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NASA NTRS2012

GOT C+ Survey of [CII] 158 Micrometer Emission: Atomic to Molecular Cloud Transitions in the Inner Galaxy

We present the results of the distribution of CO-dark H2 gas in a sample of 2200 interstellar clouds in the inner Galaxy (l = 90 deg to +57 deg) detected in the velocity resolved [CII] spectra observed in the GOT C+ survey using the Herschel HIFI. We analyze the [CII] intensities along with the ancillary HI, (12)CO and (13)CO data for each cloud to determine their evolutionary state and to derive the H2 column densities in the C(+) and C(+)/CO transition layers in the cloud. We discuss the overall Galactic distribution of the [CII] clouds and their properties as a function Galactic radius. GOT C+ results on the global distribution of [CII] clouds and CO-dark H2 gas traces the FUV and star formation rates in the Galactic disk.

structure↗

NASA NTRS1975

Interstellar clouds containing optically thin H2

The theory of Black and Delgarno that the relative populations of the excited rotational levels of H2 can be understood in terms of cascading following absorption in the Lyman and Werner bands is employed to infer the gas densities and radiation fields within diffuse interstellar clouds containing H2 that is optically thin in those bands. The procedure is described for computing the populations of the different rotation levels, the relative distribution among the different rotation levels of newly formed H2 is determined on the basis of five simplified models, and the rate of H2 formation is estimated. The results are applied to delta Ori, two components of iota Ori, the second components of rho Leo and zeta Ori, tau Sco, gamma Vel, and zeta Pup. The inferred parameters are summarized for each cloud.

Jura, M.↗

NASA NTRS1986

On the interpretation of the beta(sub p) relation in interstellar clouds

Troland and Heiles (1986) have recently presented an updated compilation of observational data concerning the relationship between the interstellar magnetic field strength B and the gas density rho (or, equivalently, the particle density n). One of the main findings of their survey was that B remains constant over the density range 0.1 - approx. 100 cu. cm and shows evidence for increase only a higher densities. They compared this result with theoretical predictions based on the Parker-instability scenario for the formation and evolution of interstellar clouds in the presence of the galactic magnetic field. In this picture, low-density gas is driven by the magnetic Rayleigh-Taylor instability into magnetic valleys, where it accumulates into denser concentrations. The gas initially flows along the magnetic field lines and there is little increase of the field strength with density; B only starts to rise when n becomes large enough for self-gravity to begin competing with the magnetic stresses. For a cloud mass of approx. 1,000 sub M and the measured background field strength, the critical density for contraction in approx. 75 cu. cm. Troland and Heiles therefore concluded that this scenario is basically consistent with the observations. This conclusion is debated.

Koenigl, A.↗

DOE OSTI2011

THE INTERSTELLAR MEDIUM IN DISTANT STAR-FORMING GALAXIES: TURBULENT PRESSURE, FRAGMENTATION, AND CLOUD SCALING RELATIONS IN A DENSE GAS DISK AT z = 2.3

We have used the Institut de Radioastronomie Millimetrique (IRAM) Plateau de Bure Interferometer and the Expanded Very Large Array to obtain a high-resolution map of the CO(6-5) and CO(1-0) emission in the lensed, star-forming galaxy SMM J2135-0102 at z = 2.32. The kinematics of the gas are well described by a model of a rotationally supported disk with an inclination-corrected rotation speed, v{sub rot} = 320 {+-} 25 km s{sup -1}, a ratio of rotational-to-dispersion support of v/{sigma} = 3.5 {+-} 0.2, and a dynamical mass of (6.0 {+-} 0.5) Multiplication-Sign 10{sup 10} M{sub Sun} within a radius of 2.5 kpc. The disk has a Toomre parameter, Q = 0.50 {+-} 0.15, suggesting that the gas will rapidly fragment into massive clumps on scales of L{sub J} {approx} 400 pc. We identify star-forming regions on these scales and show that they are {approx}10 Multiplication-Sign denser than those in quiescent environments in local galaxies, and significantly offset from the local molecular cloud scaling relations (Larson's relations). The large offset compared to local molecular cloud line-width-size scaling relations implies that supersonic turbulence should remain dominant on scales {approx}100 Multiplication-Sign smaller than in the kinematically quiescent interstellar medium (ISM) of the Milky Way, while the molecular gas in SMM J2135 is expected to be {approx}50 Multiplication-Sign denser than that in the Milky Way on all scales. This is most likely due to the high external hydrostatic pressure we measure for the ISM, P{sub tot}/k{sub B} {approx} (2 {+-} 1) Multiplication-Sign 10{sup 7} K cm{sup -3}. In such highly turbulent ISM, the subsonic regions of gravitational collapse (and star formation) will be characterized by much higher critical densities, n{sub crit} > = 10{sup 8} cm{sup -3}, a factor {approx}>1000 Multiplication-Sign more than the quiescent ISM of the Milky Way.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2014

Laboratory optical spectroscopy of the thiophenoxy radical and its profile simulation as a diffuse interstellar band based on rotational distribution by radiation and collisions

The gas-phase optical absorption spectrum of the thiophenoxy radical (C{sub 6}H{sub 5}S), a diffuse interstellar band (DIB) candidate molecule, was observed in the discharge of thiophenol using a cavity ringdown spectrometer. The ground-state rotational constants of the thiophenoxy radical were theoretically calculated, and the excited-state rotational constants were determined from the observed rotational profile. The rotational profile of a near prolate molecule having C {sub 2v} symmetry was simulated on the basis of a rotational distribution model by radiation and collisions. Although the simulated profile did not agree with the observed DIBs, the upper limit of the column density for the thiophenoxy radical in the diffuse clouds toward HD 204827 was evaluated to be 2 × 10{sup 13} cm{sup –2}. The profile simulation indicates that rotational distribution by radiation and collisions is important to reproduce a rotational profile for a DIB candidate and that the near prolate C {sub 2v} molecule is a possible candidate for DIB with a band width variation dependent on the line of sight.

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS1983

Interstellar absorption toward HD 14633

Observations are reported of optical interstellar absorption lines of Ti II, Ca II, and Na I in the spectrum of HD 14633, a probable runaway O star ejected from the Perseus spiral arm. Unusually high gas-phase abundances of the normally depleted elements Ca and Ti are found in three high-velocity interstellar line components; the light paths to only a few other stars are known to show a comparably strong Routly-Spitzer effect. The greatly reduced heavy-element depletions in these parcels of gas presumably reflect the disruption of interstellar grains in gas which may be located in the lower galactic halo at an absolute value of 2 of about 1 kpc.

Hobbs, L. M.↗

NASA NTRS1976

The interstellar matter column density to Cen X-3

The usual gas-to-dust ratio is shown to hold quite precisely for a sample of supernova remnants with available X-ray spectra and interstellar reddening observations. Supernova remnants are extended objects, tenuous enough to be optically thin in the X-ray range; it is shown that the X-ray observations can readily be interpreted in terms of a main source component and of an interstellar perturbation affecting the lower part of the spectrum. Cen X-3 is associated with an OB supergiant. The effect of the column density is easily detectable in the X-ray range. The spectrum exhibits a low energy cut-off, which is parameterized by a column of cold matter, NX. The cut-off is definitely observed to be variable. There is a suggestion that sometimes NX NH. It is concluded that on those occasions a spurious soft X-ray component is present in the source, bearing close similarity with Cyg X-1.

Ryter, C.↗

DOE OSTI2025

Directed Gas-Phase Formation of the 1-Cyanovinyl Radical (H 2 CCCN, X 2 A′) in the Interstellar Medium

The formation pathways to nitrogen-containing molecules and radicals are crucial to the understanding of the carbon–nitrogen chemistry in interstellar and atmospheric environments. While over 65 nitrogen-containing neutral species have been observed in deep space to date, their formation mechanisms─in particular, those of radical species─remain largely speculative. The crossed molecular beam technique in conjunction with electronic structure and statistical calculations was utilized to offer a detailed overview of the fundamental pathways in the gas-phase bimolecular reaction of ground-state atomic carbon (C, 3 P) with acetonitrile-d 3 (CD 3 CN, X 1 A 1 ) under single-collision conditions leading to the formation of the 1-cyanovinyl radical (D 2 CCCN, X 2 A′) coupled with deuterium atom loss. Here, the indirect reaction was initiated by barrierless carbon-atom addition, with the most probable route involving carbon addition across the carbon–nitrogen nitrile triple bond of acetonitrile, forming a three-membered ring intermediate followed by ring-opening and unimolecular decomposition via atomic deuterium loss from the C3 carbon atom. The reaction was overall exoergic, and intermediates and transition states lie lower in energy than the separated reactants, unlocking the reaction of carbon with acetonitrile in low-temperature environments such as cold molecular clouds, e.g., Taurus Molecular Cloud (TMC-1), and planetary atmospheres, e.g., Saturn’s moon Titan. In these environments, the 1-cyanovinyl radical may act as a building block for cyano-substituted polycyclic aromatic hydrocarbons and N-heterocycles, thus furthering our understanding of the complex carbon–nitrogen chemistry in deep space.

Chemical reactions↗

NASA NTRS2000

From Interstellar PAHs and Ices to the Origin of Life

Tremendous strides have been made in our understanding of interstellar material over the past twenty years thanks to significant, parallel developments in observational astronomy and laboratory astrophysics. Twenty years ago the composition of interstellar dust was largely guessed at, the concept of ices in dense molecular clouds ignored, and the notion of large, abundant, gas phase, carbon rich molecules widespread throughout the interstellar medium (ISM) considered impossible. Today the composition of dust in the diffuse ISM is reasonably well constrained to micron-sized cold refractory materials comprised of amorphous and crystalline silicates mixed with an amorphous carbonaceous material containing aromatic structural units and short, branched aliphatic chains. In dense molecular clouds, the birthplace of stars and planets, these cold dust particles are coated with mixed molecular ices whose composition is very well constrained. Lastly, the signature of carbon-rich polycyclic aromatic hydrocarbons (PAHs), shockingly large molecules by earlier interstellar chemistry standards, is widespread throughout the Universe. The first part of this lecture will describe how infrared studies of interstellar space, combined with laboratory simulations, have revealed the composition of interstellar ices (the building blocks of comets) and the high abundance and nature of interstellar PAHs. The laboratory database has now enabled us to gain insight into the identities, concentrations, and physical state of many interstellar materials. Within a dense molecular cloud, and especially in the solar nebula during the star and planet formation stage, the materials frozen into interstellar/precometary ices are photoprocessed by ultraviolet light, producing more complex molecules. The remainder of the presentation will focus on the photochemical evolution of these materials and the possible role of these compounds on the early Earth. As these materials are thought to be the building blocks of comets and related to the carbonaceous components of micrometeorites, they are likely to have been important sources of complex organic materials on the early Earth and their composition may be related to the origin of life.

Allamandola, Louis J.↗

NASA NTRS2004

From Interstellar Polycyclic Aromatic Hydrocarbons and Ice to the Origin of Life

Tremendous strides have been made in our understanding of interstellar material over the past twenty years thanks to significant, parallel developments in observational astronomy and laboratory astrophysics. Twenty years ago the composition of interstellar dust was largely guessed at, the concept of ices in dense molecular clouds ignored, and the notion of large, abundant, gas phase, carbon rich molecules widespread throughout the interstellar medium (ISM) considered impossible. Today the composition of dust in the diffuse ISM is reasonably well constrained to cold refractory materials comprised of amorphous and crystalline silicates mixed with an amorphous carbonaceous material containing aromatic structural units and short, branched aliphatic chains. In the dense ISM, the birthplace of stars and planets, these cold dust particles are coated with mixed molecular ices whose composition is very well constrained. Lastly, the signature of carbon-rich polycyclic aromatic hydrocarbons (PAHs), shockingly large molecules by early interstellar chemistry standards, is widespread throughout the Universe. The first part of this talk will describe how infrared studies of interstellar space, combined with laboratory simulations, have revealed the composition of interstellar ices (the building blocks of comets) and the high abundance and nature of interstellar PAHs. The laboratory database has now enabled us to gain insight into the identities, abundances, and physical state of many interstellar materials. Within a dense molecular cloud, and especially in the presolar nebula, the materials frozen into the interstellar/precometary ices are photoprocessed by ultraviolet light and produce more complex molecules. The remainder of the presentation will focus on the photochemical evolution of these materials and the possible role of these compounds on the to the carbonaceous components of micrometeorites, they are likely to have been important sources of complex materials on the early Earth and their composition may be related to the origin of life.

Allamandola, Louis↗

DOE OSTI2011

The X-factor in Galaxies: I. Dependence on Environment and Scale

Characterizing the conversion factor between CO emission and column density of molecular hydrogen, X{sub CO}, is crucial in studying the gaseous content of galaxies, its evolution, and relation to star formation. In most cases the conversion factor is assumed to be close to that of giant molecular clouds (GMCs) in the Milky Way, except possibly for mergers and star-bursting galaxies. However, there are physical grounds to expect that it should also depend on the gas metallicity, surface density, and strength of the interstellar radiation field. The X{sub CO} factor may also depend on the scale on which CO emission is averaged due to effects of limited resolution. We study the dependence of X{sub CO} on gas properties and averaging scale using a model that is based on a combination of results of sub-pc scale magneto-hydrodynamic simulations and on the gas distribution from self-consistent cosmological simulations of galaxy formation. Our model predicts X{sub CO} {approx} 2 - 4 x 10{sup 20} K{sup -1} cm{sup -2} km{sup -1} s, consistent with the Galactic value, for interstellar medium conditions typical for the Milky Way. For such conditions the predicted X{sub CO} varies by only a factor of two for gas surfaced densities in the range {Sigma}{sub H{sub 2}} {approx} 50-500 M{sub {circle_dot}} pc{sup -2}. However, the model also predicts that more generally on the scale of GMCs, X{sub CO} is a strong function of metallicity, and depends on the column density and the interstellar UV flux. We show explicitly that neglecting these dependencies in observational estimates can strongly bias the inferred distribution of H2 column densities of molecular clouds to have a narrower and offset range compared to the true distribution. We find that when averaged on {approx} kpc scales the X-factor depends only weakly on radiation field and column density, but is still a strong function of metallicity. The predicted metallicity dependence can be approximated as X{sub CO} {proportional_to} Z{sup -{gamma}} with {gamma} {approx} 0.5 - 0.8.

08 HYDROGEN↗

NASA NTRS2006

The X-ray Halo of GX5-1

Using Chandra observations we have measured the energy-resolved dust-scattered X-ray halo around the low-mass X-ray binary GX5-1, detecting for the first time multiply scattered X-rays from interstellar dust. % e compared the observed X-ray halo at various energies to predictions from a range of dust models. These fits used both smoothly-distributed dust as well as dust in clumped clouds, with CO and 21 cm observations helping to determine the position of the clouds along the line of sight. We found that the BARE-GR-B model of Zubko, Dwek & Arendt (2004) generally led to the best results, although inadequacies in both the overall model and the data limit our conclusions. We did find that the composite dust models of Zubko, Dwek & Arendt (2004), especially the "no carbon" models, gave uniformly poor results. Although models using cloud positions and densities derived naively from CO and 21 cm data gave generally poor results, plausible adjustments to the distance of the largest cloud and the mass of a cloud in the expanding 3 kpc Arm lead to significantly improved fits. We suggest that combining X-ray halo, CO, and 21 cm observations will be a fruitful method to improve our understanding of both the gas and dust phases of the interstellar medium.

Smith, Randall K.↗

NASA NTRS1993

Hot bubbles in a magnetic interstellar medium - Another look at the soft X-ray background

An attempt is made to understand the origin and properties of an isolated local bubble of hot gas using models of explosive events in a magnetic interstellar medium which mimic the local bubble in terms of the C band X-ray surface brightness and radius. The residual bubble of hot gas reaches a maximum size with an internal pressure below ambient, and then shrinks to smaller volume and a pressure nearly equal to ambient. The X-ray brightness reaches its minimum at the time of maximum radius and rises thereafter. It is found that the bubble growth must have been confined by a probably unacceptably large external pressure in order for the hot gas in the maximally extended cavity to radiate at a rate like that observed in the soft X-ray background.

Edgar, Richard J.↗

NASA NTRS1979

Studies of interstellar absorption lines with IUE

The paper deals with a survey of interstellar absorption lines in the wavelength region from 1150 to 3200 A toward 25 stars. The spectra of 10 of these stars show very strong narrow absorption lines of Si IV and C IV. Lines of N V appear in eight directions. The N V lines can arise in the same gas that is responsible for the ubiquitous interstellar lines of O VI, observed previously with a Copernicus spectrometer, but the lines of Si IV and C IV must be formed in a cooler gas. The measured lines of Si IV and C IV can be produced by normal H II regions photoionized by the observed stars.

Black, J. H.↗

DOE OSTI2012

THE X-FACTOR IN GALAXIES. I. DEPENDENCE ON ENVIRONMENT AND SCALE

Characterizing the conversion factor between CO emission and column density of molecular hydrogen, X{sub CO}, is crucial in studying the gaseous content of galaxies, its evolution, and relation to star formation. In most cases the conversion factor is assumed to be close to that of giant molecular clouds (GMCs) in the Milky Way, except possibly for mergers and star-bursting galaxies. However, there are physical grounds to expect that it should also depend on the gas metallicity, surface density, and strength of the interstellar radiation field. The X{sub CO} factor may also depend on the scale on which CO emission is averaged due to effects of limited resolution. We study the dependence of X{sub CO} on gas properties and averaging scale using a model that is based on a combination of results of sub-parsec scale magnetohydrodynamic simulations and on the gas distribution from self-consistent cosmological simulations of galaxy formation. Our model predicts X{sub CO} Almost-Equal-To (2-4) Multiplication-Sign 10{sup 20} K{sup -1} cm{sup -2} km{sup -1} s, consistent with the Galactic value, for interstellar medium conditions typical for the Milky Way. For such conditions the predicted X{sub CO} varies by only a factor of two for gas surface densities in the range {Sigma}{sub H{sub 2}}{approx}50-500 M{sub sun}pc{sup -2}. However, the model also predicts that more generally on the scale of GMCs, X{sub CO} is a strong function of metallicity and depends on the column density and the interstellar UV flux. We show explicitly that neglecting these dependencies in observational estimates can strongly bias the inferred distribution of H{sub 2} column densities of molecular clouds to have a narrower and offset range compared to the true distribution. We find that when averaged on {approx}kiloparsec scales the X-factor depends only weakly on radiation field and column density, but is still a strong function of metallicity. The predicted metallicity dependence can be approximated as X{sub CO}{proportional_to}Z{sup -{gamma}} with {gamma} Almost-Equal-To 0.5-0.8.

79 ASTRONOMY AND ASTROPHYSICS↗

DOE OSTI2012

FORMATION OF MOLECULAR OXYGEN AND OZONE ON AMORPHOUS SILICATES

Oxygen in the interstellar medium is seen in the gas phase, in ices (incorporated in H{sub 2}O, CO, and CO{sub 2}), and in grains such as (Mg{sub x} Fe{sub 1-x} )SiO{sub 3} or (Mg{sub x} Fe{sub 1-x} ){sub 2}SiO{sub 4}, 0 < x < 1. In this investigation, we study the diffusion of oxygen atoms and the formation of oxygen molecules and ozone on the surface of an amorphous silicate film. We find that ozone is formed at low temperature (<30 K), and molecular oxygen forms when the diffusion of oxygen atoms becomes significant, at around 60 K. This experiment, besides being the first determination of the diffusion energy barrier (1785 {+-} 35 K) for oxygen atoms on a silicate surface, suggests bare silicates as a possible storage place for oxygen atoms in low-A{sub v} environments.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗

DOE OSTI2015

HD/H{sub 2} AS A PROBE OF THE ROLES OF GAS, DUST, LIGHT, METALLICITY, AND COSMIC RAYS IN PROMOTING THE GROWTH OF MOLECULAR HYDROGEN IN THE DIFFUSE INTERSTELLAR MEDIUM

We modeled recent observations of UV absorption of HD and H{sub 2} in the Milky Way and toward damped/subdamped Lyα systems at z = 0.18 and z >1.7. N(HD)/N(H{sub 2}) ratios reflect the separate self-shieldings of HD and H{sub 2} and the coupling introduced by deuteration chemistry. Locally, observations are explained by diffuse molecular gas with 16 cm{sup –3} ≲ n(H) ≲ 128 cm{sup –3} if the cosmic-ray ionization rate per H nucleus ζ {sub H} =2 × 10{sup –16} s{sup –1}, as inferred from H{sub 3} {sup +} and OH{sup +}. The dominant influence on N(HD)/N(H{sub 2}) is the cosmic-ray ionization rate with a much weaker downward dependence on n(H) at solar metallicity, but dust extinction can drive N(HD) higher as with N(H{sub 2}). At z > 1.7, N(HD) is comparable to the Galaxy but with 10 times smaller N(H{sub 2}) and somewhat smaller N(H{sub 2})/N(H I). Comparison of our Galaxy with the Magellanic Clouds shows that smaller H{sub 2}/H is expected at subsolar metallicity, and we show by modeling that HD/H{sub 2} increases with density at low metallicity, opposite to the Milky Way. Observations of HD would be explained with higher n(H) at low metallicity, but high-z systems have high HD/H{sub 2} at metallicity 0.04 ≲ Z ≲ 2 solar. In parallel, we trace dust extinction and self-shielding effects. The abrupt H{sub 2} transition to H{sub 2}/H ≈ 1%-10% occurs mostly from self-shielding, although it is assisted by extinction for n(H) ≲ 16 cm{sup –3}. Interior H{sub 2} fractions are substantially increased by dust extinction below ≲ 32 cm{sup –3}. At smaller n(H), ζ {sub H}, small increases in H{sub 2} triggered by dust extinction can trigger abrupt increases in N(HD)

79 ASTRONOMY AND ASTROPHYSICS↗

NASA NTRS2006

Recent Selected Ion Flow Tube (SIFT) Studies Concerning the Formation of Amino Acids in the Gas Phase

Recently the simplest amino acid, glycine, has been detected in interstellar clouds, ISC, although this has since been contested. In order to substantiate either of these claims, plausible routes to amino acids need to be investigated. For gas phase synthesis, the SIFT technique has been employed to study simple amino acids via ion-molecule reactions of several ions of interstellar interest with methylamine, ethylamine, formic acid, acetic acid, and methyl formate. Carboxylic acid type ions were considered in the reactions involving the amines. In reactions where the carboxylic acid and methyl formate neutrals were studied, the reactant ions were primarily amine ion fragments. It was observed that the amines and acids preferentially fragment or accept a proton whenever energetically possible. NH3(+), however, uniquely reacted with the neutrals via atom abstraction to form NH4(+). These studies yielded a body of data relevant to astrochemistry, supplementing the available literature. However, the search for gas phase routes to amino acids using conventional molecules has been frustrated. Our most recent research investigates the fragmentation patterns of several amino acids and several possible routes have been suggested for future study.

Jackson, Douglas M.↗