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NASA NTRS2003

The Organic Component of Interstellar Dust

The distribution, chemical structure, and formation of organic matter in the interstellar medium are important to our understanding of the overall evolution of dust. The exchange of dust between the dense and diffuse interstellar medium, and the effects of processing on dust within dense clouds will affect the inventory of material available for incorporation into newly forming star and planetary systems. Observational ground-based studies have confirmed the widespread distribution of the 3.4 pm absorption band attributed to aliphatic hydrocarbons in the diffuse interstellar medium of our own galaxy, and in the dusty spectra of a few nearby galaxies, while space based observations from IS0 probed the signatures of corresponding mid-infrared features. Laboratory experiments which utilize both thermal processes and energetic processing by high energy photons and cosmic rays, produce candidate materials which offer close matches to the observed diffuse interstellar medium and extragalactic hydrocarbon absorption features. Through an analysis of the 4000 to 1000 cm (2.5 to 10 micrometers) region of the spectrum of diffuse interstellar medium (DISM) dust compared with the spectra of thirteen chemical entities produced in the laboratory which serve as analogs to the interstellar material, significant constraints have been placed on the applicability of proposed candidate materials to explain the interstellar features. The results indicate that the organic refractory material in the diffuse interstellar medium is predominantly hydrocarbon in nature, possessing little nitrogen or oxygen, with the carbon distributed between the aromatic and aliphatic forms. Long alkane chains H3C-(CH2),- with n much greater than 4 or 5 are not major constituents of this material. Comparisons to laboratory analogs indicate the DISM organic material resembles plasma processed pure hydrocarbon residues much more so than energetically processed ice residues. This result is consistent with a birthsite for the carrier of the 3.4 micrometers band in the outflow region of evolved carbon stars. Comparisons of dust from our own galaxy with that of distant galaxies suggests that the organic component of interstellar dust is widespread and may be an important universal reservoir of prebiotic organic carbon.

Pendleton, Yvonne↗

NASA NTRS1989

New insights in the photochemistry of grain mantles: The identification of the 4.62 and 6.87 micron bands

The mid-IR spectral region of molecular clouds is known to show the fingerprints of molecules frozen in the icy mantles of the interstellar grains. To study the complex chemical and physical interactions on the ice mantles accreted on grains in molecular clouds numerous UV irradiation and diffusion experiments were performed. The irradiation of binary ices was studied. Using isotopic labelling on NH3/CO and NH3/O2 ices numerous compounds were identified, of which OCN(-), NO2(-), NO3(-), and NH4(+) ions reveal a new type of chemical reactions. It appeared that these compounds were formed by proton transfer reactions induced by the interaction between an acid (HNCO, HNO2, HNO3) and a base (NH3) through a hydrogen bond. This mechanism was confirmed by a study of photolyzed diluted argon mixtures. The main astrophysically relevant data from the overall study are presented. The 4.62 micron band in W33A can be reproduced with NH3/CO containing irradiated ices and was identified with OCN(-). The 6.87 micron band in W33A and other photostellar objects is reproduced with NH3/O2 containing ices and is identified with NH4(+).

Grim, Ruud↗

NASA NTRS1981

Supernova remnant revolution in an inhomogeneous medium. I - Numerical models

The first numerical simulations of supernova remnant evolution in an inhomogeneous gas are presented. Evolution in the lowest density substrate (the intercloud) is assumed to be spherically symmetric with a large intercloud filling factor and many dense regions (clouds) within the remnant; however, mass momentum and energy transfer between cloud and intercloud are included and the position and morphology of individual clouds tracked. Evolution is considered in several different models of the interstellar medium, both those in which the intercloud gas is diffuse (0.001 to 0.01/cu cm) and those in which it is relatively dense (n approximately 0.3/cu cm) under a variety of assumptions about the efficiency of thermal evaporation from the clouds into the intercloud medium.

Cowie, L. L.↗

NASA NTRS1997

The Submillimeter Wave Astronomy Satellite: Science objectives and instrument description

The submillimeter wave astronomy satellite (SWAS) mission is dedicated to the investigation of star formation and interstellar chemistry. In order to perform the mission, SWAS will survey dense molecular clouds within the Milky Way Galaxy in either the ground state or a low-lying transition of five astrophysically-significant species: H2O, H2(18)O, O2, C I and (13)CO. The observation of these lines will: test theories that predict that these species are dominant coolants of molecular clouds during early stages of their collapse to form stars and planets, and supply information concerning the abundance of species central to the chemical models of dense interstellar gas. The SWAS will use two independent Schottky barrier diode mixers and a 53 x 68 sq cm, off-axis Cassegrain antenna.

Melnick, G. J.↗

NASA NTRS1982

Dynamical history of the Oort Cloud

Oort's hypothesis for the origin of long-period comets is reviewed, along with evidence for the existence of the Oort Cloud and recent research in stellar perturbations of cometary orbits. A Monte Carlo model is used to analyze the dynamics of comets in the Oort Cloud, to estimate the present and original populations of the cloud, and to examine implications for various theories of the origin of comets. The problem of interstellar comets is briefly considered. It is concluded that: (1) the weight of evidence favors Oort's hypothesis of a cloud of a trillion comets surrounding the solar system and extending out to about 100,000 AU; (2) the dynamical evolution of the cloud is consistent with observational data on long-period comets; and (3) estimates of the population necessary to produce the observed flux of dynamically new comets lead to physically reasonable values for the current and initial cloud masses.

Weissman, P. R.↗

DOE OSTI2022

Cool circumgalactic gas in galaxy clusters: connecting the DESI legacy imaging survey and SDSS DR16 Mg ιι absorbers

Herein we investigate the cool gas absorption in galaxy clusters by cross-correlating Mg II absorbers detected in quasar spectra from data release 16 of the Sloan Digital Sky Survey (SDSS) with galaxy clusters identified in the Dark Energy Spectroscopic Instrument (DESI) survey. We find significant covering fractions (⁠1–5percent within r 500 , depending on the chosen redshift interval), ~4–5 times higher than around random sightlines. While the covering fraction of cool gas in clusters decreases with increasing mass of the central galaxy, the total Mg II mass within r 500 is none the less ~10 times higher than for SDSS luminous red galaxies. The Mg II covering fraction versus impact parameter is well described by a power law in the inner regions and an exponential function at larger distances. The characteristic scale of the transition between these two regimes is smaller for large equivalent width absorbers. Cross-correlating Mg II absorption with photo–z selected cluster member galaxies from DESI reveals a statistically significant connection. The median projected distance between Mg II absorbers and the nearest cluster member is ~200 kpc, compared to ~500 kpc in random mocks with the same galaxy density profiles. We do not find a correlation between Mg II strength and the star formation rate of the closest cluster neighbour. This suggests that cool gas in clusters, as traced by Mg II absorption, is: (i) associated with satellite galaxies, (ii) dominated by cold gas clouds in the intracluster medium, rather than by the interstellar medium of galaxies, and (iii) may originate in part from gas stripped from these cluster satellites in the past.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS1972

The interstellar line spectra of zeta Ophiuchi and zeta Persei and their relation to the short wavelength microwave background radiation

Thirty-one high dispersion Coude spectrograms of zeta Ophiuchi and seven of zeta Persei were numerically synthesized to produce high resolution, low noise spectra in the interval 3650 A to 4350 that yield data on atomic and molecular absorption in well-defined regions of the interstellar medium. The detection threshold is improved by as much as a factor 5 over single plates. Several interstellar lines were discovered in the zeta Oph - 15km/sec cloud and the zeta Per + 13 km/sec cloud.

Bortolot, V. J., Jr.↗

NASA NTRS1977

Grain disruption in interstellar hydromagnetic shocks

The observed abundance variations of Ca, Fe, Si, and Ti in intermediate-velocity interstellar gas suggest that grains have been disrupted in clouds with velocities as low as 20 to 50 km/s. A simplified hydromagnetic shock model for such clouds is described; the dynamical equations for charged grains in the postshock region are derived, including the collisional drag and 'betatron-acceleration' effect of a magnetic-field gradient; and the fraction of grains destroyed in evaporative collisions with other grains is calculated. It is found that for shocks of 20 to 50 km/s in which the fractional H-ionization remains low, 3-10% of the grain material may be destroyed - sufficient to explain in part the heavy-element depletion pattern in intermediate velocity clouds and the well-known correlation of N(Ca II)/N(Na I) with cloud velocity.

Shull, J. M.↗

NASA NTRS1977

Radio detection of interstellar N2D/+/

Line emission from the J = 1-0 transition of interstellar N2D(+) has been detected toward the cool dust cloud L134 N. The N2H(+)/N2D(+) total-projected-density ratio in this cloud is estimated to be about 2.22. Upper limits are obtained for the fractional ionization as well as the CO and N2 abundances in L134 N. The results are shown to support the previously discovered enhancement of the abundance of deuterated molecules in cool dust clouds.

Snyder, L. E.↗

NASA NTRS1983

Interstellar ice

The 3250/cm 'ice' band observed in absorption and polarization in interstellar spectra is studied in the laboratory in low-temperature solid mixtures of H2O and other molecules. General empirical rules are derived which relate the shape and relative intensity of the H2O infrared absorptions to the degree of H2O dilution and hydrogen-bonding capacity of the dilutant. The chemical composition of mantles accreting on interstellar grains inside dense (1000-100,000/cu cm) molecular clouds has been calculated numerically. The reaction scheme comprises gas-phase as well as grain surface reactions. The results show that in most circumstances grain mantles consist of the molecules H2O, H2CO, NH3, N2, O2, CO, and CO2. The expected infrared characteristics of the calculated grain mantles are discussed with an emphasis on the observed 3250/cm 'ice' band. Grain mantles accreted at a density of about 10,000/cu cm contain large concentrations of H2O (about 60 percent) and produce a broad 3250/cm 'ice' band.

Tielens, A. G. G. M.↗

NASA NTRS1990

The effect of catastrophic collisional fragmentation and diffuse medium accretion on a computational interstellar dust system

The effects of catastrophic collisional fragmentation and diffuse medium accretion on a the interstellar dust system are computed using a Monte Carlo computer model. The Monte Carlo code has as its basis an analytic solution of the bulk chemical evolution of a two-phase interstellar medium, described by Liffman and Clayton (1989). The model is subjected to numerous different interstellar processes as it transfers from one interstellar phase to another. Collisional fragmentation was found to be the dominant physical process that shapes the size spectrum of interstellar dust. It was found that, in the diffuse cloud phase, 90 percent of the refractory material is locked up in the dust grains, primarily due to accretion in the molecular medium. This result is consistent with the observed depletions of silicon. Depletions were found to be affected only slightly by diffuse cloud accretion.

Liffman, Kurt↗

NASA NTRS1991

Astrophysical dust grains in stars, the interstellar medium, and the solar system

Studies of astrophysical dust grains in circumstellar shells, the interstellar medium, and the solar system may provide information about stellar evolution and about physical conditions in the primitive solar nebula. The following subject areas are covered: (1) the cycling of dust in stellar evolution and the formation of planetary systems; (2) astrophysical dust grains in circumstellar environments; (3) circumstellar grain formation and mass loss; (4) interstellar dust grains; (5) comet dust and the zodiacal cloud; (6) the survival of dust grains during stellar evolution; and (7) establishing connections between stardust and dust in the solar system.

Gehrz, Robert D.↗

NASA NTRS1991

Energy degradation of fast electrons in hydrogen gas

An equation is derived for calculating the energy distribution of fast electrons in a partially ionized gas and a method is provided to solve for the electron degradation spectrum and the energy deposition in different forms (ionization, excitation, or heating). As an example, the energy degradation of fast electrons in a gas of pure hydrogen is calculated, considering excitations to the lowest 10 atomic levels. The Bethe approximation and the continuous slowing-down approximation are discussed and it is concluded that these approximations are accurate to the order of 20 percent for electrons with initial energy of greater than about keV. The method and results can be used to determine heating, excitations, and ionizations by high-energy photoelectrons or cosmic-ray particles in various astrophysical circumstances, such as the interstellar medium, supernova envelopes, and QSO emission-line clouds.

Xu, Yueming↗

NASA NTRS1993

The origin and evolution of dust in interstellar and circumstellar environments

This status report covers the period from the commencement of the research program on 1 Jul. 1992 through 30 Apr. 1993. Progress is reported for research in the following areas: (1) grain formation in circumstellar envelopes; (2) photochemistry in circumstellar envelopes; (3) modeling ice features in circumstellar envelopes; (4) episodic dust formation in circumstellar envelopes; (5) grain evolution in the diffuse interstellar medium; and (6) grain evolution in dense molecular clouds.

Whittet, Douglas C. B.↗

NASA NTRS1993

Dust near luminous ultraviolet stars

This report describes research activities related to the Infrared Astronomical Satellite (IRAS) sky survey. About 745 luminous stars were examined for the presence of interstellar dust heated by a nearby star. The 'cirrus' discovered by IRAS is thermal radiation from interstellar dust at moderate and high galactic latitudes. The IRAS locates the dust which must (at some level) scatter ultraviolet starlight, although it was expected that thermal emission would be found around virtually every star, most stars shown no detectable emission. And the emission found is not uniform. It is not that the star is embedded in 'an interstellar medium', but rather what is found are discrete clouds that are heated by starlight. An exception is the dearth of clouds near the very hottest stars, implying that the very hottest stars play an active role with respect to destroying or substantially modifying the dust clouds over time. The other possibility is simply that the hottest stars are located in regions lacking in dust, which is counter-intuitive. A bibliography of related journal articles is attached.

Henry, Richard C.↗

NASA NTRS1995

Vibrationally excited H2, HCl, and NO(+) in the diffuse clouds toward Zeta Ophiuchi

Absorption lines from vibrationally excited H2 and from HCl were detected in the ultraviolet spectrum of Zeta Oph with the Hubble Space Telescope (HST). Improved upper limits on NO(+) absorption were obtained as well. The data were analyzed with an updated version of a chemical model for diffuse clouds (van Dishoeck & Black 1986) as a guide to understanding this gaseous environment. The comparisons suggest that the flux of ultraviolet radiation impinging on the cloud surface is approximately 1-2 times the average interstellar radiation field, which is lower than once believed.

Federman, S. R.↗

NASA NTRS1998

Spatially Resolved Spectroscopy of the SNR IC443

IC 443 is a supernova remnant of intermediate age, i.e. a few thousand years. It is especially interesting because part of its periphery is expanding into a molecular cloud while other sections are expanding into a typical interstellar medium of much lower density. Since the evolution of a supernova remnant through its various phases is affected by the density of the medium it expands into with the reasonable assumption that the supernova explosion was approximately symmetric we have an opportunity to observe a single object in two phases simultaneously. It was observed by ASCA in April, 1993 for a short period during the PV phase and more thoroughly in a 42 ksec exposure in March, 1994. The latter measurement provides most of the results that have been reported. Most of the analysis took place after the grant ended but is included here for completeness. The data was sent simultaneously to US and Japanese Pls. We worked independently. The software set of FTOOLs was used to construct images and spectra. They were judged to be rather unintuitive and not at all user friendly. I found I was using one FTOOL to read the header to obtain information that would only be provided to another FTOOL. The Japanese investigators were more successful. They analyzed the data and published results more rapidly. The scientific results summarized below are based primarily on their publications. Since IC 443 is an interesting example of a middle aged SNR in which a variety of processes are occurring it is one of a class. IC 443 exhibits shell-like emission in hard X-rays and extended soft X-rays with thin thermal spectra. It resembles SN 1006 in these respects. IC 443 contains hard X-rays in a semi-circular shell surrounding the thermal component. The total hard X-ray flux in the ASCA FOV is only a half of the Ginga hard component; which suggests that the hard X-rays are not confined only in the shell but some are extended larger than the ASCA FOV of eq 1 degree diameter. Japanese investigators examined the spatial structure of the thermal component and analyzed the GIS spectra with a non-equilibrium plasma model, and found no systematic variation of the interstellar absorption across the remnant. Evidence for shock acceleration of cosmic rays to high energies (10 TeV) was found by Keohane. X-ray imaging spectroscopy with ASCA reveals two regions of particularly hard emission: an unresolved source embedded in an extended emission region, and a ridge of emission coincident with the southeastern rim. Both features are located on part of the radio shell where the shock wave is interacting with molecular gas, and together they account for a majority of the emission at 7 keV. Though we would not have noticed it a priori, the unresolved feature is coincident with one resolved by the ROSAT HRI. The ASCA measurements were combined with higher energy data from the XTE and GRO missions and with radio and TeV gamma-ray data to produce a nonthermal multiwavelength spectrum for IC 443 which was fit with a cosmic ray interaction model. This model calculates the cynchrotron, bremsstrahlung, invers Compton, and neutral pion decay emission produced by locally accelerated cosmic ray interacting with ambient matter, soft photon fields, and magnetic fields.

Gorenstein, P.↗

NASA NTRS2004

Polycyclic Aromatic Hydrocarbon Ionization Energy Lowering in Water Ices

In studying various interstellar and solar system ice analogs, we have recently found that upon vacuum ultraviolet photolysis, polycyclic aromatic hydrocarbons (PAHs) frozen in water ice at low temperatures are easily ionized and indefinitely stabilized as trapped ions (Gudipati; Gudipati & Allamandola). Here we report the first experimental study that shows that PAH ionization energy is significantly lowered in PAH/H2O ices, in agreement with recent theoretical work (Woon & Park). The ionization energy (IE) of the PAH studied here, quaterrylene (C40H20, IE = 6.11 eV), is lowered by up to 2.11 eV in water ice. PAH ionization energy reduction in low-temperature water ice substantially expands the astronomical regions in which trapped ions and electrons may be important. This reduction in ionization energy should also hold for other types of trapped species in waterrich interstellar, circumstellar, and solar system ices. Subject headings: ISM: clouds - methods: laboratory - molecular processes - radiation mechanisms: nonthermal -ultraviolet: ISM - ultraviolet: solar system

Gudipati, Murthy S.↗