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At least 649 records · Page 36
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NASA NTRS1992

Evolution of near UV Halley's spectrum in the inner coma

A direct way to observe the photodissociation of water vapor in a cometary coma is to detect the OH prompt emission. This emission is shifted of delta lambda = 4 nm with respect to the OH 309 nm fluorescence band. The extended data set obtained with the three-channel spectrometer on-board Vega 2 reveals at short distance of the nucleus (i.e., less than 600 km) an excess of emission on the right wing of the OH band which may be interpreted as being mainly due to prompt emission.

Rousselot, Phillippe↗
NASA NTRS1993

The ISM in the M82 starburst

We have observed (O 1) (63 microns) and (Si 2) (35 microns) in the central 700 pc of the starburst galaxy M82. The luminosities in these transitions are 7.1 x 10(exp 7) solar luminosity and 6.2 x 10(exp 7) solar luminosity, respectively, which are each approx. 0.15% of the bolometric luminosity from this region. The ratios of (O 1) line luminosity to (O 3), (Si 2) (35 microns), and to bolometric luminosities in M82 are similar to those in M42, M17, and Sgr A. These similarities, and the association of the bulk of the (O 1) and (Si 2) emission with the ionized emission, suggest that the dominant emission mechanism for (O 1) and (Si 2) in M82 is the same as in these Galactic regions, namely warm gas photodissociated by UV flux from the OB stars responsible for the nearby H 2 regions. We argue that shock or x ray heated gas or H 2 plasma is a minor contributor to the intensities of these fine structure lines. Both the (O 1) (53 microns) and the (Si 2) (35 microns) spectrum show an asymmetric line profile indistinguishable in shape from those of the (O 3) (52 and 88 microns) and (N 3) (57 microns) lines and similar to that of the more extended (C 2) 158 micron line measured previously in M82. We detect two distinct velocity components, which we attribute to emission from two regions at either end of the central bar, where the bar connects to an orbiting torus of neutral gas seen in H 1 and CO J = 1-0. We model separately the two velocity components and derive the physical conditions in these two regions. The clouds in these regions are small, R approx. 1-2 pc, have warm neutral gas surfaces, T approx. 200 K, and are concentrated with volume filling factors of approx. 0.02 and area filling factors of 1-5. The entire central region (R approx. 700 pc) is characterized by a large number, approx. 5 x 10(exp 4), of 2 x 10(exp 3) solar mass clouds with surface densities of approx. 3 x 10(exp 4) cm(exp -3), illuminated by FUV fluxes 10(exp 4) times the average local interstellar value for the Milky Way. These clouds reside in the harsh conditions of a starburst nucleus, with photoevaporation times of 10(exp 6) yr, and collision timescales only about an order of magnitude longer.

Lord, Steven↗
NASA NTRS1992

Fractionation of CO in the diffuse clouds toward Zeta Ophiuchi

An analysis of CO A-X bands in diffuse clouds toward Zeta Ophiuchi is presented. The results provide isotopic ratios that constitute the strongest observational evidence for highly localized isotopic fractionation of CO in diffuse portions of an interstellar cloud. The value of the fractionation implies that selective photodissociation is the controlling influence of the fractionation. The molecule's excitation temperature is discussed and a lower limit to the O-16/O-18 ratio is provided. The absence of CO lines from the gas that provides broad CH+ and CH lines commonly attributed to warm gas behind a shock front is addressed.

Sheffer, Yaron↗
NASA NTRS1991

Ultraviolet spectroscopy of cometary comae

During the past decade, vacuum ultraviolet spectra of over 30 comets have been obtained with the IUE satellite observatory. With few exceptions, the spectra of these comets appear to be similar, with OH and H produced by the photodissociation of water being the dominant species and emissions of C, O, S, CS and CO2(+) usually present. Although signs of variabiity of many kinds in comet spectra appear, the evidence from the UV observations suggests that all comets have the same basic chemical composition and that observed differences are due to evolution and ageing processes. During the 1985-86 apparition of Comet Halley, spectra were also obtained by other spacecraft and by sounding rocket instruments, including a long-slit imaging spectrograph.

Feldman, Paul D.↗
NASA NTRS1992

Infrared line diagnostics of active galactic nuclei

The CLOUDY photoionization code of Ferland (1991) is employed to predict intensities and ratios of the near-, mid-, and far-IR ionic fine-structure lines from AGN of the LINER and Seyfert types, and starbursts. The depths of varying the important model parameters, including ionization parameter, density, and optical depth, are investigated. The way in which detailed IR spectroscopy can reveal the presence of density inhomogeneities and abundance anomalies is shown, and the slope of the ionizing continuum and the amount of reddening and the reddening law are determined. The predictions of the present model and others are compared to the observed line ratios of the starburst galaxy M82 and of the Galactic center. No single-zone model adequately predicted all of the observed lines, but a simple combination of a starburst plus photodissociation region of higher density have a good fit for M82. The present model predictions are used to estimate the detectability of IR fine-structure lines in various active galaxies.

Spinoglio, Luigi↗
NASA NTRS1992

Mid-infrared rotational line emission from interstellar molecular hydrogen

The line emission from the v = 0-0 S(0), S(2), and S(3), and the v = 1-0 and v = 2-1 S(1) transitions of molecular hydrogen in clouds exposed to high FUV fluxes and in shocks is modeled. Particular attention is given to the lowest pure rotational H2 transitions at 20 and 17 microns, respectively. It is found that, in photodissociation regions (PDRs), the emission comes from warm (greater than about 100 k) molecular gas, situated at optical depths greater than about 1, beyond the hot atomic surface layer of the clouds. For FUV fields, G0 = 1000 to 100,000 times the average interstellar field densities n = 10 exp 3 - 10 exp 7/cu cm, the typical line intensities are in the range 10 exp -6 to 10 exp -4 ergs/s sq cm sr. The predictions for the line intensities from both C-type and J-type shock models are compared. The results are applied to recent observations of the 0-0 S(1) transition in both the PDR and the shocked gas in Orion.

Burton, Michael G.↗
NASA NTRS1992

Aeronomy of the current Martian atmosphere

The thermal structure of the Martian atmosphere, which varies diurnally, seasonally and episodically, is discussed. The atomic oxygen airglow at 1304 A is used to determine the density of atomic oxygen, and the 1216-A Lyman-alpha line is used to calculate the density of atomic hydrogen and, when coupled with the temperature measurement, the escape flux of atomic hydrogen. The most intense airglow is the IR atmospheric band of O2 at 1.27 micron that results from the photodissociation of ozone. The escape mechanism for atomic hydrogen is thermal, or Jeans, escape, while the atomic oxygen escape is caused by a nonthermal process, namely, the dissociative recombination of O2(+). The ratio of deuterium to hydrogen is enriched by a factor of 6. Three-dimensional models of the Mars thermospheric circulation show that planetary rotation has a significant effect on the wind, composition, and temperature structure.

Barth, C. A.↗
NASA NTRS1993

Long-slit ultraviolet spectroscopy of Comet Austin (1990 V)

The spatial distributions of the brightest features in the far-UV spectrum of Comet Austin were measured, and it is found that they are generally similar to those found for Comet Halley during two rocket observations in 1986. As in Halley, the CO profile can be fit to a radial outflow profile for a source whose size is smaller than the instrumental spatial resolution. Neither the shape nor the magnitude of the extended atomic carbon profiles, however, can be satisfactorily explained by the photodissociation of CO. No evidence is found for an extended source of CO in the coma; thus, suggestions that the additional carbon can come directly from grains are difficult to accept since Austin is a low-dust comet. The presence of the O I 1356 A feature implies that there is a photoelectron excitation source in the coma. The measured O I 1304 A distribution is consistent with H2O being the dominant source of oxygen in the coma. The atomic sulfur profile matches that expected for a daughter product of relatively short-lived parents.

Sahnow, David J.↗
NASA NTRS1993

Galactic fine-structure lines - Morphologies of the warm ionized interstellar medium

Far-infrared ionic fine-structure emission lines associated with the large-scale structure of the Galactic ionized medium are interpreted in terms of a heuristic model in which three distinct source morphologies are considered: ionization-bounded H II regions, an extended, low-density medium, and externally ionized surfaces of neutral clouds. The model reveals that the overall luminosity of the Galaxy in the fine-structure lines of both C II and N II is due, in comparable proportions, to diffuse medium and the boundaries between ionized and neutral regions. Limits are set on the luminosity of the diffuse component in the plane of the Galaxy. Some individual lines of sight of the COBE FIRAS instrument are dominated, in these spectral lines, by H II regions, even with its 7 deg field of view. Average densities of both H II regions and photodissociation regions are estimated on a Galactic scale.

Petuchowski, S. J.↗
NASA NTRS1993

The production and escape of nitrogen atoms on Mars

Updated rate coefficients and a revised ionosphere-thermosphere model are used to compute the production rates and densities of odd nitrogen species in the Martian atmosphere. Computed density profiles for N(4S), N(2D), N(2P), and NO are presented. The model NO densities are found to be about a factor of 2-3 less than those measured by the Viking 1 mass spectrometer. Revised values for the escape rates of N atoms from dissociative recombination and ionospheric reactions are also computed. Dissociative recombination is found to be comparable in importance to photodissociation at low solar activity, but it is still the most important escape mechanism for N-14 at high solar activity.

Fox, J. L.↗
NASA NTRS1993

N(2P) in the dayglow - Measurement and theory

During the ATLAS-1 mission, the first dayglow altitude profiles were obtained of the N(2P) emission at 3466 A. These observations were made in the sunlit thermosphere using the Imaging Spectrometric Observatory. As all previous work on this emission has been done under auroral conditions, this study represents the first examination of the photochemical sources and sinks of N(2P) in the normal daytime thermospheric, with comparison with measurements. We find that the observations are explained by a model in which the major source is photodissociation of N2, and quenching by O is the principal low-altitude loss process, with radiative decay to N(2D) dominating above 200 km. As the dominant loss processes are likely to result in the production of N(2D), N(2P) could be a moderate source of N(2D) and may be a factor to be taken into consideration in modeling NO.

Torr, Marsha R.↗
NASA NTRS1993

Laser-induced fluorescence detection strategies for sodium atoms and compounds in high-pressure combustors

A variety of laser-induced fluorescence schemes were examined experimentally in atmospheric pressure flames to determine their use for sodium atom and salt detection in high-pressure, optically thick environments. Collisional energy transfer plays a large role in fluorescence detection. Optimum sensitivity, at the parts in 10 exp 9 level for a single laser pulse, was obtained with the excitation of the 4p-3s transition at 330 nm and the detection of the 3d-3p fluorescence at 818 nm. Fluorescence loss processes, such as ionization and amplified spontaneous emission, were examined. A new laser-induced atomization/laser-induced fluorescence detection technique was demonstrated for NaOH and NaCl. A 248-nm excimer laser photodissociates the salt molecules present in the seeded flames prior to atom detection by laser-induced fluorescence.

Weiland, Karen J. R.↗
NASA NTRS1993

Activation of extended red emission photoluminescence in carbon solids by exposure to atomic hydrogen and UV radiation

We report on new laboratory results which relate directly to the observation of strongly enhanced extended red emission (ERE) by interstellar dust in H2 photodissociation zones. The ERE has been attributed to photoluminescence by hydrogenated amorphous carbon (HAC). We are demonstrating that exposure to thermally dissociated atomic hydrogen will restore the photoluminescence efficiency of previously annealed HAC. Also, pure amorphous carbon (AC), not previously photoluminescent, can be induced to photoluminesce by exposure to atomic hydrogen. This conversion of AC into HAC is greatly enhanced by the presence of UV irradiation. The presence of dense, warm atomic hydrogen and a strong UV radiation field are characteristic environmental properties of H2 dissociation zones. Our results lend strong support to the HAC photoluminescence explanation for ERE.

Furton, Douglas G.↗
NASA NTRS1993

Investigation of the feasibility of optical diagnostic measurements at the exit of the SSME

Under Contract NAS8-36861 sponsored by NASA Marshall Space Flight Center, the United Technologies Research Center is conducting an investigation of the feasibility of remote optical diagnostics to measure temperature, species concentration and velocity at the exit of the Space Shuttle Main Engine (SSME). This is a two phase study consisting of a conceptual design phase followed by a laboratory experimental investigation. The first task of the conceptual design studies is to screen and evaluate the techniques which can be used for the measurements. The second task is to select the most promising technique or techniques, if as expected, more than one type of measurement must be used to measure all the flow variables of interest. The third task is to examine in detail analytically the capabilities and limitations of the selected technique(s). The results of this study are described in the section of this report entitled Conceptual Design Investigations. The conceptual design studies identified spontaneous Raman scattering and photodissociative flow-tagging for measurements respectively of gas temperature and major species concentration and for velocity. These techniques and others that were considered are described in the section describing the conceptual design. The objective of the second phase of investigations was to investigate experimentally the techniques identified in the first phase. The first task of the experimental feasibility study is to design and assemble laboratory scale experimental apparatus to evaluate the best approaches for SSME exit optical diagnostics for temperature, species concentrations and velocity, as selected in the Phase I conceptual design study. The second task is to evaluate performance, investigate limitations, and establish actual diagnostic capabilities, accuracies and precision for the selected optical systems. The third task is to evaluate design requirements and system trade-offs of conceptual instruments. Spontaneous Raman scattering excited by a KrF excimer laser pulse was investigated for SSME exit plane temperature and major species concentration measurements. The relative concentrations of molecular hydrogen and water vapor would be determined by measuring the integrated Q-branch scattering signals through narrow bandpass filters in front of photomultipliers. The temperature would be determined by comparing the signal from a single hydrogen rotational Raman line to the total hydrogen Q-branch signal. The rotational Raman line would be isolated by a monochromator and detected with a PMT.

Shirley, John A.↗
NASA NTRS1993

Molecular processes in comets

Classical trajectory calculations of the cross sections for vibrational and rotational energy exchange in direct and reactive collisions of hydrogen atoms and hydrogen molecules have been carried out. To test the sensitivity, three potential energy surfaces have been used. For the exchange transitions which occur at small internuclear distances, the rate coefficients for the three surfaces agree quite well. For the direct transitions, there are significant differences for the pure rotational transitions from j=0 to 2 and from j=1 to j=3 in which there is no change in vibration. For higher j the differences tend to disappear, suggesting that the rotational angular momentum can couple to the orbital angular momentum to overcome the centrifugal barrier. Complete numerically exact quantum mechanical calculations for the process in which vJ changes have been performed. Dr. M. A'Hearn has provided data on the fluorescent population of the NH rotational and fine-structure levels from which we should be able to predict accurate photodissociation lifetimes. The distribution rate of C2 is being investigated. A review of H3(+) in terrestrial and extraterrestrial environments was prepared for a volume of Advances in Atomic, Molecular and Optical Physics.

Dalgarno, A.↗
NASA NTRS1993

Requirements for the early atmosphere of Mars from nitrogen isotope ratios

The N escape models of Fox and Dalgarno and Fox required the presence of a dense, early CO2 atmosphere to inhibit fractionation of the N isotopes N-15 and N-14. The computed photochemical escape fluxes are so large at the present that the isotope ratio measured by Viking (about 1.62x terrestrial) can be produced in about 1.5 b.y. This model was refined in several ways. It was updated to incorporate the variation of the escape fluxes with increases in the solar fluxes at earlier times according to the model of Zahnle and Walker. As expected, this exacerbates the problem with overfractionation, but not greatly. Most of the escape and fractionation of the N occurs in the last 1.5 b.y., when the solar flux was only slightly different from the present. The dense early atmosphere must persist only a bit longer in order to reproduce the measured isotope ratio. The model was also modified to take into account changes in the O mixing ratio with time in the past, assuming that the O abundance is proportional to the square root of the solar flux. Although the production rate of O from photodissociation of CO2 scales as the solar flux, the strength of the winds and other mixing processes also increases with the solar flux, resulting in possibly more effective transport of O to the lower atmosphere where it is destroyed by catalytic and three-body recombination mechanisms.

Fox, J. L.↗
NASA NTRS1993

Possible solutions to the problem of channel formation on early Mars

A warm climate on early Mars would provide a natural, although not unique, explanation for the presence of fluvial networks on the ancient, heavily cratered terrains. Explaining how the climate could have been kept warm, however, is not easy. The idea that the global average surface temperature, T(sub s), could have been kept warm by a dense, CO2 atmosphere supplied by volcanism or impacts is no longer viable. It was shown that CO2 cloud formation should have kept T(sub s) well below freezing until approximately 2 b.y. ago, when the Sun had brightened to at least 86 percent of its present value. Warm equatorial regions on an otherwise cold planet seem unlikely because atmospheric CO2 would probably condense out at the poles. Warming by impact-produced dust in the atmosphere seems unlikely because the amount of warming expected for silicate dust particles is relatively small. Greenhouse warming by high altitude CO2 ice clouds seems unlikely because such are poor absorbers of infrared radiation at most wavelengths. Warming by atmospheric NH3 seems unlikely because NH3 is readily photodissociated and because N may have been in short supply as consequence of impact erosion and the high solubility of NH3. A brighter, mass-losing young Sun seems unlikely because stellar winds of the required strength were not observed on other solar-type stars. In short, most of the explanations for a warm Martian paleoclimate that were proposed in the past seem unlikely. One possibility that seems feasible from radiative/photochemical standpoint is that CH4 and associated hydrocarbon gases and particles contributed substantially to the greenhouse effect on early Mars. Methane is photochemically more stable than NH3 and the gases and particles that can be formed from it are all good absorbers of infrared radiation. The idea of a CH4-rich Martian paleoatmosphere was suggested a long time ago but has fallen out of favor because of perceived difficulties in maintaining a CH4-rich atmosphere. In particular, it is not obvious where the CH4 might come from, since volcanic gases (on Earth, at least) contain very little CH4. This difficulty could be largely overcome if early Mars was inhabited by microorganisms. Then, methanogenic bacteria living in sediments could presumably have supplied CH4 to the atmosphere in copious quantities.

Kasting, J. F.↗
NASA NTRS1994

Theoretical determination of chemical rate constants using novel time-dependent methods

The work completed within the grant period 10/1/91 through 12/31/93 falls primarily in the area of reaction dynamics using both quantum and classical mechanical methodologies. Essentially four projects have been completed and have been or are in preparation of being published. The majority of time was spent in the determination of reaction rate coefficients in the area of hydrocarbon fuel combustion reactions which are relevant to NASA's High Speed Research Program (HSRP). These reaction coefficients are important in the design of novel jet engines with low NOx emissions, which through a series of catalytic reactions contribute to the deterioration of the earth's ozone layer. A second area of research studied concerned the control of chemical reactivity using ultrashort (femtosecond) laser pulses. Recent advances in pulsed-laser technologies have opened up a vast new field to be investigated both experimentally and theoretically. The photodissociation of molecules adsorbed on surfaces using novel time-independent quantum mechanical methods was a third project. And finally, using state-of-the-art, high level ab initio electronic structure methods in conjunction with accurate quantum dynamical methods, the rovibrational energy levels of a triatomic molecule with two nonhydrogen atoms (HCN) were calculated to unprecedented levels of agreement between theory and experiment.

Dateo, Christopher E.↗