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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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NASA NTRS1984

Interstellar abundances and depletions inferred from observations of neutral atoms

Data on neutral atomic species are analyzed for the purpose of inferring relative elemental abundances and depletions in diffuse cloud cores, where it is assumed that densities are enhanced in comparison with mean densities over integrated lines of sight. Column densities of neutral atoms are compared to yield relative column densities of singly ionized species, which are assumed dominant in cloud cores. This paper incorporates a survey of literature data on neutral atomic abundances with the result that no systematic enhancement in the depletions of calcium or iron in cloud cores is found, except for zeta Ophiuchi. This may imply that depletions are not influenced by density, but other data argue against this interpretation. It is concluded either that in general all elements are depleted together in dense regions so that their relative abundances remain constant, or that typical diffuse clouds do not have significant cores, but instead are reasonably homogeneous. The data show a probable correlation between cloud-core depletion and hydrogen-molecular fraction, supporting the assumption that overall depletions are a function of density.

Snow, T. P.↗
NASA NTRS1989

Analysis of interstellar fragmentation structure based on IRAS images

The goal of this project was to develop new tools for the analysis of the structure of densely sampled maps of interstellar star-forming regions. A particular emphasis was on the recognition and characterization of nested hierarchical structure and fractal irregularity, and their relation to the level of star formation activity. The panoramic IRAS images provided data with the required range in spatial scale, greater than a factor of 100, and in column density, greater than a factor of 50. In order to construct a densely sampled column density map of a cloud complex which is both self-gravitating and not (yet?) stirred up much by star formation, a column density image of the Taurus region has been constructed from IRAS data. The primary drawback to using the IRAS data for this purpose is that it contains no velocity information, and the possible importance of projection effects must be kept in mind.

Scalo, John M.↗
NASA NTRS1996

The alpha Centauri Line of Sight: D/H Ratio, Physical Properties of Local Interstellar Gas, and Measurement of Heated Hydrogen (The 'Hydrogen Wall') Near the Heliopause

We analyze high-resolution spectra of the nearby (1.34 pc) stars alpha Cen A (G2 V) and alpha Cen B (K1 V), which were obtained with the Goddard High Resolution Spectrograph on the Hubble Space Telescope. The observations consist of echelle spectra of the Mg II 2800 A and Fe II 2599 A resonance lines and the Lyman-alpha lines of hydrogen and deuterium. The interstellar gas has a velocity (v = - 18.0 +/- 0.2 km/s) consistent with the local flow vector proposed for this line of sight by Lailement & Berlin (1992). The temperature and nonthermal velocity inferred from the Fe II, Mg II, and D I line profiles are T = 5400 +/- 500 K and xi = 1.20 +/- 0.25 km/s, respectively. However, single-component fits to the H I Lyman-alpha lines yield a Doppler parameter (b(sub HI) = 11.80 km/s) that implies a significantly warmer temperature of 8350 K, and the velocity of the H I absorption (v = - 15.8 +/- 0.2 km/s) is redshifted by about 2.2 km/s with respect to the Fe II, Mg II, and D I lines. The one-component model of the interstellar gas suggests natural logarithm N base HI = 18.03 +/- 0.01 and D/H = (5.7 +/- 0.2) x 10(exp -6) . These parameters lead to a good fit to the observed spectra, but this model does not explain the higher temperature and redshift of H I relative to the other interstellar lines. The most sensible way to resolve the discrepancy between H(I) and the other lines is to add a second absorption component to the H(I) lines. This second component is hotter (T approx. equals 30,000 K), is redshifted relative to the primary component by 2-4 km/s, and has a column density too low to be detected in the Fe(II), Mg(II), and D(I) lines. We propose that the gas responsible for this component is located near the heliopause, consisting of the heated H I gas from the interstellar medium that is compressed by the solar wind. This so-called 'hydrogen wall' is predicted by recent multifluid gasdynamical models of the interstellar gas and solar wind interaction. Our data provide the first measurements of the temperature and column density of H(I) in the hydrogen wall. After considering the effects that a corresponding hydrogen wall around alpha Cen would have on our analysis, our best estimates for the parameters of the solar hydrogen wall are natural log N(sup (2))(H(I)) = 14.74 +/- 0.24, b(sup (2))(H(I)) = 21.9 +/- 1.7 km/s (corresponding to T = 29,000 +/- 5000 K), and v(sup (2))(H(I)) greater than -16km/s. Unfortunately, the existence of this heated H(I) reduces our ability to compute the H(I) column density of the interstellar medium accurately because, with slight alterations to our assumed stellar Lyman-alpha profiles, we discovered that acceptable two-component fits also exist with natural log N(H(I))approx. 17.6. We, therefore, quote large error bars for the H I column density along the alpha Cen line of sight, natural log N(H(I)) = 17.80 +/- 0.30. For this range in N(H(I)), n(H(I)) = 0.15 /cu.cm (+/- a factor of 2) and D/H = (0.5-1.9) x 10(exp -5). This is the first direct measurement of the H(I) density in a local cloud and allows us to predict the distance from the Sun to the edge of the local cloud along various lines of sight. This range in D/H is consistent with the value D/H = 1.6 x 10(exp -5) previously derived for the Capella and Procyon lines of sight. We cannot tell whether D/H ratio varies or is constant in the local interstellar medium, but we do find that the D(I)/Mg(II) ratio for the alpha Cen line of sight is about 4 times smaller than for the Capella and Procyon lines of sight. Therefore, either D/H or the Mg depletion varies significantly over distance scales of only a few parsecs.

Linsky, Jeffrey L.↗
NASA NTRS1991

First results from the Goddard High-Resolution Spectrograph - Elemental abundances in the diffuse clouds toward Xi Persei

An analysis is presented of high-resolution observations of ultraviolet interstellar absorption lines of dominant ion stages arising in the diffuse clouds toward Xi Per obtained with the Goddard High-Resolution Spectrograph. A detection of the weak intersystem line of semiforbidden C II 2325 A is reported. For C, O, and Mg, column densities are derived from both weak lines and the damping wings of strongly saturated lines. For Mg, the derived column densities differ by a factor three. Comparison of apparent column densities profiles, between lightly depleted and heavily depleted species indicates a transition from substantial depletions at v(solar) roughly 7.5 km/s to a decrease in depletion of a factor two or more for v(solar) = 7.5-20 km/s.

Cardelli, Jason A.↗
NASA NTRS1978

Star formation in the NGC 7129 region - A CO molecular-line and far-infrared continuum study

Results are presented for a joint radio/IR study of the region around NGC 7129, a reflection nebula seen against a large dark cloud. The results reported include high-angular-resolution (C-12)O and (C-13)O line measurements, high-angular-resolution observations in the range from 35 to 200 microns, and observations at 10 to 20 microns of selected objects in the nebula. Important constraints are placed on the total stellar luminosities, dust densities and temperatures, and heating and cooling processes occurring in the nebula. Specifically, it is found that: (1) the (C-13)O and dust column densities increase significantly from west to east across the nebula; (2) the (C-13)O column density in the vicinity of the two brightest visible stars decreases relative to the column density north and south of those stars; (3) kinetic temperature increases along the whole boundary of the nebulosity; and (4) the strongest far-IR source in NGC 7129 is coincident with the pre-main-sequence star LkH-alpha 234.

Bechis, K. P.↗
NASA NTRS1988

Einstein spectra of quasars

The results of the initial stage of the CfA survey of quasar energy distributions are reviewed. Einstein imaging proportional counter spectra of 33 quasars have been studied by fitting a single power law slope and absorption by an equivalent column density of neutral hydrogen. Comparison with the higher energy HEAO-A2 data leads to a two-component model for the X-ray spectrum. The X-ray column density is systematically lower than the 21-cm measured Galactic column density along the same line of sight.

Wilkes, Belinda J.↗
NASA NTRS1976

Calcium abundance variations in diffuse interstellar clouds

Observations are discussed which show that the Na I/H2 column-density ratio is relatively constant among different clouds where H2 is optically thin, but the Na I/Ca II column-density ratio varies. This implies that the gas-phase abundance of calcium is variable. It is proposed that the wide variation in the gas-phase calcium abundance can be explained by small variations in the calcium content of grains. The observed enhancement of the Ca II/Na I column-density ratio with cloud velocity is attributed to grain destruction in grain-grain collisions within the same cloud. The structure of shock waves propagating through the interstellar gas is computed, and the grain-destruction model is compared with observational data.

Jura, M.↗
NASA NTRS2021

Are Compton-thin AGNs Globally Compton Thin?

We select eight nearby active galactic nuclei (AGNs) which, based on previous work, appear to be Compton-thin in the line of sight. We model with MYTORUS their broadband X-ray spectra from 20 individual observations with Suzaku, accounting self-consistently for Fe Kα line emission, as well as direct and scattered continuum from matter with finite column density and solar Fe abundance. Our model configuration allows us to measure the global, out of the line of sight, equivalent hydrogen column density separately from that in the line of sight. For 5 out of 20 observations (in 3 AGNs) we find that the global column density is in fact ≳1.5 × 1024 per sq.cm, consistent with the distant scattering matter being Compton-thick. For a fourth AGN, two out of five observations are also consistent with being Compton-thick, although with large errors. Some of these AGNs have been reported to host relativistically broadened Fe Kα emission. Based on our modeling, the Fe Kα emission line is not resolved in all but two Suzaku observations, and the data can be fitted well with models that only include a narrow Fe Kα emission line.

P. Tzanavaris↗
NASA NTRS1977

Nitric oxide gamma band emission rate factor

Molecular fluorescence emission rate factors for the strong bands of the nitric oxide gamma system have been calculated using recent branching ratios and different temperatures. For the 1-0 gamma band the effects of self-absorption by NO have been taken into account, and effective emission rate factors were calculated for a range of NO column densities at several different temperatures. For column densities near 10 to the 16th/sq cm self-absorption corrections to the emission rate factor can be significant. The emission rate factor for the 1-0 gamma band is 7.19 or 7.68 x 10 to the -6th photons/molecule/sec depending upon the use of experimental or theoretical branching ratios.

Cravens, T. E.↗
NASA NTRS1987

High-resolution ultraviolet observations of interstellar lines toward Zeta Persei observed with the balloon-borne ultraviolet stellar spectrometer

The balloon-borne ultraviolet stellar spectrometer payload has been used to obtain high-resolution data on interstellar absorption lines toward Zeta Per. The only lines clearly present in the 2150-2450 region were several Fe II features, which show double structure. The two velocity components were sufficiently well separated that it was possible to construct separate curves of growth to derive the Fe II column densities for the individual components. These column densities and the component velocity separation were then used to compute a realistic two-component curve of growth for the line of sight to Zeta Per, which was then used to reanalyze existing ultraviolet data from Copernicus. The results were generally similar to an earlier two-component analysis of the Copernicus data, with the important exception that the silicon depletion increased from near zero to about 1 dex. This makes the Zeta Per depletion pattern quite similar to those derived for other reddened lines of sight, supporting the viewpoint that the general diffuse interstellar medium has a nearly constant pattern of depletions.

Snow, Theodore P.↗
NASA NTRS1993

The abundance of CH(+) in translucent molecular clouds - Further tests of shock models

Observations of interstellar absorption lines of CH(+) in the (0, 0) and (1, 0) bands of the A 1Pi - X 1Sigma(+) system are presented for 17 stars with reddenings up to E(B-V) about 1.5 mag. Complementary data on interstellar CH in the (0, 0) bands of the A 2Delta - X 2Pi and B 2Sigma(-) - X 2Pi systems and C2 in the A 1Pi(u) - X 1Sigma(g)(+) red system have been obtained as well. The derived CH(+) column densities continue to increase with total column density, and values up to 10 exp 14/sq cm are reported for highly-reddened lines of sight. In most cases, the CH(+) and CH absorptions are dominated by a single strong component, with weaker features displaced by a few km/s. No significant velocity difference is found between CH(+) and neutral species such as CH and CN for this sample of randomly oriented lines of sight. In contrast with CN, the CH(+) abundance does not increase with the density in the cloud as derived from the observed C2 excitation. For the two clouds with the largest density, HD 62542 and HD 94413, no CH(+) absorption is found with CH(+)/CH less than 0.03 and 0.14, respectively. The CH(+) findings do not support a single-shock origin for the formation of the ion.

Gredel, R.↗
NASA NTRS1994

Intersystem transitions of interstellar carbon monoxide toward zeta Ophiuchi

Absorption from seven intersystem (triplet-singlet) transitions of interstellar (12)CO were detected in ultraviolet spectra of zeta Oph. The observed equivalent widths are approximately consistent with the transitions' predicted f-values and the (12) CO column density derived from the weakest of the observed A-X bands. These unsaturated intersystem transitions provide the opportunity to measure the (12)CO column density for heavily reddened (dense) sight lines. Laboratory measurements of oscillator strengths more precise than available ones will be needed to derive accurate column densities.

Federman, S. R.↗
NASA NTRS1987

Rapid ionization of the environment of SN 1987A

It has been suggested by some authors that IUE observations of the supernova SN 1987A show the presence of a strong component of the interstellar C IV 1550 and Si IV 1393 absorption lines at a velocity that approximately corresponds to the velocity of the LMC. It is possible that this component might come from originally neutral (or at least not very highly ionized) gas which has been photoionized by the initially very strong ionizing radiation field of the supernova. Theoretical considerations of this scenario lead to the study of fast (with velocities of about c) ionization fronts. It is shown that for reasonable model parameters it is possible to obtain considerably large C IV column densities, in agreement with the IUE observations. On the other hand, the models do not so easily predict the large Si IV column densities that are also obtained from the IUE observations. It is found that only models in which the interstellar medium surrounding SN 1987A is initially composed of already ionized hydrogen and helium predict substantial Si IV column densities. This result provides an interesting prediction of the ionization state of the environment of the presupernova star.

Raga, A. C.↗
NASA NTRS2002

A Multi-Wavelength Study of the Hot Component of the Interstellar Medium

The goals of this research are as follows: (1) Using the large number of lines of sight available in the ME database, identify the lines of sight with high-velocity components in interstellar lines, from neutral species through Si VI, C IV, and N V; (2) Compare the column density of the main components (i.e. low velocity components) of the interstellar lines with distance, galactic longitude and latitude, and galactic radial position. Derive statistics on the distribution of components in space (e.g. mean free path, mean column density of a component). Compare with model predictions for the column densities in the walls of old SNR bubbles and superbubbles, in evaporating cloud boundaries and in turbulent mixing layers; (3) For the lines of sight associated with multiple high velocity, high ionization components, model the shock parameters for the associated superbubble and SNR to provide more accurate energy input information for hot phase models and galactic halo models. Thus far 49 lines of sight with at least one high velocity component to the C IV lines have been identified; and (4) Obtain higher resolution data for the lines of sight with high velocity components (and a few without) to further refine these models.

Nichols, Joy↗
NASA NTRS1990

The extragalactic radiation field and sharp edges to HI disks in galaxies

Observations of neutral hydrogen (HI) are widely used as a probe of the interstellar medium in galaxies and of galactic kinematics and dynamics. The 21-cm line can be used to determine galactic rotation curves far beyond the optical disk, and is one of the prime sources of evidence for the existence of dark haloes in spiral galaxies. However, a recent attempt to measure the 21-cm emission from NGC 3198 to very low column densities (N sub H approx. equals 5 times 10 to the 18th power) found that the HI disk is very sharply truncated at a column density of a few times 10 to the 19th power (Sancisi 1989, private communication). This discovery reinforces the earlier suggestion (Briggs et al. 1980) that extended, low-column density envelopes of neutral hydrogen are not common around galaxies. Here the author suggests that the sharp edge seen in the HI disk in NGC 3198 is consistent with a model in which a self-gravitating neutral HI disk is photoionized by the extragalactic radiation field. The possibility that the extragalactic background would produce sharp edges to HI disks was first suggested by Silk and Sunyaev (1976).

Maloney, Philip Richard↗
NASA NTRS1999

A New Probe of the Molecular Gas Content in Galaxies: Application to M101

Recent studies of nearby spiral galaxies suggest that photodissoiation regions (PDRS) are capable of producing much of the observed HI in galaxy disks. In that case, measurements of the observed HI column density and the far-ultraviolet (FUV) photon flux responsible for the photodissociation process provide a new probe of the volume density of the local underlying molecular hydrogen. We develop the method and apply it to the giant Scd spiral M101. The HI column density and amount of FUV emission have been measured for a sample of 35 candidate PDRs located throughout the disk of M101 using the Very Large Array and the Ultraviolet Imaging Telescope. We find that, after correction for the known gradient of metallicity in the Interstellar Medium (ISM) of M101 and for the extinction of the ultraviolet emission, molecular gas with a narrow range of density from 30-1000/ cubic cm is found near star-forming regions at all radii in the disk of M101 out to a distance of 12 seconds approximately equals 26 kpc, close to the photometric limit of R(sub 25) approximately equals 13.5 seconds. In this picture, the ISM is virtually all molecular in the inner parts of M101. The strong decrease of the HI column density in the inner disk of the galaxy at R(sub G) < 10 kpc is a consequence of a strong increase in the dust-to-gas ratio there, resulting in an increase of the H(sub 2) formation rate on grains and a corresponding disappearance of hydrogen in its atomic form.

Smith, Denise A.↗
NASA NTRS1999

Is There an Arctic Ozone Hole?

Total ozone observations from the Total Ozone Mapping Spectrometer (TOMS) instruments during March 1997 revealed an extensive region of low column densities in the Arctic region centered near the north pole. Values were below 250 Dobson units for nearly a two week period during this period, and were correlated with the position of the northern lower stratospheric polar vortex. The March 1997 average total ozone column densities were more than 30% lower than the average of column densities observed during the 1979-1982 March period. Both the northern spring seasons of 1998 and 1999 have shown much higher levels of total ozone. In this presentation, we will discuss the causes of the low total ozone values in 1997 and contrast those low values with the higher 1998 and 1999 observations. The reason for my travel to the University of Valparaiso is to give this seminar and provide information to my colleagues on our work here at GSFC. The benefit to NASA is to interact with my Univ. of Valparaiso colleagues, gain their insights and input into this research, and establish collaborations with the current research efforts at the University.

Newman, Paul A.↗
NASA NTRS2000

Is There an Arctic Ozone Hole?

Total ozone observations from the Total Ozone Mapping Spectrometer (TOMS) instruments during March 1997 revealed an extensive region of low column densities in the Arctic region centered near the north pole. Values were below 250 Dobson units for nearly a two week period during this period, and were correlated with the position of the northern lower stratospheric polar vortex. The March 1997 average total ozone column densities were more than 30% lower than the average of column densities observed during the 1979-1982 March period. Both the northern spring seasons of 1998 and 1999 have shown much higher levels of total ozone. In this presentation, we will discuss the causes of the low total ozone values in 1997 and contrast those low values with the higher 1998 and 1999 observations. I will additionally discuss the just completed SOLVE campaign and the March 2000 ozone values.

Newman, Paul A.↗