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NASA NTRS1989

Si II mapping of the neutral gas ring in the Galactic center - Evidence for dust destruction

The distribution of Si II emission in the Galactic center has been measured and found to extend beyond 4 pc from the center. The observed forbidden Si II 34.8 micron line, which is thermally excited by H I and H2 collisions, originates from the photodissociation region formed in the neutral gas ring surrounding the ionized core. Two peaks in the Si II emission are found, one lying 20 arcsec northeast of Sgr A-asterisk along the Galactic plane and the other located about 80 arcsec north of Sgr A-asterisk. The latter peak is located beyond the northern arm seen in the ionized gas. The ratio of Si II to dust mass density varies in the ring, indicating changes in the gas-to-dust ratio. The mass density of Si II and dust are anticorrelated, indicating that Si II tracks the destruction of dust.

Herter, T.↗

NASA NTRS1991

The 158 micron forbidden C II line - A measure of global star formation activity in galaxies

Some 158 micron forbidden C II fine structure line observations from a sample of fourteen gas rich galaxies are reported. These measurements confirm and generalize previous basic results that the C II line is bright amounting to approximately 0.1 to 1 percent of the FIR luminosity of the nuclear regions of galaxies; the C II line is formed in the warm (temperature of the gas is greater than 200 K), dense (n sub H greater than 1000/cu cm) photodissociated gas at the interfaces between giant molecular clouds and ionized gas regions and is therefore associated with the molecular gas component in spiral galaxies; the C II line tracks the FIR continuum in a manner consistent with the PDR models; the integrated C II to isotope (C-12)D (transition 1 to 0) line ratio is large (greater than or equal to 1000) in all galaxies studied, and is similarly large for galactic molecular clouds; the C II line is therefore energetically very important for the study of giant molecular clouds. Conclusions obtained from these results are given.

Stacey, G. J.↗

NASA NTRS1992

SAGE II stratospheric density and temperature retrieval experiment

This paper describes a stratospheric density and temperature retrieval experiment based on the solar occultation measurement of the Stratospheric Aerosol and Gas Experiment (SAGE II). The entire retrieval analysis involves two inversion steps: the vertical structure inversion, which derives the profile of local atmospheric extinction from SAGE II limb optical depth data, and the species inversion, which inverts the concentration of air molecules, aerosols, ozone, and nitrogen dioxide from the derived atmospheric extinction at five SAGE II short wavelengths (0.385, 0.448, 0.453, 0.525, and 0.600 microns). The derived density profile is then used to infer the temperature distribution, assuming that the atmosphere is in hydrostatic equilibrium and obeys the ideal gas law. The temperature profiles retrieved from the SAGE II observations are compared with near-coincident, in both time and space, French Rayleigh lidar and NASA Wallops Flight Facility rocket datasonde soundings as well as the National Meteorological Center (NMC) data analyses. The results indicate that the mean SAGE II temperature agrees with the mean lidar measurements to within 2 C at altitudes from 30.5 to 52.5 km. The SAGE II and datasonde observations agree to within about 4 C in approximately the same altitude region.

Wang, Pi-Huan↗

NASA NTRS1992

The H II regions of the galaxy M101

The physical properties and composition of the H II region population of the spiral galaxy M101 (NGC 5457) were examined using spectrophotometrically calibrated CCD imagery of two overlapping 16-arcmin fields in the M101, in which individual H II regions were identified and mapped using an automated algorithm. The characteristics obtained for the H II region population of M101 include the H II region luminosity function and radial variations in extinction; the forbidden O III line/H-beta, O/H, and forbidden S II line/H-alpha ratios; the ionization parameter U; and the numbers of ionizing photons. Global variations of physical properties of the H II regions in M101 are compared to model expectations derived for other galaxies. The evident star formation in the M101 galaxy is discussed.

Scowen, P. A.↗

NASA NTRS1993

A comparison of the Stratospheric Aerosol and Gas Experiment II tropospheric water vapor to radiosonde measurements

Results are presented of a comparison beteen observations of the upper-tropospheric water vapor data obtained from the Stratospheric Aerosol and Gas Experiment II (SAGE II) instrument and radiosonde observations for 1987 and radiosonde-based climatologies. Colocated SAGE II-radiosonde measurement pairs are compared individually and in a zonal mean sense. A straight comparison of monthly zonal means between SAGE II and radiosondes for 1987 and Global Atmospheric Statistics (1963-1973) indicates that the clear-sky SAGE II climatology is approximately half the level of clear/cloudy sky of both radiosonde climatologies. Annual zonal means calculated from the set of profile pairs again showed SAGE II to be significantly drier in many altitude bands.

Larsen, J. C.↗

NASA NTRS1993

Extreme optical Fe II emission in luminous IRAS active galactic nuclei

Results of a program of studies and observations of strong optical Fe II emission in luminous and ultraluminous IRAS AGN are presented. New spectroscopic observations and studies of three known ultraluminous IRAS AGN with extreme optical Fe II emission, the discovery that PHL 1092 is a new ultraluminous IRAS AGN, and the detection of two new AGN with strongly variable flux in the optical Fe II emission lines are reported. These results are used to test the correlations between the Fe II emission and properties at other wavelengths such as the L(IR) and the radio emission. IR AGN with extreme Fe II emission are found to belong to a very important group of AGN, whose properties provide insight into the origin of the extreme Fe II emission and into the relation between the starburst and AGN phenomena.

Lipari, Sebastian↗

NASA NTRS1993

Detection of the N II 122 and 205 micron lines - Densities in G333.6-0.2

Measurements of the G333.6-0.2 H II region which include the first detection of the N II 122 micron forbidden line in an astronomical force and the first measurement of the N II 205 micron forbidden line in a discrete source are presented. Also considered are fine structure lines of forbidden S III, forbidden Fe III, forbidden Si II, forbidden Ne III, forbidden O III, forbidden N III, forbidden O I, and forbidden C II from 19 to 206 microns. It is concluded that the N II 122 and 205 microns forbidden line pair in a discrete astronomical source was detected for the first time. The emission in transitions is produced largely by low-ioninzation, low-density material not easily probed by other lines. Other FIR line pairs generally originate in higher density regions closer to the exciting force.

Colgan, Sean W. J.↗

NASA NTRS1990

Subcritical crack growth in soda-lime glass in combined mode I and mode II loading

Subcritical crack growth under mixed-mode loading was studied in soda-lime glass. Pure mode I, combined mode I and mode II, and pure mode II loadings were achieved in precracked disk specimens by loading in diametral compression at selected angles with respect to the symmetric radial crack. Crack growth was monitored by measuring the resistance changes in a microcircuit grid consisting of parallel, electrically conducting grid lines deposited on the surface of the disk specimens by photolithography. Subcritical crack growth rates in pure mode I, pure mode II, and combined mode I and mode II loading could be described by an exponential relationship between crack growth rate and an effective crack driving force derived from a mode I-mode II fracture toughness envelope. The effective crack driving force was based on an empirical representation of the noncoplanar strain energy release rate. Stress intensities for kinked cracks were assessed using the method of caustics and an initial decrease and a subsequent increase in the subcritical crack growth rates of kinked cracks were shown to correlate with the variations of the mode I and the mode II stress intensities.

Singh, Dileep↗

NASA NTRS1994

Classical H II regions in the Magellanic Clouds. 1: Physical properties

In the first in a series of papers I present a detailed examination of the basic physical properties of a sample of classical H II regions in the Magellanic Clouds. Narrow band H alpha imaging is used to show that such objects are representative of the most common H II regions in the Clouds and that they display a wide range of morphological characteristics. Spectroscopic data show that, as a result of the low metal content, classical H II regions in the Clouds are systematically hotter than their Galactic counterparts. The data also suggest the existence of a correlation between the excitation of an H II region and its emission measure. The spectra of one object, DEM 243, indicates the presence of a low velocity shock passing through this H II region. I also discuss the characteristics of the extinction towards these objects and speculate that it arises from dust local to the H II regions themselves.

Wilcots, Eric M.↗

NASA NTRS1994

Aerosol effect on Umkehr ozone profiles using Stratospheric Aerosol and Gas Experiment II measurements

This study examines 1211 cases of coincident ozone profiles derived from 1164 Umkehrs and 928 Stratospheric Aerosol and Gas Experiment II (SAGE II) profiles within 1000 km and 12 hours between October 1984 and April 1989 to study the stratospheric-aerosol effect on Umkehr ozone profiles. Because of the close correspondence of stratospheric aerosol optical depth at the SAGE II-measured 0.525-micrometer wavelength and the extrapolated 0.32 Umkehr wavelength determined in this study we use the 0.525-micrometer data to determine the aerosol effect on Umkehr profiles. At the 95% confidence level, we find the following errors to the Umkehr ozone amounts: in Umkehr layer 9 (-2.9 +/- 2.1), layer 8 (-2.3 +/- 1.1), layer 7 (0.1 +/- 1.1), layer 6 (2.2 +/- 1.0), layer 5 (-1.5 +/- 0.8), and layer 4 (-2.4 +/- 1.7) in percent ozone amount per 0.01 stratospheric aerosol optical depth. These results agree with previous theoretical and empirical studies within their respective error bounds in layers 9, 8, and 7. The results in layers 6, 5, and 4 differ significantly from those in previous works. Using only those eight stations with more than 47 coincidences results in mean aerosol effects that are not significantly different from the 14-station results. Because SAGE II and Umkehr produce different ozone retrievals in layer 9 and because the intralayer correlation of SAGE II ozone and aerosol in layer 9 is nonzero, one must exercise some caution in attributing the entire SAGE II-Umkehr difference in this layer to an aerosol effect.

Newchurch, M. J.↗

NASA NTRS1994

Classical H II regions in the Magellanic Clouds. 2: Stellar content

In this second in a series of papers on the nature of classical H II regions in the Magellanic Clouds I investigate the properties of the underlying stellar content of the nebulae. Particular emphasis is placed on identifying and classifying the ionizing source(s) for each H II region. With the exception of the LMC H II regions DEM 20 and DEM 8c, I find that all of the objects in this sample are ionized by more than one O or B star. Even the faintest H II regions reflect the formation of a handful of massive, albeit early B type, stars. Typically, one star accounts for 60% - 70% of the ionizing photons and 2 - 5 less massive stars provide the remaining 30% - 40%. From the statistics of the hottest stars in these H II regions, and from considering all the blue stars contained within each region, the distribution of massive stars with spectral type is consistent with results found in similar galactic H II regions.

Wilcots, Eric M.↗

NASA NTRS1994

A kinematic search for supernova remnants in giant extragalactic H II regions

We have obtained velocity fields of the Giant H II complexes NGC 5471 in M101, NGC 2363 in NGC 2366, and the largest H II region in NGC 2403 from H-alpha observations using the TAURUS imaging Fabry-Perot interferometer. We have detected five H-alpha sources with velocity profiles which are broad when compared with the surrounding H II region. Region B in NGC 5471 has been previously determined to contain a supernova remnant by the presence of nonthermal radio continuum radiation and enhanced (O I) and (S II) emission (Skillman 1985) and broad H-alpha emission (Chu & Kennicutt 1986). Two broad H-alpha sources in NGC 2363 coincide with regions where strong splitting has been found in the (O III) line (Roy et al. 1991). Two more broad H-alpha sources have been identified in the largest H II region in NGC 2403. Very Large Array (VLA) radio continuum observations with a resolution of 2 sec at lambda(6) and lambda(20) cm of all 3 H II complexes are presented. In addition, high resolution (subarcsecond) VLA images of NGC 5471 were made at lambda(2) and lambda(6) cm. The presence of a nonthermal source in region NGC 5471 B was confirmed while region NGC 5471 A appears to be dominated by thermal emission. The nonthermal spectral index in NGC 2363 A indicates the existence of none or more supernova remnants at the position of a large velocity width source detected in H-alpha emission. No similar nonthermal sources were detected in NGC 2403 #1. Supernovae explosions and stellar winds are considered as causes for these large velocity width sources (LVWS). If the emission from the LVWSs is attributed to single supernova remnants, they are unusually luminous in both nonthermal radio continuum and H-alpha emision. The very large H-alpha luminosities could be a result of high velocity gas being ionized by the neighboring stellar cluster.

Yang, Hui↗

NASA NTRS1994

Interstellar detection of the intersystem line Si II lambda 2335 toward zeta Ophiuchi

We report on the detection of the weak intersystem transistion of Si II lambda 2335 A in the sight line toward zeta Oph using the Ech-B mode (3.5 km/s resolution) of the Goddard High Resolution Spectrograph. The high-quality spectrum is characterized by an empirically measured signal-to-noise of 450, in excellent agreement with that expected from photon-statistics. The measured equivalent width of the Si II line is W(sub lambda) = 0.48 +/- 0.12 mA. Using the new experimental f-value of Calamai, Smith, and Bergeson, we find a Si II column density of 2.34 (+/- 0.58) x 10(exp 15) atoms/sq cm and (Si/H)(sub zeta Oph) = 1.78 (+/- 0.44) x 10(exp -6) for the principal absorbing component(s) at v(sub sun) approx. = -15 km/s. Analysis of the Si II lambda 1808 absorption over the same velocity range using the new experimental f-value of Bergeson & Lawler yields a column density (corrected for saturation) that is consistent within the weak line errors and confirms the relative accuracies of these new f-values. Furthermore, these results indicate that accurate abundances can now be derived for Si II, particularly from the weak Si II lambda 2335 A since it is free of saturation effects. For the zeta Oph v(sub sun) approx. = -15 km/s component(s), we find that greater than 95% of the available cosmic abundance (i.e. the 1989 meteoritic abundances of Anders & Grevesse) of Mg, Fe, and Si is 'missing' from the gas phase and is presumably locked up in the dust. These elements are present in the dust grains in ratios of Fe/Si approximately equals 0.9 and Mg/Si approximately equals 1.1, consistent with the ratio of their cosmic abundances. These ratios are in sharp contrast to more diffuse clouds like those seen toward the high-latitude halo star HD 93521 where in the dust Fe/Si approximately equals 1.8 and Mg/Si approximately equals 2.1.

Cardelli, Jason A.↗

NASA NTRS1995

Near-infrared (Fe II) and Pa Beta imaging and spectroscopy of Arp 220

We have imaged the ultraluminous infrared galaxy Arp 220 in light of the near-infrared (Fe II) 1.257 micron and Pa-beta lines, and have obtained spectra in the J- and H-band atmospheric windows. Arp 220 is a strong source of (Fe II) and Pa-beta emission, with luminosities of 1.3 x 10(exp 41) and 9.2 x 10(exp 40) ergs/s, respectively. The (Fe II) and Pa-beta emission are both extended over the central 2 sec-3 sec, but with different morphologies. We suggest that the extended (Fe II) emission is produced through the interaction of fast shocks with ambient gas in the interstellar medium (ISM) at the base of the outflowing, supernovae-driven superwind mapped by Heckman et al. (1987). The bolometric luminosity of the starburst required to power this wind is estimated to be at least 2 x 10(exp 11) solar luminosity. If the spatially unresolved (Fe II) emission is produced via a large number of supernova remnants, the implied rate is approximately 0.6/yr. The overall luminosity of such a starburst could account for a large fraction (1/2-1/3) of the Arp 220 energy budget, but the large deficit of ionizing photons (as counted by the Pa-beta luminosity) requires that the starburst be rapidly declining and/or have a low upper mass cutoff. Alternatively, dust may effectively compete with the gas for ionizing photons, or much of the ionizing radiation may escape through 'holes' in the ISM. It is also possible that a buried active galactic nuclei (AGN) produces a large fraction of the unresolved (Fe II) and Pa-beta emission. We briefly discuss these possibilities in light of these new imaging and spectroscopic data.

Armus, L.↗

NASA NTRS1994

Comparisons of the MG II index products from the NOAA-9 and NOAA-11 SBUV/2 instruments

The Mg II index is a proxy indicator of solar UV activity which is produced from measurements of the chromospheric Mg II absortion line at 280 nm. Mg II index data sets have been derived from the NOAA-9 and NOAA-11 SBUV/2 irradiance data sets using both discrete scan measurements about the Mg II line and continuous scan (sweep) measurements over the UV spectrum from 160 - 400 nm. This paper will discuss the rationale behind the creation of the different Mg II index products, and make a quantitative assessment of the differences between these products. Recommendations for future use of the Mg II index will also be presented.

Deland, M. T.↗

NASA NTRS1995

Ultraviolet imaging telescope and optical emission-line observations of H II regions in M81

Images of the type Sab spiral galaxy M81 were obtained in far-UV and near-UV bands by the Ultraviolet Imaging Telescope (UIT) during the Astro-1 Spacelab mission of 1990 December. Magnitudes in the two UV bands are determined for 52 H II regions from the catalog of Petit, Sivan, & Karachentsev (1988). Fluxes of the H-alpha and H-beta emission lines are determined from CCD images. Extinctions for the brightest H II regions are determined from observed Balmer decrements. Fainter H II regions are assigned the average of published radio-H-alpha extinctions for several bright H II regions. The radiative transfer models of Witt, Thronson, & Capuano (1992) are shown to predict a relationship between Balmer Decrement and H-alpha extinction consistent with observed line and radio fluxes for the brightest 7 H II regions and are used to estimate the UV extinction. Ratios of Lyman continuum with ratios predicted by model spectra computed for initial mass function (IMF) slope equal to -1.0 and stellar masses ranging from 5 to 120 solar mass. Ages and masses are estimated by comparing the H-alpha and far-UV fluxes and their ratio with the models. The total of the estimated stellar masses for the 52 H II regions is 1.4 x 10(exp 5) solar mass. The star-formation rate inferred for M81 from the observed UV and H-alpha fluxes is low for a spiral galaxy at approximately 0.13 solar mass/yr, but consistent with the low star-formation rates obtained by Kennicutt (1983) and Caldwell et al. (1991) for early-type spirals.

Hill, Jesse K.↗

NASA NTRS1995

Atomic far-IR fine-structure line mapping of L1630, M17, and W3: Comparison of (O I) and (C II) distributions

We mapped the distribution of atomic far-IR line emission from (O I) and (C II) over parsec scales in the Galactic star-forming regions L1630, M17, and W3 using the MPE Far-Infrared Fabry-Perot Imaging spectrometer (FIFI) on board the NASA Kuiper Airborne Observatory. The lines mapped include (O I) 63 microns, (O I) 146 microns, and (C II) 158 microns. Comparison of the intensities and ratios of these lines with models of photodissociation regions (e.g., Tielens & Hollenbach 1985, ApJ, 344, 770) allows us to derive temperatures and densities of the primarily neutral atomic gas layers lying on the surfaces of UV-illuminated molecular gas. In general, the (C II) line arises ubiquitously throughout the molecular clouds while the (O I) lines are mainly confined to warm, dense gas (T is greater than 100 K, n is greater than 10(exp 4)/cu cm) near the sites of O and B stars. The distribution of (C II) in the star-forming clouds implies that the (C II) emission arises on the surfaces of molecular clumps throughout the clouds, rather than only at the boundary layer between molecular gas and H II regions.

Howe, J. E.↗

NASA NTRS1998

A Study of the Vertical Structure of Tropical (20 deg S-20 deg N) Optically Thin Clouds from SAGE II Observations

The tropical cloud data obtained by the satellite instrument of the Stratospheric Aerosol and Gas Experiment (SAGE) II from October 1984 to May 1991 have been used to study cloud vertical distribution, including thickness and multilayer structure, and to estimate cloud optical depth. The results indicate that the SAGE-II-observed clouds are generally optically thin clouds, corresponding to a range of optical depth between approximately 8 x 10(exp -4) and 3 x 10(exp -1) with a mean of about 0.035. Two-thirds are classified as subvisual cirrus and one-third thin cirrus. Clouds between 2- to 3-km thick occur most frequently. Approximately 30% of the SAGE II cloud measurements are isolated single-layer clouds, while 65% are high clouds contiguous with an underlying opaque cloud that terminates the SAGE II profile. Thin clouds above detached opaque clouds at altitudes greater than 6.5 km occur less often. Only about 3% of the SAGE II single-layer clouds are located above the tropopause, while 58% of the cloud layers never reach the tropopause. More than one-third of the clouds appear at the tropopause. This study also shows that clouds occur more frequently and extend higher above the tropopause over the western Pacific than than over the eastern Pacific, especially during northern winter. The uncertainty of the derived results due to the SAGE II sampling constraints, data processing, and cloud characteristics is discussed.

Wang, Pi-Huan↗