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At least 73 records · Page 4
All sourcesNASA NTRSDOE OSTI
NASA NTRS2011

A New View on Interstellar Dust - High Fidelity Studies of Interstellar Dust Analogue Tracks in Stardust Flight Spare Aerogel

In 2000 and 2002 the Stardust Mission exposed aerogel collector panels for a total of about 200 days to the stream of interstellar grains sweeping through the solar system. The material was brought back to Earth in 2006. The goal of this work is the laboratory calibration of the collection process by shooting high speed [5 - 30km/s] interstellar dust (ISD) analogues onto Stardust aerogel flight spares. This enables an investigation into both the morphology of impact tracks as well as any structural and chemical modification of projectile and collector material. First results indicate a different ISD flux than previously assumed for the Stardust collection period.

Zolensky, Michael E.↗
DOE OSTI2013

ANOMALOUS DIFFUSE INTERSTELLAR BANDS IN THE SPECTRUM OF HERSCHEL 36. II. ANALYSIS OF RADIATIVELY EXCITED CH{sup +}, CH, AND DIFFUSE INTERSTELLAR BANDS

Absorption spectra toward Herschel 36 (Her 36) for the A-bar{sup 1}{Pi} Leftwards-Open-Headed-Arrow X-tilde{sup 1}{Sigma} transitions of CH{sup +} in the J = 1 excited rotational level and for the A-bar{sup 2}{Delta} Leftwards-Open-Headed-Arrow X-tilde{sup 2}{Pi} transitions of CH in the J = 3/2 excited fine structure level have been analyzed. These excited levels are above their ground levels by 40.1 K and {approx}25.7 K and indicate high radiative temperatures of the environment of 14.6 K and 6.7 K, respectively. The effect of the high radiative temperature is more spectacular in some diffuse interstellar bands (DIBs) observed toward Her 36; remarkable extended tails toward red (ETRs) were observed. We interpret these ETRs as being due to a small decrease of the rotational constants upon excitation of the excited electronic states. Along with radiative pumping of a great many high-J rotational levels, this causes the ETRs. In order to study this effect quantitatively, we have developed a model calculation in which the effects of collisions and radiation are treated simultaneously. The simplest case of linear molecules is considered. It has been found that the ETR is reproduced if the fraction of the variation of the rotational constant, {beta} {identical_to} (B' - B)/B, is sufficiently high (3%-5%) and the radiative temperature is high (T{sub r} > 50 K). Although modeling for general molecules is beyond the scope of this paper, the results indicate that the prototypical DIBs {lambda}5780.5, {lambda}5797.1, and {lambda}6613.6 which show the pronounced ETRs are due to polar molecules that are sensitive to the radiative excitation. The requirement of high {beta} favors relatively small molecules with three to six heavy atoms. DIBs {lambda}5849.8, {lambda}6196.0, and {lambda}6379.3 that do not show the pronounced ETRs are likely due to non-polar molecules or large polar molecules with small {beta}.

74 ATOMIC AND MOLECULAR PHYSICS↗
DOE OSTI2017

On the formation of nitrogen-substituted polycyclic aromatic hydrocarbons (NPAHs) in circumstellar and interstellar environments

The chemical evolution of extraterrestrial environments leads to the formation of polycyclic aromatic hydrocarbons (PAHs) via gas phase radical mediated aromatization reactions. Here, we review that these de facto barrierless reactions are capable of forming prebiotic molecules such as nitrogen substituted PAHs (NPAHs), which represent the missing link between nitrogen bearing acyclic molecules and prebiotic nucleobases along with vitamins found in meteorites. Crucial routes leading to the incorporation of nitrogen atoms into the aromatic ring have been exposed. Pyridine can be formed from the reaction of abundant vinyl cyanide and its radical or via cyano radicals reacting with 1,3-butadiene. The NPAHs 1,4-dihydro(iso)quinoline and (iso)quinoline can be synthesized through reaction of pyridyl radicals with 1,3-butadiene or sequentially with two acetylene molecules, respectively. The inclusion of nitrogen into an aromatic system and their growth can fill the mechanistic gaps missing leading from acyclic nitrogen-bearing molecules via pyridine to NPAH-type molecules in the interstellar medium.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1975

Interstellar reddening and gamma-ray emission as tracers of interstellar matter

It is shown that the gas to dust ratio in our galaxy is almost constant, even in regions of very different densities. The detailed comparison of the latitude distribution of the gamma-ray intensity with reddening data shows that in the solar vicinity the latter gives the best estimate of the total amount of interstellar gas. This conclusion is used to compute the nearby contributions to the gamma-rays longitude distribution observed by SAS II. Some implications for the large scale structure of the galaxy are examined.

Puget, J. L.↗
NASA NTRS1980

Molecule formation and infrared emission in fast interstellar shocks. II - Dissociation speeds for interstellar shock waves

The postshock destruction of molecules is examined, including the processes of (1) collisions with neutral hydrogen atoms and molecules, (2) electronic collisions, and (3) neutral chemical reactions with atoms, particularly atomic hydrogen. By using conservative estimates of collisional dissociation rates from individual vibrational states, it is found that process (1) leads to the destruction of molecular hydrogen behind shocks with speeds equal to or greater than 25 km/s if the preshock molecular gas has hydrogen nucleus densities of equal to or greater than 10 to the 4th/cu cm. At lower densities (100 per cu cm), destruction occurs for shock speeds equal to or greater than 50 km/s and process (2) dominates. Dissociation of molecules such as CO, H2O, and O2 follows the destruction of H2, as the resultant hydrogen atoms chemically dissociate the metal atoms from their bonds (process 3) in the hot postshock gas. These results demonstrate that many of the observed high-speed interstellar molecules, if shock accelerated, must have dissociated and reformed in the postshock gas.

Hollenbach, D.↗
NASA NTRS1991

The dissipation of magnetohydrodynamic turbulence responsible for interstellar scintillation and the heating of the interstellar medium

The present reexamination of the problem of ISM heating via the damping of plasma irregularities responsible for interstellar scintillations of radio sources gives attention to the effects of recent observational results on the hot plasma of the irregularities and on developments of in the theory of MHD waves. The damping mechanisms considered are linear Landau damping, ion-neutral collisional damping, nonlinear steepening of wave packets, a parametric decay instability, and nonlinear Landau damping. The heat input from all damping mechanisms can be accommodated by the 'fluctifer' region containing the irregularities, if it has features similar to the extended H II envelopes.

Spangler, Steven R.↗
NASA NTRS1991

Interstellar solid CO - Polar and nonpolar interstellar ices

Observations of the solid-CO lines corresponding to particular protostars are compared with respect to peak position and width and contrasted with the solid CO bands derived experimentally for astrophysical mixtures. Most lines of sight reveal narrow and broad observational solid-CO components; the narrow band appears when nonpolar molecules are dominant. When the concentration of CO is greater than 0.3 the 'surface modes' of the protostars are responsible for the position and shape of the CO fundamental. Some variations of the parameters are related to the composition and/or morphology of the grains particularly at the interstellar 2140/cm line. Grain mantles are theorized to be composed of both a polar and a nonpolar mixtures which reflect the chemical variations associated with accretion.

Tielens, A. G. G. M.↗
NASA NTRS1993

Optical studies of interstellar material in low density regions of the Galaxy. I - A survey of interstellar Na I and Ca II absorption toward 57 distant stars

We present high-resolution spectra of the Na I D and Ca II K lines toward 57 late-O and early-B stars along extended (d greater than 1 kpc) low-density paths through the Milky Way disk and halo. The sight lines preferentially sample diffuse gas in the interstellar medium (ISM) along interarm, Galactic center, and high latitude directions. We measure equivalent widths, apparent column densities, and absorption component structure. The Ca II to Na I ratios presented as a function of velocity for each sight line exhibit variations due to elemental depletion, ionization, and density enhancements. Absorption along high latitude sight lines is kinematically simpler than it is along interarm and Galactic center sight lines. Galactic rotation noticeably broadens the absorption profiles of distant stars located in these latter directions. Along several sight lines, we see Ca II absorption at velocities corresponding to large distances (/z/ about 1 kpc) from the Galactic plane. The effects of differences in the Ca II and Na I scale heights and nonzero velocity dispersions are readily apparent in the data. Brief notes are given for several sight lines with interesting absorption properties.

Sembach, K. R.↗
DOE OSTI2013

THE CARRIERS OF THE INTERSTELLAR UNIDENTIFIED INFRARED EMISSION FEATURES: CONSTRAINTS FROM THE INTERSTELLAR C-H STRETCHING FEATURES AT 3.2-3.5 μm

The unidentified infrared emission (UIE) features at 3.3, 6.2, 7.7, 8.6, and 11.3 μm, commonly attributed to polycyclic aromatic hydrocarbon (PAH) molecules, have recently been ascribed to mixed aromatic/aliphatic organic nanoparticles. More recently, an upper limit of <9% was placed on the aliphatic fraction (i.e., the fraction of carbon atoms in aliphatic form) of the UIE carriers based on the observed intensities of the 3.4 μm and 3.3 μm emission features by attributing them to aliphatic and aromatic C-H stretching modes, respectively, and assuming A{sub 3.4}/A{sub 3.3} ≈ 0.68 derived from a small set of aliphatic and aromatic compounds, where A{sub 3.4} and A{sub 3.3} are, respectively, the band strengths of the 3.4 μm aliphatic and 3.3 μm aromatic C-H bonds. To improve the estimate of the aliphatic fraction of the UIE carriers, here we analyze 35 UIE sources exhibiting both the 3.3 μm and 3.4 μm C-H features and determine I{sub 3.4}/I{sub 3.3}, the ratio of the power emitted from the 3.4 μm feature to that from the 3.3 μm feature. We derive the median ratio to be (I{sub 3.4}/I{sub 3.3}) ≈ 0.12. We employ density functional theory to compute A{sub 3.4}/A{sub 3.3} for a range of methyl-substituted PAHs. The resulting A{sub 3.4}/A{sub 3.3} ratio well exceeds ∼1.4, with an average ratio of A{sub 3.4}/A{sub 3.3} ≈ 1.76. By attributing the 3.4 μm feature exclusively to aliphatic C-H stretch (i.e., neglecting anharmonicity and superhydrogenation), we derive the fraction of C atoms in aliphatic form from I{sub 3.4}/I{sub 3.3} ≈ 0.12 and A{sub 3.4}/A{sub 3.3} ≈ 1.76 to be ∼2%. We therefore conclude that the UIE emitters are predominantly aromatic.

75 CONDENSED MATTER PHYSICS, SUPERCONDUCTIVITY AND↗
NASA NTRS1991

The interstellar C-H stretching band near 3.4 microns - Constraints on the composition of organic material in the diffuse interstellar medium

The composition and history of dust in the diffuse ISM was studied using 3600-2700/cm absorption spectra of objects which have widely varying amounts of visual extinctions along different lines of sight. The 3300/cm and 2950/cm features are attributed to O-H and C-H stretching vibrations, respectively. The O-H feature in OH 32.8-0.3 is suggestive of circumstellar water ice and is probably not due to material in the diffuse ISM. The features in the 3100-2700/cm region are attributed either to C-H vibrations or to M stars. The spectra of the latter show a series of narrow features in this region that are identified with photospheric OH. Objects in which these bands are seen include OH 01-477, T629-5, and the Galactic center source IRS 7. The C-H stretch feature of diffuse ISM dust has subpeaks which fall within 5/cm of C-H stretching vibrations in the -CH2- and -CH3 groups of saturated aliphatic hydrocarbons.

Sandford, S. A.↗
NASA NTRS1992

Comet Halley as an aggregate of interstellar dust and further evidence for the photochemical formation of organics in the interstellar medium

Photolysis of mixtures of CO:NH3:H2O at 12 K results in the formation of an organic residue which is not volatile in high vacuum at room temperature. Analysis of this fraction by GC-MS resulted in the detection of C2-C3 hydroxy acids and hydroxy amides, glycerol, urea, glycine, hexamethylene tetramine, formamidine and ethanolamine. Use of isotopically labeled gases made it possible to establish that the observed products were not contaminants. The reaction pathways for the formation of these products were determined from the position of the isotopic labels in the mass spectral fragments. The significance of these findings to the composition of comets and the origins of life is discussed.

Briggs, R.↗
NASA NTRS1993

Interstellar matter near the Pleiades. IV - The wake of the Pleiades through the interstellar medium in Taurus

A large emission 'cavity' whose bright rims extend about 5 deg eastward from the Pleiades, and is pressurized by the soft-UV radiation of the cluster, has been revealed by a mosaic of IRAS images; the emission cavity delineates the wake of the Pleiades as it moves supersonically through the ISM. Photoelectric heating is identified as the most likely agent of the cluster-cloud interaction generating a shock wave, and prompts the hypothesis that transverse expansion of heated gas near the cluster plays a crucial role in driving the shock. The cloud trajectory can be traced back to an origin in Gould's Belt some 15 Myr ago, in a blowout of gas into the Galactic halo.

White, Richard E.↗