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NASA NTRS2010

Spitzer Observations of Dust Destruction in the Puppis A Supernova Remnant

The interaction of the Puppis A supernova remnant (SNR) with a neighboring molecular cloud provides a unique opportunity to measure the amount of grain destruction in an SNR shock. Spitzer Space Telescope MIPS imaging of the entire SNR at 24, 70, and 160 micrometers shows an extremely good correlation with X-ray emission, indicating that the SNR's IR radiation is dominated by the thermal emission of swept-up interstellar dust, collisionally heated by the hot shocked gas. Spitzer IRS spectral observations targeted both the Bright Eastern Knot (BEK) of the SNR where a small cloud has been engulfed by the supernova blast wave and outlying portions of the associated molecular cloud that are yet to be hit by the shock front. Modeling the spectra from both regions reveals the composition and the grain size distribution of the interstellar dust, both in front of and behind the SNR shock front. The comparison shows that the ubiquitous polycyclic aromatic hydrocarbons of the interstellar medium are destroyed within the BEK, along with nearly 25% of the mass of graphite and silicate dust grains.

Arendt, Richard G.↗

NASA NTRS1987

The large scale gas and dust distribution in the galaxy: Implications for star formation

Infrared Astronomy Observations are presented for the diffuse infrared (IR) emissions from the galactic plane at wavelengths of 60 and 100 microns and the total far infrared intensity and its longitudinal variations in the disk were derived. Using available CO, 5 GHz radio-continuum, and HI data, the IR luminosity per hydrogen mass and the ingrared excess (IRE) ratio in the Galaxy were derived. The longitudinal profiles of the 60 and 100 micron emission were linearly decomposed into three components that are associated with molecular (H2), neutral (HI), and ionized (HII) phases in the interstellar medium (ISM), and the relevant dust properties were derived in each phase. Implications of the findings for various models of the diffuse IR emisison and for star formation in the galactic disk are discussed.

Sodroski, T. J.↗

NASA NTRS1989

Circumstellar gas associated with HL Tauri - Evidence for a remnant infalling envelope

Molecular carbon absorption lines in the spectrum of HL Tau has been discovered near 8775 A. These C2 lines have a heliocentric radial velocity of 43 + or - 3 km/s, redshifted by 23 + or - 3 km/s relative to the star and the associated molecular cloud. This velocity difference suggests that the molecular carbon absorption arises in an infalling envelope. Since KI and diffuse interstellar bands are much weaker than expected, the chemical composition and/or excitation conditions in the HL Tau envelope appear to differ substantially from those in the interstellar medium.

Grasdalen, Gary L.↗

NASA NTRS2014

Spectroscopic Measurement Techniques for Aerospace Flows

The conditions that characterize aerospace flows are so varied, that a single diagnostic technique is not sufficient for its measurement. Fluid dynamists use knowledge of similarity to help categorize and focus on different flow conditions. For example, the Reynolds number represents the ratio of inertial to viscous forces in a flow. When the velocity scales, length scales, and gas density are large and the magnitude of the molecular viscosity is low, the Reynolds number becomes large. This corresponds to large scale vehicles (e.g Airbus A380), fast moving objects (e.g. artillery projectiles), vehicles in dense fluids (e.g. submarine in water), or flows with low dynamic viscosity (e.g. skydiver in air). In each of these cases, the inertial forces dominate viscous forces, and unsteady turbulent fluctuations in the flow variables are observed. In contrast, flows with small length scales (e.g. dispersion of micro-particles in a solid rocket nozzle), slow moving objects (e.g. micro aerial vehicles), flows with low density gases (e.g. atmospheric re-entry), or fluids with a large magnitude of viscosity (e.g. engine coolant flow), all have low Reynolds numbers. In these cases, viscous forces become very important and often the flows can be steady and laminar. The Mach number, which is the ratio of the velocity to the speed of sound in the medium, also helps to differentiate types of flows. At very low Mach numbers, acoustic waves travel much faster than the object, and the flow can be assumed to be incompressible (e.g. Cessna 172 aircraft). As the object speed approaches the speed of sound, the gas density can become variable (e.g. flow over wing of Learjet 85). When the object speed is higher than the speed of sound (Ma > 1), the presences of shock waves and other gas dynamic features can become important to the vehicle performance (e.g. SR-71 Blackbird). In the hypersonic flow regime (Ma > 5), large changes in temperature begin to affect flow properties, causing real-gas effects to occur (e.g. X-43 Scramjet). At even higher Mach numbers, chemistry and nonequilibrium effects come into play (e.g. Startdust re-entry capsule), further complicating the measurement. These limits can be predicted by calculating the ratio of chemical and thermal relaxation time to the flow time scales. Other non-dimensional numbers can be used to further differentiate types of aerospace flows.

Danehy, Paul M.↗

NASA NTRS2000

The Hummingbird GC-IMS: In Situ Analysis of a Cometary Nucleus

Comets are of enormous scientific interest for many reasons. They are primitive bodies that date back to the earliest stages of solar system formation and, because of their small size and because they have been stored in the outer reaches of the solar system, their pristine nature has been preserved better than for any other class of body. They are extremely rich in highly volatile elements, many in the form of ices, and are richer in organic matter than any other known solar system body. It is strongly suspected that in addition to their content of primordial solar nebular material, they also incorporate unprocessed matter from the interstellar medium. Impacts by comets occur onto all the planets and satellites, often with major consequences (e.g., the dinosaur extinction event at the KIT boundary), or sometimes just providing a spectacular cosmic event (e.g., the collision of comet Shoemaker-Levy 9 with Jupiter). A mission to analyze a cometary nucleus must be capable of detecting and identifying over 30 molecular species among several different chemical groups. The Hummingbird Mission will rendezvous with, orbit, characterize, and make multiple descents to the nucleus of a comet. Hummingbird will employ a Gas Chromatograph - Ion Mobility Spectrometer (GC-IMS) as part-of a suite of sophisticated instruments for a comprehensive in situ elemental, molecular, and isotopic analysis of the comet.

Kojiro, Daniel R.↗

NASA NTRS1995

Hot shocked gas along the jets of NGC 4258 (M106)

The ROSAT High Resolution Interferometer (HRI) and Position Sensitive Proportional Counter (PSPC) detectors have been used to study the soft X-ray emission of the nearby SABbc galaxy NGC 4258 (M106). The helically twisted, nuclear jets, previously detected in radio continuum and visual emission-line studies, are found to dominate the X-ray images. After correcting pointing errors of up to +/- 7 sec in our HRI observation, we find that the southeast jet is spatially unresolved transversely along much of its 2.5 min (5 kpc) length. The northwest jet is more diffuse, and the extensive radio 'plateaus' also emit X-rays. The energy spectrum of the jets from the PSPC exposure is noisy but is best fitted as a Raymond-Smith plasma with kT = 0.3 keV, log N(sub H) approximately = 20.0/sq cm, and luminosity 1.6 x 10(exp 40) ergs/s between 0.1 and 2.4 keV. Gas at this temperature can arise from planar shocks of 500 km/s and may have been entrained as the jets scrape along the molecular clous that are known to be adjacent to the jets. The range of radial velocities from our published visual emission-line spectra of the jet are consistent with such a shock speed. We estimate that the mass of hot gas in the jets is 6 x 10(exp 4) (1/cc/n(sub e)) solar mass, with n(sub e) the average electron density. The inferred pressure of hot gas in the southeast jet may be similar to the radio equipartition pressure and to the ambient pressure of the interstellar medium (ISM). The X-ray spectral fit is poor above 0.7 keV and requires an additional hard component that peaks near the nucleus.

Cecil, G.↗

NASA NTRS1992

The molecular interstellar medium in two giant H II regions in M33 - NGC 604 and NGC 595

This study reports single-dish and interferometric CO observations for the two largest H II regions in M33, NGC 604, and NGC 595. Six individual molecular clouds are identified in the H II regions. Molecular and virial masses for three resolved clouds in NGC 604 agree very well, from which it is inferred that the CO-to-H2 conversion factor in this region is the same as it is in the inner disk of M33 and in the Galaxy, despite a metallicity in NGC 604 that is three times below solar. The total gas mass of the molecular complex associated with NGC 604 estimated from single-dish data is dissimilar to those of molecular complexes in the inner disk of M33, while the molecular gas mass of NGC 505 is roughly an order or magnitude smaller. Both regions contain a large mass fraction of atomic hydrogen, which suggests that photodissociation of molecular hydrogen is an important process in these energetic H II regions.

Wilson, Christine D.↗

NASA NTRS1984

Infrared spectroscopy of interstellar shocks

Infrared emission lines from interstellar shocks provide valuable diagnostics for violent events in the interstellar medium, such as supernova remnants and mass outflow from young stellar objects. There are two types of interstellar shocks: in J shocks, gas properties 'jump' from their preshock to their postshock values in a shock front with a thickness equal to or less than one mean free path; radiation is emitted behind the shock front, primarily in the visible and ultraviolet, but with a few strong infrared lines, such as OI(63 microns). Such shocks occur in ionized or neutral atomic gas, or at high velocities (equal to or greater than 50 km/s) in molecular gas. In C shocks, gas is accelerated and heated by collisions between charged particles, which have a low concentration and are coupled to the magnetic field, and neutral particles; radiation is generated throughout the shock and is emitted almost entirely in infrared emission lines. Such shocks occur in weakly ionized molecular gas for shock velocities below about 50 km/s.

Mckee, C. F.↗

NASA NTRS1988

The Magellanic irregular galaxy NGC 4214 - Star formation and the interstellar medium

Near- and far-IR data and mm-wave CO observations are used to study the star-forming properties of the Magellanic irregular galaxy NGC 4214 and its interstellar medium. The galaxy is forming stars at present at about 0.5-1 solar mass/yr in its central few arcmin, if the new stars follow the Salpeter initial mass function from 0.1 to 100 solar masses. This has been maintained for around a Hubble time, although some regions of the galaxy may undergo periods of enhanced and depressed star formation. Based on the mass of dust in emission at 160 microns, a plausible star formation efficiency of about 1.5 percent is estimated, if the H I gas does not take part in forming stars. Molecular masses derived using the H + 1 - 0 line and a conversion to M(H2) appropriate to spiral galaxies produces a mass estimate that is too low by a factor of five to ten.

Thronson, Harley A., Jr.↗

NASA NTRS2006

Astrochemistry: Recent Advances in the Study of Carbon Molecules in Space

Carbon molecules and ions play an important role in space. Polycyclic Aromatic Hydrocarbons (PAHs) are the best-known candidates to account for the infrared emission bands (UIR bands) and PAH spectral features are now being used as probes of the interstellar medium in Galactic and extra-galactic environments. PAHs are also thought to be among the carriers of the diffuse interstellar absorption bands (DIBs). In the model dealing with the interstellar spectral features, PAHs are present as a mixture of radicals, ions and neutral species. PAH ionization states reflect the ionization balance of the medium while PAH size, composition, and structure reflect the energetic and chemical history of the medium. A major challenge for laboratory Astrochemistry is to reproduce (in a realistic way) the physical conditions that exist in the emission and absorption interstellar zones. An extensive laboratory program has been developed in various laboratories to characterize the physical and chemical properties of PAHs in astrophysical environments and to describe how they influence the radiation and energy balance in space and the interstellar chemistry. In particular, laboratory experiments provide measurements of the spectral characteristics of interstellar PAH analogs from the ultraviolet and visible range to the infrared range for comparison with astronomical data. The harsh physical conditions of the interstellar medium - characterized by a low temperature, an absence of collisions and strong ultraviolet radiation fields - are simulated in the laboratory by associating a molecular beam with an ionizing discharge to generate a cold plasma expansion. PAH ions are formed from the neutral precursors in an isolated environment at low temperature (of the order of 100 K). The spectra of neutral and ionized PAHs are measured using the high sensitivity methods of cavity ring down spectroscopy (CRDS). These experiments provide unique information on the spectra of free, cold large carbon molecules and ions in the gas phase.

Salama, Farid↗

NASA NTRS2008

Middle UV to Near-IR Spectrum of Electron-Excited SO2

We investigated the electron impact-induced fluorescence spectrum of SO2 to provide excitation cross sections for modeling Io's mission spectrum and analyzing Cassini Imaging Science Subsystem observations. The electron-excited middle-ultraviolet visible optical near-infrared (VOIR) emission spectrum of SO2 gas was generated in the laboratory and studied from 2000 to 11,000 A at a resolution of (Delta)(lamda) approximately 2.5 A full width at half maximum (FWHM). The VOIR laboratory spectrum longward of 6000 A consists entirely of S I, II and O I, II multiplets for electron impact energies above approximately 15 eV. Between 2000 and 6000 A, we find previously identified molecular bands from both SO and SO2. This work represents a significant improvement in spectral resolution over our earlier work done at 18 A FWHM. From a measurement of the medium-resolution spectrum, we provide detailed 25- and 100-eV emission cross sections for spectral features from 2000 to 11,000 A . On the basis of these data, we suggest future ground-based and satellite telescopic observations in the VOIR that are of promise for understanding Io's atmosphere.

sulpher dioxide↗

NASA NTRS1986

Chemical evolution in spiral and irregular galaxies

A brief review of models of chemical evolution of the interstellar medium in our galaxy and other galaxies is presented. These models predict the time variation and radial dependence of chemical composition in the gas as function of the input parameters; initial mass function, stellar birth rate, chemical composition of mass lost by stars during their evolution (yields), and the existence of large scale mass flows, like infall from the halo, outflow to the intergalactic medium or radial flows within a galaxy. At present there is a considerable wealth of observational data on the composition of HII regions in spiral and irregular galaxies to constrain the models. Comparisons are made between theory and the observed physical conditions. In particular, studies of helium, carbon, nitrogen and oxygen abundances are reviewed. In many molecular clouds the information we have on the amount of H2 is derived from the observed CO column density, and a standard CO/H2 ratio derived for the solar neighborhood. Chemical evolution models and the observed variations in O/H and N/O values, point out the need to include these results in a CO/H2 relation that should be, at least, a function of the O/H ratio. This aspect is also discussed.

Torres-Peimbert, S.↗

NASA NTRS1987

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↗

NASA NTRS2009

Theoretical Modeling of Interstellar Chemistry

The chemistry of complex interstellar organic molecules will be described. Gas phase processes that may build large carbon-chain species in cold molecular clouds will be summarized. Catalytic reactions on grain surfaces can lead to a large variety of organic species, and models of molecule formation by atom additions to multiply-bonded molecules will be presented. The subsequent desorption of these mixed molecular ices can initiate a distinctive organic chemistry in hot molecular cores. The general ion-molecule pathways leading to even larger organics will be outlined. The predictions of this theory will be compared with observations to show how possible organic formation pathways in the interstellar medium may be constrained. In particular, the success of the theory in explaining trends in the known interstellar organics, in predicting recently-detected interstellar molecules, and, just as importantly, non-detections, will be discussed.

Charnley, Steven↗

NASA NTRS1985

Molecular spectroscopy from the Kuiper Airborne Observatory

Interstellar and circumstellar molecules are investigated through medium-resolution infrared spectrosocpy of the vibration-rotation and pure rotational transitions. A primary goal was the construction and improvement of instrumentation for the near and middle infrared regions, wavelengths between 2 and 10 microns. The main instrument was a cooled grating spectrometer with an interchangeable detector focal plane which could be used on the Kuiper Airborne Observatory (KAO) for airborne observations, and also at ground-based facilities. Interstellar shock waves were investigated by H2 emission from the Orion Nebula, W51, and the proto-planetary nebulae CRL 2688 and CRL 618. The observations determined the physical conditions in shocked molecular gas near these objects. From these it was possible to characterize the energetic history of mass loss from both pre- and post-main sequence stars in the regions.

Beckwith, S.↗

NASA NTRS1976

Local and large scale galactic gamma-ray emission

A study of the gas-to-dust ratio suggests that interstellar reddening may provide an accurate way of assessing column densities in various directions in the Galaxy. A gamma ray intensity of 7.25 x 10 to the -5th E sub B-V photon/sq cm/s/sterad is predicted for regions of the Galaxy where the medium cosmic ray density is equal to that observed close to the sun. It is found that in the longitude range of about 0-180 degrees, the large scale cosmic ray distribution producing the gamma rays follows that of extreme population I stars put in evidence by giant H II regions, and that of molecular hydrogen traced by carbon monoxide emission. The gamma-ray production exhibits a maximum at R roughly equal to 5 kpc, and practically vanishes at R not less than 11 kpc, beyond the location of the outermost H II regions.

Puget, J. L.↗

NASA NTRS1986

A molecular cloud in the local, hot interstellar medium

Echelle spectra recorded at the D lines of Na I are reported for nine A or F stars. Lying at approximate distances ranging from 25 to 230 pc, the stars are projected on or near the high-latitude molecular cloud MBM 12 at l = 159 deg, b = -34 deg. Among a subgroup of five of these stars separated by no more than 1.2 deg on the sky, four which are located at distances d more than 70 pc show strong interstellar D line absorption near the radial velocity of the CO emission observed in this general direction. The fifth star, at roughly 60 pc, shows no detectable absorption. MBM 12 therefore probably lies at roughly 65 pc, within the local region filled primarily by very hot, low-density gas, a conclusion supported by the large internal velocity dispersion of the molecular cloud complex.

Hobbs, L. M.↗

NASA NTRS1994

Interstellar gas and dust in the young cluster IC 348

We have completed a multiband absorption- and emission-line study of a star embedded in the young cluster IC 348, to determine the environmental effect of star formation on the interstellar medium (ISM) local to the region. The hottest and youngest star in IC 348 is BD +31 deg 643, a B5 V star which samples the inner bright nebular region. The nearby star omicron Per, which lies only 8 min to the north and is thought to lie beyond IC 348, samples the gas and dust which has not been processed by very recent star formation. We speculate that the ISM throughout the region was originally the same as that currently seen toward omicron Per, but now the constrasting environmental conditions due to the recent star formation have led to marked differences in the atomic, molecular, and dust characteristics of IC 348. These contrasts include what we have termed a 'composite' UV extinction curve for BD +31 deg 643, evidence for enhanced density and enhanced depletions within IC 348 and very different molecular abundances in the interstellar sightline to BD +31 deg 643. Toward BD +31 deg 643, we find a higher column density of CH, but lower CN and very much higher CH(+) than measured toward omicron Per. We conclude that the physical and chemical state of the gas and dust has been altered by local processes and conditions within IC 348. The characteristics of the ISM in IC 348, via our study of the star BD +31 deg 643, closely resembles those seen toward rho Oph, another sight line passing through a bright nebular region. However, the stars are not as hot in IC 348 as in Ophiuchus, so their effect on the local ISM is not as severe.

Snow, Theodore P.↗