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NASA NTRS1995

Heating And Cooling of the Interstellar Medium

This talk will review the various heating and cooling processes in the interstellar medium. The most important heating processes include the photoelectric effect on dust grains and PAH molecules. Cooling of the gas in the interstellar medium is dominated by emission in the far infrared fine structure lines of OI, CII, SiII, and CI, and the rotational transitions of CO. Many of these lines have been observed from air borne platform, in particular the Kuiper Airborne Observatory. Which of these heating and cooling processes dominates in a given region depends on its physical and especially chemical conditions, which in turn depend themselves on the interstellar UV radiation field. As a result of the interplay of these processes, the interstellar medium is organized in various structures with distinctly different physical conditions (ie., temperature, density, and degree of ionization). Over the last decade, the dominant cooling lines of the neutral interstellar medium have been studied using the Kuiper Airborne Observatory. Because of sensitivity, these observations have concentrated on dense regions illuminated by strong UV fields from nearby stars, so-called Photo Dissociation Regions (PDRs). These observations allow us to study in detail the interaction of UV photons and the interstellar gas and dust. Detailed theoretical models have been developed for PDRs. These will be reviewed and compared to the observations. Finally, anticipated crucial results of future space-based missions (SIRTF, ISO) will be pointed out.

Tielens, Alexander G. G.↗

NASA NTRS1996

Cooling of the Interstellar Medium

This talk will review the various heating and cooling processes in the interstellar medium. The most important heating processes include the photoelectric effect on dust grains and PAH molecules. Cooling of the gas in the interstellar medium is dominated by emission in the far infrared fine-structure lines of OI, CII, SiII, and CI, and the rotational transitions of CO. Which of these heating and cooling processes dominates in a given region depends on its physical and especially chemical conditions, which in turn depend themselves on the interstellar UV radiation field. As a result of the interplay of these processes, the interstellar medium is organized in various structures with distinctly different physical conditions (i.e., temperature, density, and degree of ionization). The dominant cooling lines of the neutral interstellar medium have been studied using the Kuiper Airborne Observatory, balloon-borne instruments, and recently space-based missions (IRTS, ISO). These observations have concentrated on dense regions illuminated by strong UV fields from nearby stars, so-called PhotoDissociation Regions (PDRs). These observations allow us to study in detail the interaction of UV photons observations allow us to study in detail the interaction of UV photons and the interstellar gas and dust. Detailed theoretical models have been developed for PDRs. These will be reviewed and compared to the observations. Recent space based missions have measured the dominant cooling line of the galaxy, the [CII] 158 micron line. These observations and their implications will be reviewed.

Tielens, A. G. G. M.↗

NASA NTRS1997

Interfacing Relativistic and Nonrelativistic Methods: A Systematic Sequence of Approximations

A systematic sequence of approximations for the introduction of relativistic effects into nonrelativistic molecular finite-basis set calculations is described. The theoretical basis for the approximations is the normalized elimination of the small component (ESC) within the matrix representation of the modified Dirac equation. The key features of the normalized method are the retention of the relativistic metric and the ability to define a single matrix U relating the pseudo-large and large component coefficient matrices. This matrix is used to define a modified set of one- and two-electron integrals which have the same appearance as the integrals of the Breit-Pauli Hamiltonian. The first approximation fixes the ratios of the large and pseudo-large components to their atomic values, producing an expansion in atomic 4-spinors. The second approximation defines a local fine-structure constant on each atomic centre, which has the physical value for centres considered to be relativistic and zero for nonrelativistic centres. In the latter case, the 4-spinors are the positive-energy kinetic al ly-balanced solutions of the Levy-Leblond equation, and the integrals involving pseudo-large component basis functions on these centres, are set to zero. Some results are presented for test systems to illustrate the various approximations.

Dyall, Ken↗

NASA NTRS2002

Triggered Nucleation in Ni60Nb40 Using an Electrostatic Levitator

External triggering of nucleation in an undercooled liquid metal has been performed for the first time in an electrostatically levitated sample of undercooled eutectic Ni60Nb40. With the Electrostatic Levitator (ESL) facility at NASA/MSFC, NiNb alloys were externally nucleated at various undercooling increments. Observation of the resulting microstructure indicates that only one nucleation site is initiated with a trigger if the undercooling is sufficiently high. Well-defined transition zones exist in all the samples. An "anomalous" fine structured eutectic is initially grown from the nucleation point into the undercooled liquid followed by an unexpected variable width transition zone leading to a coarse structured region of dendrites. Our study tends to corroborate the findings of other studies on highly undercooled eutectics regarding a coupled growth zone within which the lamellar structure is formed and outside of which the anomalous is found.

Rathz, T.↗

NASA NTRS2002

Atomic Data and Spectral Line Intensities for Ne III

A number of satellites and rockets have been launched to observe radiation from the Sun and other astrophysical objects. Line radiation is emitted when the electron impact excited levels decay to the lower levels by photon emission. From this radiation, the physical parameters such as electron temperature and density of the astrophysical plasma, elemental abundance, and opacity can be inferred. Ne III lines have been observed in H II regions, Ne-rich filaments in supernovae, and planetary nebulae. The allowed line at 489.50 Angstroms due to the transition 2s(sup 2) 2p(sup 5) (sup 3) P2 (goes to) 2s(sup 2)2p(sup 4)(sup 3)P2 has been identified in the solar spectrum by Vernazza and Reeves using Skylab observations. Other Ne III lines in the solar EUV spectrum have been reported by Thomas and Neupert based on observations from the Solar EUV Rocket Telescope and Spectrograph (SERTS) instrument. Atomic data for Ne III have been calculated by using a set of programs developed at, University College, London. The Superstructure and Distorted Wave (DW) programs have been updated over the years. In the Superstructure program, configuration interaction can be taken into account and radial functions are calculated in a modified Thomas-Fermi-Amaldi potential. This is a statistical potential and depends on parameters lambda 1 which are determined by optimizing the weighted sum of term energies. They are found to be lambda(sub 0)=1.2467, lambda(sub 1)=1.1617, and lambda(sub 2)=1.0663. The relativistic corrections are included by using the Breit-Pauli Hamiltonian as a perturbation to the nonrelativistic Hamiltonian. The same potential is used to calculate reactance matrices in the DW approximation in LS coupling. Collision strengths in intermediate coupling are obtained by using term coupling coefficients obtained from the Superstructure program. In this calculation, the configurations used are 2s(sup 2)2p(sup 4), 2s2p(sup 5), 2s(sup 2)2p(sup 3)3s, 2s(sup 2)p(sup 3)3d giving rise to 57 fine-structure levels in intermediate coupling.

Bhatia, A. K.↗

NASA NTRS1999

Photodissociation Regions in the Interstellar Medium of Galaxies

The interstellar medium of galaxies is the reservoir out of which stars are born and into which stars inject newly created elements as they age. The physical properties of the interstellar medium are governed in part by the radiation emitted by these stars. Far-ultraviolet (6 eV less than h(nu) less than 13.6 eV) photons from massive stars dominate the heating and influence the chemistry of the neutral atomic gas and much of the molecular gas in galaxies. Predominantly neutral regions of the interstellar medium in which the heating and chemistry are regulated by far ultraviolet photons are termed Photo-Dissociation Regions (PDRs). These regions are the origin of most of the non-stellar infrared (IR) and the millimeter and submillimeter CO emission from galaxies. The importance of PDRs has become increasingly apparent with advances in IR and submillimeter astronomy. The IR emission from PDRs includes fine structure lines of C, C+, and O; rovibrational lines of H2, rotational lines of CO; broad middle features of polycyclic aromatic hydrocarbons; and a luminous underlying IR continuum from interstellar dust. The transition of H to H2 and C+ to CO occurs within PDRs. Comparison of observations with theoretical models of PDRs enables one to determine the density and temperature structure, the elemental abundances, the level of ionization, and the radiation field. PDR models have been applied to interstellar clouds near massive stars, planetary nebulae, red giant outflows, photoevaporating planetary disks around newly formed stars, diffuse clouds, the neutral intercloud medium, and molecular clouds in the interstellar radiation field-in summary, much of the interstellar medium in galaxies. Theoretical PDR models explain the observed correlations of the [CII] 158 microns with the COJ = 1-0 emission, the COJ = 1-0 luminosity with the interstellar molecular mass, and the [CII] 158 microns plus [OI] 63 microns luminosity with the IR continuum luminosity. On a more global scale, MR models predict the existence of two stable neutral phases of the interstellar medium, elucidate the formation and destruction of star-forming molecular clouds, and suggest radiation-induced feedback mechanisms that may regulate star formation rates and the column density of gas through giant molecular clouds.

Hollenbach, David J.↗

NASA NTRS2002

Atomic Data for the K-Vacancy States of Fe XXIV

As part of a project to compute improved atomic data for the spectral modeling of iron K lines, we report extensive calculations and comparisons of atomic data for K-vacancy states in Fe XXIV. The data sets include: (i) energy levels, line wavelengths, radiative and Auger rates; (ii) inner-shell electron impact excitation rates and (iii) fine structure inner-shell photoionization cross sections. The calculations of energy levels and radiative and Auger rates have involved a detailed study of orbital representations, core relaxation, configuration interaction, relativistic corrections, cancellation effects and semi-empirical corrections. It is shown that a formal treatment of the Breit interaction is essential to render the important magnetic correlations that take part in the decay pathways of this ion. As a result, the accuracy of the present A-values is firmly ranked at better than 10% while that of the Auger rates at only 15%. The calculations of collisional excitation and photoionization cross sections take into account the effects of radiation and spectator Auger dampings. In the former, these effects cause significant attenuation of resonances leading to a good agreement with a simpler method where resonances are excluded. In the latter, resonances converging to the K threshold display symmetric profiles of constant width that causes edge smearing.

Bautista, M. A.↗

NASA NTRS2003

Decay Properties of K-Vacancy States in Fe X-Fe XVII

We report extensive calculations of the decay properties of fine-structure K-vacancy levels in Fe X-Fe XVII. A large set of level energies, wavelengths, radiative and Auger rates, and fluorescence yields has been computed using three different standard atomic codes, namely Cowan's HFR, AUTOSTRUCTURE and the Breit-Pauli R-matrix package. This multi-code approach is used to the study the effects of core relaxation, configuration interaction and the Breit interaction, and enables the estimate of statistical accuracy ratings. The Ksigma and KLL Auger widths have been found to be nearly independent of both the outer-electron configuration and electron occupancy keeping a constant ratio of 1.53 +/- 0.06. By comparing with previous theoretical and measured wavelengths, the accuracy of the present set is determined to be within 2 m Angstrom. Also, the good agreement found between the different radiative and Auger data sets that have been computed allow us to propose with confidence an accuracy rating of 20% for the line fluorescence yields greater than 0.01. Emission and absorption spectral features are predicted finding good correlation with measurements in both laboratory and astrophysical plasmas.

Mendoza, C.↗

NASA NTRS2003

Modeling of Iron K Lines: Radiative and Auger Decay Data for Fe II-Fe IX

A detailed analysis of the radiative and Auger de-excitation channels of K-shell vacancy states in Fe II-Fe IX has been carried out. Level energies, wavelengths, A-values, Auger rates and fluorescence yields have been calculated for the lowest fine-structure levels populated by photoionization of the ground state of the parent ion. Different branching ratios, namely K alpha 2/K alpha 1, K beta/K alpha, KLM/KLL, KMM/KLL, and the total K-shell fluorescence yields, omega(sub k), obtained in the present work have been compared with other theoretical data and solid-state measurements, finding good general agreement with the latter. The Kalpha 2/K alpha l ratio is found to be sensitive to the excitation mechanism. From these comparisons it has been possible to estimate an accuracy of approx.10% for the present transition probabilities.

Palmeri, P.↗

NASA NTRS2003

Intercontinental Transport of Ozone from Tropical Biomass Burning

Researchers have been looking at the connection between tropical biomass burning and ozone formation and long-range transport for roughly 15 years. One can see the linkage and transport patterns from satellite though aircraft and/or balloon-sonde profiles are required to observe the fine structure (ozone transport over thousands of km often happens in thin layers). In this review, I survey the pyrogenic ozone transport in the large oceanic basins - Indian Ocean, Pacific and Atlantic. Mechanistic complexities are discussed and examples shown from satellite, aircraft and soundings, including NASA results from TOMS, the GTE experiments and the SHADOZ sounding program. Experiments referred to include SAFARI-92, TRACE-A, INDOEX, PEM-Tropics and TRACE-P. augmented by subsidence, a variable tropopause height, and lightning - even ozone pollution from the Indian Ocean has been implicated. Over the Indian Ocean, pollution interacts with convection and the monsoon cycle.

Thompson, A. M.↗

NASA NTRS2003

Atomic Data and Spectral Line Intensities for S XI

Electron impact collision strengths, energy levels, oscillator strengths and spontaneous radiative decay rates are calculated for S XI. The configurations included are 2s(sup 2)2psup 2), 2s2p(sup 3), 2p(sup 4), 2s(sup 2)2p3l and 2s(sup 2)2p4l (I = s , p , d) giving rise to 72 fine-structure levels in intermediate coupling. Collision strengths are calculated at five incident energies, 32, 60, 90, 120, and 150 Ry. Excitation rate coefficients are calculated as a function of electron temperature by assuming a Maxwellian electron velocity distribution. Using the excitation rate coefficients and the radiative transition rates, statistical equilibrium equations for level populations are solved: The effects of resonances, proton rates, photoexcitation and cascades on level populations have been investigate. The predicted S XI line intensities are compared with EUV and UV observations of the quiet and active Sun.

Landi, E.↗

NASA NTRS2004

Oxygen Fugacity of Mare Basalts and the Lunar Mantle Application of a New Microscale Oxybarometer Based on the Valence State of Vanadium

The ability to estimate oxygen fugacities for mare basalts and to extend these observations to the lunar mantle is limited using bulk analysis techniques based on buffering assemblages or the valence state of iron. These limitations are due to reequilibration of mineral assemblages at subsolidus conditions, deviations of mineral compositions from thermodynamic ideality, size requirements, and the limits of the iron valence at very low fO2. Still, these approaches have been helpful and indicate that mare basalts crystallized at fO2 between the iron-w stite buffer (IW) and the ilmenite breakdown reaction (ilmenite = rutile + iron). It has also been inferred from these estimates that the lunar mantle is also highly reduced lying at conditions below IW. Generally, these data cannot be used to determine if the mare basalts become increasingly reduced during transport from their mantle source and eruption at the lunar surface and if there are differences in fO2 among mare basalts or mantle sources. One promising approach to determining the fO2 of mare basalts is using the mean valence of vanadium (2+, 3+, 4+, 5+) determined on spots of a few micrometers in diameter using synchrotron x-ray absorption fine structure (XAFS) spectroscopy. The average valence state of V in basaltic glasses is a function of fO2, temperature, V coordination, and melt composition. Here, we report the initial results of this approach applied to lunar pyroclastic glasses.

Shearer, C. K.↗

NASA NTRS2004

Atomic Data and Spectral Line Intensities for Ne III

Electron impact collision strengths, energy levels, oscillator strengths, and spontaneous radiative decay rates are calculated for Ne III. The configurations used are 2s(sup 2)2p(sup 4), 2s2p(sup 5), 2p(sup 6), 2s(sup 2)2p(sup 3)3(sub s), 2s(sup 2)2p(sup 3)3p, and 2s(sup 2)2p(sup 3)3d giving rise to 86 fine-structure levels in intermediate coupling. Collision strengths are calculated at five incident energies, 5.2, 10, 15, 20 and 25 Ry. Excitation rate coefficients are calculated as a function of electron temperature by assuming a Maxwellian electron velocity distribution. Using the excitation rate coefficients and the radiative transition rates, statistical equilibrium equations for level populations are solved at electron densities covering the range of 10(exp 8) - 10(exp 14)/cc at an electron temperature of logTe/K = 5.0, corresponding to maximum abundance of Ne III. Relative spectral line intensities are calculated.

Landi, E.↗

NASA NTRS2004

Far-Infrared and Submillimeter Spectroscopy of Low to Moderate Redshift Normal and Active Galaxies

Broadband surveys with planned observatories such as SIRTF, ASTRO-F, NGST, Herschel, SMA, and ALMA will provide large samples of dust-enshrouded galaxies undergoing varying proportions of star formation and accretion onto supermassive black holes. Recent evidence suggests that the mass of supermassive central black holes are proportional to central velocity dispersions of their host galaxy bulges, but it is not yet known how these properties are evolutionarily connected. To trace the evolution of these components during the important luminous, dusty merger episodes as a function of redshift, sensitive mid- to far-infrared spectroscopic diagnostic observations from space will be necessary. The high sensitivity and spatial resolution of a large aperture, natural background limited far-infrared/submillimeter telescope such as SAFAIR will allow detection and identification of the far-infrared/submillimeter components of galaxies and of a multitude of rest-frame mid- to far-infrared recombination, fine-structure, and molecular lines toward dusty galactic nuclei. The line diagnostics beginning to be developed from ISO work will help determine important parameters such as the intrinsic spectral energy distribution, ionization parameters, metallicities, densities, extinction to the embedded energy sources, and ultimately the star-formation and accretion rates in these galaxies. Ultimately, the higher spatial resolution provided by a cold, far-infrared submillimeter interferometer is needed to view the distribution of structure in galaxies in order to unravel the as yet untold story of the evolution of galaxies over the lifetime of the Universe.

Fischer, Jacqueline↗

NASA NTRS1997

Nitrogen sulfide in giant molecular clouds

We report a survey for nitrogen sulfide (NS) toward regions of massive star formation. NS was observed by means of its 2 pi 1/2, J = 3/2 --> 1/2, J = 5/2 --> 3/2, and J = 7/2 --> 5/2 transitions at 69, 115, and 161 GHz, respectively, and was detected toward 12 of 14 giant molecular clouds (GMCs) observed. Analysis of the hyperfine component relative line strengths suggests that NS emission is optically thin toward these sources, with the possible exception of Sgr B2(M). The fractional abundance of NS relative to molecular hydrogen is best defined for the Orion molecular cloud, where it is typically (1-4) x 10(-10), which is about an order of magnitude larger than found by some recent gas-phase chemistry models developed for quiescent clouds. Toward OMC-1, the NS integrated intensity is strongly peaked toward KL, but also extends all along the Orion ridge, resembling the distribution of SO and CH3OH. We have identified a spectral feature seen toward several sources as the ortho-NKK = 4(04) --> 3(13) J = 3 --> 2, fine-structure component of methylene (CH2; cf. Hollis, Jewell, & Lovas). We also report the first detection of the SO+ 2 pi 1/2, J = 3/2 --> 1/2, parity-e transition toward W51(MS) and L134N.

NASA Discipline Exobiology↗

NASA NTRS1984

Cytological and ultrastructural studies on root tissues

The anatomy and fine structure of roots from oat and mung bean seedlings, grown under microgravity conditions for 8 days aboard the Space Shuttle, was examined and compared to that of roots from ground control plants grown under similar conditions. Roots from both sets of oat seedlings exhibited characteristic monocotyledonous tissue organization and normal ultrastructural features, except for cortex cell mitochondria, which exhibited a 'swollen' morphology. Various stages of cell division were observed in the meristematic tissues of oat roots. Ground control and flight-grown mung bean roots also showed normal tissue organization, but root cap cells in the flight-grown roots were collapsed and degraded in appearance, especially at the cap periphery. At the ultrastructural level, these cells exhibited a loss of organelle integrity and a highly-condensed cytoplasm. This latter observation perhaps suggests a differing tissue sensitivity for the two species to growth conditions employed in space flight. The basis for abnormal root cap cell development is not understood, but the loss of these putative gravity-sensing cells holds potential significance for long term plant growth orientation during space flight.

NASA Program Space Biology↗

NASA NTRS1984

Prochloron--a status report

Prochloron is a genus of prokaryotic algae with photosynthetic pigments like those of chlorophytes. Prochlorophytes are almost invariably found associated as symbionts with marine protochordates (didemnid ascidians), and so far none has been successfully grown in sustained culture away from in host. Based on materials collected from nature, information of various sorts (biochemical, physiological, cytological and fine-structural) has been obtained, indicating many resemblances (and probably close phylogenetic affinities) between prochlorophytes and cyanophytes. Nevertheless they are distinguished by certain unique combinations of characters. Some of the data support the symbiogenesis theory for the origin of green-plant chloroplasts. Other possibilities are briefly discussed.

Review, Tutorial↗

NASA NTRS1999

An infrared spectral match between GEMS and interstellar grains

Infrared spectral properties of silicate grains in interplanetary dust particles (IDPs) were compared with those of astronomical silicates. The approximately 10-micrometer silicon-oxygen stretch bands of IDPs containing enstatite (MgSiO3), forsterite (Mg2SiO4), and glass with embedded metal and sulfides (GEMS) exhibit fine structure and bandwidths similar to those of solar system comets and some pre-main sequence Herbig Ae/Be stars. Some GEMS exhibit a broad, featureless silicon-oxygen stretch band similar to those observed in interstellar molecular clouds and young stellar objects. These GEMS provide a spectral match to astronomical "amorphous" silicates, one of the fundamental building blocks from which the solar system is presumed to have formed.

NASA Discipline Exobiology↗