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At least 1,549 records · Page 86

Enhanced effects of starlight on the interstellar medium

The photodesorption of molecules and atoms from the surfaces of interstellar grains can be an important source of heating for the interstellar medium and the origin of instabilities which may separate grains and gas. For low densities, the force exerted on the grains is proportional to the gas density and independent of the radiation intensity; for high densities, it is proportional to the radiative flux and independent of the gas density. This force may act differently on grains of different sizes. The photoelectric effect may also be an efficient mechanism for the separation of gas and dust in diffuse clouds.

Gerola, H.↗

Warm dense gas in the reflection nebula NGC 2023

A photodissociation region (PDR) with a known UV excitation source was studied by examining emission from atomic and molecular material. In agreement with models of externally excited PDRs, the photodissociation region southwest of HD 37903 contains both atomic and excited molecular material and arises close to the exciting star than does the bulk of the molecular cloud. The C(+) emission arises closest to the star but is 1.5 x 10 to the 17th cm or less from the peaks in the excited CO and fluorescent H2 emission regions. The (C-18)O J = 2 - 1 line, which traces molecular column density, has its emission peak about 4 x 10 to the 17th cm beyond the C(+) peak. Even in a source with an incident UV flux as low as about 1000 times the mean interstellar radiation field, there is a significant amount of CO 7 - 6 emission arising from regions with T(gas) greater than 85 K which is substantially greater than T(dust).

Jaffe, D. T.↗

Discovery of molecular hydrogen fluorescence in the diffuse interstellar medium

The first detection of molecular hydrogen fluorescence in the diffuse interstellar medium is reported. Using the Berkeley UVX Shuttle Spectrometer, H2 Lyman band fluorescence has been observed in four directions, each with high significance. Molecular hydrogen fluorescence is detected in all directions that have previously been found to contain significant CO emission. A simple equilibrium model has been developed that includes attenuation of the incident UV radiation field by H2 line and dust continuum absorption. Evidence is found that the gas in the CO emission portions of the clouds may be clumpy, with a filling factor less than 0.2 and an average density greater than 30/cu cm in most cases.

Martin, Christopher↗

GEMS Revealed: Spectrum Imaging of Aggregate Grains in Interplanetary Dust

Anhydrous interplanetary dust particles (IDPs) of cometary origin contain abundant materials that formed in the early solar nebula. These materials were transported outward and subsequently mixed with molecular cloud materials and presolar grains in the region where comets accreted [1]. GEMS (glass with embedded metal and sulfides) grains are a major component of these primitive anhydrous IDPs, along with crystalline Mg-rich silicates, Fe-Ni sulfides, carbonaceous material, and other trace phases. Some GEMS grains (~5%) are demonstrably presolar based on their oxygen isotopic compositions [2]. However, most GEMS grains are isotopically solar and have bulk chemical compositions that are incompatible with inferred compositions of interstellar dust, suggesting a solar system origin [3]. An alternative hypothesis is that GEMS grains represent highly irradiated interstellar grains whose oxygen isotopic compositions were homogenized through processing in the interstellar medium (ISM) [4]. We have obtained the first quantitative X-ray maps (spectrum images) showing the distribution of major and minor elements in individual GEMS grains. Nanometer-scale chemical maps provide critical data required to evaluate the differing models regarding the origin of GEMS grains.

Keller, L. P.↗

The AstroBiology Explorer (ABE) MIDEX Mission Concept: Using Infrared Spectroscopy to Identify Organic Molecules in Space

One of the principal means by which organic compounds are detected and identified in space is by infrared spectroscopy. Past IR telescopic and laboratory studies have shown that much of the carbon in the interstellar medium (ISM) is in complex organic species but the distribution, abundance and evolutionary relationships of these materials are not well understood. The Astrobiology Explorer (ABE) is a MIDEX mission concept designed to conduct IR spectroscopic observations to detect and identify these materials and address outstanding problems in astrobiology, astrochemistry, and astrophysics. ABE's core science program includes observations of planetary nebulae and stellar outflows, protostellar objects, Solar System objects, and galaxies, and lines of sight through dense molecular clouds and the diffuse ISM. ABE is a cryogenically-cooled 60 cm diameter space telescope equipped with 3 cross-dispersed R-2000 spectrometers that share a single common slit. Each spectrometer measures one spectral octave and together cover the entire 2.5-20 micron region simultaneously. The spectrometers use state-of-the-art InSb and Si:As 1024x1024 pixel detectors. ABE would operate in a heliocentric, Earth drift-away orbit and have a core science mission lasting approximately 1.5 years. ABE is currently under study at NASA's Ames Research Center in collaboration with Ball Aerospace and Technologies Corp.

Sandford, Scott A.↗

Radiative thermal conduction fronts

The discovery of the O VI interstellar absorption lines in our Galaxy by the Copernicus observatory was a turning point in our understanding of the Interstellar Medium (ISM). It implied the presence of widespread hot (approx. 10 to the 6th power K) gas in disk galaxies. The detection of highly ionized species in quasi-stellar objects' absorption spectra may be the first indirect observation of this hot phase in external disk galaxies. Previous efforts to understand extensive O VI absorption line data from our Galaxy were not very successful in locating the regions where this absorption originates. The location at interfaces between evaporating ISM clouds and hot gas was favored, but recent studies of steady-state conduction fronts in spherical clouds by Ballet, Arnaud, and Rothenflug (1986) and Bohringer and Hartquist (1987) rejected evaporative fronts as the absorption sites. Researchers report here on time-dependent nonequilibrium calculations of planar conductive fronts whose properties match well with observations, and suggest reasons for the difference between the researchers' results and the above. They included magnetic fields in additional models, not reported here, and the conclusions are not affected by their presence.

Borkowski, Kazimierz J.↗

The (C2) 158 micrometer emission from the Horsehead nebula

The C II 158 micrometer line and the several rotational lines of CO, CO-13, and CS toward selected positions in the Horsehead extinction region in IC 434 are mapped. The observations show that the region has a gas density of about 10,000 cu cm and an external UV (Ultraviolet) flux to 20 to 100 times the average interstellar UV field. Although this is a regime where the C+ emission varies rapidly with UV intensity, fine structure line emission from gas with this range of physical conditions were not investigated previously. Comparisons of results with models of photodissociation regions show that existing plane parallel photodissociation region models are in general agreement with the observed intensity. It is not necessary to invoke a clumpy structure in the boundary layer to explain the observations, but the overall geometry of the cloud is important in determining the distribution of C+ emission.

Zhou, S.↗

The detection of X-ray emission from the OB associations of the Large Magellanic Cloud

A systematic study of the X-ray properties of OB associations in the Large Magellanic Cloud has been carried out using data from the Einstein Observatory. An excess of young, X-ray-bright supernova remnants is found in the vicinity of the associations. In addition, diffuse X-ray emission is detected from over two dozen other associations; luminosities in the 0.16-3.5 keV band range from 2 x 10 to the 34th (the detection threshold) to 10 to the 36th ergs/s. For several of the more luminous examples, it is shown that emission from interstellar bubbles created by the OB stellar winds alone is insufficient to explain the emission. It is concluded that transient heating of the bubble cavities by recent supernovae may be required to explain the observed X-rays and that such a scenario is consistent with the number of X-ray-bright associations and the expected supernova rate from the young stars they contain.

Wang, Q.↗

The depletion of pre-interstellar matter and an examination of Ni II oscillator strengths

IUE absorption-line data obtained at 110-320 nm with resolution 10-30 pm toward the B-star component of the multiple system of Alpha Sco are presented in graphs and tables and used to characterize the circumstellar shell around the M1.5 Iab component. Resonant lines of Ni II are identified at strengths in excess of 4 pm and fit to a growth curve constructed using the Fe II, Si II, and Mn II lines, and the results are found to confirm the oscillator strengths determined by Kurucz and Peytremann (1975) to within 50 percent. Shell depletions 2-20 times smaller than those for dense clouds such as the surrounding Oph complex are calculated and attributed to the formation of circumstellar dust. The implications of the results for the condensation of various elements in dense and diffuse clouds are explored.

Cardelli, J. A.↗

Composition, structure, and chemistry of interstellar dust

Different dust components present in the interstellar medium (IM) such as amorphous carbon, polycyclic aromatic hydrocarbons, and those IM components which are organic refractory grains and icy grain mantles are discussed as well as their relative importance. The physical properties of grain surface chemistry are discussed with attention given to the surface structure of materials, the adsorption energy and residence time of species on a grain surface, and the sticking probability. Consideration is also given to the contribution of grains to the gas-phase composition of molecular clouds.

Tielens, A. G. G. M.↗

The X-ray properties of supernova remnants in the Large Magellanic Cloud

Einstein Observatory instruments were used to survey the generic properties of LMC supernova remnants (SNR). The study covered the diffuse optical bar, the 30 Dor nebula, the surrounding H I cloud, and a high fraction of the spiral arms. Twelve SNRs were identified as X-ray sources and two SNR, N157B and 0540 - 69.3, were located from radio emissions. Filamentary H-alpha emissions also revealed five other objects. X-ray intensity data indicated that density levels of the matter surrounding SNR decrease with SNR diameter. The data suggest that Sedov models for SN may not be accurate for LMC SN, which exhibit too great a luminosity scatter in the surrounding interstellar material. The emissions may be accounted for by ejecta from the SN or by multiphase interstellar material.

Long, K. S.↗

PRISM project optical instrument

The scientific goal of the Passively-cooled Reconnaissance of the InterStellar Medium (PRISM) project is to map the emission of molecular hydrogen at 17.035 micrometers and 28.221 micrometers. Since the atmosphere is opaque at these infrared wavelengths, an orbiting telescope is being studied. The availability of infrared focal plane arrays enables infrared imaging spectroscopy at the molecular hydrogen wavelengths. The array proposed for PRISM is 128 pixels square, with a pixel size of 75 micrometers. In order to map the sky in a period of six months, and to resolve the nearer molecular clouds, each pixel must cover 0.5 arcminutes. This sets the focal length at 51.6 cm. In order for the pixel size to be half the diameter of the central diffraction peak at 28 micrometers would require a telescope aperture of 24 cm; an aperture of 60 cm has been selected for the PRISM study for greater light gathering power.

Taylor, Charles R.↗

The effect of ram-pressure stripping on dwarf galaxies

ABSTRACT Ram-pressure stripping (RPS) is a well observed phenomenon of massive spiral galaxies passing through the hot intracluster medium (ICM) of galaxy clusters. For dwarf galaxies (DGs) within a cluster, the transformation from gaseous to gas-poor systems by RPS is not easily observed and must happen in the outskirts of clusters. In a few objects in close by galaxy clusters and the field, RPS has been observed. Since cluster early-type DGs also show a large variety of internal structures (unexpected central gas reservoirs, blue stellar cores, composite radial stellar profiles), we aim in this study to investigate how ram pressure (RP) affects the interstellar gas content and therefore the star formation (SF) activity. Using a series of numerical simulations, we quantify the dependence of the stripped-off gas on the velocity of the infalling DGs and on the ambient ICM density. We demonstrated that SF can be either suppressed or triggered by RP depending on the ICM density and the DGs mass. Under some conditions, RP can compress the gas, so that it is unexpectedly retained in the central DG region and forms stars. When gas clouds are still bound against stripping but lifted from a thin disc and fall back, their new stars form an ellipsoidal (young) stellar population already with a larger velocity dispersion without the necessity of harassment. Most spectacularly, star clusters can form downstream in stripped-off massive gas clouds in the case of strong RP. We compare our results to observations.

Steyrleithner, P.↗

The distribution of interstellar gas within 50pc of the Sun

Previous Copernicus determinations of H I column densities in nearby stars are reviewed. These results are compared with recent ultraviolet (IUE) data and data acquired at other wavelengths. From this combined data set a coherent picture for the distribution of neutral hydrogen within 50pc of the Sun emerges. For directions away from the galactic center, the total H I column density for a 50pc line of sight is typically approximately 10 to the 18th power/sq cm, while toward the galactic center it may be 1 to 2 x 10 to the 19th power/sq cm or higher. A tentative model is proposed which suggests that the bulk of the H I resides in a cloud in a direction toward L(II) approximately 0 deg. The data are consistent with the Sun being embedded, but near the edge of the cloud.

Bruhweiler, F. C.↗

Supershells and propagating star formation

Correlated supernovae from an OB association can carve large cavities (greater than 100 pc) in the interstellar medium (ISM), and can punch holes completely through the disk of a spiral galaxy. Supernova remnant energy within such a cavity is thermalized before the shock reaches the supershell. Thus stellar wind theory may be used to model these superbubbles. We describe how the evolution of the superbubble depends on the density distribution of the galactic disk gas and the rate of supernovae in the OB association. At a radius of 100 to 300 pc, the supershell becomes gravitationally unstable, forming giant molecular clouds which are the sites for new star formation. This gravitational instability of the supershells provides a physical mechanism for propagating star formation and may account for the observation of bursts of star formation in galaxies.

Maclow, M. M.↗

The AstroBiology Explorer (ABE) MIDEX Mission Concept: Using Infrared Spectroscopy to Identify Organic Molecules in Space

One of the principal means by which organic compounds are detected and identified in space is by infrared spectroscopy. Past IR studies (telescopic and laboratory) have demonstrated that much of the carbon in the interstellar medium (ISM) is in complex organic species of a variety of types, but the distribution, abundance, and evolutionary relationships of these materials are not well understood. The Astrobiology Explorer (ABE) is a MIDEAST mission concept designed to conduct IR spectroscopic observations to detect and identify these materials to address outstanding important problems in astrobiology, astrochemistry, and astrophysics. Systematic studies include the observation of planetary nebulae and stellar outflows, protostellar objects, Solar System Objects, and galaxies, and multiple lines of sight through dense molecular clouds and the diffuse ISM. ABE will also search for evidence of D enrichment in complex molecules in all these environments. The mission is currently under study at NASA's Ames Research Center in collaboration with Ball Aerospace and Technologies Corp. ABE is a cryogenically-cooled 60 cm diameter space telescope equipped with 3 cryogenic cross-dispersed spectrographs that share a single common slit. The 3 spectrometers each measure single spectral octaves (2.5-5, 5-10, 10-20 microns) and together cover the entire 2.5 - 20 micron region simultaneously. The spectrometers use state-of-the-art 1024x1024 pixel detectors, with a single InSb array for the 2.5-5 micron region and two Si:As arrays for the 5-10 and 10-20 micron regions. The spectral resolution is wavelength dependent but is greater than 2000 across the entire spectral range. ABE would operate in a heliocentric, Earth drift-away orbit and is designed to take maximum advantage of this environment for cooling, thermal stability, and mission lifetime. ABE would have a core science mission lasting approximately 1.5 years.

Sandford, Scott A.↗

Supershells and propagating star formation

Stellar winds and repeated supernovae from an OB association will create a cavity of coronal gas in the interstellar medium, with radius greater than 100 pc, surrounded by a dense, expanding shell of cool interstellar gas. If the association has a typical initial mass function, its supernovae explosions will inject energy into the supershell at a nearly constant rate for about 50 Myr. The supershell loses its interior pressure and enters the snowplow phase when radiative cooling becomes important or when the shell bursts through the gas disk of a galaxy, typically after a few times 10 Myr and with a radius of 100-300 pc. At approximately the same time, the supershell becomes gravitationally unstable, forming giant molecular clouds which are sites for new star formation. There is widespread evidence for supershells in the Galaxy and other spiral and irregular galaxies from 21-cm emission-line surveys, optical emission-line surveys, and studies of supernova remnants. The gravitational instability of the supershells provides a physical mechanism for induced star formation and may account for bursts of star formation, especially in irregular galaxies.

Mccray, Richard↗

CCD observations of additional interstellar lines in stars associated with the Vela Remnant and Eta Carinae nebulosity

Nine stars from the Vela Remnant and seven stars in the Eta Carinae complex are examined with CCD spectrograms with high signal-to-noise ratios for data regarding the optical interstellar lines. These data are supplemented by observations of objects with known high-velocity components and substantial interstellar Ti II lines. The CCD spectrograms have signal-to-noise ratios of 100-300, a 2-pixel resolving power of 20,000, and cover features of Na I, Ti II, Ca I, CH(+), and CH in the 3200-4400-A range. The Ca I line confirms the high H density in Vela, and the Ti II lines show evidence of shock-induced grain evaporation. Weak shocks are thought to contribute to the CH(+) component, and a high H I density in some clouds is inferred from the CH line. The high H I density is supported by IUE data on Ca I and on the fine-structure carbon lines. The data point to a significant difference in spectroscopic morphology between the two groups of stars, and the need for theoretical support is underscored.

Wallerstein, George↗