the following three factors on the evolution of rainfall: (1) urban surface roughness, (2) magnitude of the UHI temperature anomaly, and (3) physical size of the UHI temperature anomaly. The sensitivity experiments are achieved by inserting a slab of land with urban properties (e.g. roughness length, albedo, thermal character) within a rural surface environment and varying the appropriate lower boundary condition parameters. Early analysis suggests that urban surface roughness (through turbulence and low-level convergence) may control timing and initial location of UHI-induced convection. The magnitude of the heat island appears to be closely linked to the total rainfall amount with minor impact on timing and location. The physical size of the city may predominantly impact on the location of UHI-induced rainfall anomaly. The UHI factor parameter space will be thoroughly investigated with respect to their effects on rainfall amount, location, and timing. This study extends prior numerical investigations of the impact of urban surfaces on meteorological processes, particularly rainfall development. The work also contains several novel aspects, including the application of a high-resolution (less than I km) cloud-mesoscale model to investigate urban-induce rainfall process; investigation of thermal magnitude of the UHI on rainfall process; and investigation of UHI physical size on rainfall processes.

Shepherd, J. Marshall↗
NASA NTRS1978

Physics of the primitive solar nebula and of giant gaseous protoplanets

It has been proposed that the supernova responsible for injecting Al-26 into the early solar system was in fact responsible for triggering the collapse of an interstellar cloud in order to produce a system of stars, one of which would be the solar system. Details concerning the mechanism involved in such a process are discussed. Attention is given to the evolution of the primitive solar nebula, the instabilities in the primitive solar nebula, and the giant gaseous protoplanets. The principal conclusion to be drawn from the material presented is that the primitive solar nebula was a rather chaotic place, highly turbulent, with the multiple formation of giant gaseous protoplanets.

Cameron, A. G. W.↗
NASA NTRS2001

CFD Simulation of Liquid Rocket Engine Injectors

Detailed design issues associated with liquid rocket engine injectors and combustion chamber operation require CFD methodology which simulates highly three-dimensional, turbulent, vaporizing, and combusting flows. The primary utility of such simulations involves predicting multi-dimensional effects caused by specific injector configurations. SECA, Inc. and Engineering Sciences, Inc. have been developing appropriate computational methodology for NASA/MSFC for the past decade. CFD tools and computers have improved dramatically during this time period; however, the physical submodels used in these analyses must still remain relatively simple in order to produce useful results. Simulations of clustered coaxial and impinger injector elements for hydrogen and hydrocarbon fuels, which account for real fluid properties, is the immediate goal of this research. The spray combustion codes are based on the FDNS CFD code' and are structured to represent homogeneous and heterogeneous spray combustion. The homogeneous spray model treats the flow as a continuum of multi-phase, multicomponent fluids which move without thermal or velocity lags between the phases. Two heterogeneous models were developed: (1) a volume-of-fluid (VOF) model which represents the liquid core of coaxial or impinger jets and their atomization and vaporization, and (2) a Blob model which represents the injected streams as a cloud of droplets the size of the injector orifice which subsequently exhibit particle interaction, vaporization, and combustion. All of these spray models are computationally intensive, but this is unavoidable to accurately account for the complex physics and combustion which is to be predicted, Work is currently in progress to parallelize these codes to improve their computational efficiency. These spray combustion codes were used to simulate the three test cases which are the subject of the 2nd International Workshop on-Rocket Combustion Modeling. Such test cases are considered by these investigators to be very valuable for code validation because combustion kinetics, turbulence models and atomization models based on low pressure experiments of hydrogen air combustion do not adequately verify analytical or CFD submodels which are necessary to simulate rocket engine combustion. We wish to emphasize that the simulations which we prepared for this meeting are meant to test the accuracy of the approximations used in our general purpose spray combustion models, rather than represent a definitive analysis of each of the experiments which were conducted. Our goal is to accurately predict local temperatures and mixture ratios in rocket engines; hence predicting individual experiments is used only for code validation. To replace the conventional JANNAF standard axisymmetric finite-rate (TDK) computer code 2 for performance prediction with CFD cases, such codes must posses two features. Firstly, they must be as easy to use and of comparable run times for conventional performance predictions. Secondly, they must provide more detailed predictions of the flowfields near the injector face. Specifically, they must accurately predict the convective mixing of injected liquid propellants in terms of the injector element configurations.

Farmer, Richard↗
NASA NTRS2005

It's a Sooty Problem: Black Carbon and Aerosols from Space

Our knowledge of atmospheric aerosols (smoke, pollution, dust or sea salt particles, small enough to be suspended in the air), their evolution, composition, variability in space and time and interaction with solar radiation, clouds and precipitation is lacking despite decades of research. Just recently we recognized that understanding the global aerosol system is fundamental for progress in climate change and hydrological cycle research. While a single instrument was used to demonstrate 50 yrs ago that the global CO2 levels are rising, posing thread to our climate, we need an may of satellites, surface networks of radiometers, elaborated laboratory and field experiments coupled with chemical transport models to understand the global aerosol system. This complexity of the aerosol problem results from their short lifetime (1 week), variability of the chemical composition and complex chemical and physical processes in the atmosphere. The result is a heterogeneous distribution of aerosol and their properties. The new generation of satellites and surface networks of radiometers provides exciting opportunities to measure the aerosol properties and their interaction with clouds and climate. However farther development in the satellite capability, aerosol chemical models and climate models is needed to fully decipher the aerosol secrets with accuracy required to predict future climates.

Kaufman, Yoram J.↗
NASA NTRS2004

A Meteorological Model's Dependence on Radiation Update Frequency

Numerical weather models are used to simulate circulations in the atmosphere including clouds and precipitation by applying a set of mathematical equations over a three-dimensional grid. The grid is composed of discrete points at which the meteorological variables are defined. As computing power continues to rise these models are being used at finer grid spacing, but they must still cover a wide range of scales. Some of the physics that must be accounted for in the model cannot be explicitly resolved, and their effects, therefore, must be estimated or "parameterized". Some of these parameterizations are computationally expensive. To alleviate the problem, they are not always updated at the time resolution of the model with the assumption being that the impact will be small. In this study, a coupled land-atmosphere model is used to assess the impact of less frequent updates of the computationally expensive radiation physics for a case on June 6, 2002, that occurred during a field experiment over the central plains known as International H20 Project (IHOP). The model was tested using both the original conditions, which were dry, and with modified conditions wherein moisture was added to the lower part of the atmosphere to produce clouds and precipitation (i.e., a wet case). For each of the conditions (i.e., dry and wet), four set of experiments were conducted wherein the model was run for a period of 24 hours and the radiation fields (including both incoming solar and outgoing longwave) were updated every 1, 3, 10, and 100 time steps. Statistical tests indicated that average quantities of surface variables for both the dry and wet cases were the same for the various update frequencies. However, spatially the results could be quite different especially in the wet case after it began to rain. The near-surface wind field was found to be different most of the time even for the dry case. In the wet case, rain intensities and average vertical profiles of heating associated with cloudy areas were found to differ for the various radiation update frequencies. The latter implies that the mean state of the model could be different as a result of not updating the radiation fields every time step and has important implications for longer term climate studies

Eastman, Joseph L.↗
NASA NTRS1997

Laboratory Plasma Source as an MHD Model for Astrophysical Jets

The significance of the work described herein lies in the demonstration of Magnetized Coaxial Plasma Gun (MCG) devices like CPS-1 to produce energetic laboratory magneto-flows with embedded magnetic fields that can be used as a simulation tool to study flow interaction dynamic of jet flows, to demonstrate the magnetic acceleration and collimation of flows with primarily toroidal fields, and study cross field transport in turbulent accreting flows. Since plasma produced in MCG devices have magnetic topology and MHD flow regime similarity to stellar and extragalactic jets, we expect that careful investigation of these flows in the laboratory will reveal fundamental physical mechanisms influencing astrophysical flows. Discussion in the next section (sec.2) focuses on recent results describing collimation, leading flow surface interaction layers, and turbulent accretion. The primary objectives for a new three year effort would involve the development and deployment of novel electrostatic, magnetic, and visible plasma diagnostic techniques to measure plasma and flow parameters of the CPS-1 device in the flow chamber downstream of the plasma source to study, (1) mass ejection, morphology, and collimation and stability of energetic outflows, (2) the effects of external magnetization on collimation and stability, (3) the interaction of such flows with background neutral gas, the generation of visible emission in such interaction, and effect of neutral clouds on jet flow dynamics, and (4) the cross magnetic field transport of turbulent accreting flows. The applicability of existing laboratory plasma facilities to the study of stellar and extragalactic plasma should be exploited to elucidate underlying physical mechanisms that cannot be ascertained though astrophysical observation, and provide baseline to a wide variety of proposed models, MHD and otherwise. The work proposed herin represents a continued effort on a novel approach in relating laboratory experiments to astrophysical jet observation. There exists overwhelming similarity among these flows that has already produced some fascinating results and is expected to continue a high pay off in future flow similarity studies.

Mayo, Robert M.↗
NASA NTRS1979

The radiation field wave forms produced by intracloud lightning discharge processes

The large-amplitude radiation field pulses produced by intracloud lightning discharge processes have been recorded with submicrosecond time resolution. The wave forms are distinctly different from those produced by return strokes in cloud-to-ground lightning, yet they are surprisingly alike within a discharge and in different discharges. The shapes tend to be bipolar, with two or three narrow, fast-rising pulses superimposed on the initial half cycle. Pulses with a positive initial polarity are usually produced in the several tens of milliseconds prior to the first return stroke in a cloud-to-ground discharge. Positive pulses tend to occur at regular intervals and have a mean full width of about 40 + or - 13 microsec. Negative pulses are usually produced during isolated cloud discharges at more random intervals and have shapes similar to the positive pulses but with more variability. The implications of the field shapes and polarities for the physics of intracloud discharge processes are discussed.

Weidman, C. D.↗
NASA NTRS1983

Water vapor profile retrievals from the HIRS/MSU sounder

The present physically based water vapor profile retrieval system for the HIRS/MSU sounding instrument is capable of producing accurate atmospheric and surface temperature retrievals, as well as accurate measurements of cloud fields on the basis of radiance values. In order to establish the extent of useful water vapor information in channels 8, 10, 11 and 12 of the HIRS instrument, studies have been conducted in which simultaneous temperature/humidity retrievals were performed on the basis of radiances simulated for clear and cloudy conditions.

Reuter, D.↗
NASA NTRS1986

Far-infrared spectral studies from the G. P. Kuiper Airborne Observatory

The grant to carry out far-infrared spectral studies of astronomical sources from the G. P. Kuiper Airborne Observatory was first funded in October 1978 and officially ended on October 31, 1986. During these eight years, A variety of previously undetected spectral lines was discovered, and these were used to determine the physical and chemical conditions prevailing in regions of active star formation. They served to clarify the nature of the ionized hydrogen regions often adjacent to star-forming clouds, the characteristics of shocks which may signify the onset of star formation, and the nature of coolants that channel away energy thus enabling a cloud to collapse to form new stars.

Harwit, M.↗
NASA NTRS2001

Multivariate Analysis of Solar Spectral Irradiance Measurements

Principal component analysis is used to characterize approximately 7000 downwelling solar irradiance spectra retrieved at the Southern Great Plains site during an Atmospheric Radiation Measurement (ARM) shortwave intensive operating period. This analysis technique has proven to be very effective in reducing a large set of variables into a much smaller set of independent variables while retaining the information content. It is used to determine the minimum number of parameters necessary to characterize atmospheric spectral irradiance or the dimensionality of atmospheric variability. It was found that well over 99% of the spectral information was contained in the first six mutually orthogonal linear combinations of the observed variables (flux at various wavelengths). Rotation of the principal components was effective in separating various components by their independent physical influences. The majority of the variability in the downwelling solar irradiance (380-1000 nm) was explained by the following fundamental atmospheric parameters (in order of their importance): cloud scattering, water vapor absorption, molecular scattering, and ozone absorption. In contrast to what has been proposed as a resolution to a clear-sky absorption anomaly, no unexpected gaseous absorption signature was found in any of the significant components.

Pilewskie, P.↗
NASA NTRS2011

Spectral and Vertical Distribution Properties of Titan's Particulates from Thermal-IR CIRS Data: Physical Implications

Analyses of far-IR spectra between 20 and 560/cm (500 and 18 micron) recorded by the Cassini Composite Infrared Spectrometer (CIRS) yield the spectral dependence and the vertical distribution of Titan's photochemical aerosol and stratospheric ice clouds. Below the stratopause (approx. 300 km) the aerosol appears to be incompletely mixed for the following reasons: 1) the altitude dependence of the aerosol mass absorption coefficient is larger at higher altitudes than at lower altitudes, 2} the aerosol scale height varies with altitude, which implies some kind of layering effect, and 3) the aerosol abundance varies with latitude. The spectral shape of the aerosol opacity appears to be independent in altitude and latitude below the stratopause, even though inhomogeneities in the abundance appear to be prevalent throughout this altitude region. This implies that aerosol chemistry is restricted to altitude regions above the stratopause, where pressures are less than approx 0.1 mbar. The aerosol exhibits an extremely broad emisSion feature with a spectral peak at 140/cm (71 micron), which is not evident in laboratory simulated Titan aerosols (tholin) that are created at pressures greater than 0.1 mbar. A strong broad emission feature centered roughly around 160 cm-1 corresponds very closely to those found in nitrile ice spectra. This feature is pervasive throughout the region from high northern to high southern latitudes. The inference of nitrile ices is consistent with the highly restricted altitude ranges over which these features are observed, and appear to be dominated by HCN and HC3N. At low and moderate latitudes these clouds are observed to be located between 60 and 100 km, whereas at high northern latitudes during northern winter these clouds are observed at altitudes between 150 and 165 km. The ubiquitous nature of these nitrile ice clouds is inconsistent with a simple meridional circulation concept, suggesting that the true dynamical situation is more complex.

Anderson, Carrie M.↗
NASA NTRS2012

Diurnal Variation of Precipitation During MC3E Campaign

The diurnal variation of precipitation processes in the United States (US) is well recognized but incompletely understood (Cabone et al. 2002). The diurnal cycle of precipitation has been studied using surface rainfall data, radar reflectivity data, and satellite-derived cloudiness and precipitation (Wallace 1975; Dai et al. 1999; Carbone et al. 2002; Carbone and Tuttle, 2008; Parker and Ahijevych, 2007; Matsui et al. 2010 and others). These observations indicate that the summer-time precipitation most of the North America and typically feature late-afternoon precipitation maxima. These diurnal variation of precipitation can also be generally categorized into three different types: 1) afternoon rainfall maxima due to mesoscale and local circulations over the south and east of the Mississippi and Ohio valleys, 2) nocturnal rainfall maxima from eastward-propagating mesoscale convective systems (MCSs) over the Lee side of Rocky Mountain regions and 3) afternoon rainfall maxima in the Appalachian Mountains, and then propagate eastward toward the coast. The main objective of this paper is to use a regional cloud-scale model with very high-resolution (i.e., WRF) to examine the WRF ability to simulate diurnal variation of precipitation. Specifically, the study will (1) identify the physical processes responsible to diurnal variation of precipitation, (2) examine the sensitivity of resolution (2, 6, 18, and 30 km) to model simulated diurnal variation of precipitation and (3) identify the relationships between microphysics and cumulus parameterization schemes.

Tao, Wei-Kuo↗