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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 343 records · Page 19
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NASA NTRS1998

The Spectral Energy Distribution of z = 4.7 Quasar: BR1202-0727

BR 1202-0721 is a z = 4.1 quasar which is a 4" double source in the mm, possibly lensed, and has a Lyman alpha emission companion 2" away. The mm spectrum shows a large mass of cool dust ( l0Ell Msun). We report on ground based optical and near-infrared (IR) and space-based, Infrared Space Observatory (ISO) mid-IR observations of this source, combined with data from the literature. The resulting spectral energy distribution show the source to be about two orders-of-magnitude over luminous in the rest-frame near-IR in comparison with typical low-redshift quasars.

Wilkes, Belinda↗
NASA NTRS1999

Proceedings of the Workshop on the Spectrophotometric Dating of Stars and Galaxies

In the past decade, we have seen an avalanche of new observational results from space observatories and ground-based observatories. These observations have revealed young globular clusters in the cores of merger galaxies, elliptical galaxies at redshifts up to z=1.5, and starburst galaxies at high redshift. Analyses of the detailed spectra or color- magnitude diagrams of these systems promise to give a new understanding of evolutionary processes and to provide a check on cosmological ages. At the same time, these new spectro-photometric data present new challenges to current methods of spectral analysis and modeling.At the Workshop, we will discuss these new opportunities and challenges on spectro-photometric dating of stars and galaxies.

Hubeny, Ivan↗
NASA NTRS1999

ISO Mid-Infrared Observations of Giant HII Regions in M33

We present Infrared Space Observatory Camera (ISOCAM) Circular Variable Filter scans of three giant HII regions in M33. IC 133, NGC 595, and CC 93 span a wide range of metallicity, luminosity, nebular excitation, and infrared excess; three other emission regions (CC 43, CC 99, and a region to the northeast of the core of NGC 595) are luminous enough in the mid-infrared to be detected in the observed fields. ISOCAM CVF observations provide spatially resolved observations (5'') of 151 wavelengths between 5.1 and 16.5 microns with a spectral resolution R = 35 to 50. We observe atomic emission lines ([Ne II], [Ne III], and [S IV]), several "unidentified infrared bands" (UIBs; 6.2, 7.7, 8.6, 11.3, 12.0, and 12.7 microns), and in some cases a continuum which rises steeply at longer wavelengths. We conclude that the spectra of these three GHRs are well explained by combinations of ionized gas, PAHs, and very small grains in various proportions and with different spatial distributions. Comparisons between observed ratios of the various UIBs with model ratios indicate that the PAHs in all three of the GHRs are dehydrogenated and that the small PAHs have been destroyed in IC 133 but have survived in NGC 595 and CC 93. The [Ne III]/[Ne II] ratios observed in IC 133 and NGC 595 are consistent with their ages of 5 and 4.5 Myr, respectively; the deduced ionization parameter is higher in IC 133, consistent with its more compact region of emission.

Skelton, B. P.↗
NASA NTRS1999

Detection of C2H4 Neptune from ISO/PHT-S Observations

The 6-12 micrometer spectrum of Neptune has been recorded with the PHT-S instrument of the Infrared Space Observatory (ISO) at a resolution of 0.095 micrometer. In addition to the emissions of CH4, CH3D and C2H6 previously identified, the spectrum shows the first firm identification of ethylene C2H4. The inferred column density above the 0.2-mbar level is in the range (1.1 - 3) x 10(exp 14) molecules/cm. To produce this low amount, previous photochemical models invoked rapid mixing between the source and sink regions of C2H4. We show that this requirement can be relaxed if recent laboratory measurements of CH4 photolysis branching ratios at Lyman alpha are used.

Schulz, B.↗
NASA NTRS1999

Titan

With a launch in December 2001, Space Infrared Telescope Facility (SIRTF) can observe Titan in the interval after Infrared Space Observatory (ISO) but before the onset of observations by Cassini. By virtue of its broad spectral coverage in the thermal infrared, 10-180 micron, its moderately high spectral resolution, approaching lambda/delta lambda=600 over part of this wavelength range, and the very high sensitivity of its helium- cooled detectors, the Infrared Spectrometer (IRS) and MIPS on SIRTF can address several issues raised through earlier observations by the Voyager IRIS experiment and by ISO. These include, for example, a better characterization of the vertical distribution of water in Titan's middle and upper atmospheres and the discovery of new compounds, such as allene or proprionitrile. This talk will address the temperature- and composition-sounding capabilities of SIRTF, particularly in the context of how they will complement Cassini observations and aid in their planning.

Flasar, F. M.↗
NASA NTRS1999

Final Report

This multi-year grant partially supported the Infrared Space Observatory (ISO) data analysis and related activities of three Guest Investigators (Prof. Dan Clemens, Prof. Thomas Bania, and Prof. James Jackson) at Boston University for ten distinct ISO investigations. Support under this grant covered "Years 1 & 2" of the ISO activities at Boston University. "Year 3" activities support is currently in force under a separate JPL contract, with Prof. James Jackson as the Block Grant PI; "Year 4" support has been requested from JPL and that proposal is currently pending.

Clemens, Dan↗
NASA NTRS2000

Photochemistry of Saturn's Atmosphere: Hydrocarbon Chemistry and Comparisons with ISO Observations - 1

To investigate the details of hydrocarbon photochemistry on Saturn, we have developed a one-dimensional diurnally averaged model that couples hydrocarbon and oxygen photochemistry, molecular and eddy diffusion, radiative transfer, and condensation. The model results are compared with observations from the Infrared Space Observatory (ISO) to place tighter constraints on molecular abundances, to better define Saturn's eddy diffusion coefficient profile, and to identify important chemical schemes that control the abundances of the observable hydrocarbons in Saturn's upper atmosphere. From the ISO observations, we determine that the column 12 densities of CH3, CH3C2H, and C4H2 above 10 mbar are 4 (sup +2) (sub -1.5) x 10 (exp 13) cm (sup -2), (1.1 plus or minus 0.3) x 10 (exp 15) cm (exp -2), and (1.2 plus or minus 0.3) x 10 (exp 14) cm (sup -2), respectively. The observed ISO emission features also indicate C2H2 mixing ratios of 1.2 (sup +0.9) (sub -0.6) x 10 (exp -6) at 0.3 mbar and (2.7 plus or minus 0.8) x 10 (exp -7) at 1.4 mbar, and a C2H6 mixing ratio of (9 plus or minus 2.5) x 10 (exp -6) at 0.5 mbar. Upper limits are provided for C2H4, CH2CCH2, C3H8, and C6H2 sensitivity of the model results to variations in the eddy diffusion coefficient profile, the solar flux, the CH4 photolysis branching ratios, the atomic hydrogen influx, and key reaction rates are discussed in detail. We find that C4H2 and CH3C2H are particularly good tracers of important chemical processes and physical conditions in Saturn's upper atmosphere, and C2H6 is a good tracer of the eddy diffusion coefficient in Saturn's lower stratosphere. The eddy diffusion coefficient must be smaller than approximately 3 x 10 (exp 4) sq cm s (sup -1) at pressures greater than 1 mbar in order to reproduce the C2H6 abundance inferred from ISO observations. The eddy diffusion coefficients in the upper stratosphere could be constrained by observations of CH3 radicals if the low-temperature chemistry of CH3 were better understood. We also discuss the implications of our modeling for aerosol formation in Saturn's lower stratosphere-diacetylene, butane, and water condense between approximately 1 and 300 mbar in our model and will dominate stratospheric haze formation at nonauroral latitudes. Our photochemical models will be useful for planning observational sequences and for analyzing data from the upcoming Cassini mission.

Moses, Julianne I.↗
NASA NTRS2000

Photochemistry Saturn's Atmosphere: Effects of an Influx of External Oxygen - 2

We use a one-dimensional diurnally averaged model of photochemistry and diffusion in Saturn's stratosphere to investigate the influence of extraplanetary debris on atmospheric chemistry. In particular, we consider the effects of an influx of oxygen from micrometeoroid ablation or from ring-particle diffusion; the contribution from cometary impacts, satellite debris, or ring vapor is deemed to be less important. The photochemical model results are compared directly with Infrared Space Observatory (ISO) observations to constrain the influx of extraplanetary oxygen to Saturn. From the ISO observations, we determine that the column densities of CO2 and H2O above 10 mbar in Saturn's atmosphere are (6.3 +/- 1) x 10(exp 14) and (1.4 +/- 0.4) x 10(exp 15)/ square cm, respectively; our models indicate that a globally averaged oxygen influx of (4+/-2) x 10(exp 6) O atoms /sq cm/s is required to explain these observations. Models with a locally enhanced influx of H20 operating over a small fraction of the projected area do not provide as good a fit to the ISO H2O observations. If volatile oxygen compounds comprise one-third to one-half of the exogenic source by mass, then Saturn is currently being bombarded with (3 +/- 2) x 10(exp -16) g/square cm/s of extraplanetary material. To reproduce the observed CO2/H2O ratio in Saturn's stratosphere, some of the exogenic oxygen must arrive in the form of a carbon-oxygen bonded species such as CO or CO2. An influx consistent with the composition of cometary ices fails to reproduce the high observed CO2/H2O ratio, suggesting that (i) the material has ices that are slightly more carbon-rich than is typical for comets, (ii) a contribution from an organic-rich component is required, or (iii) some of the hydrogen-oxygen bonded material is converted to carbon-oxygen bonded material without photochemistry (e.g., during the ablation process). We have also reanalyzed the 5-micron CO observations of Noll and Larson and determine that the CO lines are most sensitive to the 100- to 400-mbar column density for which we derive a range of (0.7-1.5) x 10(exp 17)/square cm; the CO observations do not allow us to distinguish between an external or internal source of CO on Saturn. If we assume that all the extraplanetary oxygen derives from a micrometeoroid source, then the unfocused dust flux at 9.5 AU must be (i) (1 +/- 0.7) x 10(exp -16) g/square cm/s if interstellar grains are the source of the external oxygen on Saturn, (ii) (4 +/- 3) x 10 (exp -17) g/ sq. cm./s if IDPs on randomly inclined, highly eccentric orbits (e.g., Halley-type comet grains) are the source of the external oxygen, or (iii) (2+/- 1.4) x 10(exp -18)/g/sq cm/s if IDPs on low inclination, low eccentricity orbits (e.g., Kuiper-belt grains) are the source of the external oxygen. These estimates can be used in combination with future Cassini dust detection data to determine the ultimate source of the dust at Saturn's distance from the Sun.

Moses, Julianne I.↗
NASA NTRS1996

Far-Infrared Water Emissions from Magnetohydrodynamic Shock Waves

Nondissociative, magnetohydrodynamic, C-type shock waves are expected to be a prodigious source of far-infrared water emissions in dense interstellar regions. We have constructed a model to calculate the farinfrared H20 line spectra that emerge from such shocks. Using the best estimates currently available for the radiative cooling rate and the degree of ion-neutral coupling within the shocked gas, we modeled the temperature structure of MHD shocks using standard methods in which the charged and neutral particles are treated separately as two weakly coupled, interpenetrating fluids. Then we solved the equations of statistical equilibrium to find the populations of the lowest 179 and 170 rotational states of ortho- and para-H2O We have completed an extensive parameter study to determine the emergent H2O line luminosities as a function of preshock density in the range n(H2) equals 10(exp 4) - 10(sup 6.5)/cc and shock velocity in the range upsilon(sub s) = 5 - 40 km/ s. We find that numerous rotational transitions of water are potentially observable using the Infrared Space Observatory and the Submillimeter Wave Astronomy Satellite and may be used as diagnostics of the shocked gas. We have also computed the rotational and ro-vibrational emissions expected from H2, CO, and OH, and we discuss how complementary observations of such emissions may be used to further constrain the shock conditions. In common with previous studies, we come close to matching the observed H2, and high-J CO emissions from the Orion-KL star-forming region on the basis of a single shock model. We present our predictions for the strengths of H2O line emission from the Orion shock, and we show how our results may be scaled to other regions where molecular shocks are likely to be present.

Kaufman, Michael J.↗
NASA NTRS1995

Far-Infrared Water Line Emissions from Circumstellar Outflows

We have modeled the far-infrared water line emission expected from circumstellar outflows from oxygen-rich late-type stars, as a function of the mass-loss rate and the terminal outflow velocity. For each mass-loss rate and terminal outflow velocity considered, we computed self-consistently the gas density, temperature, outflow velocity, and water abundance as a function of distance from the star. We then used an escape probability method to solve for the equilibrium level populations of 80 rotational states of water and thereby obtained predictions for the luminosity of a large number of far-infrared rotational transitions of water. In common with previous models, our model predicts that water will be copiously produced in the warm circumstellar gas and that water rotational emission will dominate the radiative cooling. However, our use of a realistic radiative cooling function for water leads to a lower gas temperature than that predicted in previous models. Our predictions for the far-infrared water line luminosities are consequently significantly smaller than those obtained in previous studies. Observations to be carried out by the Infrared Space Observatory will provide a crucial test of the models presented here.

Chen, Wesley↗
NASA NTRS1993

Radiative Cooling of Warm Molecular Gas

We consider the radiative cooling of warm (T >= 100 K), fully molecular astrophysical gas by rotational and vibrational transitions of the molecules H2O, CO, and H2. Using an escape probability method to solve for the molecular level populations, we have obtained the cooling rate for each molecule as a function of temperature, density, and an optical depth parameter. A four-parameter expression proves useful in fitting the run of cooling rate with density for any fixed values of the temperature and optical depth parameter. We identify the various cooling mechanisms which are dominant in different regions of the astrophysically relevant parameter space. Given the assumption that water is very abundant in warm regions of the interstellar medium, H2O rotational transitions are found to dominate the cooling of warm interstellar gas over a wide portion of the parameter space considered. While chemical models for the interstellar medium make the strong prediction that water will be produced copiously at temperatures above a few hundred degrees, our assumption of a high water abundance has yet to be tested observationally. The Infrared Space Observatory and the Submillimeter Wave Astronomy Satellite will prove ideal instruments for testing whether water is indeed an important coolant of interstellar and circumstellar gas.

Neufeld, David A.↗
NASA NTRS1995

Thermal Balance in Dense Molecular Clouds: Radiative Cooling Rates and Emission-Line Luminosities

We consider the radiative cooling of fully shielded molecular astrophysical gas over a wide range of temperatures ( 10 K < T < 2500 K) and H2 densities ( 10(exp 3)/cc < n(H2) < 10(exp 10)/cc). Our model for the radiative cooling of molecular gas includes a detailed treatment of the interstellar chemistry that determines the abundances of important coolant molecules, and a detailed treatment of the excitation of the species H2, CO, H2O, HCl, O2, C, O, and their isotopic variants where important. We present results for the total radiative cooling rate and for the cooling rate due to individual coolant species, as a function of the gas temperature, density, and optical depth. We have also computed the individual millimeter, submillimeter, and far-infrared line strengths that contribute to the total radiative cooling rate, and we have obtained example spectra for the submillimeter emission expected from molecular cloud cores. Many of the important cooling lines will be detectable using the Infrared Space Observatory and the Submillimeter Wave Astronomy Satellite.

Neufeld, David A.↗
NASA NTRS1997

Detection of Thermal Water Vapor Emission from W Hydrae

We have detected four far-infrared emission lines of water vapor toward the evolved star W Hydrae, using the Short Wavelength Spectrometer (SWS) of the Infrared Space Observatory (ISO). This is the first detection of thermal water vapor emission from a circumstellar outflow.

Neufeld, David A.↗
NASA NTRS2000

The Dust Content and Opacity of Actively Star-Forming Galaxies

We present far-infrared (FIR) photometry at 150 and 205 micron(s) of eight low-redshift starburst galaxies obtained with the Infrared Space Observatory (ISO) ISOPHOT. Five of the eight galaxies are detected in both wave bands, and these data are used, in conjunction with IRAS archival photometry, to model the dust emission at lambda approximately greater than 40 microns. The FIR spectral energy distributions (SEDs) are best fitted by a combination of two modified Planck functions, with T approx. 40 - 55 K (warm dust) and T approx. 20-23 K (cool dust) and with a dust emissivity index epsilon = 2. The cool dust can be a major contributor to the FIR emission of starburst galaxies, representing up to 60% of the total flux. This component is heated not only by the general interstellar radiation field, but also by the starburst itself. The cool dust mass is up to approx. 150 times larger than the warm dust mass, bringing the gas-to-dust ratios of the starbursts in our sample close to Milky Way values, once resealed for the appropriate metallicity. The ratio between the total dust FIR emission in the range 1-1000 microns and the IRAS FIR emission in the range 40 - 120 microns is approx. 1.75, with small variations from galaxy to galaxy. This ratio is about 40% larger than previously inferred from data at millimeter wavelengths. Although the galaxies in our sample are generally classified as "UV bright," for four of them the UV energy emerging shortward of 0.2 microns is less than 15% of the FIR energy. On average, about 30% of the bolometric flux is coming out in the UV-to-near-IR wavelength range; the rest is emitted in the FIR. Energy balance calculations show that the FIR emission predicted by the dust reddening of the UV-to-near-IR stellar emission is within a factor of approx. 2 of the observed value in individual galaxies and within 20% when averaged over a large sample. If our sample of local starbursts is representative of high-redshift (z approx. greater than 1), UV - bright star-forming galaxies, these galaxies' FIR emission will be generally undetected in submillimeter surveys, unless: (1) their bolometric luminosity is comparable to or larger than that of ultraluminous FIR galaxies and (2) their FIR SED contains a cool dust component.

Calzetti, Daniela↗
NASA NTRS1997

Model For Dense Molecular Cloud Cores

We present a detailed theoretical model for the thermal balance, chemistry, and radiative transfer within quiescent dense molecular cloud cores that contain a central protostar. In the interior of such cores, we expect the dust and gas temperatures to be well coupled, while in the outer regions CO rotational emissions dominate the gas cooling and the predicted gas temperature lies significantly below the dust temperature. Large spatial variations in the gas temperature are expected to affect the gas phase chemistry dramatically; in particular, the predicted water abundance varies by more than a factor of 1000 within cloud cores that contain luminous protostars. Based upon our predictions for the thermal and chemical structure of cloud cores, we have constructed self-consistent radiative transfer models to compute the line strengths and line profiles for transitions of (12)CO, (13)CO, C(18)O, ortho- and para-H2(16)O, ortho- and para-H2(18)O, and O I. We carried out a general parameter study to determine the dependence of the model predictions upon the parameters assumed for the source. We expect many of the far-infrared and submillimeter rotational transitions of water to be detectable either in emission or absorption with the use of the Infrared Space Observatory (ISO) and the Submillimeter Wave Astronomy Satellite. Quiescent, radiatively heated hot cores are expected to show low-gain maser emission in the 183 GHz 3(sub 13)-2(sub 20) water line, such as has been observed toward several hot core regions using ground-based telescopes. We predict the (3)P(sub l) - (3)P(sub 2) fine-structure transition of atomic oxygen near 63 micron to be in strong absorption against the continuum for many sources. Our model can also account successfully for recent ISO observations of absorption in rovibrational transitions of water toward the source AFGL 2591.

Doty, Steven D.↗
NASA NTRS1997

Discovery of Interstellar Hydrogen Fluoride

We report the first detection of interstellar hydrogen fluoride. Using the Long Wavelength Spectrometer of the Infrared Space Observatory (ISO), we have detected the 121.6973 micron J = 2-1 line of HF in absorption toward the far-infrared continuum source Sagittarius B2. The detection is statistically significant at the 13 sigma level. On the basis of our model for the excitation of HF in Sgr B2, the observed line equivalent width of 1.0 nm implies a hydrogen fluoride abundance of about 3 x 10 (exp -10) relative to H, If the elemental abundance of fluorine in Sgr B2 is the same as that in the solar system, then HF accounts for about 2% of the total number of fluorine nuclei. We expect hydrogen fluoride to be the dominant reservoir of gas-phase fluorine in Sgr B2, because it is formed rapidly in exothermic reactions of atomic fluorine with either water or molecular hydrogen; thus, the measured HF abundance suggests a substantial depletion of fluorine onto dust grains. Similar conclusions regarding depletion have previously been reached for the case of chlorine in dense interstellar clouds. We also find evidence at a lower level of statistical significance (about 5 sigma) for an emission feature at the expected position of the 4(sub 32)-4(sub 23) 121.7219 micron line of water. The emission-line equivalent width of 0.5 mm for the water feature is consistent with the water abundance of 5 x 10(exp -6) relative to H, that has been inferred previously from observations of the hot core of Sgr B2.

Neufeld, David A.↗
NASA NTRS2000

Implications of SWAS Observations for Interstellar Chemistry and Star Formation

A long standing prediction of steady state gas-phase chemical theory is that H2O and O2 are important reservoirs of elemental oxygen and major coolants of the interstellar medium. Analysis of SWAS observations has set sensitive upper limits on the abundance Of O2 and has provided H2O abundances toward a variety of star forming regions. Based on these results, we show that gaseous H2O and O2 are not dominant carriers of elemental oxygen in molecular clouds. Instead the available oxygen is presumably frozen on dust grains in the form of molecular ices, with a significant portion potentially remaining in atomic form, along with CO, in the gas phase. H2O and O2 are also not significant coolants for quiescent molecular gas. In the case of H2O, a number of known chemical processes can locally elevate its abundance in regions with enhanced temperatures, such as warm regions surrounding young stars or in hot shocked gas. Thus, water can be a locally important coolant. The new information provided by SWAS, when combined with recent results from the Infrared Space Observatory, also provide several hard observational constraints for theoretical models of the chemistry in molecular clouds and we discuss various models that satisfy these conditions.

Bergin, Edwin A.↗
NASA NTRS1998

Thermal Water Vapor Emission from Shocked Regions in Orion

Using the Long Wavelength Spectrometer on board the Infrared Space Observatory, we have observed thermal water vapor emission from a roughly circular field of view approximately 75" in diameter centered on the Orion BN-KL region. The Fabry-Perot line strengths, line widths, and spectral line shifts observed in eight transitions between 71 and 125 micron show good agreement with models of thermal emission arising from a molecular cloud subjected to a magnetohydrodynamic C-type shock. Both the breadth and the relative strengths of the observed lines argue for emission from a shock rather than from warm quiescent gas in the Orion core. Although one of the eight transitions appears anomalously strong and may be subject to the effects of radiative pumping, the other seven indicate an H2O/H2 abundance ratio on the order of 5 x 10(exp -4) and a corresponding gas-phase oxygen-to-hydrogen abundance ratio on the order of 4 x 10(exp -4). Given current estimates of the interstellar, gas-phase, oxygen and carbon abundances in the solar vicinity, this value is consistent with theoretical shock models that predict the conversion into water of all the gas-phase oxygen that is not bound as CO. The overall cooling provided by rotational transitions of H2O in this region appears to be comparable to the cooling through rotational lines of CO but is an order of magnitude lower than cooling through H2 emission. However, the model that best fits our observations shows cooling by H2O and CO dominant in that portion of the postshock region where temperatures are below approximately 800 K and neither vibrational nor rotational radiative cooling by H2 is appreciable.

Harwitt, Martin↗