Chemical Simulations of Prebiotic Molecules: Interstellar Ethanimine Isomers

NASA Astrophysics Data System (ADS)

Quan, Donghui; Herbst, Eric; Corby, Joanna F.; Durr, Allison; Hassel, George

2016-06-01

The E- and Z-isomers of ethanimine (CH3CHNH) were recently detected toward the star-forming region Sagittarius (Sgr) B2(N) using the Green Bank Telescope PRIMOS cm-wave spectral data, and imaged by the Australia Telescope Compact Array. Ethanimine is not reported in the hot cores of Sgr B2, but only in gas that absorbs at +64 and +82 km s-1 in the foreground of continuum emission generated by H II regions. The ethanimine isomers can serve as precursors of the amino acid alanine and may play important roles in forming biological molecules in the interstellar medium. Here we present a study of the chemistry of ethanimine using a gas-grain simulation based on rate equations, with both isothermal and warm-up conditions. In addition, the density, kinetic temperature, and cosmic ray ionization rate have been varied. For a variety of physical conditions in the warm-up models for Sgr B2(N) and environs, the simulations show reasonable agreement with observationally obtained abundances. Isothermal models of translucent clouds along the same line of sight yield much lower abundances, so that ethanimine would be much more difficult to detect in these sources despite the fact that other complex molecules have been detected there.

Boundary conditions for the paleoenvironment: Chemical and physical processes in the pre-solar nebula. [molecular clouds, interstellar matter, and abundance

NASA Technical Reports Server (NTRS)

Irvine, W. M.; Schloerb, F. P.

1985-01-01

Two additional hyperfine components of the interstellar radical C3H were detected. In addition, methanol was discovered in interstellar clouds. The abundance of HCCN and various chemical isomers in molecular clouds was investigated.

Abundances of Neutral and Ionized PAH Along The Lines-of-Sight of Diffuse and Translucent Interstellar Clouds

NASA Astrophysics Data System (ADS)

Salama, Farid; Galazutdinov, G.; Krelowski, J.; Biennier, L.; Beletsky, Y.; Song, I.

2013-06-01

The spectra of neutral and ionized PAHs isolated in the gas phase at low temperature have been measured in the laboratory under conditions that mimic interstellar conditions and are compared with a set of astronomical spectra of reddened, early type stars. The comparisons of astronomical and laboratory data provide upper limits for the abundances of neutral PAH molecules and ions along specific lines-of-sight. Something that is not attainable from infrared observations. We present the characteristics of the laboratory facility (COSmIC) that was developed for this study and discuss the findings resulting from the comparison of the laboratory data with high resolution, high S/N ratio astronomical observations. COSmIC combines a supersonic jet expansion with discharge plasma and cavity ringdown spectroscopy and provides experimental conditions that closely mimic the interstellar conditions. The column densities of the individual PAH molecules and ions probed in these surveys are derived from the comparison of the laboratory data with high resolution, high S/N ratio astronomical observations. The comparisons of astronomical and laboratory data lead to clear conclusions regarding the expected abundances for PAHs in the interstellar environments probed in the surveys. Band profile comparisons between laboratory and astronomical spectra lead to information regarding the molecular structures and characteristics associated with the DIB carriers in the corresponding lines-of-sight. These quantitative surveys of neutral and ionized PAHs in the optical range open the way for quantitative searches of PAHs and complex organics in a variety of interstellar and circumstellar environments. Acknowledgements: F.S. acknowledges the support of the Astrophysics Research and Analysis Program of the NASA Space Mission Directorate and the technical support provided by R. Walker at NASA ARC. J.K. acknowledges the financial support of the Polish State. The authors are deeply grateful to the ESO archive as well as to the ESO staff members for their active support.

Time-dependent interstellar chemistry

NASA Technical Reports Server (NTRS)

Glassgold, A. E.

1985-01-01

Some current problems in interstellar chemistry are considered in the context of time-dependent calculations. The limitations of steady-state models of interstellar gas-phase chemistry are discussed, and attempts to chemically date interstellar clouds are reviewed. The importance of studying the physical and chemical properties of interstellar dust is emphasized. Finally, the results of a series of studies of collapsing clouds are described.

Interstellar clouds - From a dynamical perspective on their chemistry

NASA Technical Reports Server (NTRS)

Prasad, S. S.

1985-01-01

The possibility is examined that in the course of its dynamical evolution, a single mass of interstellar gas would exhibit properties of diffuse clouds, dense clouds and finally also of clouds perturbed by shocks or intense UV or X-ray radiation generated by a star of its own creation. This concept provides a common thread through the bewildering diversity of physical and chemical compositional properties shown by interstellar clouds. From this perspective, instead of being static objects, interstellar clouds are possibly incessantly evolving from initially diffuse to later dense state and then to star formation which ultimately restructures or disperses the remaining cloud material to begin the whole evolutionary process once again. Based on a simplified study of interstellar chemistry from a dynamical perspective, the ideas are presented as an heuristic: to encourage thought on the future direction of molecular astrophysics and the need to consider the chemical behavior of interstellar clouds in conjunction with, rather than in isolation from, their dynamical behavior. A physical basis must be sought for the semiempirical temperature formula which has been given a critical role in the collapse of diffuse clouds. Self-shielding effects in the chemistry of CO were neglected and this drawback should be removed; the ability of the model to explain the fractional abundances of more complex molecules, such as cyanopolyynes, should be examined.

ESO Diffuse Interstellar Bands Large Exploration Survey (EDIBLES) - Merging Observations and Laboratory Data

NASA Technical Reports Server (NTRS)

Salama, Farid

2016-01-01

The Diffuse Interstellar Bands (DIBs) are a set of 500 absorption bands that are detected in the spectra of stars with interstellar clouds in the line of sight. DIBs are found from the NUV to the NIR in the spectra of reddened stars spanning different interstellar environments in our local, and in other galaxies. DIB carriers are a significant part of the interstellar chemical inventory. They are stable and ubiquitous in a broad variety of environments and play a unique role in interstellar physics/chemistry. It has long been realized that the solving of the DIB problem requires a strong synergy between astronomical observations, laboratory astrophysics, and astrophysical modeling of line-of-sights. PAHs are among the molecular species that have been proposed as DIB carriers. We will present an assessment of the PAH-DIB model in view of the progress and the advances that have been achieved over the past years through a series of studies involving astronomical observations of DIBs, laboratory simulation of interstellar analogs for neutrals and ionized PAHs, theoretical calculations of PAH spectra and the modelization of diffuse and translucent interstellar clouds. We will present a summary of what has been learned from these complementary studies, the constraints that can now be derived for the PAHs as DIB carriers in the context of the PAH-DIB model and how these constraints can be applied to the EDIBLES project. The spectra of several neutral and ionized PAHs isolated in the gas phase at low temperature have been measured in the laboratory under experimental conditions that mimic interstellar conditions and are compared with an extensive set of astronomical spectra of reddened, early type stars. The comparisons of astronomical and laboratory data provide upper limits for the abundances of specific neutral PAH molecules and ions along specific lines-of-sight. Something that is not attainable from infrared observations alone. We present the characteristics of the laboratory facilities, MIS and COSmIC, that have been developed for this study and discuss the findings resulting from the comparison of the laboratory data with high resolution, high S/N ratio astronomical observations. MIS stands for Matrix Isolation Spectroscopy, a well-proven technique for isolating cold molecular species in inert solid environments. COSmIC stands for Cosmic Simulation Chamber. It combines a supersonic free jet expansion with discharge plasma and high-sensitivity cavity ringdown spectroscopy and time-of-flight mass spectrometry detection tools for the generation and the detection of cold, isolated gas-phase molecules and ions under experimental conditions that closely mimic interstellar conditions. The column densities of the individual neutral PAH molecules and ions probed in these surveys are derived from the comparison of these unique laboratory data with high resolution, high S/N ratio astronomical observations. The comparisons of astronomical and laboratory data lead to clear and unambiguous conclusions regarding the expected abundances for PAHs of various sizes and charge states in the interstellar environments probed in the surveys. Band profile comparisons between laboratory and astronomical spectra lead to information regarding the molecular structures and characteristics associated with the DIB carriers in the corresponding lines-of-sight. These quantitative surveys of neutral and ionized PAHs in the optical range open the way for unambiguous quantitative searches of PAHs and complex organics in a variety of interstellar and circumstellar environments.

Interstellar molecules and dense clouds.

NASA Technical Reports Server (NTRS)

Rank, D. M.; Townes, C. H.; Welch, W. J.

1971-01-01

Current knowledge of the interstellar medium is discussed on the basis of recent published studies. The subjects considered include optical identification of interstellar molecules, radio molecular lines, interstellar clouds, isotopic abundances, formation and disappearance of interstellar molecules, and interstellar probing techniques. Diagrams are plotted for the distribution of galactic sources exhibiting molecular lines, for hydrogen molecule, hydrogen atom and electron abundances due to ionization, for the densities, velocities and temperature of NH3 in the direction of Sagitarius B2, for the lower rotational energy levels of H2CO, and for temporal spectral variations in masing H2O clouds of the radio source W49. Future applications of the maser and of molecular microscopy in this field are visualized.

Boundary Conditions for the Paleoenvironment: Chemical and Physical Processes in the Pre-Solar Nebula

NASA Technical Reports Server (NTRS)

Irvine, William M.; Schloerb, F. Peter

1997-01-01

The basic theme of this program is the study of molecular complexity and evolution in interstellar clouds and in primitive solar system objects. Research has included the detection and study of a number of new interstellar molecules and investigation of reaction pathways for astrochemistry from a comparison of theory and observed molecular abundances. The latter includes studies of cold, dark clouds in which ion-molecule chemistry should predominate, searches for the effects of interchange of material between the gas and solid phases in interstellar clouds, unbiased spectral surveys of particular sources, and systematic investigation of the interlinked chemistry and physics of dense interstellar clouds. In addition, the study of comets has allowed a comparison between the chemistry of such minimally thermally processed objects and that of interstellar clouds, shedding light on the evolution of the biogenic elements during the process of solar system formation.

Molecules in interstellar clouds. [physical and chemical conditions of star formation and biological evolution

NASA Technical Reports Server (NTRS)

Irvine, W. M.; Hjalmarson, A.; Rydbeck, O. E. H.

1981-01-01

The physical conditions and chemical compositions of the gas in interstellar clouds are reviewed in light of the importance of interstellar clouds for star formation and the origin of life. The Orion A region is discussed as an example of a giant molecular cloud where massive stars are being formed, and it is pointed out that conditions in the core of the cloud, with a kinetic temperature of about 75 K and a density of 100,000-1,000,000 molecules/cu cm, may support gas phase ion-molecule chemistry. The Taurus Molecular Clouds are then considered as examples of cold, dark, relatively dense interstellar clouds which may be the birthplaces of solar-type stars and which have been found to contain the heaviest interstellar molecules yet discovered. The molecular species identified in each of these regions are tabulated, including such building blocks of biological monomers as H2O, NH3, H2CO, CO, H2S, CH3CN and H2, and more complex species such as HCOOCH3 and CH3CH2CN.

Magnetic seismology of interstellar gas clouds: Unveiling a hidden dimension.

PubMed

Tritsis, Aris; Tassis, Konstantinos

2018-05-11

Stars and planets are formed inside dense interstellar molecular clouds by processes imprinted on the three-dimensional (3D) morphology of the clouds. Determining the 3D structure of interstellar clouds remains challenging because of projection effects and difficulties measuring the extent of the clouds along the line of sight. We report the detection of normal vibrational modes in the isolated interstellar cloud Musca, allowing determination of the 3D physical dimensions of the cloud. We found that Musca is vibrating globally, with the characteristic modes of a sheet viewed edge on, not the characteristics of a filament as previously supposed. We reconstructed the physical properties of Musca through 3D magnetohydrodynamic simulations, reproducing the observed normal modes and confirming a sheetlike morphology. Copyright © 2018 The Authors, some rights reserved; exclusive licensee American Association for the Advancement of Science. No claim to original U.S. Government Works.

Boundary conditions for the paleoenvironment: Chemical and physical processes in the pre-solar nebula

NASA Technical Reports Server (NTRS)

Irvine, William M.; Schloerb, F. Peter

1987-01-01

Detailed study of the first interstellar hydrocarbon ring, cyclopropenylidene (C3H2), is continuing. The singly deuterated isotope of this molecule, C3HD, was observed in several cold interstellar clouds. The results of a large survey for C3H2 in galactic sources of various types will soon be completed. It appears that cyclopropenylidene is present in virtually all interstellar clouds of at least moderate density. In order to make the first determinations of the CO2/CO abundance ratio in interstellar sources, observations of protonated CO2 were pursued. The spectrum from 18.5 to 22 GHz for several interstellar clouds is being systematically measured. Particular attention is being given to the cold, dark clouds TMC-1 and L124N, which may be formation sites for solar mass stars. The phenomena of maser emission from molecules of methanol is being studied in certain interstellar clouds. A comparison of 1 millimeter continuum emission from dust with the column density of carbon monoxide as determined from the rare C(18)O isotope for 4 molecular clouds in the Galaxy is nearing completion. Papers published during the period of this report are listed.

Comets, carbonaceous chondrites, and interstellar clouds: Condensation of carbon

NASA Technical Reports Server (NTRS)

Field, G. B.

1979-01-01

Comets, carbonaceous chondrites, and interstellar clouds are discussed in relation to information on interstellar dust. The formation and presence of carbon in stars, comets, and meteorites is investigated. The existence of graphite in the interstellar medium, though it is predicted from thermodynamic calculations, is questioned and the form of carbon contained in comets is considered.

Morphology and ionization of the interstellar cloud surrounding the solar system.

PubMed

Frisch, P C

1994-09-02

The first encounter between the sun and the surrounding interstellar cloud appears to have occurred 2000 to 8000 years ago. The sun and cloud space motions are nearly perpendicular, an indication that the sun is skimming the cloud surface. The electron density derived for the surrounding cloud from the carbon component of the anomalous cosmic ray population in the solar system and from the interstellar ratio of Mg(+) to Mg degrees toward Sirius support an equilibrium model for cloud ionization (an electron density of 0.22 to 0.44 per cubic centimeter). The upwind magnetic field direction is nearly parallel to the cloud surface. The relative sun-cloud motion indicates that the solar system has a bow shock.

EVALUATING THE MORPHOLOGY OF THE LOCAL INTERSTELLAR MEDIUM: USING NEW DATA TO DISTINGUISH BETWEEN MULTIPLE DISCRETE CLOUDS AND A CONTINUOUS MEDIUM

DOE Office of Scientific and Technical Information (OSTI.GOV)

Redfield, Seth; Linsky, Jeffrey L., E-mail: sredfield@wesleyan.edu, E-mail: jlinsky@jila.colorado.edu

Ultraviolet and optical spectra of interstellar gas along the lines of sight to nearby stars have been interpreted by Redfield and Linsky and previous studies as a set of discrete warm, partially ionized clouds each with a different flow vector, temperature, and metal depletion. Recently, Gry and Jenkins proposed a fundamentally different model consisting of a single cloud with nonrigid flows filling space out to 9 pc from the Sun that they propose better describes the local ISM. Here we test these fundamentally different morphological models against the spatially unbiased Malamut et al. spectroscopic data set, and find that themore » multiple cloud morphology model provides a better fit to both the new and old data sets. The detection of three or more velocity components along the lines of sight to many nearby stars, the presence of nearby scattering screens, the observed thin elongated structures of warm interstellar gas, and the likely presence of strong interstellar magnetic fields also support the multiple cloud model. The detection and identification of intercloud gas and the measurement of neutral hydrogen density in clouds beyond the Local Interstellar Cloud could provide future morphological tests.« less

The interstellar N2 abundance towards HD 124314 from far-ultraviolet observations.

PubMed

Knauth, David C; Andersson, B-G; McCandliss, Stephan R; Moos, H Warren

2004-06-10

The abundance of interstellar molecular nitrogen (N2) is of considerable importance: models of steady-state gas-phase interstellar chemistry, together with millimetre-wavelength observations of interstellar N2H+ in dense molecular clouds predict that N2 should be the most abundant nitrogen-bearing molecule in the interstellar medium. Previous attempts to detect N2 absorption in the far-ultraviolet or infrared (ice features) have hitherto been unsuccessful. Here we report the detection of interstellar N2 at far-ultraviolet wavelengths towards the moderately reddened star HD 124314 in the constellation of Centaurus. The N2 column density is larger than expected from models of diffuse clouds and significantly smaller than expected for dense molecular clouds. Moreover, the N2 abundance does not explain the observed variations in the abundance of atomic nitrogen (N I) towards high-column-density sightlines, implying that the models of nitrogen chemistry in the interstellar medium are incomplete.

The state of clouds in a violent interstellar medium

NASA Astrophysics Data System (ADS)

Heathcote, S. R.; Brand, P. W. J. L.

1983-04-01

A highly approximate but simple model is developed which describes the interaction of a supernova blast wave with an interstellar cloud. The behavior of a cloud when exposed to conditions prevalent in a violent interstellar medium is examined using this model. Results show that after a cloud has been shocked it is rarely allowed sufficient time to return to pressure equilibrium with its surroundings before encountering a second shock. Thus, significant departures from pressure equilibrium are inevitable. It is determined that the disruption of a cloud by its passage through a blast wave is quite effective and the half life of clouds cannot greatly exceed the mean interval between shocks striking a given cloud. In addition, it is found that composite core-envelope clouds are not viable under typical conditions.

Interaction of a supernova shock with two interstellar clouds

NASA Astrophysics Data System (ADS)

Hansen, J. F.; McKee, C. F.

2005-10-01

The interaction of supernova shocks and interstellar clouds is an important astrophysical phenomenon since it can result in stellar and planetary formation. Our experiments attempt to simulate this mass-loading as it occurs when a shock passes through interstellar clouds. We drive a strong shock using a 5 kJ laser into a foam-filled cylinder with embedded Al spheres (diameter D=120 μm) simulating interstellar clouds. The density ratio between Al and foam is ˜9. We have previously reported on the interaction between shock and a single cloud, and the ensuing Kelvin-Helmholtz and Widnall instabilities. We now report on experiments under way in which two clouds are placed side by side. Cloud separation (center to center) is either 1.2xD or 1.5xD. Initial results for 1.2xD show that cloud material merges and travels further downstream than in the single cloud case. For 1.5xD, material does not merge, but the clouds tilt toward each other. Work performed under the auspices of the Department of Energy by the Lawrence Livermore National Laboratory under contract number W-7405-ENG-48.

Newly detected molecules in dense interstellar clouds

NASA Astrophysics Data System (ADS)

Irvine, William M.; Avery, L. W.; Friberg, P.; Matthews, H. E.; Ziurys, L. M.

Several new interstellar molecules have been identified including C2S, C3S, C5H, C6H and (probably) HC2CHO in the cold, dark cloud TMC-1; and the discovery of the first interstellar phosphorus-containing molecule, PN, in the Orion "plateau" source. Further results include the observations of 13C3H2 and C3HD, and the first detection of HCOOH (formic acid) in a cold cloud.

The Physics and Chemistry of Small Translucent Molecular Clouds. VIII. HCN and HNC

NASA Astrophysics Data System (ADS)

Turner, B. E.; Pirogov, L.; Minh, Y. C.

1997-07-01

We have conducted a survey of HCN and HNC (two rotational transitions each) in our standard sample of 11 cirrus cores and 27 Clemens-Barvainis translucent cores whose structures and chemistry have been studied earlier in this series. Both species are seen in all 38 objects. HCNH+ has been searched in three objects. These results are modeled in terms of our previous hydrostatic equilibrium and n ~ r-α structures together with other chemical and physical properties derived earlier. A detailed program has been written to handle the complex radiative transfer of the hyperfine splitting (hfs) of HCN. It is shown that serious errors are made in deriving HCN abundances by methods that ignore the hfs. Both HCN and HNC abundances are high, typically 1(-8) in most sources. The chemically important ratio HCN/HNC is found to be ~2.5 if these species are spatially centrally peaked and ~6 if not. Both species abundances increase monotonically with increasing extinction in the 1.2-2.7 mag range (edge to center), thus displaying the same characteristic transition between diffuse and dense cloud chemistry as do most other species we have studied. HCN/HNC decreases with increasing extinction to a value of 1.3 at Av0 ~ 10, approaching the expected value of 1.0 for dense clouds. Two types of ion-molecule chemistry models have been carried out: a full model using the Standard Model rate file and comprising 409 species (by Lee and Herbst), and a simplified model comprising 21 nitrogen-bearing species for conditions relevant to translucent clouds. Good agreement between observations and chemistry models is achieved throughout the translucent extinction range. Important conclusions are that (1) neutral-neutral reactions such as N + CH2 dominate the chemistry of HCN; (2) low ion-polar reaction rates are strongly favored over high ones; (3) the reaction C+ + NH3 --> H2NC+ --> HNC is unimportant, thus largely uncoupling the CN and NH chemistries; (4) the ratio HCN/HNC is not a particularly important diagnostic of the CN chemistry; (5) model NH3 abundances are at least a factor 100 lower than observed in translucent clouds, even if the reaction N+H+3-->NH+2 is permitted at Langevin rate.

Boundary Conditions for the Paleoenvironment: Chemical and Physical Processes in Dense Interstellar Clouds: Summary of Research

NASA Technical Reports Server (NTRS)

Irvine, William M.

1999-01-01

The basic theme of this program was the study of molecular complexity and evolution for the biogenic elements and compounds in interstellar clouds and in primitive solar system objects. Research included the detection and study of new interstellar and cometary molecules and investigation of reaction pathways for astrochemistry from a comparison of theory and observed molecular abundances. The latter includes studies of cold, dark clouds in which ion-molecule chemistry should predominate, searches for the effects of interchange of material between the gas and solid phases in interstellar clouds, unbiased spectral surveys of particular sources, and systematic investigation of the interlinked chemistry and physics of dense interstellar clouds. In addition, the study of comets has allowed a comparison between the chemistry of such minimally thermally processed objects and that of interstellar clouds, shedding light on the evolution of the biogenic elements during the process of solar system formation. One PhD dissertation on this research was completed by a graduate student at the University of Massachusetts. An additional 4 graduate students at the University of Massachusetts and 5 graduate students from other institutions participated in research supported by this grant, with 6 of these thus far receiving PhD degrees from the University of Massachusetts or their home institutions. Four postdoctoral research associates at the University of Massachusetts also participated in research supported by this grant, receiving valuable training.

Recent observations of organic molecules in nearby cold, dark interstellar clouds

NASA Technical Reports Server (NTRS)

Suzuki, H.; Ohishi, M.; Morimoto, M.; Kaifu, N.; Friberg, P.

1985-01-01

Recent investigations of the organic chemistry of relatively nearby cold, dark interstellar clouds are reported. Specifically, the presence of interstellar tricarbon monoxide (C3O) in Taurus Molecular Cloud 1 (TMC-1) is confirmed. The first detection in such regions of acetaldehyde (CH3CHO), the most complex oxygen-containing organic molecule yet found in dark clouds is reported, as well as the first astronomical detection of several molecular rotational transitions, including the J = 18-17 and 14-13 transitions of cyanodiacetylene (HC5N), the 1(01)-0(00) transition of acetaldehyde, and the J = 5-4 transition of C3O. A significant upper limit is set on the abundance of cyanocarbene (HCCN) as a result of the first reported interstellar search for this molecule.

A reanalysis of the HCO(+)/HOC(+) abundance ratio in dense interstellar clouds

NASA Technical Reports Server (NTRS)

Jarrold, M. F.; Bowers, M. T.; Defrees, D. J.; Mclean, A. D.; Herbst, E.

1986-01-01

New theoretical and experimental results have prompted a reinvestigation of the HCO(+)/HOC(+) abundance ratio in dense interstellar clouds. These results pertain principally but not exclusively to the reaction between HOC(+) and H2, which was previously calculated by DeFrees et al. (1984) to possess a large activation energy barrier. New calculations, reported here, indicate that this activation energy barrier is quite small and may well be zero. In addition, experimental results at higher energy and temperature indicate strongly that the reaction proceeds efficiently at interstellar temperatures. If HOC(+) does indeed react efficiently with H2 in interstellar clouds, the calculated HCO(+)/HOC(+) abundance ratio rises to a substantially greater value under standard dense cloud conditions than is deduced via the tentative observation of HOC(+) in Sgr B2.

Collisional excitation of molecules in dense interstellar clouds

NASA Technical Reports Server (NTRS)

Green, S.

1985-01-01

State transitions which permit the identification of the molecular species in dense interstellar clouds are reviewed, along with the techniques used to calculate the transition energies, the database on known molecular transitions and the accuracy of the values. The transition energies cannot be measured directly and therefore must be modeled analytically. Scattering theory is used to determine the intermolecular forces on the basis of quantum mechanics. The nuclear motions can also be modeled with classical mechanics. Sample rate constants are provided for molecular systems known to inhabit dense interstellar clouds. The values serve as a database for interpreting microwave and RF astrophysical data on the transitions undergone by interstellar molecules.

First Observation of the Submillimeter Polarization Spectrum in a Translucent Molecular Cloud

NASA Astrophysics Data System (ADS)

Ashton, Peter C.; Ade, Peter A. R.; Angilè, Francesco E.; Benton, Steven J.; Devlin, Mark J.; Dober, Bradley; Fissel, Laura M.; Fukui, Yasuo; Galitzki, Nicholas; Gandilo, Natalie N.; Klein, Jeffrey; Korotkov, Andrei L.; Li, Zhi-Yun; Martin, Peter G.; Matthews, Tristan G.; Moncelsi, Lorenzo; Nakamura, Fumitaka; Netterfield, Calvin B.; Novak, Giles; Pascale, Enzo; Poidevin, Frédérick; Santos, Fabio P.; Savini, Giorgio; Scott, Douglas; Shariff, Jamil A.; Soler, Juan D.; Thomas, Nicholas E.; Tucker, Carole E.; Tucker, Gregory S.; Ward-Thompson, Derek

2018-04-01

Polarized emission from aligned dust is a crucial tool for studies of magnetism in the ISM, but a troublesome contaminant for studies of cosmic microwave background polarization. In each case, an understanding of the significance of the polarization signal requires well-calibrated physical models of dust grains. Despite decades of progress in theory and observation, polarized dust models remain largely underconstrained. During its 2012 flight, the balloon-borne telescope BLASTPol obtained simultaneous broadband polarimetric maps of a translucent molecular cloud at 250, 350, and 500 μm. Combining these data with polarimetry from the Planck 850 μm band, we have produced a submillimeter polarization spectrum, the first for a cloud of this type. We find the polarization degree to be largely constant across the four bands. This result introduces a new observable with the potential to place strong empirical constraints on ISM dust polarization models in a previously inaccessible density regime. Compared to models by Draine & Fraisse, our result disfavors two of their models for which all polarization arises due only to aligned silicate grains. By creating simple models for polarized emission in a translucent cloud, we verify that extinction within the cloud should have only a small effect on the polarization spectrum shape, compared to the diffuse ISM. Thus, we expect the measured polarization spectrum to be a valid check on diffuse ISM dust models. The general flatness of the observed polarization spectrum suggests a challenge to models where temperature and alignment degree are strongly correlated across major dust components.

Copernicus observations of C I and CO in diffuse interstellar clouds

NASA Technical Reports Server (NTRS)

Jenkins, E. B.; Jura, M.; Loewenstein, M.

1980-01-01

Copernicus was used to observe absorption lines of C I in its ground state and excited fine structure levels and CO toward 29 stars. We use the C I data to infer densities and pressures within the observed clouds, and because our results are of higher precision than previous work, much more precise estimates of the physical conditions in clouds are obtained. In agreement with previous work, the interstellar thermal pressure appears to be variable, with most clouds having values of p/k between 1000/cu cm K and 10,000/cu cm K, but there are some clouds with p/k as high as 100,000/cu cm K. Our results are consistent with the view that the interstellar thermal pressure is so variable that the gas undergoes continuous dynamic evolution. Our observations provide useful constraints on the physical processes on the surfaces of grains. In particular, we find that grains are efficient catalysts of interstellar H2 in the sense that at least half of the hydrogen atoms that strike grains come off as part of H2. Results place strong constraints on models for the formation and destruction of interstellar CO. In many clouds, an order of magnitude less CO than predicted in some models was found.

Physical conditions in CaFe interstellar clouds

NASA Astrophysics Data System (ADS)

Gnaciński, P.; Krogulec, M.

2008-01-01

Interstellar clouds that exhibit strong Ca I and Fe I lines are called CaFe clouds. Ionisation equilibrium equations were used to model the column densities of Ca II, Ca I, K I, Na I, Fe I and Ti II in CaFe clouds. We find that the chemical composition of CaFe clouds is solar and that there is no depletion into dust grains. CaFe clouds have high electron densities, n_e≈1 cm-3, that lead to high column densities of neutral Ca and Fe.

A detailed investigation of proposed gas-phase syntheses of ammonia in dense interstellar clouds

NASA Technical Reports Server (NTRS)

Herbst, Eric; Defrees, D. J.; Mclean, A. D.

1987-01-01

The initial reactions of the Herbst and Klemperer (1973) and the Dalgarno (1974) schemes (I and II, respectively) for the gas-phase synthesis of ammonia in dense interstellar clouds were investigated. The rate of the slightly endothermic reaction between N(+) and H2 to yield NH(+) and H (scheme I) under interstellar conditions was reinvestigated under thermal and nonthermal conditions based on laboratory data. It was found that the relative importance of this reaction in synthesizing ammonia is determined by how the laboratory data at low temperature are interpreted. On the other hand, the exothermic reaction between N and H3(+) to form NH2(+) + H (scheme II) was calculated to possess significant activation energy and, therefore, to have a negligible rate coefficient under interstellar conditions. Consequently, this reaction cannot take place appreciably in interstellar clouds.

Optical observations of nearby interstellar gas

NASA Astrophysics Data System (ADS)

Frisch, P. C.; York, D. G.

1984-11-01

Observations indicated that a cloud with a heliocentric velocity of approximately -28 km/s and a hydrogen column density that possibly could be on the order of, or greater than, 5 x 10 to the 19 power/square cm is located within the nearest 50 to 80 parsecs in the direction of Ophiuchus. This is a surprisingly large column density of material for this distance range. The patchy nature of the absorption from the cloud indicates that it may not be a feature with uniform properties, but rather one with small scale structure which includes local enhancements in the column density. This cloud is probably associated with the interstellar cloud at about the same velocity in front of the 20 parsec distant star alpha Oph (Frisch 1981, Crutcher 1982), and the weak interstellar polarization found in stars as near as 35 parsecs in this general region (Tinbergen 1982). These data also indicate that some portion of the -14 km/s cloud also must lie within the 100 parsec region. Similar observations of both Na1 and Ca2 interstellar absorption features were performed in other lines of sight. Similar interstellar absorption features were found in a dozen stars between 20 and 100 parsecs of the Sun.

Optical Observations of Nearby Interstellar Gas

NASA Technical Reports Server (NTRS)

Frisch, P. C.; York, D. G.

1984-01-01

Observations indicated that a cloud with a heliocentric velocity of approximately -28 km/s and a hydrogen column density that possibly could be on the order of, or greater than, 5 x 10 to the 19 power/square cm is located within the nearest 50 to 80 parsecs in the direction of Ophiuchus. This is a surprisingly large column density of material for this distance range. The patchy nature of the absorption from the cloud indicates that it may not be a feature with uniform properties, but rather one with small scale structure which includes local enhancements in the column density. This cloud is probably associated with the interstellar cloud at about the same velocity in front of the 20 parsec distant star alpha Oph (Frisch 1981, Crutcher 1982), and the weak interstellar polarization found in stars as near as 35 parsecs in this general region (Tinbergen 1982). These data also indicate that some portion of the -14 km/s cloud also must lie within the 100 parsec region. Similar observations of both Na1 and Ca2 interstellar absorption features were performed in other lines of sight. Similar interstellar absorption features were found in a dozen stars between 20 and 100 parsecs of the Sun.

Large Interstellar Polarisation Survey. II. UV/optical study of cloud-to-cloud variations of dust in the diffuse ISM

NASA Astrophysics Data System (ADS)

Siebenmorgen, R.; Voshchinnikov, N. V.; Bagnulo, S.; Cox, N. L. J.; Cami, J.; Peest, C.

2018-03-01

It is well known that the dust properties of the diffuse interstellar medium exhibit variations towards different sight-lines on a large scale. We have investigated the variability of the dust characteristics on a small scale, and from cloud-to-cloud. We use low-resolution spectro-polarimetric data obtained in the context of the Large Interstellar Polarisation Survey (LIPS) towards 59 sight-lines in the Southern Hemisphere, and we fit these data using a dust model composed of silicate and carbon particles with sizes from the molecular to the sub-micrometre domain. Large (≥6 nm) silicates of prolate shape account for the observed polarisation. For 32 sight-lines we complement our data set with UVES archive high-resolution spectra, which enable us to establish the presence of single-cloud or multiple-clouds towards individual sight-lines. We find that the majority of these 35 sight-lines intersect two or more clouds, while eight of them are dominated by a single absorbing cloud. We confirm several correlations between extinction and parameters of the Serkowski law with dust parameters, but we also find previously undetected correlations between these parameters that are valid only in single-cloud sight-lines. We find that interstellar polarisation from multiple-clouds is smaller than from single-cloud sight-lines, showing that the presence of a second or more clouds depolarises the incoming radiation. We find large variations of the dust characteristics from cloud-to-cloud. However, when we average a sufficiently large number of clouds in single-cloud or multiple-cloud sight-lines, we always retrieve similar mean dust parameters. The typical dust abundances of the single-cloud cases are [C]/[H] = 92 ppm and [Si]/[H] = 20 ppm.

Composition, structure and chemistry of interstellar dust

NASA Technical Reports Server (NTRS)

Tielens, Alexander G. G. M.; Allamandola, Louis J.

1986-01-01

The observational constraints on the composition of the interstellar dust are analyzed. The dust in the diffuse interstellar medium consists of a mixture of stardust (amorphous silicates, amorphous carbon, polycyclic aromatic hydrocarbons, and graphite) and interstellar medium dust (organic refractory material). Stardust seems to dominate in the local diffuse interstellar medium. Inside molecular clouds, however, icy grain mantles are also important. The structural differences between crystalline and amorphous materials, which lead to differences in the optical properties, are discussed. The astrophysical consequences are briefly examined. The physical principles of grain surface chemistry are discussed and applied to the formation of molecular hydrogen and icy grain mantles inside dense molecular clouds. Transformation of these icy grain mantles into the organic refractory dust component observed in the diffuse interstellar medium requires ultraviolet sources inside molecular clouds as well as radical diffusion promoted by transient heating of the mantle. The latter process also returns a considerable fraction of the molecules in the grain mantle to the gas phase.

A search for pre-main sequence stars in the high-latitude molecular clouds. II - A survey of the Einstein database

NASA Technical Reports Server (NTRS)

Caillault, Jean-Pierre; Magnani, Loris

1990-01-01

The preliminary results are reported of a survey of every EINSTEIN image which overlaps any high-latitude molecular cloud in a search for X-ray emitting pre-main sequence stars. This survey, together with complementary KPNO and IRAS data, will allow the determination of how prevalent low mass star formation is in these clouds in general and, particularly, in the translucent molecular clouds.

Evaluating the Morphology of the Local Interstellar Medium: Using New Data to Distinguish between Multiple Discrete Clouds and a Continuous Medium

NASA Astrophysics Data System (ADS)

Redfield, Seth; Linsky, Jeffrey L.

2015-10-01

Ultraviolet and optical spectra of interstellar gas along the lines of sight to nearby stars have been interpreted by Redfield & Linsky and previous studies as a set of discrete warm, partially ionized clouds each with a different flow vector, temperature, and metal depletion. Recently, Gry & Jenkins proposed a fundamentally different model consisting of a single cloud with nonrigid flows filling space out to 9 pc from the Sun that they propose better describes the local ISM. Here we test these fundamentally different morphological models against the spatially unbiased Malamut et al. spectroscopic data set, and find that the multiple cloud morphology model provides a better fit to both the new and old data sets. The detection of three or more velocity components along the lines of sight to many nearby stars, the presence of nearby scattering screens, the observed thin elongated structures of warm interstellar gas, and the likely presence of strong interstellar magnetic fields also support the multiple cloud model. The detection and identification of intercloud gas and the measurement of neutral hydrogen density in clouds beyond the Local Interstellar Cloud could provide future morphological tests. Based on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Data Archive at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS AR-09525.01A. These observations are associated with programs #11568.

Chemical Complexity in Local Diffuse and Translucent Clouds: Ubiquitous Linear C3H and CH3CN, a Detection of HC3N and an Upper Limit on the Abundance of CH2CN

NASA Astrophysics Data System (ADS)

Liszt, Harvey; Gerin, Maryvonne; Beasley, Anthony; Pety, Jerome

2018-04-01

We present Jansky Very Large Array observations of 20–37 GHz absorption lines from nearby Galactic diffuse molecular gas seen against four cosmologically distant compact radio continuum sources. The main new observational results are that l-C3H and CH3CN are ubiqitous in the local diffuse molecular interstellar medium at {\\text{}}{A}{{V}} ≲ 1, while HC3N was seen only toward B0415 at {\\text{}}{A}{{V}} > 4 mag. The linear/cyclic ratio is much larger in C3H than in C3H2 and the ratio CH3CN/HCN is enhanced compared to TMC-1, although not as much as toward the Horsehead Nebula. More consequentially, this work completes a long-term program assessing the abundances of small hydrocarbons (CH, C2H, linear and cyclic C3H and C3 {{{H}}}2, and C4H and C4H‑) and the CN-bearing species (CN, HCN, HNC, HC3N, HC5N, and CH3CN): their systematics in diffuse molecular gas are presented in detail here. We also observed but did not strongly constrain the abundances of a few oxygen-bearing species, most prominently HNCO. We set limits on the column density of CH2CN, such that the anion CH2CN‑ is only viable as a carrier of diffuse interstellar bands if the N(CH2CN)/N(CH2CN‑) abundance ratio is much smaller in this species than in any others for which the anion has been observed. We argue that complex organic molecules (COMS) are not present in clouds meeting a reasonable definition of diffuse molecular gas, i.e., {\\text{}}{A}{{V}} ≲ 1 mag. Based on observations obtained with the NRAO Jansky Very Large Array (VLA).

Interstellar gas in the Gum Nebula

NASA Technical Reports Server (NTRS)

Wallerstein, G.; Jenkins, E. B.; Silk, J.

1980-01-01

A survey of the interstellar gas near the Gum Nebula by optical observation of 67 stars at Ca II, 42 stars at Na I, and 14 stars in the UV with the Copernicus satellite provided radial velocities and column densities for all resolved absorption components. Velocity dispersions for gas in the Gum Nebula are not significantly larger than in the general interstellar medium; the ionization structure is predominantly that of an H II region with moderately high ionization. Denser, more highly ionized clouds are concentrated toward the Gum Nebula; these clouds do not show the anomalously high ionization observed in the Vela remnant clouds.

The physics of interstellar shock waves

NASA Technical Reports Server (NTRS)

Shull, J. Michael; Draine, Bruce T.

1987-01-01

This review discusses the observations and theoretical models of interstellar shock waves, in both diffuse cloud and molecular cloud environments. It summarizes the relevant gas dynamics, atomic, molecular and grain processes, radiative transfer, and physics of radiative and magnetic precursors in shock models. It then describes the importance of shocks for observations, diagnostics, and global interstellar dynamics. It concludes with current research problems and data needs for atomic, molecular and grain physics.

DESPOTIC - a new software library to Derive the Energetics and SPectra of Optically Thick Interstellar Clouds

NASA Astrophysics Data System (ADS)

Krumholz, Mark R.

2014-01-01

I describe DESPOTIC, a code to Derive the Energetics and SPectra of Optically Thick Interstellar Clouds. DESPOTIC represents such clouds using a one-zone model, and can calculate line luminosities, line cooling rates, and in restricted cases line profiles using an escape probability formalism. It also includes approximate treatments of the dominant heating, cooling and chemical processes for the cold interstellar medium, including cosmic ray and X-ray heating, grain photoelectric heating, heating of the dust by infrared and ultraviolet radiation, thermal cooling of the dust, collisional energy exchange between dust and gas, and a simple network for carbon chemistry. Based on these heating, cooling and chemical rates, DESPOTIC can calculate clouds' equilibrium gas and dust temperatures, equilibrium carbon chemical state and time-dependent thermal and chemical evolution. The software is intended to allow rapid and interactive calculation of clouds' characteristic temperatures, identification of their dominant heating and cooling mechanisms and prediction of their observable spectra across a wide range of interstellar environments. DESPOTIC is implemented as a PYTHON package, and is released under the GNU General Public License.

Photodissociation Regions in the Interstellar Medium of Galaxies

NASA Technical Reports Server (NTRS)

Hollenbach, David J.; Tielens, A. G. G. M.; DeVincenzi, Donald L. (Technical Monitor)

1999-01-01

The interstellar medium of galaxies is the reservoir out of which stars are born and into which stars inject newly created elements as they age. The physical properties of the interstellar medium are governed in part by the radiation emitted by these stars. Far-ultraviolet (6 eV less than h(nu) less than 13.6 eV) photons from massive stars dominate the heating and influence the chemistry of the neutral atomic gas and much of the molecular gas in galaxies. Predominantly neutral regions of the interstellar medium in which the heating and chemistry are regulated by far ultraviolet photons are termed Photo-Dissociation Regions (PDRs). These regions are the origin of most of the non-stellar infrared (IR) and the millimeter and submillimeter CO emission from galaxies. The importance of PDRs has become increasingly apparent with advances in IR and submillimeter astronomy. The IR emission from PDRs includes fine structure lines of C, C+, and O; rovibrational lines of H2, rotational lines of CO; broad middle features of polycyclic aromatic hydrocarbons; and a luminous underlying IR continuum from interstellar dust. The transition of H to H2 and C+ to CO occurs within PDRs. Comparison of observations with theoretical models of PDRs enables one to determine the density and temperature structure, the elemental abundances, the level of ionization, and the radiation field. PDR models have been applied to interstellar clouds near massive stars, planetary nebulae, red giant outflows, photoevaporating planetary disks around newly formed stars, diffuse clouds, the neutral intercloud medium, and molecular clouds in the interstellar radiation field-in summary, much of the interstellar medium in galaxies. Theoretical PDR models explain the observed correlations of the [CII] 158 microns with the COJ = 1-0 emission, the COJ = 1-0 luminosity with the interstellar molecular mass, and the [CII] 158 microns plus [OI] 63 microns luminosity with the IR continuum luminosity. On a more global scale, MR models predict the existence of two stable neutral phases of the interstellar medium, elucidate the formation and destruction of star-forming molecular clouds, and suggest radiation-induced feedback mechanisms that may regulate star formation rates and the column density of gas through giant molecular clouds.

Diamonds in dense molecular clouds - A challenge to the standard interstellar medium paradigm

NASA Technical Reports Server (NTRS)

Allamandola, L. J.; Sandford, S. A.; Tielens, A. G. G. M.; Herbst, T. M.

1993-01-01

Observations of a newly discovered infrared C-H stretching band indicate that interstellar diamond-like material appears to be characteristic of dense clouds. In sharp contrast, the spectral signature of dust in the diffuse interstellar medium is dominated by -CH2- and -CH3 groups. This dichotomy in the aliphatic organic component between the dense and diffuse media challenges standard assumptions about the processes occurring in, and interactions between, these two media. The ubiquity of this interstellar diamond-like material rules out models for meteoritic diamond formation in unusual circumstellar environments and implies that the formation of the diamond-like material is associated with common interstellar processes or stellar types.

A new interstellar molecule - Tricarbon monoxide

NASA Technical Reports Server (NTRS)

Matthews, H. E.; Irvine, W. M.; Friberg, P.; Brown, R. D.; Godfrey, P. D.

1984-01-01

The C3O molecule, whose pure rotational spectrum has only recently been studied in the laboratory, has been detected in the cold, dark interstellar Taurus Molecular Cloud 1. Since C3O is the first interstelar carbon chain molecule to contain oxygen, its existence places an important new constraint on chemical schemes for cold interstellar clouds. The abundance of C3O can be understood in terms of purely gas-phase ion-molecule chemistry.

CaFe interstellar clouds

NASA Astrophysics Data System (ADS)

Bondar, A.; Kozak, M.; Gnaciński, P.; Galazutdinov, G. A.; Beletsky, Y.; Krełowski, J.

2007-07-01

A new kind of interstellar cloud is proposed. These are rare (just a few examples among ~300 lines of sight) objects with the CaI 4227-Å, FeI 3720-Å and 3860-Å lines stronger than those of KI (near 7699 Å) and NaI (near 3302 Å). We propose the name `CaFe' for these clouds. Apparently they occupy different volumes from the well-known interstellar HI clouds where the KI and ultraviolet NaI lines are dominant features. In the CaFe clouds we have not found either detectable molecular features (CH, CN) or diffuse interstellar bands which, as commonly believed, are carried by some complex, organic molecules. We have found the CaFe clouds only along sightlines toward hot, luminous (and thus distant) objects with high rates of mass loss. In principle, the observed gas-phase interstellar abundances reflect the combined effects of the nucleosynthetic history of the material, the depletion of heavy elements into dust grains and the ionization state of these elements which may depend on irradiation by neighbouring stars. Based on data collected using the Maestro spectrograph at the Terskol 2-m telescope, Russia; and on data collected using the ESO Feros spectrograph; and on data obtained from the ESO Science Archive Facility acquired with the UVES spectrograph, Chile. E-mail: `arctur'@rambler.ru (AB); marizak@astri.uni.torun.pl (MK); pg@iftia.univ.gda.pl (PG); gala@boao.re.kr (GAG); ybialets@eso.org (YB); jacek@astri.uni.torun.pl (JK)

New Insights Concerning the Local Interstellar medium

NASA Astrophysics Data System (ADS)

Linsky, Jeffrey L.; Redfield, Seth

2015-08-01

We have been analyzing HST high-resolution ultraviolet spectra of nearby stars to measure the radial velocities, turbulence, temperature, and depletions on warm diffuse interstellar gas within a few parsecs of the Sun. These data reveal a picture of many partially-ionized warm gas clouds, each with their own vector velocity and physical characteristics. This picture has been recently challenged by Gry and Jenkins (2014), who argue for a single nonrigid cloud surrounding the Sun. We present a test of these two very different morphological structure by checking how well each predicts the radial velocities in a new data set (Malamut et al. 2014) that was not available when both models were constructed. We find that the multicloud model (Redfield & Linsky 2008) provides a much better fit to the new data. We compare the new IBEX results for the temperature and velocity of inflowing He gas (McComas et al. 2015) with the properties of the Local Interstellar Cloud and the G cloud. We also show a preliminary three-dimensional model for the local interstellar medium.

Interplay of dust alignment, grain growth, and magnetic fields in polarization: lessons from the emission-to-extinction ratio

NASA Astrophysics Data System (ADS)

Fanciullo, L.; Guillet, V.; Boulanger, F.; Jones, A. P.

2017-06-01

Context. Polarized extinction and emission from dust in the interstellar medium (ISM) are hard to interpret, as their dependence on dust optical properties, grain alignment, and magnetic field orientation is complex. This is particularly true in molecular clouds. The aforementioned phenomena are usually considered independently in polarization studies, while it is likely that they all contribute and their effects have yet to be disentangled. Aims: The data available today are not yet used to their full potential. The combination of emission and extinction, in particular, provides information not available from either of them alone. We combine data from the scientific literature on polarized dust extinction with Planck data on polarized emission, and we use them to constrain the possible variations in dust and environmental conditions inside molecular clouds, and especially translucent lines of sight, taking the magnetic field orientation into account. Methods: We focused on the dependence between λmax (the wavelength of maximum polarization in extinction) and other observables such as the extinction polarization, the emission polarization, and the ratio between the two. We set out to reproduce these correlations using Monte Carlo simulations in which we varied the relevant quantities in a dust model, which are grain alignment, size distribution, and magnetic field orientation, to mimic the diverse conditions that are expected inside molecular clouds. Results: None of the quantities we chose can explain the observational data on their own: the best results are obtained when all quantities vary significantly across and within clouds. However, some of the data, most notably the stars with a low ratio of polarization in emission to polarization in extinction, are not reproduced by our simulation. Conclusions: Our results suggest not only that dust evolution is necessary to explain polarization in molecular clouds, but that a simple change in size distribution is not sufficient to explain the data. Our results also point the way for future and more sophisticated models.

Silicon chemistry in interstellar clouds

NASA Technical Reports Server (NTRS)

Langer, William D.; Glassgold, A. E.

1990-01-01

A new model of interstellar silicon chemistry is presented that explains the lack of SiO detections in cold clouds and contains an exponential temperature dependence for the SiO abundance. A key aspect of the model is the sensitivity of SiO production by neutral silicon reactions to density and temperature, which arises from the dependence of the rate coefficients on the population of the excited fine-structure levels of the silicon atom. As part of the explanation of the lack of SiO detections at low temperatures and densities, the model also emphasizes the small efficiencies of the production routes and the correspondingly long times needed to reach equilibrium. Measurements of the abundance of SiO, in conjunction with theory, can provide information on the physical properties of interstellar clouds such as the abundance of oxygen bearing molecules and the depletion of interstellar silicon.

The impact of different interstellar medium structures on the dynamical evolution of supernova remnants

NASA Astrophysics Data System (ADS)

Wang, Yueyang; Bao, Biwen; Yang, Chuyuan; Zhang, Li

2018-05-01

The dynamical properties of supernova remnants (SNRs) evolving with different interstellar medium structures are investigated through performing extensive two-dimensional magnetohydrodynamic (MHD) simulations in the cylindrical symmetry. Three cases of different interstellar medium structures are considered: the uniform medium, the turbulent medium and the cloudy medium. Large-scale density and magnetic fluctuations are calculated and mapped into the computational domain before simulations. The clouds are set by random distribution in advance. The above configuration allows us to study the time-dependent dynamical properties and morphological evolution of the SNR evolving with different ambient structures, along with the development of the instabilities at the contact discontinuity. Our simulation results indicate that remnant morphology deviates from symmetry if the interstellar medium contains clouds or turbulent density fluctuations. In the cloudy medium case, interactions between the shock wave and clouds lead to clouds' fragmentation. The magnetic field can be greatly enhanced by stretching field lines with a combination of instabilities while the width of amplification region is quite different among the three cases. Moreover, both the width of amplification region and the maximum magnetic-field strength are closely related to the clouds' density.

The influence of Oort clouds on the mass and chemical balance of the interstellar medium

NASA Technical Reports Server (NTRS)

Stern, S. Alan; Shull, J. Michael

1990-01-01

The contribution of stellar encounters and interstellar erosion to comet cloud mass injection to the ISM is calculated. It is shown that evaporative mass loss from passing stars and SNe results in an average Galactic mass injection rate of up to 10 to the -5th solar mass/yr if such clouds are frequent around solar-type stars. Cometary erosion by interstellar grains produces an injection rate of 10 to the -5th to 10 to the -4th solar mass/yr. An injection rate of 2 x 10 to the -5th solar mass/yr is calculated. Each of these rates could be increased by a factor of about 15 if the comet clouds contain a significant amount of smaller debris. It is concluded that the total mass injection rate of material to the ISM by comet clouds is small compared to other ISM mass injection sources. Comet cloud mass loss to the ISM could be responsible for a sizeable fraction of the metal and dust abundances of the ISM if Oort clouds are common.

A cloud/particle model of the interstellar medium - Galactic spiral structure

NASA Technical Reports Server (NTRS)

Levinson, F. H.; Roberts, W. W., Jr.

1981-01-01

A cloud/particle model for gas flow in galaxies is developed that incorporates cloud-cloud collisions and supernovae as dominant local processes. Cloud-cloud collisions are the main means of dissipation. To counter this dissipation and maintain local dispersion, supernova explosions in the medium administer radial snowplow pushes to all nearby clouds. The causal link between these processes is that cloud-cloud collisions will form stars and that these stars will rapidly become supernovae. The cloud/particle model is tested and used to investigate the gas dynamics and spiral structures in galaxies where these assumptions may be reasonable. Particular attention is given to whether large-scale galactic shock waves, which are thought to underlie the regular well-delineated spiral structure in some galaxies, form and persist in a cloud-supernova dominated interstellar medium; this question is answered in the affirmative.

The determination of cloud masses and dust characteristics from submillimetre thermal emission

NASA Technical Reports Server (NTRS)

Hildebrand, R. H.

1983-01-01

The principles by which the dust and masses and total masses of interstellar clouds and certain characteristics of interstellar dust grains can be derived from observations of far infrared and submillimeter thermal emission are reviewed. To the extent possible, the discussion will be independent of particular grain models.

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

NASA Technical Reports Server (NTRS)

Liffman, Kurt

1990-01-01

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.

Theoretical studies of the extraterrestrial chemistry of biogenic elements and compounds

NASA Technical Reports Server (NTRS)

Defrees, D. J.

1991-01-01

Organic compounds, molecules related to those in living systems, are found in many different extraterrestrial environments. The study of organic astrochemistry is important to exobiology both because it demonstrates the ubiquity of processes which led to life on Earth and because the dust clouds where molecules are found are analogs of the solar nebula from which the Earth formed. In the long chain of events leading from the Big Bang, and a universe composed of atomic hydrogen and helium, to the emergence of life on Earth, molecular interstellar clouds are an early link, the most primitive objects which display any significant organic chemistry. One such cloud was the direct precursor to the solar system and to all objects which it contains. Theoretical methods are ideally suited to studying interstellar cloud chemistry. They have been applied to determine spectroscopic constants of candidate interstellar molecules, mechanisms of ion-molecule reactions, and composition of dust grains. Accurate predictions of rotational constants and dipole moments of long-chain carbon molecules HC13N, HC15N, and C5O have been made to aid in determining the size limit of gas-phase interstellar molecules. Models of gas-phase interstellar chemistry use reaction rate constants measured at room temperature and extrapolated to interstellar temperatures. The temperature dependence of NH3(+)+H2 yields NH4(+)+H is anomalous, however, with a minimum rate at about 100K, casting doubt on the extrapolation procedures. The temperature dependence has now been explained.

Influence of galactic arm scale dynamics on the molecular composition of the cold and dense ISM. I. Observed abundance gradients in dense clouds

NASA Astrophysics Data System (ADS)

Ruaud, M.; Wakelam, V.; Gratier, P.; Bonnell, I. A.

2018-04-01

Aim. We study the effect of large scale dynamics on the molecular composition of the dense interstellar medium during the transition between diffuse to dense clouds. Methods: We followed the formation of dense clouds (on sub-parsec scales) through the dynamics of the interstellar medium at galactic scales. We used results from smoothed particle hydrodynamics (SPH) simulations from which we extracted physical parameters that are used as inputs for our full gas-grain chemical model. In these simulations, the evolution of the interstellar matter is followed for 50 Myr. The warm low-density interstellar medium gas flows into spiral arms where orbit crowding produces the shock formation of dense clouds, which are held together temporarily by the external pressure. Results: We show that depending on the physical history of each SPH particle, the molecular composition of the modeled dense clouds presents a high dispersion in the computed abundances even if the local physical properties are similar. We find that carbon chains are the most affected species and show that these differences are directly connected to differences in (1) the electronic fraction, (2) the C/O ratio, and (3) the local physical conditions. We argue that differences in the dynamical evolution of the gas that formed dense clouds could account for the molecular diversity observed between and within these clouds. Conclusions: This study shows the importance of past physical conditions in establishing the chemical composition of the dense medium.

Recommended Rest Frequencies for Observed Interstellar Molecular Microwave Transitions - 2002 Revision

National Institute of Standards and Technology Data Gateway

SRD 116 NIST Recommended Rest Frequencies for Observed Interstellar Molecular Microwave Transitions - 2002 Revision (Web, free access)   Critically evaluated transition frequencies for the molecular transitions detected in interstellar and circumstellar clouds are presented.

Two γ-ray bursts from dusty regions with little molecular gas.

PubMed

Hatsukade, B; Ohta, K; Endo, A; Nakanishi, K; Tamura, Y; Hashimoto, T; Kohno, K

2014-06-12

Long-duration γ-ray bursts are associated with the explosions of massive stars and are accordingly expected to reside in star-forming regions with molecular gas (the fuel for star formation). Previous searches for carbon monoxide (CO), a tracer of molecular gas, in burst host galaxies did not detect any emission. Molecules have been detected as absorption in the spectra of γ-ray burst afterglows, and the molecular gas is similar to the translucent or diffuse molecular clouds of the Milky Way. Absorption lines probe the interstellar medium only along the line of sight, so it is not clear whether the molecular gas represents the general properties of the regions where the bursts occur. Here we report spatially resolved observations of CO line emission and millimetre-wavelength continuum emission in two galaxies hosting γ-ray bursts. The bursts happened in regions rich in dust, but not particularly rich in molecular gas. The ratio of molecular gas to dust (<9-14) is significantly lower than in star-forming regions of the Milky Way and nearby star-forming galaxies, suggesting that much of the dense gas where stars form has been dissipated by other massive stars.

Unusually high rotational temperature of the CN radical

NASA Astrophysics Data System (ADS)

Krełowski, J.; Galazutdinov, G.; Beletsky, Y.

2011-07-01

We analyse a high-resolution, high signal-to-noise spectrogram of the hot reddened star Trumpler 16 112 to find relationships between the physical parameters of the intervening interstellar medium (e.g., the rotational temperature of the CN radical) and the intensities of interstellar lines/bands. We report on the discovery of an interstellar cloud that shows an exceptionally high rotational temperature of CN (4.5 K) and unusually strong Ca I and Fe I interstellar lines. This rare CaFe-type cloud seemingly contains no diffuse band carriers. Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere, Chile. Programs 073.D-0609(A) and 082.C-0566(A).

Evolutionary models of interstellar chemistry

NASA Technical Reports Server (NTRS)

Prasad, Sheo S.

1987-01-01

The goal of evolutionary models of interstellar chemistry is to understand how interstellar clouds came to be the way they are, how they will change with time, and to place them in an evolutionary sequence with other celestial objects such as stars. An improved Mark II version of an earlier model of chemistry in dynamically evolving clouds is presented. The Mark II model suggests that the conventional elemental C/O ratio less than one can explain the observed abundances of CI and the nondetection of O2 in dense clouds. Coupled chemical-dynamical models seem to have the potential to generate many observable discriminators of the evolutionary tracks. This is exciting, because, in general, purely dynamical models do not yield enough verifiable discriminators of the predicted tracks.

Putting a Ring on it: Light Echoes from X-ray Transients as Probes of Interstellar Dust and Galactic Structure

NASA Astrophysics Data System (ADS)

Heinz, Sebastian

2017-09-01

When an X-ray transient exhibits a bright flare, scattering by interstellar dust clouds can give rise to a light echo in the form of concentric rings. To date, three such echoes have been detected, each leading to significant discoveries and press attention. We propose a Target-of-Opportunity campaign to observe future echoes with the aim to follow the temporal evolution of the echo in order to (a) map the 3D distribution interstellar dust along the line of sight to parsec accuracy, (b) constrain the composition and grain size distribution of ISM dust in each of the clouds towards the source, (c) measure the distance to the X-ray source, (d) constrain the velocity dispersion of molecular clouds and (e) search for evidence of streaming velocities by combing X-ray and CO data on the clouds.

Lower mass limit of an evolving interstellar cloud and chemistry in an evolving oscillatory cloud

NASA Technical Reports Server (NTRS)

Tarafdar, S. P.

1986-01-01

Simultaneous solution of the equation of motion, equation of state and energy equation including heating and cooling processes for interstellar medium gives for a collapsing cloud a lower mass limit which is significantly smaller than the Jeans mass for the same initial density. The clouds with higher mass than this limiting mass collapse whereas clouds with smaller than critical mass pass through a maximum central density giving apparently similar clouds (i.e., same Av, size and central density) at two different phases of its evolution (i.e., with different life time). Preliminary results of chemistry in such an evolving oscillatory cloud show significant difference in abundances of some of the molecules in two physically similar clouds with different life times. The problems of depletion and short life time of evolving clouds appear to be less severe in such an oscillatory cloud.

Goulds Belt, Interstellar Clouds, and the Eocene-Oligocene Helium-3 Spike

NASA Technical Reports Server (NTRS)

Rubincam, David Parry

2015-01-01

Drag from hydrogen in the interstellar cloud which formed Gould's Belt may have sent small meteoroids with embedded helium to the Earth, perhaps explaining part or all of the (sup 3) He spike seen in the sedimentary record at the Eocene-Oligocene transition. Assuming the Solar System passed through part of the cloud, meteoroids in the asteroid belt up to centimeter size may have been dragged to the resonances, where their orbital eccentricities were pumped up into Earth-crossing orbits.

Dust evolution, a global view I. Nanoparticles, nascence, nitrogen and natural selection … joining the dots

NASA Astrophysics Data System (ADS)

Jones, A. P.

2016-12-01

The role and importance of nanoparticles for interstellar chemistry and beyond is explored within the framework of The Heterogeneous dust Evolution Model for Interstellar Solids (THEMIS), focusing on their active surface chemistry, the effects of nitrogen doping and the natural selection of interesting nanoparticle sub-structures. Nanoparticle-driven chemistry, and in particular the role of intrinsic epoxide-type structures, could provide a viable route to the observed gas phase OH in tenuous interstellar clouds en route to becoming molecular clouds. The aromatic-rich moieties present in asphaltenes probably provide a viable model for the structures present within aromatic-rich interstellar carbonaceous grains. The observed doping of such nanoparticle structures with nitrogen, if also prevalent in interstellar dust, could perhaps have important and observable consequences for surface chemistry and the formation of precursor pre-biotic species.

Organic Chemistry in Interstellar Ices: Connection to the Comet Halley Results

NASA Technical Reports Server (NTRS)

Schutte, W. A.; Agarwal, V. K.; deGroot, M. S.; Greenberg, J. M.; McCain, P.; Ferris, J. P.; Briggs, R.

1997-01-01

Mass spectroscopic measurements on the gas and dust in the coma of Comet Halley revealed the presence of considerable amounts of organic species. Greenberg (1973) proposed that prior to the formation of the comet UV processing of the ice mantles on grains in dense clouds could lead to the formation of complex organic molecules. Theoretical predictions of the internal UV field in dense clouds as well as the discovery in interstellar ices of species like OCS and OCN- which have been formed in simulation experiments by photoprocessing of interstellar ice analogues point to the importance of such processing. We undertook a laboratory simulation study of the formation of organic molecules in interstellar ices and their possible relevance to the Comet Halley results.

Interstellar Dust: Contributed Papers

NASA Technical Reports Server (NTRS)

Tielens, Alexander G. G. M. (Editor); Allamandola, Louis J. (Editor)

1989-01-01

A coherent picture of the dust composition and its physical characteristics in the various phases of the interstellar medium was the central theme. Topics addressed included: dust in diffuse interstellar medium; overidentified infrared emission features; dust in dense clouds; dust in galaxies; optical properties of dust grains; interstellar dust models; interstellar dust and the solar system; dust formation and destruction; UV, visible, and IR observations of interstellar extinction; and quantum-statistical calculations of IR emission from highly vibrationally excited polycyclic aromatic hydrocarbon (PAH) molecules.

Analysis of ultraviolet spectrophotometric data from Copernicus

NASA Technical Reports Server (NTRS)

Snow, T. P., Jr.

1979-01-01

Ultraviolet spectral data from the OAO 3 satellite are being used to study interstellar absorption lines and stellar and circumstellar lines in hot stars. The interstellar data are beneficial in analyzing the depletions of heavy elements from the gas phase and in elucidating how these depletions depend on physical conditions. Abundances in separate velocity components were determined from line profiles. Observations were carried out for interstellar abundances, both atomic and molecular, towards a number of stars. The better quality data are being analyzed for profile information and the lesser data are being used in curve-of-growth analyses. Molecular observations were carried out as well, N2 was sought; interstellar C2 was detected and its rotational excitation utilized to establish limits in interstellar cloud temperatures. An extensive search for H2O resulted in a tentative identification which will produce new information on chemical reaction rates. Interstellar depletions and grain properties in the rho Ophiuchi cloud, stellar wind variability, and circumstellar lines are also under study.

Photometry and Classification of Stars around the Reflection Nebula NGC 7023 IN Cepheus. II. Interstellar Extinction and Cloud Distances

NASA Astrophysics Data System (ADS)

Zdanavičius, K.; Zdanavičius, J.; Straižys, V.; Maskoliūnas, M.

Interstellar extinction is investigated in a 1.5 square degree area in the direction of the reflection nebula NGC 7023 at ℓ = 104.1\\degr, b = +14.2\\degr. The study is based on photometric classification and the determination of interstellar extinctions and distances of 480 stars down to V = 16.5 mag from photometry in the Vilnius seven-color system published in Paper I (2008). The investigated area is divided into five smaller subareas with slightly different dependence of the extinction on distance. The distribution of reddened stars is in accordance with the presence of two dust clouds at 282 pc and 715 pc, however in some directions the dust distribution can be continuous or more clouds can be present.

Gas Phase Spectroscopy of Cold PAH Ions: Contribution to the Interstellar Extinction and the Diffuse Interstellar Bands

NASA Technical Reports Server (NTRS)

Biennier, L.; Salama, F.; Allamandola, L. J.; Scherer, J. J.; OKeefe, A.

2002-01-01

Polycyclic Aromatic Hydrocarbon molecules (PAHs) are ubiquitous in the interstellar medium (ISM) and constitute the building blocks of interstellar dust grains. Despite their inferred important role in mediating the energetic and chemical processes in thc ISM, their exact contribution to the interstellar extinction, and in particular to the diffuse interstellar bands (DIBs) remains unclear. The DIBs are spectral absorption features observed in the line of sight of stars that are obscured by diffuse interstellar clouds. More than 200 bands have been reported to date spanning from the near UV to the near IR with bandwidths ranging from 0.4 to 40 Angstroms (Tielens & Snow 1995). The present consensus is that the DIBs arise from free flying, gas-phase, organic molecules and/or ions that are abundant under the typical conditions reigning in the diffuse ISM. PAHs have been proposed as possible carriers (Allamandola et al. 1985; Leger & DHendecourt 1985). The PAH hypothesis is consistent with the cosmic abundance of Carbon and Hydrogen and with the required photostability of the DIB carriers against the strong VUV radiation field in the diffuse interstellar clouds. A significant fraction of PAHs is expected to be ionized in the diffuse ISM.

The interstellar line spectra of zeta Ophiuchi and zeta Persei and their relation to the short wavelength microwave background radiation. Ph.D. Thesis - N. Y. Univ.

NASA Technical Reports Server (NTRS)

Bortolot, V. J., Jr.

1972-01-01

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.

Chemistry and Evolution of Interstellar Clouds

NASA Technical Reports Server (NTRS)

Wooden, D. H.; Charnley, S. B.; Ehrenfreund, P.

2003-01-01

In this chapter we describe how elements have been and are still being formed in the galaxy and how they are transformed into the reservoir of materials present at the time of formation of our protosolar nebula. We discuss the global cycle of matter, beginning at its formation site in stars, where it is ejected through winds and explosions into the diffuse interstellar medium. In the next stage of the global cycle occurs in cold, dense molecular clouds, where the complexity of molecules and ices increases relative to the diffuse ISM.. When a protostar forms in a dense core within a molecular cloud, it heats the surrounding infalling matter warms and releases molecules from the solid phase into the gas phase in a warm, dense core, sponsoring a rich gas-phase chemistry. Some material from the cold and warm regions within molecular clouds probably survives as interstellar matter in the protostellar disk. For the diffuse ISM, for cold, dense clouds, and for dense-warm cores, the physio-chemical processes that occur within the gas and solid phases are discussed in detail.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Asahina, Yuta; Kawashima, Tomohisa; Furukawa, Naoko

The formation mechanism of CO clouds observed with the NANTEN2 and Mopra telescopes toward the stellar cluster Westerlund 2 is studied by 3D magnetohydrodynamic simulations, taking into account the interstellar cooling. These molecular clouds show a peculiar shape composed of an arc-shaped cloud on one side of the TeV γ -ray source HESS J1023-575 and a linear distribution of clouds (jet clouds) on the other side. We propose that these clouds are formed by the interaction of a jet with clumps of interstellar neutral hydrogen (H i). By studying the dependence of the shape of dense cold clouds formed bymore » shock compression and cooling on the filling factor of H i clumps, we found that the density distribution of H i clumps determines the shape of molecular clouds formed by the jet–cloud interaction: arc clouds are formed when the filling factor is large. On the other hand, when the filling factor is small, molecular clouds align with the jet. The jet propagates faster in models with small filling factors.« less

On the Surface Formation of NH3 and HNCO in Dark Molecular Clouds - Searching for Wöhler Synthesis in the Interstellar Medium

NASA Astrophysics Data System (ADS)

Fedoseev, Gleb; Lamberts, Thanja; Linnartz, Harold; Ioppolo, Sergio; Zhao, Dongfeng

Despite its potential to reveal the link between the formation of simple species and more complex molecules (e.g., amino acids), the nitrogen chemistry of the interstellar medium (ISM) is still poorly understood. Ammonia (NH _{3}) is one of the few nitrogen-bearing species that have been observed in interstellar ices toward young stellar objects (YSOs) and quiescent molecular clouds. The aim of the present work is to experimentally investigate surface formation routes of NH _{3} and HNCO through non-energetic surface reactions in interstellar ice analogues under fully controlled laboratory conditions and at astrochemically relevant cryogenic temperatures. This study focuses on the formation of NH _{3} and HNCO in CO-rich (non-polar) interstellar ices that simulate the CO freeze-out stage in interstellar dark cloud regions, well before thermal and energetic processing start to become predominant. Our work confirms the surface formation of ammonia through the sequential addition of three hydrogen/deuterium atoms to a single nitrogen atom at low temperature. The H/D fractionation of the formed ammonia is also shown. Furthermore, we show the surface formation of solid HNCO through the interaction of CO molecules with NH radicals - one of the intermediates in the formation of solid NH _{3}. Finally, we discuss the implications of HNCO in astrobiology, as a possible starting point for the formation of more complex prebiotic species.

From Interstellar PAHs and Ices to the Origin of Life

NASA Technical Reports Server (NTRS)

Allamandola, Louis J.; DeVincenzi, Donald L. (Technical Monitor)

2000-01-01

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.

Application of Stochastic and Deterministic Approaches to Modeling Interstellar Chemistry

NASA Astrophysics Data System (ADS)

Pei, Yezhe

This work is about simulations of interstellar chemistry using the deterministic rate equation (RE) method and the stochastic moment equation (ME) method. Primordial metal-poor interstellar medium (ISM) is of our interest and the socalled “Population-II” stars could have been formed in this environment during the “Epoch of Reionization” in the baby universe. We build a gas phase model using the RE scheme to describe the ionization-powered interstellar chemistry. We demonstrate that OH replaces CO as the most abundant metal-bearing molecule in such interstellar clouds of the early universe. Grain surface reactions play an important role in the studies of astrochemistry. But the lack of an accurate yet effective simulation method still presents a challenge, especially for large, practical gas-grain system. We develop a hybrid scheme of moment equations and rate equations (HMR) for large gas-grain network to model astrochemical reactions in the interstellar clouds. Specifically, we have used a large chemical gas-grain model, with stochastic moment equations to treat the surface chemistry and deterministic rate equations to treat the gas phase chemistry, to simulate astrochemical systems as of the ISM in the Milky Way, the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC). We compare the results to those of pure rate equations and modified rate equations and present a discussion about how moment equations improve our theoretical modeling and how the abundances of the assorted species are changed by varied metallicity. We also model the observed composition of H2O, CO and CO2 ices toward Young Stellar Objects in the LMC and show that the HMR method gives a better match to the observation than the pure RE method.

Interstellar Isotopes: Prospects with ALMA

NASA Technical Reports Server (NTRS)

Charnley Steven B.

2010-01-01

Cold molecular clouds are natural environments for the enrichment of interstellar molecules in the heavy isotopes of H, C, N and O. Anomalously fractionated isotopic material is found in many primitive Solar System objects, such as meteorites and comets, that may trace interstellar matter that was incorporated into the Solar Nebula without undergoing significant processing. Models of the fractionation chemistry of H, C, N and O in dense molecular clouds, particularly in cores where substantial freeze-out of molecules on to dust has occurred, make several predictions that can be tested in the near future by molecular line observations. The range of fractionation ratios expected in different interstellar molecules will be discussed and the capabilities of ALMA for testing these models (e.g. in observing doubly-substituted isotopologues) will be outlined.

Observations of the interstellar gas with the Copernicus satellite

NASA Technical Reports Server (NTRS)

Morton, D. C.

1975-01-01

Results are reviewed for Copernicus far-UV measurements of the absorption lines of H I, D I, H2, and heavier elements in the interstellar gas. Column densities along several lines of sight, as estimated from Ly-alpha absorption-line profiles, confirm that wide differences in the gas density are present in various directions. The measurement of interstellar D I implies an open universe unless alternate sources for this nuclide are found. Analysis of reddened stars for which the line of sight passes through one or more interstellar clouds indicates a depletion of several heavy elements in the gas. It is suggested that the depleted elements may be present in grains rather than molecules and that the intercloud medium may consist primarily of H II with a few small H I clouds.

Sunlight-switchable light shutter fabricated using liquid crystals doped with push-pull azobenzene.

PubMed

Oh, Seung-Won; Baek, Jong-Min; Yoon, Tae-Hoon

2016-11-14

We propose a sunlight-switchable light shutter using liquid crystal/polymer composite doped with push-pull azobenzene. The proposed light shutter is switchable between the translucent and transparent states by application of an electric field or by UV irradiation. Switching by UV irradiation is based on the change of the liquid crystal (LC) clearing point by the photo-isomerization effect of push-pull azobenzene. Under sunlight, the light shutter can be switched from the translucent to the transparent state by the nematic-isotropic phase transition of the LC domains triggered by trans-cis photo-isomerization of the push-pull azobenzene molecules. When the amount of sunlight is low because of cloud cover or when there is no sunlight at sunset, the light shutter rapidly relaxes from its transparent state back to its initial translucent state by the isotropic-nematic phase transition induced by cis-trans back-isomerization of the push-pull azobenzene molecules.

Theoretical studies in interstellar cloud chemistry

NASA Technical Reports Server (NTRS)

Chiu, Y. T.; Prasad, S. S.

1993-01-01

This final report represents the completion of the three tasks under the purchase order no. SCPDE5620,1,2F. Chemical composition of gravitationally contracting, but otherwise quiescent, interstellar clouds and of interstellar clouds traversed by high velocity shocks, were modeled in a comprehensive manner that represents a significant progress in modeling these objects. The evolutionary chemical modeling, done under this NASA contract, represents a notable advance over the 'classical' fixed condition equilibrium models because the evolutionary models consider not only the chemical processes but also the dynamical processes by which the dark interstellar clouds may have assumed their present state. The shock calculations, being reported here, are important because they extend the limited chemical composition derivable from dynamical calculations for the total density and temperature structures behind the shock front. In order to be tractable, the dynamical calculations must severely simplify the chemistry. The present shock calculations take the shock profiles from the dynamical calculations and derive chemical composition in a comprehensive manner. The results of the present modeling study are still to be analyzed with reference to astronomical observational data and other contemporary model predictions. As far as humanly possible, this analysis will be continued with CRE's (Creative Research Enterprises's) IR&D resources, until a sponsor is found.

Diffuse interstellar clouds as a chemical laboratory - The chemistry of diatomic carbon species

NASA Technical Reports Server (NTRS)

Federman, S. R.; Huntress, W. T., Jr.

1989-01-01

The chemistry of C2, CH, and CO in diffuse interstellar clouds is analyzed and compared to absorption line measurements toward background stars. Analytical expressions in terms of column densities are derived for the rate equations. The results indicate that in clouds with 4 mag of visual extinction, the abundance of C+ has to decrease by a factor of about 15 from the value traditionally used for clouds with 1 mag of extinction. The rate coefficients for the reactions C+ + CH - C2+ + H and C+ + H2 - CH2+ + h-nu need to be reduced from previous estimates. Chemical arguments are presented for the revised rate coefficients.

Molecular diagnostics of interstellar shocks

NASA Technical Reports Server (NTRS)

Hartquist, T. W.; Dalgarno, A.; Oppenheimer, M.

1980-01-01

The chemistry of molecules in shocked regions of the interstellar gas is considered and calculations are carried out for a region subjected to a shock at a velocity of 8 km/sec. Substantial enhancements are predicted in the concentrations of the molecules H2S, SO, and SiO compared to those anticipated in cold interstellar clouds.

Molecular diagnostics of interstellar shocks

NASA Astrophysics Data System (ADS)

Hartquist, T. W.; Dalgarno, A.; Oppenheimer, M.

1980-02-01

The chemistry of molecules in shocked regions of the interstellar gas is considered and calculations are carried out for a region subjected to a shock at a velocity of 8 km/sec. Substantial enhancements are predicted in the concentrations of the molecules H2S, SO, and SiO compared to those anticipated in cold interstellar clouds.

Analysis of ultraviolet spectrophotometric data from Copernicus. Final report

DOE Office of Scientific and Technical Information (OSTI.GOV)

Snow, T.P. Jr

1979-04-17

Ultraviolet spectral data from the OAO 3 satellite are being used to study interstellar absorption lines and stellar and circumstellar lines in hot stars. The interstellar data are beneficial in analyzing the depletions of heavy elements from the gas phase and in elucidating how these depletions depend on physical conditions. Abundances in separate velocity components were determined from line profiles. Observations were carried out for interstellar abundances, both atomic and molecular, towards a number of stars. The better quality data are being analyzed for profile information and the lesser data are being used in curve-of-growth analyses. Molecular observations were carriedmore » out as well; N/sup 2/ was sought, interstellar C/sup 2/ was detected and its rotational excitation utilized to establish limits in interstellar cloud temperatures. An extensive search for H/sup 2/O resulted in a tentative identification which will produce new information on chemical reaction rates. Interstellar depletions and grain properties in the rho Ophiuchi cloud, stellar wind variability, and circumstellar lines are also under study.« less

Relative Translucency of a Multilayered Ultratranslucent Zirconia Material.

PubMed

Shamseddine, Loubna; Majzoub, Zeina

2017-12-01

The aim of this study was to compare the translucency parameter (TP) of ultratranslucent multilayered (UTML) zirconia according to thickness and layer level. Rectangles of UTML zirconia with four layers [dentin layer (DEL), first transitional layer (FTL), second transitional layer (STL), and enamel layer (ENL)] and four different thicknesses (0.4, 0.6, 0.8, and 1 mm) were milled from blanks. Digital images were taken in a dark studio against white and black backgrounds under simulated daylight illumination and international commission on illumination (CIE) Lab* color values recorded using Photoshop Creative Cloud software. The TP was computed and compared according to thickness and layer level using analysis of variance (ANOVA) followed by Bonferroni post hoc analysis for multiple comparisons. Significance was set at p < 0.05. In each thickness, TP values were similar between any two layers. The significant effect of thickness on the TP was observed only in the first two layers. In the DEL, translucency was significantly greater at 0.4 mm than all other thicknesses. In the FTL, differences were significant between 0.4 and 0.8 mm and between 0.4 and 1 mm. The investigated zirconia does not seem to show gradational changes in relative translucency from dentin to enamel levels regardless of the thickness used. Thickness affected the TP only in the first two layers with better translu-cency at 0.4 mm. Since relative translucency does not seem to be significantly different between layers, clinicians can modify the apicocoronal positioning of the UTML layers within the restoration according to the desired Chroma without any implications on the clinically perceived translucency. While the thickness of 0.4 mm may be suggested for anterior esthetic veneers because of its higher translucency, the other thicknesses of 0.6 to 1 mm can be used to mask colored abutments in full contour restorations.

High-energy radiation from collisions of high-velocity clouds and the Galactic disc

NASA Astrophysics Data System (ADS)

del Valle, Maria V.; Müller, A. L.; Romero, G. E.

2018-04-01

High-velocity clouds (HVCs) are interstellar clouds of atomic hydrogen that do not follow normal Galactic rotation and have velocities of a several hundred kilometres per second. A considerable number of these clouds are falling down towards the Galactic disc. HVCs form large and massive complexes, so if they collide with the disc a great amount of energy would be released into the interstellar medium. The cloud-disc interaction produces two shocks: one propagates through the cloud and the other through the disc. The properties of these shocks depend mainly on the cloud velocity and the disc-cloud density ratio. In this work, we study the conditions necessary for these shocks to accelerate particles by diffusive shock acceleration and we study the non-thermal radiation that is produced. We analyse particle acceleration in both the cloud and disc shocks. Solving a time-dependent two-dimensional transport equation for both relativistic electrons and protons, we obtain particle distributions and non-thermal spectral energy distributions. In a shocked cloud, significant synchrotron radio emission is produced along with soft gamma rays. In the case of acceleration in the shocked disc, the non-thermal radiation is stronger; the gamma rays, of leptonic origin, might be detectable with current instruments. A large number of protons are injected into the Galactic interstellar medium, and locally exceed the cosmic ray background. We conclude that under adequate conditions the contribution from HVC-disc collisions to the galactic population of relativistic particles and the associated extended non-thermal radiation might be important.

Interstellar Antifreeze: Ethylene Glycol

NASA Technical Reports Server (NTRS)

Hollis, J. M.; Lovas, F. J.; Jewell, P. R.; Coudert, L. H.

2002-01-01

Interstellar ethylene glycol (HOCH2CH2,OH) has been detected in emission toward the Galactic center source Sagittarius B2(N-LMH) by means of several millimeter-wave rotational torsional transitions of its lowest energy conformer. The types and kinds of molecules found to date in interstellar clouds suggest a chemistry that favors aldehydes and their corresponding reduced alcohols-e.g., formaldehyde (H2CO)/methanol (CH3OH), acetaldehyde (CH3CHO)/ethanol (CH3CH2OH). Similarly, ethylene glycol is the reduced alcohol of glycolaldehyde (CH2OHCHO), which has also been detected toward Sgr B2(N-LMH). While there is no consensus as to how any such large complex molecules are formed in the interstellar clouds, atomic hydrogen (H) and carbon monoxide (CO) could form formaldehyde on grain surfaces, but such surface chemistry beyond that point is uncertain. However, laboratory experiments have shown that the gas-phase reaction of atomic hydrogen (H) and solid-phase CO at 10-20 K can produce formaldehyde and methanol and that alcohols and other complex molecules can be synthesized from cometary ice analogs when subject to ionizing radiation at 15 K. Thus, the presence of aldehyde/ reduced alcohol pairs in interstellar clouds implies that such molecules are a product of a low-temperature chemistry on grain surfaces or in grain ice mantles. This work suggests that aldehydes and their corresponding reduced alcohols provide unique observational constraints on the formation of complex interstellar molecules.

Interstellar gas in the Gum Nebula

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wallerstein, G.; Silk, J.; Jenkins, E.B.

1980-09-15

We have surveyed the interstellar gas in and around the Gum Nebula by optically observing 67 stars at Ca II, 42 stars at Na I, and 14 stars in the ultraviolet with the Copernicus satellite. Velocity dispersions for gas in the Gum Nebula, excluding the region of Vela remnant filaments, are not significantly larger than in the general interstellar medium. The ionization structure is predominantly that of an H II region with moderately high ionization, i.e., strong Si III and S III, in clouds with Vertical BarV/sub LSR/Vertical Bar> or approx. =10 km s/sup -1/. Furthermore, we find an increasemore » in fine-structure excitation with increasing component LSR velocity, suggestive of ram-pressure confinement for the intermediate-velocity clouds. These denser, more highly ionized clouds appear to be concentrated toward the inner Gum Nebula, where a somewhat higher velocity dispersion is found than in the outer regions. Clouds in the Gum Nebula do not show the anomalously high ionization seen in the Vela remnant clouds. The observational data are generally consistent with a model of the Gum Nebula as an H II region ionized by OB stars and stirred up by multiple stellar winds.« less

Summer School on Interstellar Processes: Abstracts of contributed papers

NASA Technical Reports Server (NTRS)

Hollenbach, D. J. (Editor); Thronson, H. A., Jr. (Editor)

1986-01-01

The Summer School on Interstellar Processes was held to discuss the current understanding of the interstellar medium and to analyze the basic physical processes underlying interstellar phenomena. Extended abstracts of the contributed papers given at the meeting are presented. Many of the papers concerned the local structure and kinematics of the interstellar medium and focused on such objects as star formation regions, molecular clouds, HII regions, reflection nebulae, planetary nebulae, supernova remnants, and shock waves. Other papers studied the galactic-scale structure of the interstellar medium either in the Milky Way or other galaxies. Some emphasis was given to observations of interstellar grains and

Planetary atmospheres program

NASA Technical Reports Server (NTRS)

1982-01-01

Non-solar compositional models of the troposphere of Jupiter, halide cloud condensation and volatile element inventories on Venus, and shock-wave processing of interstellar cloud materials are discussed.

A Herschel [C ii] Galactic plane survey. II. CO-dark H2 in clouds

NASA Astrophysics Data System (ADS)

Langer, W. D.; Velusamy, T.; Pineda, J. L.; Willacy, K.; Goldsmith, P. F.

2014-01-01

Context. H i and CO large scale surveys of the Milky Way trace the diffuse atomic clouds and the dense shielded regions of molecular hydrogen clouds, respectively. However, until recently, we have not had spectrally resolved C+ surveys in sufficient lines of sight to characterize the ionized and photon dominated components of the interstellar medium, in particular, the H2 gas without CO, referred to as CO-dark H2, in a large sample of interstellar clouds. Aims: We use a sparse Galactic plane survey of the 1.9 THz (158 μm) [C ii] spectral line from the Herschel open time key programme, Galactic Observations of Terahertz C+ (GOT C+), to characterize the H2 gas without CO in a statistically significant sample of interstellar clouds. Methods: We identify individual clouds in the inner Galaxy by fitting the [C ii] and CO isotopologue spectra along each line of sight. We then combine these spectra with those of H i and use them along with excitation models and cloud models of C+ to determine the column densities and fractional mass of CO-dark H2 clouds. Results: We identify1804 narrow velocity [C ii] components corresponding to interstellar clouds in different categories and evolutionary states. About 840 are diffuse molecular clouds with no CO, ~510 are transition clouds containing [C ii] and 12CO, but no 13CO, and the remainder are dense molecular clouds containing 13CO emission. The CO-dark H2 clouds are concentrated between Galactic radii of ~3.5 to 7.5 kpc and the column density of the CO-dark H2 layer varies significantly from cloud to cloud with a global average of 9 × 1020 cm-2. These clouds contain a significant fraction by mass of CO-dark H2, that varies from ~75% for diffuse molecular clouds to ~20% for dense molecular clouds. Conclusions: We find a significant fraction of the warm molecular ISM gas is invisible in H i and CO, but is detected in [C ii]. The fraction of CO-dark H2 is greatest in the diffuse clouds and decreases with increasing total column density, and is lowest in the massive clouds. The column densities and mass fraction of CO-dark H2 are less than predicted by models of diffuse molecular clouds using solar metallicity, which is not surprising as most of our detections are in Galactic regions where the metallicity is larger and shielding more effective. There is an overall trend towards a higher fraction of CO-dark H2 in clouds with increasing Galactic radius, consistent with lower metallicity there. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.

DUST AND GAS IN THE MAGELLANIC CLOUDS FROM THE HERITAGE HERSCHEL KEY PROJECT. II. GAS-TO-DUST RATIO VARIATIONS ACROSS INTERSTELLAR MEDIUM PHASES

DOE Office of Scientific and Technical Information (OSTI.GOV)

Roman-Duval, Julia; Gordon, Karl D.; Meixner, Margaret

2014-12-20

The spatial variations of the gas-to-dust ratio (GDR) provide constraints on the chemical evolution and lifecycle of dust in galaxies. We examine the relation between dust and gas at 10-50 pc resolution in the Large and Small Magellanic Clouds (LMC and SMC) based on Herschel far-infrared (FIR), H I 21 cm, CO, and Hα observations. In the diffuse atomic interstellar medium (ISM), we derive the GDR as the slope of the dust-gas relation and find GDRs of 380{sub −130}{sup +250} ± 3 in the LMC, and 1200{sub −420}{sup +1600} ± 120 in the SMC, not including helium. The atomic-to-molecular transition is locatedmore » at dust surface densities of 0.05 M {sub ☉} pc{sup –2} in the LMC and 0.03 M {sub ☉} pc{sup –2} in the SMC, corresponding to A {sub V} ∼ 0.4 and 0.2, respectively. We investigate the range of CO-to-H{sub 2} conversion factor to best account for all the molecular gas in the beam of the observations, and find upper limits on X {sub CO} to be 6 × 10{sup 20} cm{sup –2} K{sup –1} km{sup –1} s in the LMC (Z = 0.5 Z {sub ☉}) at 15 pc resolution, and 4 × 10{sup 21} cm{sup –2} K{sup –1} km{sup –1} s in the SMC (Z = 0.2 Z {sub ☉}) at 45 pc resolution. In the LMC, the slope of the dust-gas relation in the dense ISM is lower than in the diffuse ISM by a factor ∼2, even after accounting for the effects of CO-dark H{sub 2} in the translucent envelopes of molecular clouds. Coagulation of dust grains and the subsequent dust emissivity increase in molecular clouds, and/or accretion of gas-phase metals onto dust grains, and the subsequent dust abundance (dust-to-gas ratio) increase in molecular clouds could explain the observations. In the SMC, variations in the dust-gas slope caused by coagulation or accretion are degenerate with the effects of CO-dark H{sub 2}. Within the expected 5-20 times Galactic X {sub CO} range, the dust-gas slope can be either constant or decrease by a factor of several across ISM phases. Further modeling and observations are required to break the degeneracy between dust grain coagulation, accretion, and CO-dark H{sub 2}. Our analysis demonstrates that obtaining robust ISM masses remains a non-trivial endeavor even in the local Universe using state-of-the-art maps of thermal dust emission.« less

Isotope Fractionation in the Interstellar Medium

NASA Technical Reports Server (NTRS)

Charnley, Steven

2011-01-01

Anomalously fractionated isotopic material is found in many primitive Solar System objects, such as meteorites and comets. It is thought, in some cases, to trace interstellar matter that was incorporated into the Solar Nebula without undergoing significant processing. We will present the results of models of the nitrogen, oxygen, and carbon fractionation chemistry in dense molecular clouds, particularly in cores where substantial freeze-out of molecules on to dust has occurred. The range of fractionation ratios expected in different interstellar molecules will be discussed and compared to the ratios measured in molecular clouds, comets and meteoritic material. These models make several predictions that can be tested in the near future by molecular line observations, particularly with ALMA.

Iron hydrides formation in interstellar clouds

NASA Astrophysics Data System (ADS)

Bar-Nun, A.; Pasternak, M.; Barrett, P. H.

1980-07-01

A recent Moessbauer study with Fe-57 in a solid hydrogen or hydrogen-argon matrix demonstrated the formation of an iron hydride molecule (FeH2) at 2.5-5 K. Following this and other studies, the possible existence of iron hydride molecules in interstellar clouds is proposed. In clouds, the iron hydrides FeH and FeH2 would be formed only on grains, by encounters of H atoms or H2 molecules with Fe atoms which are adsorbed on the grains. The other transition metals, Sc, Ti, V, Cr, Mn, Co, N, Cd and also Cu and Ca form hydrides of the type M-H, which could be responsible, at least in part, for the depletion of these metals in clouds.

The abundance of CO in diffuse interstellar clouds - An ultraviolet survey

NASA Technical Reports Server (NTRS)

Federman, S. R.; Glassgold, A. E.; Jenkins, E. B.; Shaya, E. J.

1980-01-01

CO was detected in 17 directions and its upper limits were estimated in 21 directions by a UV survey carried out with the Copernicus satellite in the C-X 1088 A and E-X 1076 A lines toward 48 bright stars. The CO column densities range from 10 to the 12th to 10 to the 17th/sq cm and correlate with C I and H2. The tendency of the C I/CO ratio to be about 10 follows the ratio of particular atomic and molecular cross-sections and the physical parameters of interstellar clouds. Finally, the connection between UV observations in diffuse clouds and radio observations of (C-13)O in dark clouds is discussed.

A Survey of Near-infrared Diffuse Interstellar Bands

NASA Astrophysics Data System (ADS)

Hamano, Satochi; Kobayashi, Naoto; Kawakita, Hideyo; Ikeda, Yuji; Kondo, Sohei; Sameshima, Hiroaki; Arai, Akira; Matsunaga, Noriyuki; Yasui, Chikako; Mizumoto, Misaki; Fukue, Kei; Izumi, Natsuko; Otsubo, Shogo; Takenada, Keiichi

2018-04-01

We propose a study of interstellar molecules with near-infrared (NIR) high-resolution spectroscopy as a science case for the 3.6-m Devasthal Optical Telescope (DOT). In particular, we present the results obtained on-going survey of diffuse interstellar bands (DIBs) in NIR with the newly developed NIR high-resolution spectrograph WINERED, which offers a high sensitivity in the wavelength range of 0.91-1.36 µm. Using the WINERED spectrograph attached to the 1.3-m Araki telescope in Japan, we obtained high-quality spectra of a number of early-type stars in various environments, such as diffuse interstellar clouds, dark clouds and star-forming regions, to investigate the properties of NIR DIBs and constrain their carriers. As a result, we successfully identified about 50 new NIR DIBs, where only five fairly strong DIBs had been identified previously. Also, some properties of DIBs in the NIR are discussed to constrain the carriers of DIBs.

The distribution of interstellar dust in the solar neighborhood

NASA Technical Reports Server (NTRS)

Gaustad, John E.; Van Buren, Dave

1993-01-01

We surveyed the IRAS data base at the positions of the 1808 O6-B9.5 stars in The Bright Star Catalog for extended objects with excess emission at 60 microns, indicating the presence of interstellar dust at the location of the star. Within 400 pc the filling factor of the interstellar medium, for dust clouds with a density greater than 0.5/cu cm is 14.6 + or - 2.4%. Above a density of 1.0/cu cm, the density distribution function appears to follow a power law index - 1.25. When the dust clouds are mapped onto the galactic plane, the sun appears to be located in a low-density region of the interstellar medium of width about 60 pc extending at least 500 pc in the direction of longitudes 80 deg - 260 deg, a feature we call the 'local trough'.

On the cosmic ray diffusion in a violent interstellar medium

NASA Technical Reports Server (NTRS)

Bykov, A. M.; Toptygin, I. N.

1985-01-01

A variety of the available observational data on the cosmic ray (CR) spectrum, anisotropy and composition are in good agreement with a suggestion on the diffusion propagation of CR with energy below 10(15) eV in the interstellar medium. The magnitude of the CR diffusion coefficient and its energy dependence are determined by interstellar medium (ISM) magnetic field spectra. Direct observational data on magnetic field spectra are still absent. A theoretical model to the turbulence generation in the multiphase ISM is resented. The model is based on the multiple generation of secondary shocks and concomitant large-scale rarefactions due to supernova shock interactions with interstellar clouds. The distribution function for ISM shocks are derived to include supernova statistics, diffuse cloud distribution, and various shock wave propagation regimes. This permits calculation of the ISM magnetic field fluctuation spectrum and CR diffusion coefficient for the hot phase of ISM.

Vibrational Spectroscopy after OSU - From C2- to Interstellar Polycyclic Aromatic Hydrocarbons

NASA Technical Reports Server (NTRS)

Allamandola, Louis J.

2006-01-01

The composition of interstellar ice and dust provides insight into the chemical history of the interstellar medium and early solar system. It is now possible to probe this unique and unusual chemistry and determine the composition of these microscopic interstellar particles which are hundreds to many thousands of light years away thanks to substantial progress in two areas: astronomical spectroscopic techniques in the middle-infrared, the spectral region most diagnostic of chemical composition, and laboratory simulations which realistically reproduce the critical conditions in various interstellar environments. High quality infrared spectra of many different astronomical sources, some associated with giant, dark molecular clouds -the birthplace of stars and planets- and others in more tenuous, UV radiation rich regions are now available. The fundamentals of IR spectroscopy and what comparisons of astronomical IR spectra with laboratory spectra of materials prepared under realistic simulated interstellar conditions tell us about the components of these materials is the subject of this talk. These observations have shown that mixed molecular ices comprised of H2O, CH3OH, CO, NH3 and H2CO contain most of the molecular material in molecular clouds and that gas phase, ionized polycyclic aromatic hydrocarbons (PAHs) are widespread and surprisingly abundant throughout most of the interstellar medium.

Ultraviolet observations of the gas phase abundances in the diffuse clouds toward Zeta Ophiuchi at 3.5 kilometers per second resolution

NASA Technical Reports Server (NTRS)

Savage, Blair D.; Cardelli, Jason A.; Sofia, Ulysses J.

1992-01-01

Goddard High Resolution Spectrograph echelle mode measurements at 3.5 km/s resolution are presented for interstellar absorption produced by C II, O I, Mg I, Mg II, Al III, P II, Cr II, Mn II, Fe II, Ni II, Cu II, Zn II, Ga II, Ge II, and Kr I. The absorption line measurements are converted into representations of apparent column density per unit velocity in order to study the multicomponent nature of the absorption. The high spectral resolution of the measurements allows a comparative study of gas phase abundances for many species in the absorbing clouds near -27 and -15 km/s with a typical precision of about 0.05 dex. The matter absorbing near -27 km/s is situated in the local interstellar medium and has log N(H I) of about 19.74. This absorption provides information about the modest 'base' depletion associated with the lower density interstellar medium. The depletion results suggest that accretion processes are operating interstellar clouds that exhibit similar depletion efficiencies for some elements but much higher depletion efficiencies for others.

Three milieux for interstellar chemistry: gas, dust, and ice

NASA Astrophysics Data System (ADS)

Herbst, Eric

The interdisciplinary science of astrochemistry is 45 years of age, if we pinpoint its origin to have occurred when the first polyatomic molecules were detected in the interstellar gas. Since that time, the field has grown remarkably from an esoteric area of research to one that unites scientists around the globe. Almost 200 different molecules have been detected in the gas-phase of interstellar clouds, mainly by rotational spectroscopy, while dust particles and their icy mantles in colder regions can be probed by vibrational spectroscopy. Astrochemistry is exciting to scientists in a number of different fields. Astronomers are interested in molecular spectra from the heavens because such spectra are excellent probes of the physical conditions where molecules exist, while chemists are interested in the exotic molecules, their spectra, and the unusual chemical processes that produce and destroy them under conditions often very different from those on our home planet. Chemical simulations involving thousands of reactions are now used to calculate concentrations and spectra of interstellar molecules as functions of time. Even biologists share an interest in the subject, because the interstellar clouds of gas and dust, portions of which collapse to form stars and planetary systems, contain organic molecules that may become part of the initial inventory of new planets and may indeed be the precursors of life. An irresistible subject to its practitioners, astrochemistry is proving to be exciting to a much wider audience. In this perspective article, the field is first introduced, and the emphasis is then placed on the three environments in which chemistry occurs in the interstellar medium: the gas phase, the surfaces of bare dust particles, and the ice mantles that cover bare grains in cold dense interstellar clouds. What we do know and what we do not know is distinguished. The status of chemical simulations for a variety of interstellar sources having to do with stellar and planetary evolution is surveyed. An optimistic view of the future of astrochemistry ends the article.

Spectroscopy and reactions of molecules important in chemical evolution

NASA Technical Reports Server (NTRS)

Becker, R. S.

1974-01-01

The research includes: (1) hot hydrogen atom reactions in terms of the nature of products produced, mechanism of the reactions and the implication and application of such reactions for molecules existing in interstellar clouds, in planetary atmospheres, and in chemical evolution; (2) photochemical reactions that can lead to molecules important in chemical evolution, interstellar clouds and as constituents in planetary atmospheres; and (3) spectroscopic and theoretical properties of biomolecules and their precursors and where possible, use these to understand their photochemical behavior.

Isotopic Fractionation in Primitive Material: Quantifying the Contribution of Interstellar Chemistry

NASA Technical Reports Server (NTRS)

Charnley, Steven

2010-01-01

Anomalously fractionated isotopic material is found in many primitive Solar System objects, such as meteorites and comets. It is thought, in some cases, to trace interstellar matter that was incorporated into the Solar Nebula without undergoing significant processing. We will present the results of models of the nitrogen, oxygen, and carbon fractionation chemistry in dense molecular clouds, particularly in cores where substantial freeze-out of molecules on to dust has occurred. The range of fractionation ratios expected in different interstellar molecules will be discussed and compared to the ratios measured in molecular clouds, comets and meteoritic material. These models make several predictions that can be tested in the near future by molecular line observations, particularly with ALMA.

Insights into H2 formation in space from ab initio molecular dynamics

PubMed Central

Casolo, Simone; Tantardini, Gian Franco; Martinazzo, Rocco

2013-01-01

Hydrogen formation is a key process for the physics and the chemistry of interstellar clouds. Molecular hydrogen is believed to form on the carbonaceous surface of dust grains, and several mechanisms have been invoked to explain its abundance in different regions of space, from cold interstellar clouds to warm photon-dominated regions. Here, we investigate direct (Eley–Rideal) recombination including lattice dynamics, surface corrugation, and competing H-dimers formation by means of ab initio molecular dynamics. We find that Eley–Rideal reaction dominates at energies relevant for the interstellar medium and alone may explain observations if the possibility of facile sticking at special sites (edges, point defects, etc.) on the surface of the dust grains is taken into account. PMID:23572584

Chemistry in dynamically evolving clouds

NASA Technical Reports Server (NTRS)

Tarafdar, S. P.; Prasad, S. S.; Huntress, W. T., Jr.; Villere, K. R.; Black, D. C.

1985-01-01

A unified model of chemical and dynamical evolution of isolated, initially diffuse and quiescent interstellar clouds is presented. The model uses a semiempirically derived dependence of the observed cloud temperatures on the visual extinction and density. Even low-mass, low-density, diffuse clouds can collapse in this model, because the inward pressure gradient force assists gravitational contraction. In contrast, previous isothermal collapse models required the low-mass diffuse clouds to be unrealistically cold before gravitational contraction could start. Theoretically predicted dependences of the column densities of various atoms and molecules, such as C and CO, on visual extinction in diffuse clouds are in accord with observations. Similarly, the predicted dependences of the fractional abundances of various chemical species (e.g., CO, H2CO, HCN, HCO(+)) on the total hydrogen density in the core of the dense clouds also agree with observations reported to date in the literature. Compared with previous models of interstellar chemistry, the present model has the potential to explain the wide spectrum of chemical and physical properties of both diffuse and dense clouds with a common formalism employing only a few simple initial conditions.

Interstellar organic chemistry.

NASA Technical Reports Server (NTRS)

Sagan, C.

1972-01-01

Most of the interstellar organic molecules have been found in the large radio source Sagittarius B2 toward the galactic center, and in such regions as W51 and the IR source in the Orion nebula. Questions of the reliability of molecular identifications are discussed together with aspects of organic synthesis in condensing clouds, degradational origin, synthesis on grains, UV natural selection, interstellar biology, and contributions to planetary biology.

Are comets connected to the origin of life

NASA Technical Reports Server (NTRS)

Delsemme, A. H.

1981-01-01

Possible connections between comets and the origin of life on earth are discussed. The orbital evolution of comets and their origin are considered within a framework for the origin of the solar system, with particular attention given to the origin of the biosphere, and the origin of the Oort cloud. Evidence suggesting that cometary nuclei are undifferentiated throughout is considered, and a model of the average composition of a mean new comet is obtained from observational data which is similar to that of an interstellar frost. The chemistry of the model composition giving rise to the species observed in cometary spectra is considered, as well as the relations of cometary to cosmic abundances of oxygen, carbon and sulfur. The characteristics of possible sites for prebiotic chemistry, including interstellar clouds, the protosolar nebula, comets in the Oort cloud, periodic comets and the primitive earth, are examined, and a possible role of comets in bringing the interstellar prebiotic chemistry to earth is suggested.

Dust near luminous ultraviolet stars

NASA Technical Reports Server (NTRS)

Henry, Richard C.

1993-01-01

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.

Thermal Pressure in Diffuse H2 Gas Measured by Herschel [C II] Emission and FUSE UV H2 Absorption

NASA Astrophysics Data System (ADS)

Velusamy, T.; Langer, W. D.; Goldsmith, P. F.; Pineda, J. L.

2017-04-01

UV absorption studies with the Far Ultraviolet Spectroscopic Explorer (FUSE) satellite have made important observations of H2 molecular gas in Galactic interstellar translucent and diffuse clouds. Observations of the 158 μm [C II] fine-structure line with Herschel trace the same H2 molecular gas in emission. We present [C II] observations along 27 lines of sight (LOSs) toward target stars of which 25 have FUSE H2 UV absorption. Two stars have only HST STIS C II λ2325 absorption data. We detect [C II] 158 μm emission features in all but one target LOS. For three target LOSs that are close to the Galactic plane, | {\\text{}}b| < 1°, we also present position-velocity maps of [C II] emission observed by Herschel Heterodyne Instrument in the Far Infrared (HIFI) in on-the-fly spectral-line mapping. We use the velocity-resolved [C II] spectra observed by the HIFI instrument toward the target LOSs observed by FUSE to identify [C II] velocity components associated with the H2 clouds. We analyze the observed velocity integrated [C II] spectral-line intensities in terms of the densities and thermal pressures in the H2 gas using the H2 column densities and temperatures measured by the UV absorption data. We present the H2 gas densities and thermal pressures for 26 target LOSs and from the [C II] intensities derive a mean thermal pressure in the range of ˜6100-7700 K cm-3 in diffuse H2 clouds. We discuss the thermal pressures and densities toward 14 targets, comparing them to results obtained using the UV absorption data for two other tracers C I and CO. Our results demonstrate the richness of the far-IR [C II] spectral data which is a valuable complement to the UV H2 absorption data for studying diffuse H2 molecular clouds. While the UV absorption is restricted to the directions of the target star, far-IR [C II] line emission offers an opportunity to employ velocity-resolved spectral-line mapping capability to study in detail the clouds’ spatial and velocity structures.

New Large Interstellar Molecules Detected with the GBT

NASA Technical Reports Server (NTRS)

Hollis, Jan M.

2005-01-01

At present, more than 135 different molecules have been identified in interstellar clouds. The newest instrument in the interstellar molecule search arsenal is the recently commissioned Green Bank Telescope (GBT). In 2004, the large aldehydes propenal (CH2CHCHO) and propanal (CH3CH2CHO) were the first new interstellar molecules discovered with the GBT. At the same time, the GBT was used to observe interstellar glycolaldehyde (CH2OHCHO), which is the simplest possible aldehyde sugar; interstellar ethylene glycol (HOCH2CH2OH), which is the sugar alcohol of glycolaldehyde; and interstellar methylcyanodiacetylene (CH3C5N). These new GBT observations suggest that successive atomic addition reactions are common in the formation of larger related species. The observations will be presented and discussed.

Galaxy and the solar system

DOE Office of Scientific and Technical Information (OSTI.GOV)

Smoluchowski, R.; Bahcall, J.M.; Matthews, M.S.

1986-01-01

The solar-Galactic neighborhood, massive interstellar clouds and other Galactic features, the Oort cloud, perturbations of the solar system, and the existence and stability of a solar companion star are examined in chapters based on contributions to a conference held in Tucson, AZ during January 1985. The individual topics addressed include: the Galactic environment of the solar system; stars within 25 pc of the sun; the path of the sun in 100 million years; the local velocity field in the last billion years; interstellar clouds near the sun; and evidence for a local recent supernova. Also considered are: dynamic influence ofmore » Galactic tides and molecular clouds on the Oort cloud; cometary evidence for a solar companion; dynamical interactions between the Oort cloud and the Galaxy; geological periodicities and the Galaxy; giant comets and the Galaxy; dynamical evidence for Planet X; evolution of the solar system in the presence of a solar companion star; mass extinctions, crater ages, and comet showers; evidence for Nemesis, a solar companion star.« less

Goulds Belt, Interstellar Clouds, and the Eocene Oligocene Helium-3 Enhancement

NASA Technical Reports Server (NTRS)

Rubincam, David Parry

2015-01-01

Drag from hydrogen in the interstellar cloud which formed Gould's Belt may have sent interplanetary dust particle (IDPs) and small meteoroids with embedded helium to the Earth, perhaps explaining part the helium-3 flux increase seen in the sedimentary record near the Eocene-Oligocene transition. Assuming the Solar System passed through part of the cloud, IDPs in the inner Solar System may have been dragged to Earth, while dust and small meteoroids in the asteroid belt up to centimeter size may have been dragged to the resonances, where their orbital eccentricities were pumped up into Earth-crossing orbits; however, this hypotheses does not explain the Popigai and Chesapeake Bay impacts.

A search with Copernicus for interstellar N2 in diffuse clouds

NASA Technical Reports Server (NTRS)

Lutz, B. L.; Snow, T. P., Jr.; Owen, T.

1979-01-01

Multiple Copernicus scans of two N2 band regions (near 958.5 and 960.2A) of Delta Sco and Epsilon Per are reported. The observations indicate upper limits for the number of N2 molecules equal to 1.0-3.8 times 10 to the -12th/sq cm and 1.2-4.4 times 10 to the -12th/sq cm, respectively; the limits depend on the cloud temperature. It is suggested that the limits are consistent with the column densities predicted by chemical models for diffuse interstellar clouds, and the predicted relative abundances are presented in terms of the ratio of N(N2)/(2N(H2) + N(Hl)).

Search for molecular absorptions with the Fourier Transform Spectrometer

NASA Technical Reports Server (NTRS)

Knacke, Roger F.

1995-01-01

The objective of this research was a search for water molecules in the gas phase in molecular clouds. Water should be among the most abundant gases in the clouds and is of fundamental importance in gas chemistry, cloud cooling, shock wave chemistry, and gas-grain interactions of interstellar dust. Detection of water in Comet Halley in the 2.7 micron v(3) band in 1986 had shown that airborne H2O observations are feasible (ground-based observations of H2O are impossible because of the massive water content of the atmosphere). We planned to observe the v(3) band in interstellar clouds where a number of lines of this band should be in absorption. The search for H2O commenced in 1988 with a two flight program on the KAO. this resulted in a detection of interstellar H2O with S/N of 2-4 in the v(3) 1(01)-2(02) line at 3801.42/cm. A subsequent flight series of two flights in 1989 resulted in confirmation to the 3801.42/cm line detection and the detection of altogether four strong lines in the 000-001 v(3) vibration-rotation band of H2O.

Scientists Discover Sugar in Space

NASA Astrophysics Data System (ADS)

2000-06-01

The prospects for life in the Universe just got sweeter, with the first discovery of a simple sugar molecule in space. The discovery of the sugar molecule glycolaldehyde in a giant cloud of gas and dust near the center of our own Milky Way Galaxy was made by scientists using the National Science Foundation's 12 Meter Telescope, a radio telescope on Kitt Peak, Arizona. "The discovery of this sugar molecule in a cloud from which new stars are forming means it is increasingly likely that the chemical precursors to life are formed in such clouds long before planets develop around the stars," said Jan M. Hollis of the NASA Goddard Space Flight Center in Greenbelt, MD. Hollis worked with Frank J. Lovas of the University of Illinois and Philip R. Jewell of the National Radio Astronomy Observatory (NRAO) in Green Bank, WV, on the observations, made in May. The scientists have submitted their results to the Astrophysical Journal Letters. "This discovery may be an important key to understanding the formation of life on the early Earth," said Jewell. Conditions in interstellar clouds may, in some cases, mimic the conditions on the early Earth, so studying the chemistry of interstellar clouds may help scientists understand how bio-molecules formed early in our planet's history. In addition, some scientists have suggested that Earth could have been "seeded" with complex molecules by passing comets, made of material from the interstellar cloud that condensed to form the Solar System. Glycolaldehyde, an 8-atom molecule composed of carbon, oxygen and hydrogen, can combine with other molecules to form the more-complex sugars Ribose and Glucose. Ribose is a building block of nucleic acids such as RNA and DNA, which carry the genetic code of living organisms. Glucose is the sugar found in fruits. Glycolaldehyde contains exactly the same atoms, though in a different molecular structure, as methyl formate and acetic acid, both of which were detected previously in interstellar clouds. Glycolaldehyde is a simpler molecular cousin to table sugar, the scientists say. The sugar molecule was detected in a large cloud of gas and dust some 26,000 light-years away, near the center of our Galaxy. Such clouds, often many light-years across, are the material from which new stars are formed. Though very rarified by Earth standards, these interstellar clouds are the sites of complex chemical reactions that occur over hundreds of thousands or millions of years. So far, about 120 different molecules have been discovered in these clouds. Most of these molecules contain a small number of atoms, and only a few molecules with eight or more atoms have been found in interstellar clouds. The 12 Meter Telescope "Finding glycolaldehyde in one of these interstellar clouds means that such molecules can be formed even in very rarified conditions," said Hollis. "We don't yet understand how it could be formed there," he added. "A combination of more astronomical observations and theoretical chemistry work will be required to resolve the mystery of how this molecule is formed in space." "We hope this discovery inspires renewed efforts to find even more kinds of molecules, so that, with a better idea of the total picture, we may be able to deduce the details of the prebiotic chemistry taking place in interstellar clouds," Hollis said. The discovery was made by detecting faint radio emission from the sugar molecules in the interstellar cloud. Molecules rotate end-for-end, and as they change from one rotational energy state to another, they emit radio waves at precise frequencies. The "family" of radio frequencies emitted by a particular molecule forms a unique "fingerprint" that scientists can use to identify that molecule. The scientists identified glycolaldehyde by detecting six frequencies of radio emission in what is termed the millimeter-wavelength region of the electromagnetic spectrum -- a region between more-familiar microwaves and infrared radiation. The NRAO 12 Meter Telescope used to detect the sugar molecule has been a pioneer instrument in the detection of molecules in space. Built in 1967, it made the first detections of dozens of the molecules now known to exist in space, including the important first discovery of carbon monoxide, now widely used by astronomers as a signpost showing regions where stars are being formed. The 12 Meter Telescope is scheduled to be closed at the end of July, in preparation for the Atacama Large Millimeter Array, an advanced system of 64 radio-telescope antennas in northern Chile now being developed by an international partnership. The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc. Giant Molecular Cloud Near Milky Way's Center The giant molecular cloud, known as Sagittarius B2 (North), as seen by the NSF's Very Large Array (VLA) radio telescope in New Mexico. This is the cloud in which scientists using the 12 Meter Telescope detected the simple sugar molecule glycolaldehyde. This VLA image shows hydrogen gas in a region nearly 3 light-years across. In this image, red indicates stronger radio emission; blue weaker. The 12 Meter Telescope studied this region at much shorter wavelengths, which revealed the evidence of sugar molecules. CREDIT: R. Gaume, M. Claussen, C. De Pree, W.M. Goss, D. Mehringer, NRAO/AUI/NSF.

The 15 273 Å diffuse interstellar band in the dark cloud Barnard 68

NASA Astrophysics Data System (ADS)

Elyajouri, Meriem; Cox, Nick L. J.; Lallement, Rosine

2017-09-01

High obscuration of background stars behind dark clouds precludes the detection of optical diffuse interstellar bands (DIBs) and hence our knowledge of DIB carriers in these environments. Taking advantage of the reduced obscuration of starlight in the near-infrared (NIR) we used one of the strongest NIR DIBs at 15 273 Å to probe the presence and properties of its carrier throughout the nearby interstellar dark cloud Barnard 68. We measured equivalent widths (EW) for different ranges of visual extinction AV, using VLT/KMOS H-band (1.46-1.85 μm) moderate-resolution (R 4000) spectra of 43 stars situated behind the cloud. To do so, we fitted the data with synthetic stellar spectra from the APOGEE project and TAPAS synthetic telluric transmissions appropriate for the observing site and time period. The results show an increase of DIB EW with increasing AV. However, the rate of increase is much flatter than expected from the EW-AV quasi-proportionality established for this DIB in the Galactic diffuse interstellar medium. Based on a simplified inversion assuming sphericity, it is found that the volume density of the DIB carrier is 2.7 and 7.9 times lower than this expected average value in the external and central regions of the cloud, which have nH≃ 0.4 and 3.5 × 105 cm-3, respectively. Further measurements with multiplex NIR spectrographs should allow detailed modeling of such an edge effect of this DIB and other bands and help clarify its actual origin. Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere under ESO programme 096.C-0931(A).

Interstellar grain chemistry and organic molecules

NASA Technical Reports Server (NTRS)

Allamandola, L. J.; Sandford, S. A.

1990-01-01

The detection of prominant infrared absorption bands at 3250, 2170, 2138, 1670 and 1470 cm(-1) (3.08, 4.61, 4.677, 5.99 and 6.80 micron m) associated with molecular clouds show that mixed molecular (icy) grain mantles are an important component of the interstellar dust in the dense interstellar medium. These ices, which contain many organic molecules, may also be the production site of the more complex organic grain mantles detected in the diffuse interstellar medium. Theoretical calculations employing gas phase as well as grain surface reactions predict that the ices should be dominated only by the simple molecules H2O, H2CO, N2, CO, O2, NH3, CH4, possibly CH3OH, and their deuterated counterparts. However, spectroscopic observations in the 2500 to 1250 cm(-1)(4 to 8 micron m) range show substantial variation from source reactions alone. By comparing these astronomical spectra with the spectra of laboratory-produced analogs of interstellar ices, one can determine the composition and abundance of the materials frozen on the grains in dense clouds. Experiments are described in which the chemical evolution of an interstellar ice analog is determined during irradiation and subsequent warm-up. Particular attention is paid to the types of moderately complex organic materials produced during these experiments which are likely to be present in interstellar grains and cometary ices.

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

NASA Technical Reports Server (NTRS)

Allamandola, Louis

2004-01-01

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.

Following the Interstellar History of Carbon: From the Interiors of Stars to the Surfaces of Planets.

PubMed

Ziurys, L M; Halfen, D T; Geppert, W; Aikawa, Y

2016-12-01

The chemical history of carbon is traced from its origin in stellar nucleosynthesis to its delivery to planet surfaces. The molecular carriers of this element are examined at each stage in the cycling of interstellar organic material and their eventual incorporation into solar system bodies. The connection between the various interstellar carbon reservoirs is also examined. Carbon has two stellar sources: supernova explosions and mass loss from evolved stars. In the latter case, the carbon is dredged up from the interior and then ejected into a circumstellar envelope, where a rich and unusual C-based chemistry occurs. This molecular material is eventually released into the general interstellar medium through planetary nebulae. It is first incorporated into diffuse clouds, where carbon is found in polyatomic molecules such as H 2 CO, HCN, HNC, c-C 3 H 2 , and even C 60 + . These objects then collapse into dense clouds, the sites of star and planet formation. Such clouds foster an active organic chemistry, producing compounds with a wide range of functional groups with both gas-phase and surface mechanisms. As stars and planets form, the chemical composition is altered by increasing stellar radiation, as well as possibly by reactions in the presolar nebula. Some molecular, carbon-rich material remains pristine, however, encapsulated in comets, meteorites, and interplanetary dust particles, and is delivered to planet surfaces. Key Words: Carbon isotopes-Prebiotic evolution-Interstellar molecules-Comets-Meteorites. Astrobiology 16, 997-1012.

Simulating Chemistry in Star Forming Environments

NASA Astrophysics Data System (ADS)

Gong, Munan

Chemistry plays an important role in the interstellar medium (ISM), regulating the heating and cooling of the gas and determining abundances of molecular species that trace gas properties in observations. One of the most abundant and important molecules in the ISM is CO. CO is a main coolant for the molecular ISM, and the CO(J = 1 - 0) line emission is a widely used observational tracer for molecular clouds. In Chapter 2, we propose a new simplified chemical network for hydrogen and carbon chemistry in the atomic and molecular ISM. We compare results from our chemical network in detail with results from a full photodissociation region (PDR) code, and also with the Nelson & Langer (NL99) network previously adopted in the simulation literature. We show that our chemical network gives similar results to the PDR code in the equilibrium abundances of all species over a wide range of densities, temperature, and metallicities, whereas the NL99 network shows significant disagreement. We also compare with observations of diffuse and translucent clouds. We find that the CO, CHx and OHx abundances are consistent with equilibrium predictions for densities n = 100 - 1000 cm-3, but the predicted equilibrium CI abundance is higher than observations, signaling the potential importance of non-equilibrium/dynamical effects. In Chapter 3, we apply our new chemistry network to a study of the XCO conversion factor, which is used to convert the CO luminosity to the total H2 mass. We use numerical simulations to investigate how XCO depends on numerical resolution, non-equilibrium chemistry, physical environment, and observational beam size. Our study employs 3D magnetohydrodynamics (MHD) simulations of galactic disks with solar neighborhood conditions, where star formation and the three-phase interstellar medium (ISM) is self-consistently generated by the interaction between gravity and stellar feedback. Synthetic CO maps are obtained by post-processing the MHD simulations with chemistry and radiation transfer. We find that CO is only an approximate tracer of H2. Nevertheless, 〈 XCO〉 = 0.7 - 1.0 x 1020 cm-2K-1km-1 s consistent with observations, insensitive to the evolutionary ISM state or the far-ultraviolet (FUV) radiation field strength. Our numerical simulations successfully reproduce the observed variations of X CO on parsec scales, as well as the dependence of X CO on extinction and the CO excitation temperature.

Simulation of optical interstellar scintillation

NASA Astrophysics Data System (ADS)

Habibi, F.; Moniez, M.; Ansari, R.; Rahvar, S.

2013-04-01

Aims: Stars twinkle because their light propagates through the atmosphere. The same phenomenon is expected on a longer time scale when the light of remote stars crosses an interstellar turbulent molecular cloud, but it has never been observed at optical wavelengths. The aim of the study described in this paper is to fully simulate the scintillation process, starting from the molecular cloud description as a fractal object, ending with the simulations of fluctuating stellar light curves. Methods: Fast Fourier transforms are first used to simulate fractal clouds. Then, the illumination pattern resulting from the crossing of background star light through these refractive clouds is calculated from a Fresnel integral that also uses fast Fourier transform techniques. Regularisation procedure and computing limitations are discussed, along with the effect of spatial and temporal coherency (source size and wavelength passband). Results: We quantify the expected modulation index of stellar light curves as a function of the turbulence strength - characterised by the diffraction radius Rdiff - and the projected source size, introduce the timing aspects, and establish connections between the light curve observables and the refractive cloud. We extend our discussion to clouds with different structure functions from Kolmogorov-type turbulence. Conclusions: Our study confirms that current telescopes of ~4 m with fast-readout, wide-field detectors have the capability of discovering the first interstellar optical scintillation effects. We also show that this effect should be unambiguously distinguished from any other type of variability through the observation of desynchronised light curves, simultaneously measured by two distant telescopes.

Observational evidence of dust evolution in galactic extinction curves

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cecchi-Pestellini, Cesare; Casu, Silvia; Mulas, Giacomo

Although structural and optical properties of hydrogenated amorphous carbons are known to respond to varying physical conditions, most conventional extinction models are basically curve fits with modest predictive power. We compare an evolutionary model of the physical properties of carbonaceous grain mantles with their determination by homogeneously fitting observationally derived Galactic extinction curves with the same physically well-defined dust model. We find that a large sample of observed Galactic extinction curves are compatible with the evolutionary scenario underlying such a model, requiring physical conditions fully consistent with standard density, temperature, radiation field intensity, and average age of diffuse interstellar clouds.more » Hence, through the study of interstellar extinction we may, in principle, understand the evolutionary history of the diffuse interstellar clouds.« less

Copernicus observations of interstellar matter toward the Orion OB1 association. I - Epsilon and Pi-5 Orionis

NASA Technical Reports Server (NTRS)

Shull, J. M.

1979-01-01

Copernicus UV data on interstellar lines toward Epsilon Ori and Pi-5 Ori are analyzed to study abundances and physical conditions in both low- and intermediate-velocity components. Clouds at -8 and +5 km/s (LSR) toward Epsilon Ori show typical depletions of Fe, Ti, Mg, and Si in dense (H number density about 100 per cu cm) gas. Low-column-density intermediate-velocity clouds toward both stars, with low densities (hydrogen number density less than 1 per cu cm) and near-cosmic Si abundances, are consistent with a widespread pattern of high-velocity gas over a 15-deg area surrounding the Orion region. Such activity may be attributed to the repeated action of supernovae in a patchy low-density region of interstellar gas.

More on the lambda 2800 A 'interstellar extinction' feature

NASA Astrophysics Data System (ADS)

McLachlan, A.; Nandy, K.

1985-02-01

In a response made to a recent letter by Karim et al. (1984), it is shown that the examples of interstellar absorption at 2800 A that they attribute to proteinaceous material can all be attributed to overexposure of IUE detectors. It is pointed out that stars in the Large Magellanic Cloud show pronounced absorption at 2800 A which cannot be due to interstellar protein since there is no associated absorption at 2200 A; this lack of absorption cannot be due to presence of graphite, whose absorption is weak in the Cloud. The claim by Karim et al. that the spectra of eight stars show 2800 A absorption and that these spectra are saturation-free is considered, and it is shown that data processing problems at IUE ground stations make these spectra unreliable.

An interstellar cloud density from Copernicus observations of CO in the spectrum of Zeta Ophiuchi

NASA Technical Reports Server (NTRS)

Smith, A. M.; Stecher, T. P.; Krishna Swamy, K. S.

1978-01-01

Interstellar CO absorption bands in Copernicus spectra of Zeta Oph have been studied. Absorption profiles, computed under the assumption that excitation is due to collisions with H2 molecules and interaction with the 3-K background radiation field, were fitted to the reduced data of nine bands. When a gas kinetic temperature of 56 K is assumed, the best-fit condition implies a hydrogen-nucleus density of 120 per cu cm, a CO column density of 1.2 by 10 to the 15th power per sq cm, and a radial-velocity dispersion of 0.9 km/s. The relevance of these results to existing ideas concerning the Zeta Oph interstellar clouds is discussed. It is suggested that the strongest interstellar component is not circumstellar in origin but is instead part of a supernova remnant. Simple calculations are made to establish the plausibility of the supernova-remnant identification. This suggestion is also supported by Heiles's (1976) 21-cm pictures.

Correlation properties of interstellar dust: Diffuse interstellar bands

NASA Technical Reports Server (NTRS)

Somerville, W. B.

1989-01-01

Results are presented from a research program in which an attempt was made to establish the physical nature of the interstellar grains, and the carriers of the diffuse interstellar bands, by comparing relations between different observed properties; the properties used include the extinction in the optical and ultraviolet (including wavelength 2200 and the far-UV rise), cloud density, atomic depletions, and strengths of the diffuse bands. Observations and also data from literature were used, selecting particularly sight-lines where some observed property was found to have anomalous behavior.

A survey of bimolecular ion-molecule reactions for use in modeling the chemistry of planetary atmospheres, cometary comae, and interstellar clouds

NASA Technical Reports Server (NTRS)

Anicich, V. G.; Huntress, W. T., Jr.

1986-01-01

All bimolecular positive ion-molecule reactions reported from 1965 to 1985 for temperatures below 1000 K are included in the present survey of those ion-molecule reactions pertinent to the chemistries of planetary atmospheres, cometary comae, and interstellar clouds. This survey is intended as an update of the first, by Huntress (1977). The tabular presentation is organized according to reactant ion, with cross-references for both the ionic and the neutral reactants as well as the ionic and neutral products.

Dissociative recombination in interstellar clouds

NASA Technical Reports Server (NTRS)

Black, John H.; Van Dishoeck, Ewine F.

1989-01-01

Molecular ions play a significant role in the chemistry and evolution of interstellar molecular clouds, even though these regions are overwhelmingly neutral. The dissociative recombination (DR) process governs the abundances of many of these ions and of related neutral species. The gas-phase ion-molecule chemistry of the simplest species is summarized, with emphasis on those problems which are most sensitive to uncertain rates or product branching ratios of DR processes. Examples of the kinds of information needed about DR processes are presented. The importance of the H3(+) ion and prospects for its direct observation are discussed.

Astrophysics: Multi-Molecular Views of a Stellar Nursery

NASA Technical Reports Server (NTRS)

Wiseman, Jennifer; Sewilo, Marta

2017-01-01

New detectors for radio telescopes can map emissions from many different molecules simultaneously across interstellar clouds. One such pioneering study has probed a wide area of a star-forming cloud in the Orion constellation.

Astrophysics: Multi-molecular views of a stellar nursery

NASA Astrophysics Data System (ADS)

Wiseman, Jennifer; Sewilo, Marta

2017-06-01

New detectors for radio telescopes can map emissions from many different molecules simultaneously across interstellar clouds. One such pioneering study has probed a wide area of a star-forming cloud in the Orion constellation.

Term Projects on Interstellar Comets

ERIC Educational Resources Information Center

Mack, John E.

1975-01-01

Presents two calculations of the probability of detection of an interstellar comet, under the hypothesis that such comets would escape from comet clouds similar to that believed to surround the sun. Proposes three problems, each of which would be a reasonable term project for a motivated undergraduate. (Author/MLH)

Interstellar and Cometary Dust

NASA Technical Reports Server (NTRS)

Mathis, John S.

1997-01-01

'Interstellar dust' forms a continuum of materials with differing properties which I divide into three classes on the basis of observations: (a) diffuse dust, in the low-density interstellar medium; (b) outer-cloud dust, observed in stars close enough to the outer edges of molecular clouds to be observed in the optical and ultraviolet regions of the spectrum, and (c) inner-cloud dust, deep within the cores of molecular clouds, and observed only in the infrared by means of absorption bands of C-H, C=O, 0-H, C(triple bond)N, etc. There is a surprising regularity of the extinction laws between diffuse- and outer-cloud dust. The entire mean extinction law from infrared through the observable ultraviolet spectrum can be characterized by a single parameter. There are real deviations from this mean law, larger than observational uncertainties, but they are much smaller than differences of the mean laws in diffuse- and outer-cloud dust. This fact shows that there are processes which operate over the entire distribution of grain sizes, and which change size distributions extremely efficiently. There is no evidence for mantles on grains in local diffuse and outer-cloud dust. The only published spectra of the star VI Cyg 12, the best candidate for showing mantles, does not show the 3.4 micro-m band which appreciable mantles would produce. Grains are larger in outer-cloud dust than diffuse dust because of coagulation, not accretion of extensive mantles. Core-mantle grains favored by J. M. Greenberg and collaborators, and composite grains of Mathis and Whiffen (1989), are discussed more extensively (naturally, I prefer the latter). The composite grains are fluffy and consist of silicates, amorphous carbon, and some graphite in the same grain. Grains deep within molecular clouds but before any processing within the solar system are presumably formed from the accretion of icy mantles on and within the coagulated outer-cloud grains. They should contain a mineral/carbonaceous matrix, without organic refractory mantles, in between the ices. Unfortunately, they may be significantly processed by chemical processes accompanying the warming (over the 10 K of the dark cloud cores) which occurs in the outer solar system. Evidence of this processing is the chemical anomalies present in interplanetary dust particles collected in the stratosphere, which may be the most primitive materials we have obtained to date. The comet return mission would greatly clarify the situation, and probably provide samples of genuine interstellar grains.

Low-temperature surface formation of NH3 and HNCO: hydrogenation of nitrogen atoms in CO-rich interstellar ice analogues

NASA Astrophysics Data System (ADS)

Fedoseev, G.; Ioppolo, S.; Zhao, D.; Lamberts, T.; Linnartz, H.

2015-01-01

Solid-state astrochemical reaction pathways have the potential to link the formation of small nitrogen-bearing species, like NH3 and HNCO, and prebiotic molecules, specifically amino acids. To date, the chemical origin of such small nitrogen-containing species is still not well understood, despite the fact that ammonia is an abundant constituent of interstellar ices towards young stellar objects and quiescent molecular clouds. This is mainly because of the lack of dedicated laboratory studies. The aim of this work is to experimentally investigate the formation routes of NH3 and HNCO through non-energetic surface reactions in interstellar ice analogues under fully controlled laboratory conditions and at astrochemically relevant temperatures. This study focuses on the formation of NH3 and HNCO in CO-rich (non-polar) interstellar ices that simulate the CO freeze-out stage in dark interstellar cloud regions, well before thermal and energetic processing start to become relevant. We demonstrate and discuss the surface formation of solid HNCO through the interaction of CO molecules with NH radicals - one of the intermediates in the formation of solid NH3 upon sequential hydrogenation of N atoms. The importance of HNCO for astrobiology is discussed.

The efficiency of photodissociation for molecules in interstellar ices

NASA Astrophysics Data System (ADS)

Kalvāns, J.

2018-05-01

Processing by interstellar photons affects the composition of the icy mantles on interstellar grains. The rate of photodissociation in solids differs from that of molecules in the gas phase. The aim of this work was to determine an average, general ratio between photodissociation coefficients for molecules in ice and gas. A 1D astrochemical model was utilized to simulate the chemical composition for a line of sight through a collapsing interstellar cloud core, whose interstellar extinction changes with time. At different extinctions, the calculated column densities of icy carbon oxides and ammonia (relative to water ice) were compared to observations. The latter were taken from literature data of background stars sampling ices in molecular clouds. The best-fit value for the solid/gas photodissociation coefficient ratio was found to be ≈0.3. In other words, gas-phase photodissociation rate coefficients have to be reduced by a factor of 0.3 before applying them to icy species. A crucial part of the model is a proper inclusion of cosmic-ray induced desorption. Observations sampling gas with total extinctions in excess of ≈22 mag were found to be uncorrelated to modelling results, possibly because of grains being covered with non-polar molecules.

Recent observations of interstellar molecules - Detection of CCO and a limit on H2C3O

NASA Technical Reports Server (NTRS)

Brown, R. D.; Cragg, D. M.; Godfrey, P. D.; Irvine, W. M.; Mcgonagle, D.; Ohishi, M.

1992-01-01

In order to test gas-phase reaction schemes for the production of small oxides of carbon in cold, dense interstellar clouds, we have searched for the radical CCO and for propadienone (H2C3O) in Taurus Molecular Cloud 1, a nearby cloud which exhibits a rich organic chemistry. The radical CCO has been detected with a fractional abundance some two orders of magnitude less than that of CCS, about one order of magnitude less than that of H2CCO, and slightly less than that of C3O. An upper limit has been obtained on the abundance of propadienone which is slightly less than that of its isomer propynal (HC2CHO).

Morphological diagnostics of star formation in molecular clouds

NASA Astrophysics Data System (ADS)

Beaumont, Christopher Norris

Molecular clouds are the birth sites of all star formation in the present-day universe. They represent the initial conditions of star formation, and are the primary medium by which stars transfer energy and momentum back to parsec scales. Yet, the physical evolution of molecular clouds remains poorly understood. This is not due to a lack of observational data, nor is it due to an inability to simulate the conditions inside molecular clouds. Instead, the physics and structure of the interstellar medium are sufficiently complex that interpreting molecular cloud data is very difficult. This dissertation mitigates this problem, by developing more sophisticated ways to interpret morphological information in molecular cloud observations and simulations. In particular, I have focused on leveraging machine learning techniques to identify physically meaningful substructures in the interstellar medium, as well as techniques to inter-compare molecular cloud simulations to observations. These contributions make it easier to understand the interplay between molecular clouds and star formation. Specific contributions include: new insight about the sheet-like geometry of molecular clouds based on observations of stellar bubbles; a new algorithm to disambiguate overlapping yet morphologically distinct cloud structures; a new perspective on the relationship between molecular cloud column density distributions and the sizes of cloud substructures; a quantitative analysis of how projection effects affect measurements of cloud properties; and an automatically generated, statistically-calibrated catalog of bubbles identified from their infrared morphologies.

A search for pre-main-sequence stars in high-latitude molecular clouds. 3: A survey of the Einstein database

NASA Technical Reports Server (NTRS)

Caillault, Jean-Pierre; Magnani, Loris; Fryer, Chris

1995-01-01

In order to discern whether the high-latitude molecular clouds are regions of ongoing star formation, we have used X-ray emission as a tracer of youthful stars. The entire Einstein database yields 18 images which overlap 10 of the clouds mapped partially or completely in the CO (1-0) transition, providing a total of approximately 6 deg squared of overlap. Five previously unidentified X-ray sources were detected: one has an optical counterpart which is a pre-main-sequence (PMS) star, and two have normal main-sequence stellar counterparts, while the other two are probably extragalactic sources. The PMS star is located in a high Galactic latitude Lynds dark cloud, so this result is not too suprising. The translucent clouds, though, have yet to reveal any evidence of star formation.

Hot interstellar gas and ionization of embedded clouds

NASA Technical Reports Server (NTRS)

Cheng, K.-P.; Bruhweiler, F.

1990-01-01

Researchers present detailed photoionization calculations for the instellar cloud in which the Sun is embedded. They consider the EUV radiation field with contribution from discrete stellar sources and from a thermal bremsstrahlung-radiative recombination spectrum emitted from the surrounding 10 to the 6th power k coronal substrate. They establish lower limits to the fractional ionization of hydrogen and helium of 0.17 and 0.29 respectively. The high He ionization fraction results primarily from very strong line emission below 500 A originating in the surrounding coronal substrate while the H ionization is dominated by the EUV radiation from the discrete stellar sources. The dual effects of thermal conduction and the EUV spectrum of the 10 to the 6th k plasma on ionization in the cloud skin are explored. The EUV radiation field and Auger ionization have insignificant effects on the resulting ionic column densities of Si IV, C IV, N V and O VI through the cloud skin. Calculations show that the abundances of these species are dominated by collisional ionization in the thermal conduction front. Because of a low charge exchange rate with hydrogen, the ionic column density ratios of N (CIII)/N (CII) and N (NII)/N (NI) are dominated by the EUV radiation field in the local interstellar medium. These ratios should be important diagnostics for the EUV radiation field and serve as surrogate indicators of the interstellar He and H ionization fraction respectively. Spacecraft such as Lyman which is designed to obtain high resolution spectral data down to the Lyman limit at 912 A could sample interstellar lines of these ions.

Distribution of hot stars and hydrogen in the Large Magellanic Cloud

NASA Technical Reports Server (NTRS)

Page, T.; Carruthers, G. R.

1981-01-01

Imagery of the Large Magellanic Cloud (LMC), in the wavelength ranges 1050 to 1600 A and 1250 to 1600 A, was obtained by the S201 far ultraviolet camera during the Apollo 16 mission. These images were reduced to absolute far-UV intensity distributions over the area of the LMC, with 3 to 5 arc min angular resolution. Comparison of these far-UV measurements in the LMC with H sub alpha and 21 cm surveys reveals that interstellar hydrogen in the LMC is often concentrated in 100 pc clouds within 500 pc clouds. Furthermore, at least 25 associations of O-B stars in the LMC are outside the interstellar hydrogen clouds; four of them appear to be on the far side. Far-UV and mid-UV spectra were obtained of stars in 12 of these associations, using the International Ultraviolet Explorer. Equivalent widths of L alpha and six other lines, and relative intensities of the continuum at seven wavelength from 1300 A to 2900 A, were measured. These spectra are also discussed.

A flux-limited treatment for the conductive evaporation of spherical interstellar gas clouds

NASA Technical Reports Server (NTRS)

Dalton, William W.; Balbus, Steven A.

1993-01-01

In this work, we present and analyze a new analytic solution for the saturated (flux-limited) thermal evaporation of a spherical cloud. This work is distinguished from earlier analytic studies by allowing the thermal conductivity to change continuously from a diffusive to a saturated form, in a manner usually employed only in numerical calculations. This closed form solution will be of interest as a computational benchmark. Using our calculated temperature profiles and mass-loss rates, we model the thermal evaporation of such a cloud under typical interstellar medium (ISM) conditions, with some restrictions. We examine the ionization structure of the cloud-ISM interface and evaluate column densities of carbon, nitrogen, oxygen, neon, and silicon ions toward the cloud. In accord with other investigations, we find that ionization equilibrium is far from satisfied under the assumed conditions. Since the inclusion of saturation effects in the heat flux narrows the thermal interface relative to its classical structure, we also find that saturation effects tend to lower predicted column densities.

The interstellar D1 line at high resolution

NASA Technical Reports Server (NTRS)

Hobbs, L. M.; Welty, D. E.

1990-01-01

Observations at a resolving power or a velocity resolution are reported of the interstellar D(sub 1) line of Na I in the spectra of gamma Cas, delta Ori, epsilon Ori, pi Sco, delta Cyg, and alpha Cyg. An echelle grating was used in a double-pass configuration with a CCD detector in the coude spectrograph of the 2.7 m reflector at McDonald Observatory. At least 42 kinematically distinct clouds are detected along the light paths to the five more distant stars, in addition to a single cloud seen toward delta Cyg. The absorption lines arising in 13 of the clouds are sufficiently narrow and unblended to reveal clearly resolved hyperfine structure components split by 1.05 km/s. An additional 13 clouds apparently show comparably narrow, but more strongly blended, lines. For each individual cloud, upper limits T(sub max) and (v sub t)(sub max) on the temperature and the turbulent velocity, respectively, are derived by fitting the observed lines with theoretical absorption profiles.

ON THE INTERACTION OF ADENINE WITH IONIZING RADIATION: MECHANISTICAL STUDIES AND ASTROBIOLOGICAL IMPLICATIONS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Evans, Nicholas L.; Ullrich, Susanne; Bennett, Chris J.

2011-04-01

The molecular inventory available on the prebiotic Earth was likely derived from both terrestrial and extraterrestrial sources. A complete description of which extraterrestrial molecules may have seeded early Earth is therefore necessary to fully understand the prebiotic evolution which led to life. Galactic cosmic rays (GCRs) are expected to cause both the formation and destruction of important biomolecules-including nucleic acid bases such as adenine-in the interstellar medium within the ices condensed on interstellar grains. The interstellar ultraviolet (UV) component is expected to photochemically degrade gas-phase adenine on a short timescale of only several years. However, the destruction rate is expectedmore » to be significantly reduced when adenine is shielded in dense molecular clouds or even within the ices of interstellar grains. Here, biomolecule destruction by the energetic charged particle component of the GCR becomes important as it is not fully attenuated. Presented here are results on the destruction rate of the nucleobase adenine in the solid state at 10 K by energetic electrons, as generated in the track of cosmic ray particles as they penetrate ices. When both UV and energetic charged particle destructive processes are taken into account, the half-life of adenine within dense interstellar clouds is found to be {approx}6 Myr, which is on the order of a star-forming molecular cloud. We also discuss chemical reaction pathways within the ices to explain the production of observed species, including the formation of nitriles (R-C{identical_to}N), epoxides (C-O-C), and carbonyl functions (R-C=O).« less

Non-equilibrium ionization around clouds evaporating in the interstellar medium

NASA Technical Reports Server (NTRS)

Ballet, J.; Luciani, J. F.; Mora, P.

1986-01-01

It is of prime importance for global models of the interstellar medium to know whether dense clouds do or do not evaporate in the hot coronal gas. The rate of mass exchanges between phases depends very much on that. McKee and Ostriker's model, for instance, assumes that evaporation is important enough to control the expansion of supernova remnants, and that mass loss obeys the law derived by Cowie and McKee. In fact, the geometry of the magnetic field is nearly unknown, and it might totally inhibit evaporation, if the clouds are not regularly connected to the hot gas. Up to now, the only test of the theory is the U.V. observation (by the Copernicus and IUE satellites) of absorption lines of ions such as OVI or NV, that exist at temperatures of a few 100,000 K typical of transition layers around evaporating clouds. Other means of testing the theory are discussed.

Spitzer Observations of Dust Destruction in the Puppis A Supernova Remnant

NASA Technical Reports Server (NTRS)

Arendt, Richard G.; Dweek, Eli; Blair, William P.; Ghavamian, Parviz; Hwang, Una; Long, Knox X.; Petre, Robert; Rho, Jeonghee; Winkler, P. Frank

2010-01-01

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.

The Properties of Single Interstellar Clouds: Cycle 1, SIDE-2 Observations

NASA Astrophysics Data System (ADS)

Hobbs, Lewis

1990-12-01

WE PROPOSE TO USE THE ECHELLE GRATING OF THE HIGH RESOLUTION SPECTROGRAPH OVER A TWO-YEAR PERIOD TO OBSERVE THE PROFILES OF INTERSTELLAR ABSORPTION LINES. THE COLUMN DENSITES OF 18 NEUTRAL OR IONIZED FORMS OF THE ELEMENTS C,N,O,Mg,Si,P,S,Fe, AND Zn WILL BE MEASURED IN THE APPROXIMATELY 100 INDIVIDUAL INTERSTELLAR CLOUDS ALONG THE LIGHT PATHS TO 18 BRIGHT, BROAD-LINED STARS OF EARLY SPECTRAL TYPE WITHIN 1 KPC OF THE SUN. THE PRIMARY PURPOSE OF THE OBSERVATIONS IS TO DETERMINE MORE ACCURATELY THAN WAS HITHERTO POSSIBLE THE FUNDAMENTAL PHYSICAL PROPERTIES OF THE RESOLVED CLOUDS, INCLUDING LINEAR SIZE, TEMPERATURE, TOTAL DENSITY, FRACTIONAL IONIZATION AND THE RELATIVE ABUNDANCES OF THE 9 SELECTED ELEMENTS. THE REST OF THIS OBSERVING PROGRAM IS CONTAINED IN APPROVED PROPOSAL ID = 2251; THE PROGRAM EUNUMERATED HERE CONSISTS OF THAT PART OF OUR ORIGINAL PROGRAM WHICH CAN BE CARRIED OUT USING ONLY SIDE 2 OF THE GHRS. THIS PROGRAM THEREFORE CONSISTS OF ECH-B OBSERVATIONS OF EACH OF 4 STARS AT 7 WAVELENGTHS. PROGRAM 2251 SHOULD BE CONSULTED FOR ADDITIONAL DETAILS.

The Properties of Single Interstellar Clouds: Hopr Repeat Cycle 1, SIDE-2 Observations

NASA Astrophysics Data System (ADS)

Hobbs, Lewis

1990-12-01

WE PROPOSE TO USE THE ECHELLE GRATING OF THE HIGH RESOLUTION SPECTROGRAPH OVER A TWO-YEAR PERIOD TO OBSERVE THE PROFILES OF INTERSTELLAR ABSORPTION LINES. THE COLUMN DENSITES OF 18 NEUTRAL OR IONIZED FORMS OF THE ELEMENTS C,N,O,Mg,Si,P,S,Fe, AND Zn WILL BE MEASURED IN THE APPROXIMATELY 100 INDIVIDUAL INTERSTELLAR CLOUDS ALONG THE LIGHT PATHS TO 18 BRIGHT, BROAD-LINED STARS OF EARLY SPECTRAL TYPE WITHIN 1 KPC OF THE SUN. THE PRIMARY PURPOSE OF THE OBSERVATIONS IS TO DETERMINE MORE ACCURATELY THAN WAS HITHERTO POSSIBLE THE FUNDAMENTAL PHYSICAL PROPERTIES OF THE RESOLVED CLOUDS, INCLUDING LINEAR SIZE, TEMPERATURE, TOTAL DENSITY, FRACTIONAL IONIZATION AND THE RELATIVE ABUNDANCES OF THE 9 SELECTED ELEMENTS. THE REST OF THIS OBSERVING PROGRAM IS CONTAINED IN APPROVED PROPOSAL ID = 2251; THE PROGRAM EUNUMERATED HERE CONSISTS OF THAT PART OF OUR ORIGINAL PROGRAM WHICH CAN BE CARRIED OUT USING ONLY SIDE 2 OF THE GHRS. THIS PROGRAM THEREFORE CONSISTS OF ECH-B OBSERVATIONS OF EACH OF 4 STARS AT 7 WAVELENGTHS. PROGRAM 2251 SHOULD BE CONSULTED FOR ADDITIONAL DETAILS.

The Properties of Single Interstellar Clouds Cycle 2

NASA Astrophysics Data System (ADS)

Hobbs, Lewis

1991-07-01

IN THIS CONTINUATION PROPOSAL, WE PROPOSE TO USE THE ECHELLEAND 160M GRATINGS OF THE HIGH RESOLUTION SPECTROGRAPH TO OBSERVE THE PZROFILES OF INTERSTELLAR ABSORPTION LINES, DURING THE SECOND YEAR OF A TWO-YEAR PROGRAM. IN THE TWO CYCLES TOGETHER, THE COLUMN DENSITES OF 17 NEUTRAL OR IONIZED FORMS OF THE ELEMENTS C,N,O,Mg,Si,P,S,Fe, AND Zn WILL BE MEASURED IN THE APPROXIMATELY 100 INDIVIDUAL INTERSTELLAR CLOUDS ALONG THE LIGHT PATHS TO 12 BRIGHT, BROAD-LINED STARS OF EARLY SPECTRAL TYPE WITHIN 1 KPC OF THE SUN. THE PRIMARY PURPOSE OF THE OBSERVATIONS IS TO DETERMINE MORE ACCURATELY THAN WAS HITHERTO POSSIBLE THE FUNDAMENTAL PHYSICAL PROPERTIES OF THE RESOLVED CLOUDS, INCLUDING LINEAR SIZE, TEMPERATURE, TOTAL DENSITY, FRACTIONAL IONIZATION AND THE RELATIVE ABUNDANCES OF THE 9 SELECTED ELEMENTS. THIS SECOND-YEAR PROGRAM CONSISTS OF ECH-B AND G160M OBSERVATIONS OF EACH OF 4 STARS AT 21 OR MORE WAVELENGTHS, AND OF A SUBSET OF THESE OBSERVATIONS FOR A FIFTH STAR, PI SCO. PROGRAMS 2251 AND 3993 SHOULD BE CONSULTED FOR DETAILS OF THE PREVIOUS OBSERVATIONS OBTAINED DURING CYCLE 1.

The violent interstellar medium in Milky-Way like disk galaxies

NASA Astrophysics Data System (ADS)

Karoline Walch, Stefanie

2015-08-01

Molecular clouds are cold, dense, and turbulent filamentary structures that condense out of the multi-phase interstellar medium. They are also the sites of star formation. The minority of new-born stars is massive, but these stars are particularly important for the fate of their parental molecular clouds as their feedback drives turbulence and regulates star formation.I will present results from the SILCC project (SImulating the Life Cycle of molecular Clouds), in which we study the formation and dispersal of molecular clouds within the multi-phase ISM using high-performance, three-dimensional simulations of representative pieces of disk galaxies. Apart from stellar feedback, self-gravity, an external stellar potential, and magnetic fields, we employ an accurate description of gas heating and cooling as well as a small chemical network including molecule formation and (self-)shielding from the interstellar radiation field. We study the impact of the supernova rate and the positioning of the supernova explosions with respect to the molecular gas in a well defined set of simulations. This allows us to draw conclusions on structure of the multi-phase ISM, the amount of molecular gas formed, and the onset of galactic outflows. Furthermore, we show how important stellar wind feedback is for regulating star formation in these disks.

Modeling the chemistry of the dense interstellar clouds. I - Observational constraints for the chemistry

NASA Technical Reports Server (NTRS)

Federman, S. R.; Huntress, W. T., Jr.; Prasad, S. S.

1990-01-01

A search for correlations arising from molecular line data is made in order to place constraints on the chemical models of interstellar clouds. At 10 to the 21st H2/sq cm, N(CO) for dark clouds is a factor of six greater than the value for diffuse clouds. This implies that the strength of the UV radiation field where CO shields itself from dissociation is about one-half the strength of the average Galactic field. The dark cloud data indicate that the abundance of CO continues to increase with A(V) for directions with A(V) of 4 mag or less, although less steeply with N(H2) than for diffuse clouds. For H2CO, a quadratic relationship is obtained in plots versus H2 column density. The data suggest a possible turnover at the highest values for A(V). NH3 shows no correlation with H2, C(O-18), HC3N, or HC5N; a strong correlation is found between HC5N and HC3N, indicating a chemical link between the cyanopolyynes.

Cosmic ray processing of N2-containing interstellar ice analogues at dark cloud conditions

NASA Astrophysics Data System (ADS)

Fedoseev, G.; Scirè, C.; Baratta, G. A.; Palumbo, M. E.

2018-04-01

N2 is believed to lock considerable part of nitrogen elemental budget and, therefore, to be one of the most abundant ice constituent in cold dark clouds. This laboratory-based research utilizes high energetic processing of N2 containing interstellar ice analogues using 200 keV H+ and He+ ions that mimics cosmic ray processing of the interstellar icy grains. It aims to investigate the formation of (iso)cyanates and cyanides in the ice mantles at the conditions typical for cold dark clouds and prestellar cores. Investigation of cosmic ray processing as a chemical trigger mechanism is explained by the high stability of N2 molecules that are chemically inert in most of the atom- and radical-addition reactions and cannot be efficiently dissociated by cosmic ray induced UV-field. Two sets of experiments are performed to closer address solid-state chemistry occurring in two distinct layers of the ice formed at different stages of dark cloud evolution, i.e. `H2O-rich' and `CO-rich' ice layers. Formation of HNCO and OCN- is discussed in all of the performed experiments. Corresponding kinetic curves for HNCO and OCN- are obtained. Furthermore, a feature around 2092 cm-1 assigned to the contributions of 13CO, CN-, and HCN is analysed. The kinetic curves for the combined HCN/CN- abundance are derived. In turn, normalized formation yields are evaluated by interpolation of the obtained results to the low irradiation doses relevant to dark cloud stage. The obtained values can be used to interpret future observations towards cold dark clouds using James Webb Space Telescope.

THE INTERSTELLAR MEDIUM IN THE KEPLER SEARCH VOLUME

DOE Office of Scientific and Technical Information (OSTI.GOV)

Johnson, Marshall C.; Redfield, Seth; Jensen, Adam G., E-mail: mjohnson@astro.as.utexas.edu

2015-07-10

The properties of the interstellar medium (ISM) surrounding a planetary system can impact planetary climate through a number of mechanisms, including changing the size of the astrosphere (one of the major shields for cosmic rays) as well as direct deposition of material into planetary atmospheres. In order to constrain the ambient ISM conditions for exoplanetary systems, we present observations of interstellar Na i and K i absorption toward seventeen early type stars in the Kepler prime mission field of view (FOV). We identify 39 Na i and 8 K i velocity components, and attribute these to 11 ISM clouds. Sixmore » of these are detected toward more than one star, and for these clouds we put limits on the cloud properties, including distance and hydrogen number density. We identify one cloud with significant (≳1.5 cm{sup −3}) hydrogen number density located within the nominal ∼100 pc boundary of the Local Bubble. We identify systems with confirmed planets within the Kepler FOV that could lie within these ISM clouds, and estimate upper limits on the astrosphere sizes of these systems under the assumption that they do lie within these clouds. Under this condition, the Kepler-20, 42, and 445 multiplanet systems could have compressed astrospheres much smaller than the present-day heliosphere. Among the known habitable zone planet hosts, Kepler-186 could have an astrosphere somewhat smaller than the heliosphere, while Kepler-437 and KOI-4427 could have astrospheres much larger than the heliosphere. The thick disk star Kepler-444 may have an astrosphere just a few AU in radius.« less

A survey of bimolecular ion-molecule reactions for use in modeling the chemistry of planetary atmospheres, cometary comae, and interstellar clouds - 1993 supplement

NASA Technical Reports Server (NTRS)

Anicich, V. G.

1993-01-01

This is a supplement to a previous paper (Anicich & Huntress 1986). It is a survey of bimolecular positive ion-molecule reactions with potential importance to the chemistry of planetary atmospheres, cometary comae, and interstellar clouds. This supplement covers the literature from 1986 through 1991, with some additional citations missed in the original survey. Over 200 new citations are included. A table of reactions is listed by reactant ion, and cross-references are provided for both ionic and neutral reactants and also for both ionic and neutral products.

Molecular clouds in galaxies with different Z - Fragmentation of diffuse clouds driven by opacity

NASA Technical Reports Server (NTRS)

Franco, Jose; Cox, Donald P.

1986-01-01

Molecular clouds are formed from diffuse interstellar clouds when the external ultraviolet radiation field is prevented from penetrating into the cloud. The opacity is provided mainly by dust grains and the required column density to the cloud center is larger than about 5 x 10 to the 20th (solar Z/Z)/sq cm. This high-opacity criterion could have a significant impact on the radial trends observed in spiral galaxies, and on the distinctions between spiral and dwarf irregular galaxies.

VARIATIONS BETWEEN DUST AND GAS IN THE DIFFUSE INTERSTELLAR MEDIUM. II. SEARCH FOR COLD GAS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Reach, William T.; Heiles, Carl; Bernard, Jean-Philippe, E-mail: wreach@sofia.usra.edu

2017-01-01

The content of interstellar clouds, in particular the inventory of diffuse molecular gas, remains uncertain. We identified a sample of isolated clouds, approximately 100 M {sub ⊙} in size, and used the dust content to estimate the total amount of gas. In Paper I, the total inferred gas content was found significantly larger than that seen in 21 cm emission measurements of H i. In this paper we test the hypothesis that the apparent excess “dark” gas is cold H i, which would be evident in absorption but not in emission due to line saturation. The results show that theremore » is not enough 21 cm absorption toward the clouds to explain the total amount of “dark” gas.« less

Ion-molecule calculation of the abundance ratio of CCD to CCH in dense interstellar clouds

NASA Technical Reports Server (NTRS)

Herbst, Eric; Adams, Nigel G.; Smith, David; Defrees, D. J.

1987-01-01

Laboratory measurements and calculations have been performed to determine the abundance ratio of the deuterated ethynyl radical (CCD) to the normal radical (CCH) which can be achieved in dense interstellar clouds via isotopic fractionation in the C2H2(+) (HD)=C2HD(+)(H2) system of reactions. According to this limited treatment, the CCD/CCH abundance ratio which can be attained is in the range 0.02-0.03 for the Orion molecular cloud and 0.0l-0.02 for TMC-1. These ranges of numbers are in reasonable agreement with the observed values in Orion and TMC-1. However, the analysis of the CCD/CCH abundance ratio is complicated via the presence of competing fractionation mechanisms, especially in the low-temperature source TMC-1.

Researchers Use NRAO Telescope to Study Formation Of Chemical Precursors to Life

NASA Astrophysics Data System (ADS)

2006-08-01

In just two years of work, an international research team has discovered eight new complex, biologically-significant molecules in interstellar space using the National Science Foundation's Robert C. Byrd Green Bank Telescope (GBT) in West Virginia. "This is a feat unprecedented in the 35-year history of searching for complex molecules in space and suggests that a universal prebiotic chemistry is at work," said Jan M. Hollis of the NASA Goddard Space Flight Center, leader of the research team. Chemistry Cycle The Cosmic Chemistry Cycle CREDIT: Bill Saxton, NRAO/AUI/NSF Full Size Image Files Interactive Graphic With "Mouseover" Text Blocks Chemical Cycle Graphic (above image, JPEG, 129K) Graphic With Text Blocks (JPEG, 165K) High-Res TIFF (44.2M) High-Res TIFF With Text Blocks (44.2M) Green Bank Telescope and Molecule Diagrams (JPEG, 58K) Green Bank Telescope and Molecule Diagrams (TIFF, 21M) New Molecules: Chemical Diagrams (PDF, 64K) The new discoveries are helping scientists unlock the secrets of how the molecular precursors to life can form in the giant clouds of gas and dust in which stars and planets are born. "The first of the many chemical processes that ultimately led to life on Earth probably took place even before our planet was formed. The GBT has taken the leading role in exploring the origin of biomolecules in interstellar clouds," said Phil Jewell of the National Radio Astronomy Observatory (NRAO). The eight new molecules discovered with the GBT bring the total to 141 different molecular species found in interstellar space. About 90 percent of those interstellar molecules contain carbon, which is required for a molecule to be classified as organic. The newly-discovered molecules all contain carbon and are composed of 6 to 11 atoms each. These results suggest, the scientists say, that chemical evolution occurs routinely in the gas and dust from which stars and planets eventually are born. The mass of an interstellar cloud is 99 percent gas and one percent dust. The GBT discoveries have been made in just two prototypical interstellar clouds. The molecules acetamide (CH3CONH2), cyclopropenone (H2C3O), propenal (CH2CHCHO), propanal (CH3CH2CHO), and ketenimine (CH2CNH) were found in a cloud called Sagittarius B2(N), which is near the center of our Milky Way Galaxy some 26,000 light years from Earth. This star-forming region is the largest repository of complex interstellar molecules known. The molecules methyl-cyano-diacetylene (CH3C5N), methyl-triacetylene (CH3C6H), and cyanoallene (CH2CCHCN) were found in the Taurus Molecular Cloud (TMC-1), which is relatively nearby at a distance of 450 light years. The starless TMC-1 cloud is dark and cold with a temperature of only 10 degrees above absolute zero and may eventually evolve into a star-forming region. "The discovery of these large organic molecules in the coldest regions of the interstellar medium has certainly changed the belief that large organic molecules would only have their origins in hot molecular cores. It has forced us to rethink the paradigms of interstellar chemistry," said Anthony Remijan of the NRAO. These large molecules found with the GBT are built up from smaller ones, the scientists say, by two principal mechanisms. In the first, simple chemical reactions add an atom to a molecular structure residing on the surface of a dust grain. As an example of this process, the researchers cite a molecule called cyclopropenylidene (c-C3H2, where "c-" means cyclic), which contains three carbon atoms in a ring. Cyclopropenylidene was discovered in interstellar space in 1987, and is known to be highly reactive. In 2005, using the GBT, scientists discovered another molecule, cyclopropenone (c-H2C3O), which can be produced by adding an oxygen atom to cyclopropenylidene. The second method for constructing larger molecules from smaller ones involves neutral-radical reactions that can occur within the gas in an interstellar cloud. For example, in 2006, the scientists discovered acetamide (CH3CONH2), which can be formed when a previously-discovered neutral molecule called formamide (HCONH2) combines with radicals such as CH2 and CH3, also previously discovered. Acetamide is particularly interesting because it contains a peptide bond which is the means for linking amino acids together to form proteins. Once interstellar molecules are ejected from dust grains into the gas phase, presumably by shock waves, they are free to rotate end-over-end. As gas molecules change their rotational modes, they can emit or absorb radiation at precise radio frequencies, called transitions, that are unique to each type of molecule. By detecting several rotational transitions, astronomers can unambiguously identify a specific interstellar molecule. "It is important to note that likely interstellar molecule candidates are first studied in gas-phase laboratory experiments so that transition frequencies are known in advance of an interstellar experiment," said Frank Lovas of the National Institute of Standards and Technology. Along the line of sight from the interstellar cloud to the telescope, thousands of billions of molecules undergo the exact same transition, producing a signal strong enough to be detected by sensitive equipment. For this type of work, the GBT is the world's most sensitive tool that can be accurately pointed and track astronomical objects. In addition to Hollis, Jewell, Remijan, and Lovas, the research team included Lewis Snyder of the University of Illinois; Harald Mollendal of the University of Oslo, Norway; Vadim Ilyushin of the Institute of Radio Astronomy of the National Academy of Sciences of the Ukraine; and Isabell Kleiner of the Universite Paris, France. The astronomers' reports on their results appeared in 8 separate editions of the Astrophysical Journal. The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

Observational analysis of the well-correlated diffuse bands: 6196 and 6614 Å

NASA Astrophysics Data System (ADS)

Krełowski, J.; Galazutdinov, G. A.; Bondar, A.; Beletsky, Y.

2016-08-01

We confirm, using spectra from seven observatories, that the diffuse bands 6196 and 6614 are very tightly correlated. However, their strength ratio is not constant as well as profile shapes. Apparently, the two interstellar features do not react in unison to the varying physical conditions of different interstellar clouds.

CNO isotopes in red giant stars

NASA Technical Reports Server (NTRS)

Wannier, P. G.

1985-01-01

The production and distribution of the CNO nuclides is discussed in light of observed abundance ratios in red giants and in the interstellar medium. Isotope abundances have been measured in the atmospheres and in the recent ejecta of cool giants, including carbon stars, S-type stars and red supergiants as well as in oxygen-rich giants making their first ascent of the giant branch. Several of the observations suggest revision of currently accepted nuclear cross-sections and of the mixing processes operating in giant envelopes. By comparing red giant abundances with high-quality observations of the interstellar medium, conclusions are reached about the contribution of intermediate-mass stars to galactic nuclear evolution. The three oxygen isotopes, O-16, -17 and -18, are particularly valuable for such comparison because they reflect three different stages of stellar nucleosynthesis. One remarkable result comes from observations of O-17/O-18 in several classes of red giant stars. The observed range of values for red giants excludes the entire range of values seen in interstellar molecular clouds. Furthermore, both the observations of stars and interstellar clouds exclude the isotopic ratio found in the solar system.

Low-energy electron-induced chemistry of condensed methanol: implications for the interstellar synthesis of prebiotic molecules.

PubMed

Boamah, Mavis D; Sullivan, Kristal K; Shulenberger, Katie E; Soe, ChanMyae M; Jacob, Lisa M; Yhee, Farrah C; Atkinson, Karen E; Boyer, Michael C; Haines, David R; Arumainayagam, Christopher R

2014-01-01

In the interstellar medium, UV photolysis of condensed methanol (CH3OH), contained in ice mantles surrounding dust grains, is thought to be the mechanism that drives the formation of "complex" molecules, such as methyl formate (HCOOCH3), dimethyl ether (CH3OCH3), acetic acid (CH3COOH), and glycolaldehyde (HOCH2CHO). The source of this reaction-initiating UV light is assumed to be local because externally sourced UV radiation cannot penetrate the ice-containing dark, dense molecular clouds. Specifically, exceedingly penetrative high-energy cosmic rays generate secondary electrons within the clouds through molecular ionizations. Hydrogen molecules, present within these dense molecular clouds, are excited in collisions with these secondary electrons. It is the UV light, emitted by these electronically excited hydrogen molecules, that is generally thought to photoprocess interstellar icy grain mantles to generate "complex" molecules. In addition to producing UV light, the large numbers of low-energy (< 20 eV) secondary electrons, produced by cosmic rays, can also directly initiate radiolysis reactions in the condensed phase. The goal of our studies is to understand the low-energy, electron-induced processes that occur when high-energy cosmic rays interact with interstellar ices, in which methanol, a precursor of several prebiotic species, is the most abundant organic species. Using post-irradiation temperature-programmed desorption, we have investigated the radiolysis initiated by low-energy (7 eV and 20 eV) electrons in condensed methanol at - 85 K under ultrahigh vacuum (5 x 10(-10) Torr) conditions. We have identified eleven electron-induced methanol radiolysis products, which include many that have been previously identified as being formed by methanol UV photolysis in the interstellar medium. These experimental results suggest that low-energy, electron-induced condensed phase reactions may contribute to the interstellar synthesis of "complex" molecules previously thought to form exclusively via UV photons.

Laboratory absorption spectra of molecules at interstellar cloud temperatures - First measurements on CO at about 97 nm

NASA Technical Reports Server (NTRS)

Smith, P. L.; Yoshino, K.; Stark, G.; Ito, K.; Stevens, M. H.

1991-01-01

In the 91-100 nm spectral region, where absorption of photons by interstellar CO usually leads to dissociation, laboratory spectra obtained at 295 K show that most CO bands are both overlapped and perturbed. Reliable band oscillator strengths cannot be extracted from such spectra. As a consequence, synthetic extreme-ultraviolet absorption spectra for CO at the low temperatures that prevail in interstellar clouds are uncertain. A supersonic expansion technique has been used to cool CO to 30 K and three bands in the 97-nm region have been studied with high spectral resolution. The measured spectrum at 30 K is in reasonable agreement with some published modeled spectra, but the ratios of integrated cross sections are somewhat different from those determined from low resolution spectra obtained at 295 K, in which the bands are blended.

Metastability of isoformyl ions in collisions with helium and hydrogen. [in interstellar molecular clouds

NASA Technical Reports Server (NTRS)

Green, S.

1984-01-01

The stability of HOC(+) ions under conditions in interstellar molecular clouds is considered. In particular, the possibility that collisions with helium or hydrogen will induce isomerization to the stable HCO(+) form is examined theoretically. Portions of the electronic potential energy surfaces for interaction with He and H atoms are obtained from standard quantum mechanical calculations. Collisions with He atoms are found to be totally ineffective for inducing isomerization. Collisions with H atoms are found to be ineffective at low interstellar temperatures owing to a small (about 500 K) barrier in the entrance channel; at higher temperatures where this barrier can be overcome, however, collisions with hydrogen atoms do result in conversion to the stable HCO(+) form. Although detailed calculations are not presented, it is argued that low-energy collisions with H2 molecules are also ineffective in destroying the metastable ion.

The Origin of Filamentary Star Forming Clouds in Magnetised Galaxies

NASA Astrophysics Data System (ADS)

Körtgen, Bastian; Banerjee, Robi; Pudritz, Ralph E.; Schmidt, Wolfram

2018-05-01

Observations show that galaxies and their interstellar media are pervaded by strong magnetic fields with energies in the diffuse component being at least comparable to the thermal and even as large or larger than the turbulent energy. Such strong magnetic fields prevent the formation of stars because patches of the interstellar medium are magnetically subcritical. Here we present the results from global numerical simulations of strongly magnetised and self-gravitating galactic discs, which show that the buoyancy of the magnetic field due to the Parker instability leads at first to the formation of giant filamentary regions. These filamentary structures become gravitationally unstable and fragment into ˜105M⊙ clouds that attract kpc long, coherent filamentary flows that build them into GMCs. Our results thus provide a solution to the long-standing problem of how the transition from sub- to supercritical regions in the interstellar medium proceeds.

Organic Synthesis in Simulated Interstellar Ice Analogs

NASA Technical Reports Server (NTRS)

Dworkin, Jason P.; Bernstein, Max P.; Sandford, Scott A.; Allamandola, Louis J.; Deamer, David W.; Elsila, Jamie; Zare, Richard N.; DeVincenzi, Donald (Technical Monitor)

2001-01-01

Comets and carbonaceous micrometeorites may have been significant sources of organic compounds on the early Earth. Ices on grains in interstellar dense molecular clouds contain a variety of simple molecules as well as aromatic molecules of various sizes. While in these clouds the icy grains are processed by ultraviolet light and cosmic radiation which produces more complex organic molecules. ID We have run laboratory simulations to identify the types of molecules which could have been generated photolytically in pre-cometary ices. Experiments were conducted by forming various realistic interstellar mixed-molecular ices with and without polycyclic aromatic hydrocarbons (PAHs) at approx. 10 K under high vacuum irradiated with LTV light from a hydrogen plasma lamp: The residue that remained after warming to room temperature was analyzed by HPLC, and by laser desorption mass spectrometry. The residue contains several classes of compounds which may be of prebiotic significance.

Organic Synthesis in Simulated Interstellar Ice Analogs

NASA Technical Reports Server (NTRS)

Dworkin, Jason P.; Bernstein, Max P.; Sandford, Scott A.; Allamandola, Louis J.; Deamer, David W.; Elsila, Jamie; Zare, Richard N.

2001-01-01

Comets and carbonaceous micrometeorites may have been significant sources of organic compounds on the early Earth. Ices on grains in interstellar dense molecular clouds contain a variety of simple molecules as well as aromatic molecules of various sizes. While in these clouds the icy grains are processed by ultraviolet light and cosmic radiation which produces more complex organic molecules. We have run laboratory simulations to identify the types of molecules which could have been generated photolytically in pre-cometary ices. Experiments were conducted by forming various realistic interstellar mixed-molecular ices with and without polycyclic aromatic hydrocarbons (PAHs) at approx. 10 K under high vacuum irradiated with UV light from a hydrogen plasma lamp. The residue that remained after warming to room temperature was analyzed by HPLC, and by laser desorption mass spectrometry. The residue contains several classes of compounds which may be of prebiotic significance.

Laboratory Investigations of the Physical and Optical Properties of the Analogs of Individual Cosmic Dust Grains

NASA Technical Reports Server (NTRS)

Abbas, M. M.; Tankosic, D.; Craven, P. D.; Spann, J. F.; LeClair, A.; West, E. A.

2005-01-01

Microdsub-micron size cosmic dust grains play an important role in the physical and dynamical process in the galaxy, the interstellar medium, and the interplanetary and planetary environments. The dust grains in various astrophysical environments are generally charged by a variety of mechanisms that include collisional process with electrons and ions, and photoelectric emissions with UV radiation. The photoelectric emission process is believed to be the dominant process in many astrophysical environments with nearby UV sources, such as the interstellar medium, diffuse clouds, the outer regions of the dense molecular clouds, interplanetary medium, dust in planetary environments and rings, cometary tails, etc. Also, the processes and mechanisms involved in the rotation and alignment of interstellar dust grains are of great interest in view of the polarization of observed starlight as a probe for evaluation of the galactic magnetic field.

Silicon chemistry in interstellar clouds

NASA Technical Reports Server (NTRS)

Langer, William D.; Glassgold, A. E.

1989-01-01

Interstellar SiO was discovered shortly after CO but it has been detected mainly in high density and high temperature regions associated with outflow sources. A new model of interstellar silicon chemistry that explains the lack of SiO detections in cold clouds is presented which contains an exponential temperature dependence for the SiO abundance. A key aspect of the model is the sensitivity of SiO production by neutral silicon reactions to density and temperature, which arises from the dependence of the rate coefficients on the population of the excited fine structure levels of the silicon atom. This effect was originally pointed out in the context of neutral reactions of carbon and oxygen by Graff, who noted that the leading term in neutral atom-molecule interactions involves the quadrupole moment of the atom. Similar to the case of carbon, the requirement that Si has a quadrupole moment requires population of the J = 1 level, which lies 111K above the J = 0 ground state and has a critical density n(cr) equal to or greater than 10(6)/cu cm. The SiO abundance then has a temperature dependence proportional to exp(-111/T) and a quadratic density dependence for n less than n(cr). As part of the explanation of the lack of SiO detections at low temperatures and densities, this model also emphasizes the small efficiencies of the production routes and the correspondingly long times needed to reach equilibrium. Measurements of the abundance of SiO, in conjunction with theory, can provide information on the physical properties of interstellar clouds such as the abundances of oxygen bearing molecules and the depletion of interstellar silicon.

Photochemistry of polycyclic aromatic hydrocarbons in cosmic water ice. II. Near UV/VIS spectroscopy and ionization rates

NASA Astrophysics Data System (ADS)

Bouwman, J.; Cuppen, H. M.; Steglich, M.; Allamandola, L. J.; Linnartz, H.

2011-05-01

Context. Mid-infrared emission features originating from polycyclic aromatic hydrocarbons (PAHs) are observed towards photon dominated regions in space. Towards dense clouds, however, these emission features are quenched. Observations of dense clouds show that many simple volatile molecules are frozen out on interstellar grains, forming thin layers of ice. Recently, observations have shown that more complex non-volatile species, presumably including PAHs, also freeze out and contribute to the ongoing solid-state chemistry. Aims: The study presented here aims at obtaining reaction rate data that characterize PAH photochemistry upon vacuum ultraviolet (VUV) irradiation in an interstellar H2O ice analogue to explore the potential impact of PAH:H2O ice reactions on overall interstellar ice chemistry. To this end, the experimental results are implemented in a chemical model under simple interstellar cloud conditions. Methods: Time-dependent near-UV/VIS spectroscopy on the VUV photochemistry of anthracene, pyrene, benzo[ghi]perylene and coronene containing interstellar H2O ice analogs is performed at 25 and 125 K, using an optical absorption setup. Results: Near-UV/VIS absorption spectra are presented for these four PAHs and their photoproducts including cationic species trapped in H2O ice. Oscillator strengths of the cation absorption bands are derived relative to the oscillator strength of the neutral parent PAH. The loss of the parent and growth of PAH photoproducts are measured as a function of VUV dose, yielding solid state reaction constants. The rate constants are used in an exploratory astrochemical model, to assess the importance of PAH:H2O ice photoprocessing in UV exposed interstellar environments, compared with the timescales in which PAH molecules are incorporated in interstellar ices. Conclusions: All four PAHs studied here are found to be readily ionized upon VUV photolysis when trapped in H2O ice and exhibit similar rates for ionization at astronomically relevant temperatures. Depending on the relative efficiency of H2O photodesorption and PAH photoionization in H2O ice, the latter may trigger a charge induced aromatic solid state chemistry, in which PAH cations play a central role.

Interstellar Ice and Dust: The Feedstock of the Solar System

NASA Technical Reports Server (NTRS)

Allamandola, L. J.; Morrison, David (Technical Monitor)

1994-01-01

Studying the chemical and isotopic composition of interstellar ice and dust provides insight into the composition and chemical history of the solid bodies in the solar nebula and the nature of the material subsequently brought into the inner part of the solar system by comets and meteorites. It is now possible to probe the composition of these microscopic interstellar particles (some hundreds of light years away), thanks to substantial progress in two areas: astronomical spectroscopic techniques in the middle-infrared, the spectral region most diagnostic of composition; and laboratory simulations which realistically reproduce the critical conditions in various interstellar environments. High quality infrared spectra of many different astronomical sources, some associated with dark molecular clouds, and others in the diffuse interstellar medium (DISM) are now available. What comparisons of these spectra with laboratory spectra tell us about the complex organic components of these materials is the subject of this talk. Most interstellar material is concentrated in large molecular clouds where simple molecules are formed by gas phase and dust grain surface reactions. Gaseous species (except H2) striking the cold (10K) dust will stick, forming an icy grain mantle. This accretion, coupled with energetic particle bombardment and UV photolysis, will produce a complex chemical mixture containing volatile, non-volatile, and isotopically fractionated species. One can compare spectra of the diffuse and dense interstellar medium with the spectra of analogs produced in the laboratory under conditions which mimic those in these different environments. In this way one can determine the composition and abundances of the major constituents present and place general constraints on the types and relative abundances of organics coating the grains. Ices in dense clouds contain H2O, CH3OH, CO, perhaps some NH3 and H2CO, as well as nitriles and ketones or esters. There is some evidence that the later, more complex species, are also present on the grains in the DISM. The evidence for these materials, in addition to carbon rich materials such as amorphous carbon, microdiamonds, and polycyclic aromatic hydrocarbons will be reviewed and the possible connection with meteoritic organics will be discussed.

Molecular Line Studies of Ballistic Stellar Interlopers Burrowing through Dense Interstellar Clouds

NASA Astrophysics Data System (ADS)

Rosen, Anna; Sahai, R.; Claussen, M.; Morris, M.

2010-01-01

When an intermediate-mass star speeds through a dense interstellar cloud at a high velocity, it can produce a cometary or bow shock structure due to the cloud being impacted by the intense stellar wind. This class of objects, recently discovered in an HST imaging survey, has been dubbed "ballistic stellar interlopers" (Sahai et al. 2009). Using the ARO's 12m and SMT 10m millimeter-wave dishes, we have obtained molecular line emission data towards 10 stellar interloper sources, in order to identify and characterize the dense clouds with which the interlopers are interacting. We have made small "on-the-fly" maps in the 12CO (J=2-1) and 13CO (J=2-1) lines for each cloud, and obtained spectra of high-density tracers such as N2H+ (J=3-2), HCO+ (J=3-2), CN(N=2-1), and SO(J=5-4), which probe a range of physical conditions in the interstellar clouds being impacted by the interlopers. The data have been reduced and analyzed, and preliminary estimates of the cloud temperatures (9-22 K) and 13CO optical depths (0.18-0.37) have been made. The maps, which show the emission as a function of radial velocity and spatial offset from the location of the interlopers, have helped us distinguish between the clouds interacting with the interlopers, and those which are unrelated but happen to lie along the line of sight. These data will now enable us to carry out high-resolution mm-wave interferometric observations of the interlopers in the future. This research was performed at JPL under the Minority Education Initiatives program. RS and MM were funded by a Long Term Space Astrophysics award from NASA for this work. The National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. Special thanks goes to John Bieging and Bill Peters of the Arizona Radio Observatory.

Is interstellar archeology possible?

NASA Astrophysics Data System (ADS)

Carrigan, Richard A.

2012-09-01

Searching for signatures of cosmic-scale archeological artifacts such as Dyson spheres is an interesting alternative to conventional radio SETI. Uncovering such an artifact does not require the intentional transmission of a signal on the part of the original civilization. This type of search is called interstellar archeology or sometimes cosmic archeology. A variety of interstellar archeology signatures is discussed including non-natural planetary atmospheric constituents, stellar doping, Dyson spheres, as well as signatures of stellar, and galactic-scale engineering. The concept of a Fermi bubble due to interstellar migration is reviewed in the discussion of galactic signatures. These potential interstellar archeological signatures are classified using the Kardashev scale. A modified Drake equation is introduced. With few exceptions interstellar archeological signatures are clouded and beyond current technological capabilities. However SETI for so-called cultural transmissions and planetary atmosphere signatures are within reach.

Chemistry in interstellar space. [environment characteristics influencing reaction dynamics

NASA Technical Reports Server (NTRS)

Donn, B.

1973-01-01

The particular characteristics of chemistry in interstellar space are determined by the unique environmental conditions involved. Interstellar matter is present at extremely low densities. Large deviations from thermodynamic equilibrium are, therefore, to be expected. A relatively intense ultraviolet radiation is present in many regions. The temperatures are in the range from 5 to 200 K. Data concerning the inhibiting effect of small activation energies in interstellar clouds are presented in a table. A summary of measured activation energies or barrier heights for exothermic exchange reactions is also provided. Problems of molecule formation are discussed, taking into account gas phase reactions and surface catalyzed processes.

Habitable zones exposed: astrosphere collapse frequency as a function of stellar mass.

PubMed

Smith, David S; Scalo, John M

2009-09-01

Stellar astrospheres--the plasma cocoons carved out of the interstellar medium by stellar winds--are one of several buffers that partially screen planetary atmospheres and surfaces from high-energy radiation. Screening by astrospheres is continually influenced by the passage of stars through the fluctuating density field of the interstellar medium (ISM). The most extreme events occur inside dense interstellar clouds, where the increased pressure may compress an astrosphere to a size smaller than the liquid-water habitable-zone distance. Habitable planets then enjoy no astrospheric buffering from exposure to the full flux of galactic cosmic rays and interstellar dust and gas, a situation we call "descreening" or "astrospheric collapse." Under such conditions the ionization fraction in the atmosphere and contribution to radiation damage of putative coding organisms at the surface would increase significantly, and a series of papers have suggested a variety of global responses to descreening. These possibilities motivate a more careful calculation of the frequency of descreening events. Using a ram-pressure balance model, we compute the size of the astrosphere in the apex direction as a function of parent-star mass and velocity and ambient interstellar density, emphasizing the importance of gravitational focusing of the interstellar flow. The interstellar densities required to descreen planets in the habitable zone of solar- and subsolar-mass stars are found to be about 600(M/M[middle dot in circle])(-2) cm(-3) for the Sun's velocity relative to the local ISM. Such clouds are rare and small, indicating that descreening encounters are rare. We use statistics from two independent catalogues of dense interstellar clouds to derive a dependence of descreening frequency on the parent-star mass that decreases strongly with decreasing stellar mass, due to the weaker gravitational focusing and smaller habitable-zone distances for lower-mass stars. We estimate an uncertain upper limit to the absolute frequency of descreening encounters as 1-10 Gyr(-1) for solar-type stars and 10(2) to 10(9) times smaller for stars between 0.5 and 0.1 M[middle dot in circle]. Habitable-zone planets orbiting late-K to M stars are virtually never exposed to the severe consequences that have been proposed for astrospheric descreening events, but descreening events at a moderate rate may occur for stars with the Sun's mass or larger.

The chemistry of dense interstellar clouds

NASA Technical Reports Server (NTRS)

Irvine, W. M.

1991-01-01

The basic theme of this program is the study of molecular complexity and evolution in interstellar and circumstellar clouds incorporating the biogenic elements. Recent results include the identification of a new astronomical carbon-chain molecule, C4Si. This species was detected in the envelope expelled from the evolved star IRC+10216 in observations at the Nobeyama Radio Observatory in Japan. C4Si is the carrier of six unidentified lines which had previously been observed. This detection reveals the existence of a new series of carbon-chain molecules, C sub n Si (n equals 1, 2, 4). Such molecules may well be formed from the reaction of Si(+) with acetylene and acetylene derivatives. Other recent research has concentrated on the chemical composition of the cold, dark interstellar clouds, the nearest dense molecular clouds to the solar system. Such regions have very low kinetic temperatures, on the order of 10 K, and are known to be formation sites for solar-type stars. We have recently identified for the first time in such regions the species of H2S, NO, HCOOH (formic acid). The H2S abundance appears to exceed that predicted by gas-phase models of ion-molecule chemistry, perhaps suggesting the importance of synthesis on grain surfaces. Additional observations in dark clouds have studied the ratio of ortho- to para-thioformaldehyde. Since this ratio is expected to be unaffected by both radiative and ordinary collisional processes in the cloud, it may well reflect the formation conditions for this molecule. The ratio is observed to depart from that expected under conditions of chemical equilibrium at formation, perhaps reflecting efficient interchange between cold dust grains in the gas phase.

Polarization of seven MBM clouds at high Galactic latitude

NASA Astrophysics Data System (ADS)

Neha, S.; Maheswar, G.; Soam, A.; Lee, C. W.

2018-06-01

We made R-band polarization measurements of 234 stars towards the direction of the MBM 33-39 cloud complex. The distance of the MBM 33-39 complex was determined as 120 ± 10 pc using polarization results and near-infrared photometry from the 2MASS survey. The magnetic field geometry of the individual clouds inferred from our polarimetric results reveals that the field lines are in general consistent with the global magnetic field geometry of the region obtained from previous studies. This implies that the clouds in the complex are permeated by the interstellar magnetic field. Multi-wavelength polarization measurements of a few stars projected on to the complex suggest that the size of the dust grains in these clouds is similar to those found in the normal interstellar medium of the Milky Way. We studied a possible formation scenario of the MBM 33-39 complex by combining the polarization results from our study with those from the literature and by identifying the distribution of ionized, atomic and molecular (dust) components of material in the region.

The astrochemistry of H+3

NASA Astrophysics Data System (ADS)

Herbst, E.

2000-09-01

The reactions of the molecular ion H3+ are pivotal to the chemistry of dense interstellar clouds. Produced by the cosmic-ray ionizati on of molecular hydrogen, H3+ reacts with a variety of a toms and molecules to produce species that are precursors to many of the detect ed molecules in dense clouds. For example, the reaction of H3+ with atomic O leads, eventually, to the production of water, while the re action with HD leads to the production of a wide variety of deuterated isotopom ers. In this article, the chemistry of H3+ and the produc ts derived from it are discussed in the larger context of interstellar chemistr y.

The Properties of Single Interstellar Clouds: Modified Cycle 1 Observations

NASA Astrophysics Data System (ADS)

Hobbs, Lewis

1990-12-01

WE PROPOSE TO USE THE ECHELLE AND 160M GRATINGS OF THE HIGH RESOLUTION SPECTROGRAPH OVER A TWO-YEAR PERIOD TO OBSERVE THE PROFILES OF INTERSTELLAR ABSORPTION LINES. THE COLUMN DENSITES OF 18 NEUTRAL OR IONIZED FORMS OF THE ELEMENTS C,N,O,Mg,Si,P,S,Fe, AND Zn WILL BE MEASURED IN THE APPROXIMATELY 100 INDIVIDUAL INTERSTELLAR CLOUDS ALONG THE LIGHT PATHS TO 18 BRIGHT, BROAD-LINED STARS OF EARLY SPECTRAL TYPE WITHIN 1 KPC OF THE SUN. THE PRIMARY PURPOSE OF THE OBSERVATIONS IS TO DETERMINE MORE ACCURATELY THAN WAS HITHERTO POSSIBLE THE FUNDAMENTAL PHYSICAL PROPERTIES OF THE RESOLVED CLOUDS, INCLUDING LINEAR SIZE, TEMPERATURE, TOTAL DENSITY, FRACTIONAL IONIZATION AND THE RELATIVE ABUNDANCES OF THE 9 SELECTED ELEMENTS. THE REST OF THIS OBSERVING PROGRAM IS CONTAINED IN APPROVED PROPOSAL ID = 3993; THE PROGRAM ENUMERATED HERE CONSISTS OF THAT PART OF OUR ORIGINAL PROGRAM, ID = 2251, WHICH REQUIRED MODIFICATION IN ORDER TO BE CARRIED OUT USING ONLY SIDE 2 OF THE GHRS. THIS PROGRAM THEREFORE CONSISTS OF ECH-B AND G160M OBSERVATIONS OF EACH OF 8 STARS AT 14 OR MORE WAVELENGTHS. PROGRAMS 2251 AND 3993 SHOULD BE CONSULTED FOR ADDITIONAL DETAILS.

A search for the 13175 A infrared diffuse band in dense environments

NASA Technical Reports Server (NTRS)

Adamson, A. J.; Kerr, Tom H.; Whittet, D. C. B.; Duley, Walter W.

1994-01-01

Models of ionized interstellar C60 predict a strong transition in the 1.2 micrometer region, and two candidate bands have recently been detected in reddened stars. We have searched for the stronger of these bands (at 13175 A) in the Taurus dark cloud complex, to determine its response to the dark-cloud environment. None of the three lines of sight studied (two near the cloud surface, one reaching A(sub V) greater than 20(sup m)) give rise to a detectable band; in one case the equivalent width is a factor of order three below that predicted. Since such behaviour is also shown by the optical Diffuse Interstellar Bands, we suggest that the 13175 A band is a genuine DIB, but we caution against an automatic interpretation in terms of an ionic carrier.

Interstellar clouds containing optically thin H2

NASA Technical Reports Server (NTRS)

Jura, M.

1975-01-01

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.

H2CS abundances and ortho-to-para ratios in interstellar clouds

NASA Technical Reports Server (NTRS)

Minh, Y. C.; Irvine, W. M.; Brewer, M. K.

1991-01-01

Several H2CS ortho and para transitions have been observed toward interstellar molecular clouds, including cold, dark clouds and star-forming regions. H2CS fractional abundances f(H2CS) about 1-2 10 to the -9th relative to molecular hydrogen toward TMC-1, Orion A, and NGC 7538, and about 5 10 to the -10th for L134N are derived. The H2CS ortho-to-para ratios in TMC-1 are about 1.8 toward the cyanopolyyne peak and the ammonia peak, which may indicate the thermalization of H2CS on 10 K grains. A ratio of about 3, the statistical value, for Orion (3N, 1E) and NGC 7538 is derived, while a value of about 2 for Orion (KL) is found.

Discovery of pulsed OH maser emission stimulated by a pulsar.

PubMed

Weisberg, Joel M; Johnston, Simon; Koribalski, Bärbel; Stanimirovic, Snezana

2005-07-01

Stimulated emission of radiation has not been directly observed in astrophysical situations up to this time. Here we demonstrate that photons from pulsar B1641-45 stimulate pulses of excess 1720-megahertz line emission in an interstellar hydroxyl (OH) cloud. As this stimulated emission is driven by the pulsar, it varies on a few-millisecond time scale, which is orders of magnitude shorter than the quickest OH maser variations previously detected. Our 1612-megahertz spectra are inverted copies of the 1720-megahertz spectra. This "conjugate line" phenomenon enables us to constrain the properties of the interstellar OH line-producing gas. We also show that pulsar signals undergo significantly deeper OH absorption than do other background sources, which confirms earlier tentative findings that OH clouds are clumpier on small scales than are neutral hydrogen clouds.

Centrosymmetric molecules as possible carriers of diffuse interstellar bands

NASA Astrophysics Data System (ADS)

Kaźmierczak, M.; Schmidt, M. R.; Galazutdinov, G. A.; Musaev, F. A.; Betelesky, Y.; Krełowski, J.

2010-11-01

In this paper, we present new data with interstellar C2 (Phillips bands A 1 Πu-X1 Σ+g), from observations made with the Ultraviolet-Visual Echelle Spectrograph of the European Southern Observatory. We have determined the interstellar column densities and excitation temperatures of C2 for nine Galactic lines. For seven of these, C2 has never been observed before, so in this case the still small sample of interstellar clouds (26 lines of sight), where a detailed analysis of C2 excitation has been made, has increased significantly. This paper is a continuation of previous works where interstellar molecules (C2 and diffuse interstellar bands) have been analysed. Because the sample of interstellar clouds with C2 has increased, we can show that the width and shape of the profiles of some diffuse interstellar bands (6196 and 5797 Å) apparently depend on the gas kinetic and rotational temperatures of C2; the profiles are broader because of the higher values of the gas kinetic and rotational temperatures of C2. There are also diffuse interstellar bands (4964 and 5850 Å) for which this effect does not exist. Based on observations made with ESO telescopes at the Paranal Observatory under programme IDs 266.D-5655(A), 67.C-0281(A), 71.C-0513(C), 67.D-0439(A) and 082.C-0566(A) and at La Silla under programme IDs 078.C-0403(A), 076.C-0164(A) and 073.C-0337(A). Also based on observations made with the 1.8-m telescope in South Korea and the 2-m telescope at the International Centre for Astronomical and Medico-Ecological Research, Terskol, Russia. E-mail: kazmierczak@astri.uni.torun.pl (MK); schmidt@ncac.torun.pl (MRS); runizag@gmail.com (GAG); ybialets@eso.org (YB); jacek@astri.uni.torun.pl (JK)

Shedding light on the Type Ia supernova extinction puzzle: dust location found

NASA Astrophysics Data System (ADS)

Bulla, M.; Goobar, A.; Dhawan, S.

2018-06-01

The colour evolution of reddened Type Ia supernovae can place strong constraints on the location of dust and help address the question of whether the observed extinction stems from the interstellar medium or from circumstellar material surrounding the progenitor. Here we analyse BV photometry of 48 reddened Type Ia supernovae from the literature and estimate the dust location from their B - V colour evolution. We find a time-variable colour excess E(B - V) for 15 supernovae in our sample and constrain dust to distances between 0.013 and 45 pc (4 × 1016 - 1020 cm). For the remaining supernovae, we obtain a constant E(B - V) evolution and place lower limits on the dust distance from the explosion. In all the 48 supernovae, the inferred dust location is compatible with an interstellar origin for the extinction. This is corroborated by the observation that supernovae with relatively nearby dust (≲ 1 pc) are located close to the center of their host galaxy, in high-density dusty regions where interactions between the supernova radiation and interstellar clouds close by are likely to occur. For supernovae showing time-variable E(B - V), we identify a potential preference for low RV values, unusually strong sodium absorption and blue-shifted and time-variable absorption features. Within the interstellar framework, this brings evidence to a proposed scenario where cloud-cloud collisions induced by the supernova radiation pressure can shift the grain size distribution to smaller values and enhance the abundance of sodium in the gaseous phase.

High-Resolution Imaging of the Multiphase Interstellar Thick Disk in Two Edge-On Spiral Galaxies

NASA Astrophysics Data System (ADS)

Howk, J. Christopher; Rueff, K.

2009-01-01

We present broadband and narrow-band images, acquired from Hubble Space Telescope WFPC2 and WIYN 3.5 m telescope respectively, of two edge-on spiral galaxies, NGC 4302 and NGC 4013. These high-resolution images (BVI + H-alpha) provide a detailed view of the thick disk interstellar medium (ISM) in these galaxies. Both galaxies show prominent extraplanar dust-bearing clouds viewed in absorption against the background stellar light. Individual clouds are found to z 2 kpc in each galaxy. These clouds each contain >10^4 to >10^5 solar masses of gas. Both galaxies have extraplanar diffuse ionized gas (DIG), as seen in our H-alpha images and earlier work. In addition to the DIG, discrete H II regions are found at heights up to 1 kpc from both galaxies. We compare the morphologies of the dusty clouds with the DIG in these galaxies and discuss the relationship between these components of the thick disk ISM.

Detection of interstellar HCS and its metastable isomer HSC: new pieces in the puzzle of sulfur chemistry

NASA Astrophysics Data System (ADS)

Agúndez, M.; Marcelino, N.; Cernicharo, J.; Tafalla, M.

2018-03-01

We present the first identification in interstellar space of the thioformyl radical (HCS) and its metastable isomer HSC. These species were detected toward the molecular cloud L483 through observations carried out with the IRAM 30 m telescope in the λ3 mm band. We derive beam-averaged column densities of 7 × 1012 cm-2 for HCS and 1.8 × 1011 cm-2 for HSC, which translate into fractional abundances relative to H2 of 2 × 10-10 and 6 × 10-12, respectively. Although the amount of sulfur locked by these radicals is low, their detection allows placing interesting constraints on the chemistry of sulfur in dark clouds. Interestingly, the H2CS/HCS abundance ratio is found to be quite low, 1, in contrast with the oxygen analog case, in which the H2CO/HCO abundance ratio is around 10 in dark clouds. Moreover, the radical HCS is found to be more abundant than its oxygen analog, HCO. The metastable species HOC, the oxygen analog of HSC, has not yet been observed in space. These observational constraints are compared with the outcome of a recent model of the chemistry of sulfur in dark clouds. The model underestimates the fractional abundance of HCS by at least one order of magnitude, overestimates the H2CS/HCS abundance ratio, and does not provide an abundance prediction for the metastable isomer HSC. These observations should prompt a revision of the chemistry of sulfur in interstellar clouds.

Radioastronomical Searches for Instellar Biomolecules

NASA Technical Reports Server (NTRS)

Kuan, Y.-J.; Huang, H.-C.; Charnley, S. B.; Markwick, A.; Botta, O.; Ehrenfreund, P.; Kisiel, Z.; Butner, H. M.

2003-01-01

Impacts of comets and asteroids could have delivered large amounts of organic matter to the early Earth. to retain a significant interstellar signature; observations of recent bright comets indicate that they have a molecular inventory consistent with their ices being largely unmodified interstellar material. Many simple organic molecules with biochemical significance observed in circumstellar envelopes and in molecular clouds, similar to that from which the Solar System formed, may have acted as the precursors of the more complex organics found in meteorites. Therefore, there is potentially a strong link between interstellar organics and prebiotic chemical evolution. Radioastronomical observations, particularly at millimeter wavelengths, allow us to determine the chemical composition and characteristics of the molecular inventory in interstellar space. Here we report some of our recent results from extensive astronomical searches for astrobiologically-important interstellar organics.

The Laboratory Production of Complex Organic Molecules in Simulated Interstellar Ices

NASA Technical Reports Server (NTRS)

Dworkin, J. P.; Sandford, S. A.; Bernstein, M. P.; Allamandola, L. J.

2002-01-01

Much of the volatiles in interstellar dense clouds exist in ices surrounding dust grains. Their low temperatures preclude most chemical reactions, but ionizing radiation can drive reactions that produce a suite of new species, many of which are complex organics. The Astrochemistry Lab at NASA Ames studies the UV radiation processing of interstellar ice analogs to better identify the resulting products and establish links between interstellar chemistry, the organics in meteorites, and the origin of life on Earth. Once identified, the spectral properties of the products can be quantified to assist with the search for these species in space. Of particular interest are findings that UV irradiation of interstellar ice analogs produces molecules of importance in current living organisms, including quinones, amphiphiles, and amino acids.

Experimental interstellar organic chemistry: Preliminary findings

NASA Technical Reports Server (NTRS)

Khare, B. N.; Sagan, C.

1971-01-01

In a simulation of interstellar organic chemistry in dense interstellar clouds or on grain surfaces, formaldehyde, water vapor, ammonia and ethane are deposited on a quartz cold finger and ultraviolet-irradiated in high vacuum at 77K. The HCHO photolytic pathway which produces an aldehyde radical and a superthermal hydrogen atom initiates solid phase chain reactions leading to a range of new compounds, including methanol, ethanol, acetaldehyde, acetonitrile, acetone, methyl formate, and possibly formic acid. Higher nitriles are anticipated. Genetic relations among these interstellar organic molecules (e.g., the Cannizzaro and Tischenko reactions) must exist. Some of them, rather than being synthesized from smaller molecules, may be degradation products of larger organic molecules, such as hexamethylene tetramine, which are candidate consitituents of the interstellar grains. The experiments reported here may also be relevant to cometary chemistry.

Ultraviolet observations of cool stars. VII - Local interstellar hydrogen and deuterium Lyman-alpha

NASA Technical Reports Server (NTRS)

Mcclintock, W.; Henry, R. C.; Linsky, J. L.; Moos, H. W.

1978-01-01

High-resolution Copernicus spectra of Epsilon Eri and Epsilon Ind containing interstellar hydrogen and deuterium L-alpha absorption lines are presented, reduced, and analyzed. Parameters of the interstellar hydrogen and deuterium toward these two stars are derived independently, without any assumptions concerning the D/H ratio. Copernicus spectra of Alpha Aur and Alpha Cen A are reanalyzed, and limits on the D/H number-density ratio consistent with the data for all four stars are considered. A comparison of the present estimates for the parameters of the local interstellar medium with those obtained by other techniques shows that there is no compelling evidence for significant variations in the hydrogen density and D/H ratio in the local interstellar medium. On this basis the hypothesis of an approaching local interstellar cloud proposed by Vidal-Madjar et al. (1978) is rejected

Diffuse interstellar bands in reflection nebulae

NASA Technical Reports Server (NTRS)

Fischer, O.; Henning, Thomas; Pfau, Werner; Stognienko, R.

1994-01-01

A Monte Carlo code for radiation transport calculations is used to compare the profiles of the lambda lambda 5780 and 6613 Angstrom diffuse interstellar bands in the transmitted and the reflected light of a star embedded within an optically thin dust cloud. In addition, the behavior of polarization across the bands were calculated. The wavelength dependent complex indices of refraction across the bands were derived from the embedded cavity model. In view of the existence of different families of diffuse interstellar bands the question of other parameters of influence is addressed in short.

Indirect observation of unobservable interstellar molecules

NASA Technical Reports Server (NTRS)

Herbst, E.; Green, S.; Thaddeus, P.; Klemperer, W.

1977-01-01

It is suggested that the abundances of neutral non-polar interstellar molecules unobservable by radio astronomy can be systematically determined by radio observation of the protonated ions. As an example, observed N2H(+) column densities are analyzed to infer molecular nitrogen abundances in dense interstellar clouds. The chemistries and expected densities of the protonated ions of O2, C2, CO2, C2H2 and CH4 are then discussed. Microwave transition frequencies fo HCO2(+) and C2H3(+) are estimated, and a preliminary astronomical search for HCO2(+) is described.

The Physics and Chemistry of Small Translucent Molecular Clouds. VII. SO + and H 2S

NASA Astrophysics Data System (ADS)

Turner, B. E.

1996-09-01

In this third paper on sulfur species, we have conducted a survey of SO+ (two transitions) and H2S (one transition) in our standard samples of 11 cirrus cores and 27 Clemens-Barvainis translucent objects whose structures and chemistry have been studied earlier in this series. SO+(2II½, J = 3/2-1/2) is seen weakly in 12 objects, while H2S (110 -101) is detected quite strongly in 31 objects. These results are modeled in terms of our previous hydrostatic equilibrium and n ˜r-α structures together with other chemical and physical properties derived earlier. The typical H2S fractional abundance is large, ˜1 × 10-8, and increases monotonically with increasing extinction in the 1.2-2.7 mag range (edge-to-center). Thus H2S displays the same characteristic transition between diffuse and dense cloud chemistry as do SO, SO2, CS, HCS +, HCO +, and other species studied in this series. By contrast, the SO + abundances are small, 1 × 10-9, and exhibit a marginal decrease with increasing extinction. The simple ion-molecule network as used by Turner for sulfur chemistry includes the sulfur hydride species and predicts the observed parameters of SO+ but predicts an H2S abundance 2 orders of magnitude less than observed. Of the 10 species presently analyzed in detail in the translucent cores, H2S is only the second (along with H2CO) that fails to be explained in detail by quiescent cloud ion-molecule chemistry. Various catalytic models of H2S on grains are discussed. Photocatalysis of H2S is found capable of producing the observed abundances but only for sizable accreted mantles. Other types of surface chemistry are also successful but are close to the limits of possible efficiencies. We have detected OCS and H2CS in one object, CB 17, with abundances of 1 × 10-9 and 7 × 10-9 respectively. Our ion-molecule model has been expanded to include OCS and H2CS chemistry. We find that the model fits observed abundances within a factor of 3 for both species.

Radio Emission from a Young Supernova Remnant Interacting with an Interstellar Cloud: Magnetohydrodynamic Simulation with Relativistic Electrons

NASA Astrophysics Data System (ADS)

Jun, Byung-Il; Jones, T. W.

1999-02-01

We present two-dimensional MHD simulations of the evolution of a young Type Ia supernova remnant (SNR) during its interaction with an interstellar cloud of comparable size at impact. We include for the first time in such simulations explicit relativistic electron transport. This was done using a simplified treatment of the diffusion-advection equation, thus allowing us to model injection and acceleration of cosmic-ray electrons at shocks and their subsequent transport. From this information we also model radio synchrotron emission, including spectral information. The simulations were carried out in spherical coordinates with azimuthal symmetry and compare three different situations, each incorporating an initially uniform interstellar magnetic field oriented in the polar direction on the grid. In particular, we modeled the SNR-cloud interactions for a spherical cloud on the polar axis, a toroidal cloud whose axis is aligned with the polar axis, and, for comparison, a uniform medium with no cloud. We find that the evolution of the overrun cloud qualitatively resembles that seen in simulations of simpler but analogous situations: that is, the cloud is crushed and begins to be disrupted by Rayleigh-Taylor and Kelvin-Helmholtz instabilities. However, we demonstrate here that, in addition, the internal structure of the SNR is severely distorted as such clouds are engulfed. This has important dynamical and observational implications. The principal new conclusions we draw from these experiments are the following. (1) Independent of the cloud interaction, the SNR reverse shock can be an efficient site for particle acceleration in a young SNR. (2) The internal flows of the SNR become highly turbulent once it encounters a large cloud. (3) An initially uniform magnetic field is preferentially amplified along the magnetic equator of the SNR, primarily because of biased amplification in that region by Rayleigh-Taylor instabilities. A similar bias produces much greater enhancement to the magnetic energy in the SNR during an encounter with a cloud when the interstellar magnetic field is partially transverse to the expansion of the SNR. The enhanced magnetic fields have a significant radial component, independent of the field orientation external to the SNR. This leads to a strong equatorial bias in synchrotron brightness that could easily mask any enhancements to electron-acceleration efficiency near the magnetic equator of the SNR. Thus, to establish the latter effect, it will be essential to establish that the magnetic field in the brightest regions are actually tangential to the blast wave. (4) The filamentary radio structures correlate well with ``turbulence-enhanced'' magnetic structures, while the diffuse radio emission more closely follows the gas-density distribution within the SNR. (5) At these early times, the synchrotron spectral index due to electrons accelerated at the primary shocks should be close to 0.5 unless those shocks are modified by cosmic-ray proton pressures. While that result is predictable, we find that this simple result can be significantly complicated in practice by SNR interactions with clouds. Those events can produce regions with significantly steeper spectra. Especially if there are multiple cloud encounters, this interaction can lead to nonuniform spatial spectral distributions or, through turbulent mixing, produce a spectrum that is difficult to relate to the actual strength of the blast wave. (6) Interaction with the cloud enhances the nonthermal electron population in the SNR in our simulations because of additional electron injection taking place in the shocks associated with the cloud. Together with point 3, this means that SNR-cloud encounters can significantly increase the radio emission from the SNR.

Physical state of interstellar atoms. [from Copernicus satellite UV data

NASA Technical Reports Server (NTRS)

York, D. G.

1974-01-01

Brief survey of the physical conditions along the lines of sight to reddened and unreddened stars, as determined from Copernicus observation of interstellar lines between 95 and 300 nm. Differences in ionization structure and density between clouds and the local intercloud medium are discussed. Some new data for beta Centauri is used to supplement the previously available data.

Interstellar Matter

NASA Astrophysics Data System (ADS)

Savage, B.; Murdin, P.

2000-11-01

The enormous volume of space between the stars in the Milky Way Galaxy is filled with interstellar matter (ISM). The ISM plays a central role in the processes of STAR FORMATION and GALAXY EVOLUTION. Stars form from the ISM in dense molecular clouds. The radiant and mechanical energy produced by stars heats, ionizes, and produces structures in the ISM. Gradual or catastrophic mass loss from stars ...

The Production of Complex Organics from Interstellar Ices

NASA Technical Reports Server (NTRS)

Sandford, Scott A.; Allamandola, Louis; Bernstein, Max; Deamer, David; Dworkin, Jason; Zare, Richard

2001-01-01

Infrared spectroscopy of ices in interstellar dense molecular clouds has shown that they contain a variety of simple molecules, as well as aromatic hydrocarbons. While in these clouds, these ices are processed by ultraviolet light and cosmic rays. High vacuum, UV irradiation laboratory simulations conducted using various realistic approx. 10 K interstellar mixed-molecular ice analogs, both with and without polycyclic aromatic hydrocarbons (PAHs), have been carried out in NASA-Ames' Astrochemistry Laboratory. Upon warming, these irradiated ices are found to produce refractory organic residues. These residues have been analyzed using a variety of techniques, including HPLC and laser desorption mass spectrometry, and they have been shown to contain a variety of complex organic compounds. Several of these compounds may be of prebiotic significance. In particular, we will discuss the detection of quinones (substituted PAHs that are used by living systems for electron transport) and amphiphiles (molecules that self-assemble to form membranes). Laboratory simulations have also demonstrated that the organic products can show isotopic enrichments in D that provide clues for the mechanisms of their formation. Similar compounds and D enrichments are seen in the organics found in primitive meteorites, suggesting a direct link between interstellar chemistry and the delivery of organics to newly formed planets.

Properties of Cold HI Emission Clouds in the Inner-Galaxy ALFA Survey

NASA Astrophysics Data System (ADS)

Hughes, James Marcus; Gibson, Steven J.; Noriega-Crespo, Alberto; Newton, Jonathan; Koo, Bon-Chul; Douglas, Kevin A.; Peek, Joshua Eli Goldston; Park, Geumsook; Kang, Ji-hyun; Korpela, Eric J.; Heiles, Carl E.; Dame, Thomas M.

2017-01-01

Star formation, a critical process within galaxies, occurs in the coldest, densest interstellar clouds, whose gas and dust content are observed primarily at radio and infrared wavelengths. The formation of molecular hydrogen (H2) from neutral atomic hydrogen (HI) is an essential early step in the condensation of these clouds from the ambient interstellar medium, but it is not yet completely understood, e.g., what is the predominant trigger? Even more troubling, the abundance of H2 may be severely underestimated by standard tracers like CO, implying significant "dark" H2, and the quantity of HI may also be in error if opacity effects are neglected. We have developed an automated method to account for both HI and H2 in cold, diffuse clouds traced by narrow-line HI 21-cm emission in the Arecibo Inner-Galaxy ALFA (I-GALFA) survey. Our algorithm fits narrow (2-5 km/s), isolated HI line profiles to determine their spin temperature, optical depth, and true column density. We then estimate the "visible" H2 column in the same clouds with CfA and Planck CO data and the total gas column from dust emission measured by Planck, IRAS, and other surveys. Together, these provide constraints on the dark H2 abundance, which we examine in relation to other cloud properties and stages of development. Our aim is to build a database of H2-forming regions with significant dark gas to aid future analyses of coalescing interstellar clouds. We acknowledge support from NSF, NASA, Western Kentucky University, and Williams College. I-GALFA is a GALFA-HI survey observed with the 7-beam ALFA receiver on the 305-meter William E. Gordon Telescope. The Arecibo Observatory is a U.S. National Science Foundation facility operated under sequential cooperative agreements with Cornell University and SRI International, the latter in alliance with the Ana G. Mendez-Universidad Metropolitana and the Universities Space Research Association.

Are CO Observations of Interstellar Clouds Tracing the H2?

NASA Astrophysics Data System (ADS)

Federrath, Christoph; Glover, S. C. O.; Klessen, R. S.; Mac Low, M.

2010-01-01

Interstellar clouds are commonly observed through the emission of rotational transitions from carbon monoxide (CO). However, the abundance ratio of CO to molecular hydrogen (H2), which is the most abundant molecule in molecular clouds is only about 10-4. This raises the important question of whether the observed CO emission is actually tracing the bulk of the gas in these clouds, and whether it can be used to derive quantities like the total mass of the cloud, the gas density distribution function, the fractal dimension, and the velocity dispersion--size relation. To evaluate the usability and accuracy of CO as a tracer for H2 gas, we generate synthetic observations of hydrodynamical models that include a detailed chemical network to follow the formation and photo-dissociation of H2 and CO. These three-dimensional models of turbulent interstellar cloud formation self-consistently follow the coupled thermal, dynamical and chemical evolution of 32 species, with a particular focus on H2 and CO (Glover et al. 2009). We find that CO primarily traces the dense gas in the clouds, however, with a significant scatter due to turbulent mixing and self-shielding of H2 and CO. The H2 probability distribution function (PDF) is well-described by a log-normal distribution. In contrast, the CO column density PDF has a strongly non-Gaussian low-density wing, not at all consistent with a log-normal distribution. Centroid velocity statistics show that CO is more intermittent than H2, leading to an overestimate of the velocity scaling exponent in the velocity dispersion--size relation. With our systematic comparison of H2 and CO data from the numerical models, we hope to provide a statistical formula to correct for the bias of CO observations. CF acknowledges financial support from a Kade Fellowship of the American Museum of Natural History.

Laboratory and modeling studies of chemistry in dense molecular clouds

NASA Technical Reports Server (NTRS)

Huntress, W. T., Jr.; Prasad, S. S.; Mitchell, G. F.

1980-01-01

A chemical evolutionary model with a large number of species and a large chemical library is used to examine the principal chemical processes in interstellar clouds. Simple chemical equilibrium arguments show the potential for synthesis of very complex organic species by ion-molecule radiative association reactions.

Time scales for molecule formation by ion-molecule reactions

NASA Technical Reports Server (NTRS)

Langer, W. D.; Glassgold, A. E.

1976-01-01

Analytical solutions are obtained for nonlinear differential equations governing the time-dependence of molecular abundances in interstellar clouds. Three gas-phase reaction schemes are considered separately for the regions where each dominates. The particular case of CO, and closely related members of the Oh and CH families of molecules, is studied for given values of temperature, density, and the radiation field. Nonlinear effects and couplings with particular ions are found to be important. The time scales for CO formation range from 100,000 to a few million years, depending on the chemistry and regime. The time required for essentially complete conversion of C(+) to CO in the region where the H3(+) chemistry dominates is several million years. Because this time is longer than or comparable to dynamical time scales for dense interstellar clouds, steady-state abundances may not be observed in such clouds.

The formation of molecules in interstellar clouds from singly and multiply ionized atoms

NASA Technical Reports Server (NTRS)

Langer, W. D.

1978-01-01

The suggestion is considered that multiply ionized atoms produced by K- and L-shell X-ray ionization and cosmic-ray ionization can undergo ion-molecule reactions and also initiate molecule production. The role of X-rays in molecule production in general is discussed, and the contribution to molecule production of the C(+) radiative association with hydrogen is examined. Such gas-phase reactions of singly and multiply ionized atoms are used to calculate molecular abundances of carbon-, nitrogen-, and oxygen-bearing species. The column densities of the molecules are evaluated on the basis of a modified version of previously developed isobaric cloud models. It is found that reactions of multiply ionized carbon with H2 can contribute a significant fraction of the observed CH in diffuse interstellar clouds in the presence of diffuse X-ray structures or discrete X-ray sources and that substantial amounts of CH(+) can be produced under certain conditions.

The sensitivity of gas-phase models of dense interstellar clouds to changes in dissociative recombination branching ratios

NASA Technical Reports Server (NTRS)

Millar, T. J.; Defrees, D. J.; Mclean, A. D.; Herbst, E.

1988-01-01

The approach of Bates to the determination of neutral product branching ratios in ion-electron dissociative recombination reactions has been utilized in conjunction with quantum chemical techniques to redetermine branching ratios for a wide variety of important reactions of this class in dense interstellar clouds. The branching ratios have then been used in a pseudo time-dependent model calculation of the gas phase chemistry of a dark cloud resembling TMC-1 and the results compared with an analogous model containing previously used branching ratios. In general, the changes in branching ratios lead to stronger effects on calculated molecular abundances at steady state than at earlier times and often lead to reductions in the calculated abundances of complex molecules. However, at the so-called 'early time' when complex molecule synthesis is most efficient, the abundances of complex molecules are hardly affected by the newly used branching ratios.

Molecules in celestial objects. III - Study of CO in interstellar diffuse clouds

NASA Technical Reports Server (NTRS)

Tarafdar, S. P.; Krishna Swamy, K. S.

1982-01-01

The absorption lines corresponding to the A-X transition of CO have been looked for in the IUE spectra of 14 stars with varying values of the colour excess, E(B-V) and found to be present in the spectra of nine stars with E(B-V) at least 0.28. The column density of CO has been determined towards these nine stars and its upper limit towards the rest of the stars. The curve of growth analysis has been found to show that the contribution to CO absorption is possibly from a single interstellar cloud for stars with E(B-V) less than 0.4 and from more than one cloud for stars with E(B-V) greater than 0.4. The observed column density of CO as a function of E(B-V) has been found to be in good agreement with that expected from the theory of ion-molecular chemistry.

Submillimeter heterodyne detection of interstellar carbon monoxide at 434 micrometers

NASA Technical Reports Server (NTRS)

Fetterman, H. R.; Clifton, B. J.; Peck, D. D.; Tannenwald, P. E.; Koepf, G. A.; Goldsmith, P. F.; Erickson, N. R.; Buhl, D.; Mcavoy, N.

1981-01-01

Laser heterodyne observations of submillimeter emissions from carbon monoxide in the Orion molecular cloud are reported. High frequency and spatial resolution observations were made at the NASA Infrared Telescope Facility on Mauna Kea by the use of an optically pumped laser local oscillator and quasi-optical Schottky diode mixer for heterodyne detection of the J = 6 - 5 rotational transition of CO at 434 microns. Spectral analysis of the 434-micron emission indicates that the emitting gas is optically thin and is at a temperature above 180 K. Results thus demonstrate the potential contributions of ground-based high-resolution submillimeter astronomy to the study of active regions in interstellar molecular clouds.

Large Area Crop Inventory Experiment (LACIE). Detection of episodic phenomena on LANDSAT imagery. [Kansas

NASA Technical Reports Server (NTRS)

Chesnutwood, C. M. (Principal Investigator)

1976-01-01

The author has identified the following significant results. Episodic phenomena such as rainfall shortly before data pass, thin translucent clouds, cloud shadows, and aircraft condensation trails and their shadows are responsible for changes in the spectral reflectivities of some surfaces. These changes are readily detected on LANDSAT full-frame imagery. Histograms of selected areas in Kansas show a distinct decrease in mean radiance values, but also, an increase in scene contrast, in areas where recent rains had occurred. Histograms from a few individual fields indicate that the mean radiance values for winter wheat followed a different trend after a rainfall than alfalfa or grasses.

Grain Surface Chemistry and the Composition of Interstellar Ices

NASA Technical Reports Server (NTRS)

Tielens, A. G. G. M.

2006-01-01

Submicron sized dust grains are an important component of the interstellar medium. In particular they provide surface where active chemistry can take place. At the low temperatures (-10 K) of the interstellar medium, colliding gas phase species will stick, diffuse, react, and form an icy mantle on these dust grains. This talk will review the principles of grain surface chemistry and delineate important grain surface routes, focusing on reactions involving H, D, and O among each other and with molecules such as CO. Interstellar ice mantles can be studied through the fundamental vibrations of molecular species in the mid-infrared spectra of sources embedded in or located behind dense molecular clouds. Analysis of this type of data has provided a complex view of the composition of these ices and the processes involved. Specifically, besides grain surface chemistry, the composition of interstellar ices is also affected by thermal processing due to nearby newly formed stars. This leads to segregation between different ice components as well as outgassing. The latter results in the formation of a so-called Hot Core region with a gas phase composition dominated by evaporated mantle species. Studies of such regions provide thus a different view on the ice composition and the chemical processes involved. Interstellar ices can also be processed by FUV photons and high energy cosmic ray ions. Cosmic ray processing likely dominates the return of accreted species to the gas phase where further gas phase reactions can take place. These different chemical routes towards molecular complexity in molecular clouds and particularly regions of star formation will be discussed.

Galactic gamma-ray observations and galactic structure

NASA Technical Reports Server (NTRS)

Stecker, F. W.

1975-01-01

Recent observations of gamma-rays originating in the galactic disk together with radio observations, support an emerging picture of the overall structure of our galaxy with higher interstellar gas densities and star formation rates in a region which corresponds to that of the inner arms. The emerging picture is one where molecular clouds make up the dominant constituent of the interstellar gas in the inner galaxy and play a key role in accounting for the gamma-rays and phenomena associated with the production of young stars and other population 1 objects. In this picture, cosmic rays are associated with supernovae and are primarily of galactic origin. These newly observed phenomena can be understood as consequences of the density wave theories of spiral structure. Based on these new developments, the suggestion is made that a new galactic population class, Population O, be added to the standard Populations 1 and 2 in order to recognize important differences in dynamics and distribution between diffuse galactic H1 and interstellar molecular clouds.

Molecular and mass spectroscopic analysis of isotopically labeled organic residues

NASA Technical Reports Server (NTRS)

Mendoza-Gomez, Celia X.; Greenberg, J. Mayo; Mccain, P.; Ferris, J. P.; Briggs, R.; Degroot, M. S.; Schutte, Willem A.

1989-01-01

Experimental studies aimed at understanding the evolution of complex organic molecules on interstellar grains were performed. The photolysis of frozen gas mixtures of various compositions containing H2O, CO, NH3, and CH4 was studied. These species were chosen because of their astrophysical importance as deducted from observational as well as theoretical studies of ice mantles on interstellar grains. These ultraviolet photolyzed ices were warmed up in order to produce refractory organic molecules like the ones formed in molecular clouds when the icy mantles are being irradiated and warmed up either by a nearby stellar source or impulsive heating. The laboratory studies give estimates of the efficiency of production of such organic material under interstellar conditions. It is shown that the gradual carbonization of organic mantles in the diffuse cloud phase leads to higher and higher visual absorptivity - yellow residues become brown in the laboratory. The obtained results can be applied to explaining the organic components of comets and their relevance to the origin of life.

Theoretical Modeling of Interstellar Chemistry

NASA Technical Reports Server (NTRS)

Charnley, Steven

2009-01-01

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.

Observing Interstellar and Intergalactic Magnetic Fields

NASA Astrophysics Data System (ADS)

Han, J. L.

2017-08-01

Observational results of interstellar and intergalactic magnetic fields are reviewed, including the fields in supernova remnants and loops, interstellar filaments and clouds, Hii regions and bubbles, the Milky Way and nearby galaxies, galaxy clusters, and the cosmic web. A variety of approaches are used to investigate these fields. The orientations of magnetic fields in interstellar filaments and molecular clouds are traced by polarized thermal dust emission and starlight polarization. The field strengths and directions along the line of sight in dense clouds and cores are measured by Zeeman splitting of emission or absorption lines. The large-scale magnetic fields in the Milky Way have been best probed by Faraday rotation measures of a large number of pulsars and extragalactic radio sources. The coherent Galactic magnetic fields are found to follow the spiral arms and have their direction reversals in arms and interarm regions in the disk. The azimuthal fields in the halo reverse their directions below and above the Galactic plane. The orientations of organized magnetic fields in nearby galaxies have been observed through polarized synchrotron emission. Magnetic fields in the intracluster medium have been indicated by diffuse radio halos, polarized radio relics, and Faraday rotations of embedded radio galaxies and background sources. Sparse evidence for very weak magnetic fields in the cosmic web is the detection of the faint radio bridge between the Coma cluster and A1367. Future observations should aim at the 3D tomography of the large-scale coherent magnetic fields in our Galaxy and nearby galaxies, a better description of intracluster field properties, and firm detections of intergalactic magnetic fields in the cosmic web.

HD 62542: Probing the Bare, Dense Core of an Interstellar Cloud

NASA Astrophysics Data System (ADS)

Welty, Daniel; Sonnentrucker, Paule G.; Rachford, Brian; Snow, Theodore; York, Donald G.

2018-01-01

We discuss the interstellar absorption from many atomic and molecular species seen in high-resolution HST/STIS UV spectra of the moderately reddened B3-5 V star HD 62542 [E(B-V) ~ 0.35; AV ~ 1.2]. This remarkable sight line exhibits both very steep far-UV extinction and a high fraction of hydrogen in molecular form -- with strong absorption from CH, C2, CN, and CO but weak absorption from CH+ and most of the commonly observed diffuse interstellar bands. Most of the material appears to reside in a single narrow velocity component -- thus offering a rare opportunity to probe the relatively dense, primarily molecular core of a single interstellar cloud, with little associated diffuse atomic gas.Detailed analyses of the absorption-line profiles seen in the UV spectra reveal a number of properties of the main diffuse molecular cloud toward HD 62542:1) The depletions of Mg, Si, and Fe are more severe than those seen in any other sight line, but the depletions of Cl and Kr are very mild; the overall pattern of depletions differs somewhat from those derived from larger samples of Galactic sight lines.2) The rotational excitation of H2 and C2 indicates that the gas is fairly cold (Tk = 40-45 K) and moderately dense (nH > 420 cm-3) somewhat higher densities are suggested by the fine-structure excitation of neutral carbon.3) The excitation temperatures characterizing the rotational populations of both 12CO (11.7 K) and 13CO (7.7 K) are higher than those typically found for Galactic diffuse molecular clouds.4) Carbon is primarily singly ionized -- N(C+) > N(CO) > N(C).5) The relative abundances of various trace neutral atomic species reflect the effects of both the steep far-UV extinction and the severe depletions of some elements.6) Differences in line widths for the various atomic and molecular species are suggestive of differences in spatial distribution within the main cloud.Support for this study was provided by NASA, via STScI grant GO-12277.008-A.

Kinetic Monte Carlo simulations of water ice porosity: extrapolations of deposition parameters from the laboratory to interstellar space

NASA Astrophysics Data System (ADS)

Clements, Aspen R.; Berk, Brandon; Cooke, Ilsa R.; Garrod, Robin T.

2018-02-01

Using an off-lattice kinetic Monte Carlo model we reproduce experimental laboratory trends in the density of amorphous solid water (ASW) for varied deposition angle, rate and surface temperature. Extrapolation of the model to conditions appropriate to protoplanetary disks and interstellar dark clouds indicate that these ices may be less porous than laboratory ices.

Neutral gas and diffuse interstellar bands in the LMC

NASA Technical Reports Server (NTRS)

Danks, Anthony C.; Penprase, Brian

1994-01-01

Tracing the dynamics of the neutral gas and observing diffuse interstellar bands in the LMC (Large Magellanic Cloud) was the focus of this study. The S/N values, a Quartz lamp exposure, a T horium Argon Comparision lamp exposure, and spectral plots for each star observed were taken. The stars observed were selected to sample the 30 Dor vicinty. NaI absorption profiles are included.

Enhancement of Feedback Efficiency by Active Galactic Nucleus Outflows via the Magnetic Tension Force in the Inhomogeneous Interstellar Medium

DOE Office of Scientific and Technical Information (OSTI.GOV)

Asahina, Yuta; Ohsuga, Ken; Nomura, Mariko, E-mail: asahina@cfca.jp

By performing three-dimensional magnetohydrodynamics simulations of subrelativistic jets and disk winds propagating into the magnetized inhomogeneous interstellar medium (ISM), we investigate the magnetic effects on the active galactic nucleus feedback. Our simulations reveal that the magnetic tension force promotes the acceleration of the dense gas clouds, since the magnetic field lines, which are initially straight, bend around the gas clouds. In the jet models, the velocity dispersion of the clouds increases with an increase in the initial magnetic fields. The increment of the kinetic energy of the clouds is proportional to the initial magnetic fields, implying that the magnetic tensionmore » force increases the energy conversion efficiency from the jet to the gas clouds. Through simulations of the mildly collimated disk wind and the funnel-shaped disk wind, we confirm that such an enhancement of the energy conversion efficiency via the magnetic fields appears even if the energy is injected via the disk winds. The enhancement of the acceleration of the dense part of the magnetized ISM via the magnetic tension force will occur wherever the magnetized inhomogeneous matter is blown away.« less

Interactions of stars and interstellar matter in Scorpio Centaurus

NASA Technical Reports Server (NTRS)

De Geus, E. J.

1992-01-01

The interaction of the stars in the Scorpio-Centaurus OB association with the ambient interstellar medium is investigated. Large H I loops in the fourth galactic quadrant are parts of expanding shells surrounding the subgroups of the association. The energy output of the original stellar population of the subgroups is calculated. Comparison with the kinetic energy of the shells shows that the energy output of the stars in the subgroups is sufficient to form the shells. The masses of the shells are consistent with those of giant molecular clouds GMCs, suggesting that the shells consist of swept-up, original GMC material. The influence of the expanding shell around the young Upper-Scorpius subgroup on the morphology of the Ophiuchus molecular clouds is investigated. The interaction of the shell with the Ophiuchus clouds accounts for the presence of a slow shock and for the shape of the elongated dark clouds connected to the Rho Oph dense cloud. The close passage of the trajectory of the runaway star Zeta Oph by the center of the Upper-Scorpius shell, combined with the time scale of formation of the shell, strongly suggests that the star has originated in the Upper-Scorpius subgroup.

Discovery of Molecular and Atomic Clouds Associated with the Magellanic Superbubble 30 Doradus C

NASA Astrophysics Data System (ADS)

Sano, H.; Yamane, Y.; Voisin, F.; Fujii, K.; Yoshiike, S.; Inaba, T.; Tsuge, K.; Babazaki, Y.; Mitsuishi, I.; Yang, R.; Aharonian, F.; Rowell, G.; Filipović, M. D.; Mizuno, N.; Tachihara, K.; Kawamura, A.; Onishi, T.; Fukui, Y.

2017-07-01

We analyzed the 2.6 mm CO and 21 cm H I lines toward the Magellanic superbubble 30 Doradus C, in order to reveal the associated molecular and atomic gas. We uncovered five molecular clouds in a velocity range from 251 to 276 km s-1 toward the western shell. The non-thermal X-rays are clearly enhanced around the molecular clouds on a parsec scale, suggesting possible evidence for magnetic field amplification via shock-cloud interaction. The thermal X-rays are brighter in the eastern shell, where there are no dense molecular or atomic clouds, opposite to the western shell. The TeV γ-ray distribution may spatially match the total interstellar proton column density as well as the non-thermal X-rays. If the hadronic γ-ray is dominant, the total energy of the cosmic-ray protons is at least ˜ 1.2× {10}50 erg with the estimated mean interstellar proton density ˜60 cm-3. In addition, the γ-ray flux associated with the molecular cloud (e.g., MC3) could be detected and resolved by the Cherenkov Telescope Array (CTA). This should permit CTA to probe the diffusion of cosmic-rays into the associated dense ISM.

Diamond, aromatic, aliphatic components of interstellar dust grains: Random covalent networks in carbonaceous grains

NASA Astrophysics Data System (ADS)

Duley, W. W.

1995-05-01

A formalism based on the theory of random covalent networks (RCNs) in amorphous solids is developed for carbonaceous dust grains. RCN solutions provide optimized structures and relative compositions for amorphous materials. By inclusion of aliphatic, aromatic, and diamond clusters, solutions specific to interstellar materials can be obtained and compared with infrared spectral data. It is found that distinct RCN solutions corresponding to diffuse cloud and molecular cloud materials are possible. Specific solutions are derived for three representative objects: VI Cyg No. 12, NGC 7538 (IRS 9), and GC IRS 7. While diffuse cloud conditions with a preponderance of sp2 and sp3 bonded aliphatic CH species can be reproduced under a variety of RCN conditions, the presence of an abundant tertiary CH or diamond component is highly constrained. These solutions are related quantitatively to carbon depletions and can be used to provide a quantitative estimate of carbon in these various dust components. Despite the abundance of C6 aromatic rings in many RCN solutions, the infrared absorption due to the aromatic stretch at approximately 3.3 micrometers is weak under all conditions. The RCN formalism is shown to provide a useful method for tracing the evolutionary properties of interstellar carbonaceous grains.

Anomalous 13C isotope abundances in C3S and C4H observed toward the cold interstellar cloud, Taurus Molecular Cloud-1.

PubMed

Sakai, Nami; Takano, Shuro; Sakai, Takeshi; Shiba, Shoichi; Sumiyoshi, Yoshihiro; Endo, Yasuki; Yamamoto, Satoshi

2013-10-03

We have studied the abundances of the (13)C isotopic species of C3S and C4H in the cold molecular cloud, Taurus Molecular Cloud-1 (Cyanopolyyne Peak), by radioastronomical observations of their rotational emission lines. The CCCS/(13)CCCS and CCCS/C(13)CCS ratios are determined to be >206 and 48 ± 15, respectively. The CC(13)CS line is identified with the aid of laboratory microwave spectroscopy, and the range of the CCCS/CC(13)CS ratio is found to be from 30 to 206. The abundances of at least two (13)C isotopic species of C3S are thus found to be different. Similarly, it is found that the abundances of the four (13)C isotopic species of C4H are not equivalent. The CCCCH/(13)CCCCH, CCCCH/C(13)CCCH, CCCCH/CC(13)CCH, and CCCCH/CCC(13)CH ratios are evaluated to be 141 ± 44, 97 ± 27, 82 ± 15, and 118 ± 23, respectively. Here the errors denote 3 times the standard deviation. These results will constrain the formation pathways of C3S and C4H, if the nonequivalence is caused during the formation processes of these molecules. The exchange reactions after the formation of these two molecules may also contribute to the nonequivalence. In addition, we have confirmed that the (12)C/(13)C ratio of some species are significantly higher than the interstellar elemental (12)C/(13)C ratio of 60-70. The observations of the (13)C isotopic species provide us with rich information on chemical processes in cold interstellar clouds.

A survey of interstellar neutral potassium. I - Abundances and physical conditions in clouds toward 188 early-type stars

NASA Technical Reports Server (NTRS)

Chaffee, F. H., Jr.; White, R. E.

1982-01-01

Observations of interstellar absorption in the resonance doublet 7664, 7698 A of neutral potassium toward 188 early-type stars at a spectral resolution of 8 km/s are reported. The 7664 A line is successfully separated from nearly coincident telluric O2 absorption for all but a few of the 165 stars for which K I absorption is detected, making possible an abundance analysis by the doublet ratio method. The relationships between the potassium abundances and other atomic abundances, the abundance of molecular hydrogen, and interstellar reddening are investigated.

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

NASA Technical Reports Server (NTRS)

Gehrz, Robert D.

1991-01-01

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.

Massive stars: privileged sources of cosmic-rays for interstellar astrochemistry

NASA Astrophysics Data System (ADS)

De Becker, M.

2015-01-01

Massive stars can be considered as crucial engines for interstellar physics. They are indeed the main providers of UV radiation field, and constitute a substantial source of chemical enrichment. On their evolution time-scale (at most about 10 Myr), they typically stay close to their formation site, i.e. close to molecular clouds very rich in interstellar molecules. These stellar objects have also the property to be involved in particle acceleration processes leading to the production of high energy charged particles (cosmic-rays). After rejection in the interstellar medium, these particles will play a substantial role in processes such as those simulated in various facilities dedicated to experimental astrochemistry. This short contribution intends to put these particles, crucial for astrochemistry, in their adequate astrophysical context.

Investigating nearby exoplanets via interstellar radar

NASA Astrophysics Data System (ADS)

Scheffer, Louis K.

2014-01-01

Interstellar radar is a potential intermediate step between passive observation of exoplanets and interstellar exploratory missions. Compared with passive observation, it has the traditional advantages of radar astronomy. It can measure surface characteristics, determine spin rates and axes, provide extremely accurate ranges, construct maps of planets, distinguish liquid from solid surfaces, find rings and moons, and penetrate clouds. It can do this even for planets close to the parent star. Compared with interstellar travel or probes, it also offers significant advantages. The technology required to build such a radar already exists, radar can return results within a human lifetime, and a single facility can investigate thousands of planetary systems. The cost, although too high for current implementation, is within the reach of Earth's economy.

The South African Astronomical Observatory

NASA Technical Reports Server (NTRS)

1989-01-01

Topics discussed in the Overview of Year 1988 include the following: Supernova in the Large Magellanic Cloud; Galaxies; Ground based observations of celestial x ray sources; the Magellanic Clouds; Pulsating variables; Galactic structure; Binary star phenomena; The provision of photometric standards; Nebulae and interstellar matter; Stellar astrophysics; Astrometry; Solar system studies; Visitors programs; Publications; and General matters.

Astrophysical radiation environments of habitable worlds

NASA Astrophysics Data System (ADS)

Smith, David Samuel

Numerous astrophysical sources of radiation affect the environment of planets orbiting within the liquid-water habitable zone of main-sequence stars. This dissertation reaches a number of conclusions about the ionizing radiation environment of the habitable zone with respect to X-rays and gamma-rays from stellar flares and background Galactic cosmic rays. Gamma-rays and X-rays incident on terrestrial-like exoplanet atmospheres can be efficiently reprocessed into diffuse UV emission that, depending on the presence of atmospheric UV absorbers, can reach the surface. Extreme solar X-ray flares over the last 4.6 Gyr could have delivered large enough radiation doses to the Martian surface to sterilize any unprotected organisms, depending on the largest energy releases possible. These flares also pose a significant hazard to manned space missions, since a large flare can occur with little or no warning during an extravehicular activity. A flare as large as the largest observed could deliver radiation doses exceeding safety limits to an astronaut protected by only a spacesuit. With respect to particle radiation, the nature of Galactic cosmic-ray modulation by astrospheres means that habitable-zone cosmic-ray fluxes change by much larger magnitudes when passing through low- densities regions of the interstellar medium. In contrast to the popular idea that passages through dense molecular clouds are required to significantly enhance Galactic cosmic-ray fluxes and affect planets' electrical circuits, background mutation rates, and climates, we find that densities of only 0.1-10 cm -3 , the densities of most interstellar clouds, are sufficient to bring fluxes close to the full, interstellar level. Finally, passages through dense molecular clouds are necessary to shrink astrospheres to within the habitable zone, but such events produce even higher interstellar hydrogen and dust accretion rates than have been estimated because of the combination of enhanced charge-exchange rates between stellar-wind ions and interstellar neutrals and the growing importance of the central star's gravity on particle trajectories as the astrosphere shrinks.

Carbon Chemistry in Transitional Clouds from the GOT C+ Survey of CII 158 micron Emission in the Galactic Plane

NASA Astrophysics Data System (ADS)

Langer, W. D.; Velusamy, T.; Pineda, J.; Willacy, K.; Goldsmith, P. F.

2011-05-01

In understanding the lifecycle and chemistry of the interstellar gas, the transition from diffuse atomic to molecular gas clouds is a very important stage. The evolution of carbon from C+ to C0 and CO is a fundamental part of this transition, and C+ along with its carbon chemistry is a key diagnostic. Until now our knowledge of interstellar gas has been limited primarily to the diffuse atomic phase traced by HI and the dense molecular H2 phase traced by CO. However, we have generally been missing an important layer in diffuse and transition clouds, which is denoted by the warm "dark gas'', that is mostly H2 and little HI and CO, and is best traced with C+. Here, we discuss the chemistry in the transition from C+ to C0 and CO in these clouds as understood by a survey of the CII 1.9 THz (158 micron) line from a sparse survey of the inner galaxy over about 40 degrees in longitude as part of the Galactic Observations of Terahertz C+ (GOT C+) program, a Herschel Space Observatory Open Time Key Program to study interstellar clouds by sampling ionized carbon. Using the first results from GOT C+ along 11 LOSs, in a sample of 53 transition clouds, Velusamy, Langer et al. (A&A 521, L18, 2010) detected an excess of CII intensities indicative of a thick H2 layer (a significant warm H2, "dark gas'' component) around the 12CO core. Here we present a much larger, statistically significant sample of a few hundred diffuse and transition clouds traced by CII, along with auxiliary HI and CO data in the inner Galaxy between l=-30° and +30°. Our new and more extensive sample of transition clouds is used to elucidate the time dependent physical and carbon chemical evolution of diffuse to transition clouds, and transition layers. We consider the C+ to CO conversion pathways such as H++ O and C+ + H2 chemistry for CO production to constrain the physical parameters such as the FUV intensity and cosmic ray ionization rate that drive the CO chemistry in the diffuse transition clouds.

Analysis of interstellar cloud structure based on IRAS images

NASA Technical Reports Server (NTRS)

Scalo, John M.

1992-01-01

The goal of this project was to develop new tools for the analysis of the structure of densely sampled maps of interstellar star-forming regions. A particular emphasis was on the recognition and characterization of nested hierarchical structure and fractal irregularity, and their relation to the level of star formation activity. The panoramic IRAS images provided data with the required range in spatial scale, greater than a factor of 100, and in column density, greater than a factor of 50. In order to construct densely sampled column density maps of star-forming clouds, column density images of four nearby cloud complexes were constructed from IRAS data. The regions have various degrees of star formation activity, and most of them have probably not been affected much by the disruptive effects of young massive stars. The largest region, the Scorpius-Ophiuchus cloud complex, covers about 1000 square degrees (it was subdivided into a few smaller regions for analysis). Much of the work during the early part of the project focused on an 80 square degree region in the core of the Taurus complex, a well-studied region of low-mass star formation.

The Diffuse Interstellar Bands: Solving a Century Old Problem

NASA Technical Reports Server (NTRS)

Salama, Farid

2017-01-01

The Diffuse Interstellar Bands (DIBs) are a set of apporoximately 500 absorption bands that are seen in the spectra of reddened stars (i.e., stars obscured by the presence of interstellar clouds in their line of sight). The first DIBs were detected in the visible over a century ago. Diffuse Interstellar Bands are now detected from the near ultraviolet to the near infrared in the spectra of reddened stars spanning a variety of interstellar environments in our local, and in other galaxies. Although DIB carriers are a significant part of the interstellar chemical inventory as they account for a noticeable fraction of the interstellar extinction, the nature of their carriers is still unknown over a century after the detection of the first bands. DIB carriers are stable and ubiquitous in a broad variety of interstellar environments and play a unique role in interstellar physics and chemistry. It has long been realized that the solving of the DIB problem requires a strong synergy between astronomical observations, laboratory astrophysics and astrochemistry, quantum chemistry calculations and astrophysical modeling of line-of-sights. In this review, we'll present and discuss the current state of this perplexing problem. We'll review the progress and the failures that have been encountered in the long quest for the identification of the carriers of these ubiquitous interstellar bands.

Embedded Outflows from Herbig-Haro 46/47

NASA Technical Reports Server (NTRS)

2003-01-01

[figure removed for brevity, see original site] Click on image for larger view of insert

This image from NASA's Spitzer Space Telescope transforms a dark cloud into a silky translucent veil, revealing the molecular outflow from an otherwise hidden newborn star. Using near-infrared light, Spitzer pierces through the dark cloud to detect the embedded outflow in an object called HH 46/47. Herbig-Haro (HH) objects are bright, nebulous regions of gas and dust that are usually buried within dark clouds. They are formed when supersonic gas ejected from a forming protostar, or embryonic stars, interacts with the surrounding interstellar medium. These young stars are often detected only in the infrared.

The Spitzer image was obtained with the infrared array camera and is a three-color mosaic. Emission at 3.6 microns is shown as blue, emission from 4.5 and 5.8 microns has been combined as green, and 8.0 micron emission is depicted as red.

HH 46/47 is a striking example of a low mass protostar ejecting a jet and creating a bipolar, or two-sided, outflow. The central protostar lies inside a dark cloud (known as a 'Bok globule') which is illuminated by the nearby Gum Nebula. Located at a distance of 1,140 light-years and found in the constellation Vela, the protostar is hidden from view in the visible-light image (inset). With Spitzer, the star and its dazzling jets of molecular gas appear with clarity.

The 8-micron channel of the infrared array camera is sensitive to emission from polycyclic aromatic hydrocarbons. These organic molecules, comprised of carbon and hydrogen, are excited by the surrounding radiation field and become luminescent, accounting for the reddish cloud. Note that the boundary layer of the 8-micron emission corresponds to the lower right edge of the dark cloud in the visible-light picture.

Outflows are fascinating objects, since they characterize one of the most energetic phases of the formation of low-mass stars (like our Sun). The jets arising from these protostars can reach sizes of trillions of miles and velocities of hundreds of thousands miles per hour. Outflows are clear evidence of the presence of a process that creates supersonic beams of gas. This mechanism is tightly bound to the presence of circumstellar discs which surround the young stars. Such discs are likely to contain the materials from which planetary systems form. Our Sun probably underwent a similar process some 4.5 billion years ago. Hence the interest in understanding how quickly and efficiently this mass accretion and loss process takes place in protostars.

Radiative Transfer in a Translucent Cloud Illuminated by an Extended Background Source

NASA Astrophysics Data System (ADS)

Biganzoli, Davide; Potenza, Marco A. C.; Robberto, Massimo

2017-05-01

We discuss the radiative transfer theory for translucent clouds illuminated by an extended background source. First, we derive a rigorous solution based on the assumption that multiple scatterings produce an isotropic flux. Then we derive a more manageable analytic approximation showing that it nicely matches the results of the rigorous approach. To validate our model, we compare our predictions with accurate laboratory measurements for various types of well-characterized grains, including purely dielectric and strongly absorbing materials representative of astronomical icy and metallic grains, respectively, finding excellent agreement without the need to add free parameters. We use our model to explore the behavior of an astrophysical cloud illuminated by a diffuse source with dust grains having parameters typical of the classic ISM grains of Draine & Lee and protoplanetary disks, with an application to the dark silhouette disk 114-426 in Orion Nebula. We find that the scattering term modifies the transmitted radiation, both in terms of intensity (extinction) and shape (reddening) of the spectral distribution. In particular, for small optical thickness, our results show that scattering makes reddening almost negligible at visible wavelengths. Once the optical thickness increases enough and the probability of scattering events becomes close to or larger than 1, reddening becomes present but is appreciably modified with respect to the standard expression for line-of-sight absorption. Moreover, variations of the grain refractive index, in particular the amount of absorption, also play an important role in changing the shape of the spectral transmission curve, with dielectric grains showing the minimum amount of reddening.

Interstellar Polycyclic Aromatic Compounds and Astrophysics

NASA Technical Reports Server (NTRS)

Hodgins, Douglas M.; DeVincenzi, Donald (Technical Monitor)

1999-01-01

Polycyclic aromatic compounds (PACs), a class of organic molecules whose structures are characterized by the presence of two or more fused aromatic rings, have been the subject of astrophysical interest for nearly two decades. Large by interstellar standards (from as few as 20 to perhaps as many as several hundred atoms), it has been suggested that these species are among the most abundant interstellar molecules impacting a wide range of astrophysical phenomena including: the ubiquitous family of infrared emission bands observed in an ever-increasing assortment of astronomical objects; the subtle but rich array of discrete visible/near-infrared interstellar molecular absorption features known as the diffuse interstellar bands (DIBs); the broad near-infrared quasi-continuum observed in a number of nebulae known as excess red emission (ERE); the interstellar ultraviolet extinction curve and broad '2200 Angstrom bump'; the heating/cooling mechanisms of interstellar clouds. Nevertheless, until recently a lack of good-quality laboratory spectroscopic data on PACs under astrophysically relevant conditions (i.e. isolated, ionized molecules; ionized molecular clusters, etc.) has hindered critical evaluation and extension of this model

Collisional excitation of interstellar methyl cyanide

NASA Technical Reports Server (NTRS)

Green, Sheldon

1986-01-01

Theoretical calculations are used to determine the collisional excitation rates of methyl cyanide under interstellar molecular cloud conditions. The required Q(L,M) as a function of kinetic temperature were determined by averaging fixed energy IOS (infinite order sudden) results over appropriate Boltzmann distributions of collision energies. At a kinetic temperature of 40 K, rates within a K ladder were found to be accurate to generally better than about 30 percent.

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

NASA Technical Reports Server (NTRS)

White, Richard E.; Bally, John

1993-01-01

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.

Element abundances at high redshift

NASA Technical Reports Server (NTRS)

Meyer, David M.; Welty, D. E.; York, D. G.

1989-01-01

Abundances of Si(+), S(+), Cr(+), Mn(+), Fe(_), and Zn(+) are considered for two absorption-line systems in the spectrum of the QSO PKS 0528 - 250. Zinc and sulfur are underabundant, relative to H, by a factor of 10 compared to their solar and Galactic interstellar abundances. The silicon-, chromium-, iron-, and nickel-to-hydrogen ratios are less than the solar values and comparable to the local interstellar ratios. A straightforward interpretation is that nucleosynthesis in these high-redshift systems has led to only about one-tenth as much heavy production as in the gas clouds around the sun, and that the amount of the observed underabundances attributable to grain depletion is small. The dust-to-gas ratio in these clouds is less than 8 percent of the Galactic value.

Decoding IR Spectra of Cosmic Ices and Organics in the Laboratory

NASA Technical Reports Server (NTRS)

Allamandola, Louis J.

2006-01-01

Tremendous strides have been made in our understanding of interstellar material over the past twenty-five years thanks to significant developments in observational IR astronomy and dedicated laboratory experiments. Twenty-five years ago the composition of interstellar dust was largely guessed at. Today the composition of interstellar dust is reasonably well understood. In the diffuse interstellar medium (ISM) the dust population is mainly comprised of small grains of silicates and amorphous carbon. In dark molecular clouds, the birthplace of stars and planets, these cold refractory dust particles are coated with mixed molecular ices whose composition is reasonably 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. This extraordinary progress has been made possible by the close collaboration of laboratory experimentalists and theoreticians with IR astronomers using groundbased, air-borne, and orbiting telescopes.

Interferometric observations of large biologically interesting interstellar and cometary molecules

PubMed Central

Snyder, Lewis E.

2006-01-01

Interferometric observations of high-mass regions in interstellar molecular clouds have revealed hot molecular cores that have substantial column densities of large, partly hydrogen-saturated molecules. Many of these molecules are of interest to biology and thus are labeled “biomolecules.” Because the clouds containing these molecules provide the material for star formation, they may provide insight into presolar nebular chemistry, and the biomolecules may provide information about the potential of the associated interstellar chemistry for seeding newly formed planets with prebiotic organic chemistry. In this overview, events are outlined that led to the current interferometric array observations. Clues that connect this interstellar hot core chemistry to the solar system can be found in the cometary detection of methyl formate and the interferometric maps of cometary methanol. Major obstacles to understanding hot core chemistry remain because chemical models are not well developed and interferometric observations have not been very sensitive. Differentiation in the molecular isomers glycolaldehdye, methyl formate, and acetic acid has been observed, but not explained. The extended source structure for certain sugars, aldehydes, and alcohols may require nonthermal formation mechanisms such as shock heating of grains. Major advances in understanding the formation chemistry of hot core species can come from observations with the next generation of sensitive, high-resolution arrays. PMID:16894168

The peculiar behaviour of the 5780 and 5797 DIBs in HD25137

NASA Technical Reports Server (NTRS)

Porceddu, Ignazio; Benvenuti, P.

1994-01-01

The interstellar environment close to the high latitude molecular cloud Lynds 1569 (L1569, Lynds 1962), also known as MBM 18 (Magnani, Blitz and Mundy, 1985), has been analyzed by Penrase et al. (1990) and Penrase (1993). Their observations of the CH, CH(sup+), and CN molecular features, are consistent with a region having a high molecular and reduced dust content. They also observed the background star HD 24263- located 8 degrees far from the center of L1569 - reporting a CH rich line of sight and the presence of two intervening clouds from a sodium lines spectra. The infrared excess which has been revealed by the IRAS survey at 12 microns might suggest the presence of PAH's molecules, the well know candidate for the Unidentified Infrared Bands and Diffuse Interstellar Bands. This interesting scenario led to the investigation of the behavior of the diffuse interstellar bands toward HD 25137, which is supposed to be a background object for L1569 (Penrase et al., 1990); as well as the field star HD 24263. As part of a wider observational program devoted to study the HLC's special environments, the observations of the diffuse interstellar bands (DIB's) at 5780 and 5797 lambda lambda in the direction of the two above mentioned stars, HD 24263 and HD 25137 are presented here.

Synthesis and chirality of amino acids under interstellar conditions.

PubMed

Giri, Chaitanya; Goesmann, Fred; Meinert, Cornelia; Evans, Amanda C; Meierhenrich, Uwe J

2013-01-01

Amino acids are the fundamental building blocks of proteins, the biomolecules that provide cellular structure and function in all living organisms. A majority of amino acids utilized within living systems possess pre-specified orientation geometry (chirality); however the original source for this specific orientation remains uncertain. In order to trace the chemical evolution of life, an appreciation of the synthetic and evolutional origins of the first chiral amino acids must first be gained. Given that the amino acids in our universe are likely to have been synthesized in molecular clouds in interstellar space, it is necessary to understand where and how the first synthesis might have occurred. The asymmetry of the original amino acid synthesis was probably the result of exposure to chiral photons in the form of circularly polarized light (CPL), which has been detected in interstellar molecular clouds. This chirality transfer event, from photons to amino acids, has been successfully recreated experimentally and is likely a combination of both asymmetric synthesis and enantioselective photolysis. A series of innovative studies have reported successful simulation of these environments and afforded production of chiral amino acids under realistic circumstellar and interstellar conditions: irradiation of interstellar ice analogues (CO, CO2, NH3, CH3OH, and H2O) with circularly polarized ultraviolet photons at low temperatures does result in enantiomer enriched amino acid structures (up to 1.3% ee). This topical review summarizes current knowledge and recent discoveries about the simulated interstellar environments within which amino acids were probably formed. A synopsis of the COSAC experiment onboard the ESA cometary mission ROSETTA concludes this review: the ROSETTA mission will soft-land on the nucleus of the comet 67P/Churyumov-Gerasimenko in November 2014, anticipating the first in situ detection of asymmetric organic molecules in cometary ices.

The ESO Diffuse Interstellar Bands Large Exploration Survey: EDIBLES I. Project description, survey sample and quality assessment.

PubMed

Cox, Nick L J; Cami, Jan; Farhang, Amin; Smoker, Jonathan; Monreal-Ibero, Ana; Lallement, Rosine; Sarre, Peter J; Marshall, Charlotte C M; Smith, Keith T; Evans, Christopher J; Royer, Pierre; Linnartz, Harold; Cordiner, Martin A; Joblin, Christine; van Loon, Jacco Th; Foing, Bernard H; Bhatt, Neil H; Bron, Emeric; Elyajouri, Meriem; de Koter, Alex; Ehrenfreund, Pascale; Javadi, Atefeh; Kaper, Lex; Khosroshadi, Habib G; Laverick, Mike; Le Petit, Franck; Mulas, Giacomo; Roueff, Evelyne; Salama, Farid; Spaans, Marco

2017-10-01

The carriers of the diffuse interstellar bands (DIBs) are largely unidentified molecules ubiquitously present in the interstellar medium (ISM). After decades of study, two strong and possibly three weak near-infrared DIBs have recently been attributed to the [Formula: see text] fullerene based on observational and laboratory measurements. There is great promise for the identification of the over 400 other known DIBs, as this result could provide chemical hints towards other possible carriers. In an effort to systematically study the properties of the DIB carriers, we have initiated a new large-scale observational survey: the ESO Diffuse Interstellar Bands Large Exploration Survey (EDIBLES). The main objective is to build on and extend existing DIB surveys to make a major step forward in characterising the physical and chemical conditions for a statistically significant sample of interstellar lines-of-sight, with the goal to reverse-engineer key molecular properties of the DIB carriers. EDIBLES is a filler Large Programme using the Ultraviolet and Visual Echelle Spectrograph at the Very Large Telescope at Paranal, Chile. It is designed to provide an observationally unbiased view of the presence and behaviour of the DIBs towards early-spectral type stars whose lines-of-sight probe the diffuse-to-translucent ISM. Such a complete dataset will provide a deep census of the atomic and molecular content, physical conditions, chemical abundances and elemental depletion levels for each sightline. Achieving these goals requires a homogeneous set of high-quality data in terms of resolution ( R ~ 70 000 - 100 000), sensitivity (S/N up to 1000 per resolution element), and spectral coverage (305-1042 nm), as well as a large sample size (100+ sightlines). In this first paper the goals, objectives and methodology of the EDIBLES programme are described and an initial assessment of the data is provided.

The ESO Diffuse Interstellar Bands Large Exploration Survey (EDIBLES) . I. Project description, survey sample, and quality assessment

NASA Astrophysics Data System (ADS)

Cox, Nick L. J.; Cami, Jan; Farhang, Amin; Smoker, Jonathan; Monreal-Ibero, Ana; Lallement, Rosine; Sarre, Peter J.; Marshall, Charlotte C. M.; Smith, Keith T.; Evans, Christopher J.; Royer, Pierre; Linnartz, Harold; Cordiner, Martin A.; Joblin, Christine; van Loon, Jacco Th.; Foing, Bernard H.; Bhatt, Neil H.; Bron, Emeric; Elyajouri, Meriem; de Koter, Alex; Ehrenfreund, Pascale; Javadi, Atefeh; Kaper, Lex; Khosroshadi, Habib G.; Laverick, Mike; Le Petit, Franck; Mulas, Giacomo; Roueff, Evelyne; Salama, Farid; Spaans, Marco

2017-10-01

The carriers of the diffuse interstellar bands (DIBs) are largely unidentified molecules ubiquitously present in the interstellar medium (ISM). After decades of study, two strong and possibly three weak near-infrared DIBs have recently been attributed to the C60^+ fullerene based on observational and laboratory measurements. There is great promise for the identification of the over 400 other known DIBs, as this result could provide chemical hints towards other possible carriers. In an effort tosystematically study the properties of the DIB carriers, we have initiated a new large-scale observational survey: the ESO Diffuse Interstellar Bands Large Exploration Survey (EDIBLES). The main objective is to build on and extend existing DIB surveys to make a major step forward in characterising the physical and chemical conditions for a statistically significant sample of interstellar lines-of-sight, with the goal to reverse-engineer key molecular properties of the DIB carriers. EDIBLES is a filler Large Programme using the Ultraviolet and Visual Echelle Spectrograph at the Very Large Telescope at Paranal, Chile. It is designed to provide an observationally unbiased view of the presence and behaviour of the DIBs towards early-spectral-type stars whose lines-of-sight probe the diffuse-to-translucent ISM. Such a complete dataset will provide a deep census of the atomic and molecular content, physical conditions, chemical abundances and elemental depletion levels for each sightline. Achieving these goals requires a homogeneous set of high-quality data in terms of resolution (R 70 000-100 000), sensitivity (S/N up to 1000 per resolution element), and spectral coverage (305-1042 nm), as well as a large sample size (100+ sightlines). In this first paper the goals, objectives and methodology of the EDIBLES programme are described and an initial assessment of the data is provided.

The ESO Diffuse Interstellar Bands Large Exploration Survey: EDIBLES I. Project description, survey sample and quality assessment

PubMed Central

Cox, Nick L. J.; Cami, Jan; Farhang, Amin; Smoker, Jonathan; Monreal-Ibero, Ana; Lallement, Rosine; Sarre, Peter J.; Marshall, Charlotte C. M.; Smith, Keith T.; Evans, Christopher J.; Royer, Pierre; Linnartz, Harold; Cordiner, Martin A.; Joblin, Christine; van Loon, Jacco Th.; Foing, Bernard H.; Bhatt, Neil H.; Bron, Emeric; Elyajouri, Meriem; de Koter, Alex; Ehrenfreund, Pascale; Javadi, Atefeh; Kaper, Lex; Khosroshadi, Habib G.; Laverick, Mike; Le Petit, Franck; Mulas, Giacomo; Roueff, Evelyne; Salama, Farid; Spaans, Marco

2017-01-01

The carriers of the diffuse interstellar bands (DIBs) are largely unidentified molecules ubiquitously present in the interstellar medium (ISM). After decades of study, two strong and possibly three weak near-infrared DIBs have recently been attributed to the C60+ fullerene based on observational and laboratory measurements. There is great promise for the identification of the over 400 other known DIBs, as this result could provide chemical hints towards other possible carriers. In an effort to systematically study the properties of the DIB carriers, we have initiated a new large-scale observational survey: the ESO Diffuse Interstellar Bands Large Exploration Survey (EDIBLES). The main objective is to build on and extend existing DIB surveys to make a major step forward in characterising the physical and chemical conditions for a statistically significant sample of interstellar lines-of-sight, with the goal to reverse-engineer key molecular properties of the DIB carriers. EDIBLES is a filler Large Programme using the Ultraviolet and Visual Echelle Spectrograph at the Very Large Telescope at Paranal, Chile. It is designed to provide an observationally unbiased view of the presence and behaviour of the DIBs towards early-spectral type stars whose lines-of-sight probe the diffuse-to-translucent ISM. Such a complete dataset will provide a deep census of the atomic and molecular content, physical conditions, chemical abundances and elemental depletion levels for each sightline. Achieving these goals requires a homogeneous set of high-quality data in terms of resolution (R ~ 70 000 – 100 000), sensitivity (S/N up to 1000 per resolution element), and spectral coverage (305–1042 nm), as well as a large sample size (100+ sightlines). In this first paper the goals, objectives and methodology of the EDIBLES programme are described and an initial assessment of the data is provided. PMID:29151608

Probing non polar interstellar molecules through their protonated form: Detection of protonated cyanogen (NCCNH+)★

PubMed Central

Agúndez, M.; Cernicharo, J.; de Vicente, P.; Marcelino, N.; Roueff, E.; Fuente, A.; Gerin, M.; Guélin, M.; Albo, C.; Barcia, A.; Barbas, L.; Bolaño, R.; Colomer, F.; Diez, M. C.; Gallego, J. D.; Gómez-González, J.; López-Fernández, I.; López-Fernández, J. A.; López-Pérez, J. A.; Malo, I.; Serna, J. M.; Tercero, F.

2015-01-01

Cyanogen (NCCN) is the simplest member of the series of dicyanopolyynes. It has been hypothesized that this family of molecules can be important constituents of interstellar and circumstellar media, although the lack of a permanent electric dipole moment prevents its detection through radioastronomical techniques. Here we present the first solid evidence of the presence of cyanogen in interstellar clouds through the detection of its protonated form toward the cold dark clouds TMC-1 and L483. Protonated cyanogen (NCCNH+) has been identified through the J = 5 – 4 and J = 10 – 9 rotational transitions using the 40m radiotelescope of Yebes and the IRAM 30m telescope. We derive beam averaged column densities for NCCNH+ of (8.6 ± 4.4) × 1010 cm−2 in TMC-1 and (3.9 ± 1.8) × 1010 cm−2 in L483, which translate to fairly low fractional abundances relative to H2, in the range (1-10) × 10−12. The chemistry of protonated molecules in dark clouds is discussed, and it is found that, in general terms, the abundance ratio between the protonated and non protonated forms of a molecule increases with increasing proton affinity. Our chemical model predicts an abundance ratio NCCNH+/NCCN of ~ 10−4, which implies that the abundance of cyanogen in dark clouds could be as high as (1-10) × 10−8 relative to H2, i.e., comparable to that of other abundant nitriles such as HCN, HNC, and HC3N. PMID:26543239

Gas-Grain Chemical Models: Inclusion of a Grain Size Distribution and a Study Of Young Stellar Objects in the Magellanic Clouds

NASA Astrophysics Data System (ADS)

Pauly, Tyler Andrew

2017-06-01

Computational models of interstellar gas-grain chemistry have aided in our understanding of star-forming regions. Chemical kinetics models rely on a network of chemical reactions and a set of physical conditions in which atomic and molecular species are allowed to form and react. We replace the canonical single grain-size in our chemical model MAGICKAL with a grain size distribution and analyze the effects on the chemical composition of the gas and grain surface in quiescent and collapsing dark cloud models. We find that a grain size distribution coupled with a temperature distribution across grain sizes can significantly affect the bulk ice composition when dust temperatures fall near critical values related to the surface binding energies of common interstellar chemical species. We then apply the updated model to a study of ice formation in the cold envelopes surrounding massive young stellar objects in the Magellanic Clouds. The Magellanic Clouds are local satellite galaxies of the Milky Way, and they provide nearby environments to study star formation at low metallicity. We expand the model calculation of dust temperature to include a treatment for increased interstellar radiation field intensity; we vary the radiation field to model the elevated dust temperatures observed in the Magellanic Clouds. We also adjust the initial elemental abundances used in the model, guided by observations of Magellanic Cloud HII regions. We are able to reproduce the relative ice fractions observed, indicating that metal depletion and elevated grain temperature are important drivers of the envelope ice composition. The observed shortfall in CO in Small Magellanic Cloud sources can be explained by a combination of reduced carbon abundance and increased grain temperatures. The models indicate that a large variation in radiation field strength is required to match the range of observed LMC abundances. CH 3OH abundance is found to be enhanced (relative to total carbon abundance) in low-metallicity models, providing seed material for complex organic molecule formation. We conclude with a preliminary study of the recently discovered hot core in the Large Magellanic Cloud; we create a grid of models to simulate hot core formation in Magellanic Cloud environments, comparing them to models and observations of well-characterized galactic counterparts.

C+/CO Transitions in the Diffuse ISM: Transitional Cloud Sample from the GOT C+ Survey of [CII] in the inner Galaxy at l = -30deg to 30deg

NASA Astrophysics Data System (ADS)

Velusamy, T.; Pineda, J. L.; Langer, W. D.; Willacy, K.; Goldsmith, P. F.

2011-05-01

Our knowledge of interstellar gas has been limited primarily to the diffuse atomic phase traced by HI and the well-shielded molecular phase traced by CO. Recently, using the first results of the Herschel Key Project GOT C+, a HIFI C+ survey of the Galactic plane, Velusamy, Langer, Pineda et al. (A&A 521, L18, 2010) have shown that in the diffuse interstellar transition clouds a significant fraction of the carbon exists primarily as C^+ with little C^0 and CO in a warm 'dark gas' layer in which hydrogen is mostly H_2 with little atomic H, surrounding a modest 12CO-emitting core. The [CII] fine structure transition, at 1.9 THz (158 μm) is the best tracer of this component of the interstellar medium, which is critical to our understanding of the atomic to molecular cloud transitions. The Herschel Key Project GOT C+ is designed to study such clouds by observing with HIFI the [CII] line emission along 500 lines of sight (LOSs) throughout the Galactic disk. Here we present the identification and chemical status of a few hundred diffuse and transition clouds traced by [CII], along with auxiliary HI and CO data covering ~100 LOSs in the inner Galaxy between l= -30° and 30°. We identify transition clouds as [CII] components that are characterized by the presence of both HI and 12CO, but no 13CO emission. The intensities, I(CII) and I(HI), are used as measures of the visual extinction, AV, in the cloud up to the C^+/C^0/CO transition layer and a comparison with I(12CO) yields a more complete H_2 molecular inventory. Our results show that [CII] emission is an excellent tool to study transition clouds and their carbon chemistry in the ISM, in particular as a unique tracer of molecular H_2, which is not easily observed by other means. The large sample presented here will serve as a resource to study the chemical and physical status of diffuse transition clouds in a wide range of Galactic environments and constrain the physical parameters such as the FUV intensity and cosmic ray ionization rate that drive the CO chemistry in the diffuse ISM.

Multiple generations of grain aggregation in different environments preceded solar system body formation.

PubMed

Ishii, Hope A; Bradley, John P; Bechtel, Hans A; Brownlee, Donald E; Bustillo, Karen C; Ciston, James; Cuzzi, Jeffrey N; Floss, Christine; Joswiak, David J

2018-06-26

The solar system formed from interstellar dust and gas in a molecular cloud. Astronomical observations show that typical interstellar dust consists of amorphous ( a -) silicate and organic carbon. Bona fide physical samples for laboratory studies would yield unprecedented insight about solar system formation, but they were largely destroyed. The most likely repositories of surviving presolar dust are the least altered extraterrestrial materials, interplanetary dust particles (IDPs) with probable cometary origins. Cometary IDPs contain abundant submicron a- silicate grains called GEMS (glass with embedded metal and sulfides), believed to be carbon-free. Some have detectable isotopically anomalous a- silicate components from other stars, proving they are preserved dust inherited from the interstellar medium. However, it is debated whether the majority of GEMS predate the solar system or formed in the solar nebula by condensation of high-temperature (>1,300 K) gas. Here, we map IDP compositions with single nanometer-scale resolution and find that GEMS contain organic carbon. Mapping reveals two generations of grain aggregation, the key process in growth from dust grains to planetesimals, mediated by carbon. GEMS grains, some with a- silicate subgrains mantled by organic carbon, comprise the earliest generation of aggregates. These aggregates (and other grains) are encapsulated in lower-density organic carbon matrix, indicating a second generation of aggregation. Since this organic carbon thermally decomposes above ∼450 K, GEMS cannot have accreted in the hot solar nebula, and formed, instead, in the cold presolar molecular cloud and/or outer protoplanetary disk. We suggest that GEMS are consistent with surviving interstellar dust, condensed in situ, and cycled through multiple molecular clouds. Copyright © 2018 the Author(s). Published by PNAS.

The Inventory of Interstellar Materials Available for the Formation of the Solar System

NASA Technical Reports Server (NTRS)

Sandford, Scott A.; Witteborn, Fred C. (Technical Monitor)

1996-01-01

Dr. Derek Sears, the editor of the journal Meteoritics and Planetary Science, has established a policy of having each issue of the journal contain an invited review of an area that he deems to be of special cur-rent importance. Typically 20 to 25 pages of the beginning of the journal are devoted to each review. He has asked me to prepare such a review summarizing what we know about the composition and structure of interstellar materials. The attached paper is the result. This is a good time for such a review since tremendous progress has been made in the field of interstellar dust in recent years through the use of telescopic observations, theoretical studies, laboratory studies of analogs, and the study of actual interstellar samples found in meteorites. It is increasing clear that the interstellar medium (ISM) contains an enormous diversity of materials created by a wide range of chemical and physical processes. This understanding is a far cry from the picture of interstellar materials held as recently as two decades ago, a picture which incorporated only a few generic types of grains and few molecules. In the paper I review our current knowledge of the more abundant materials thought to exist in the ISM. The review concentrates on matter in interstellar dense molecular clouds since it is the materials in these environments from which new stars and planetary systems are formed, although materials in circumstellar environments and in the diffuse ISM are also discussed. The paper focuses largely on solid materials since they contain a major fraction of the heavier elements in clouds and because solids are most likely to survive incorporation into new planetary systems in identifiable form. The paper concludes with discussion of some of the implications resulting from the identification of these interstellar materials. I also present some new thoughts, the most intriguing being that meteoritic 'microdiamonds' may be the same material that modelers of the interstellar extinction call graphite! If my suggestion is correct it will help resolve the dilemma associated with the current over-subscription of cosmic C and should lead to some new approaches to both extinction models and the study of meteoritic interstellar microdiamonds.

Cosmic Carbon Chemistry: From the Interstellar Medium to the Early Earth

PubMed Central

Ehrenfreund, Pascale; Cami, Jan

2010-01-01

Astronomical observations have shown that carbonaceous compounds in the gas and solid state, refractory and icy are ubiquitous in our and distant galaxies. Interstellar molecular clouds and circumstellar envelopes are factories of complex molecular synthesis. A surprisingly large number of molecules that are used in contemporary biochemistry on Earth are found in the interstellar medium, planetary atmospheres and surfaces, comets, asteroids and meteorites, and interplanetary dust particles. In this article we review the current knowledge of abundant organic material in different space environments and investigate the connection between presolar and solar system material, based on observations of interstellar dust and gas, cometary volatiles, simulation experiments, and the analysis of extraterrestrial matter. Current challenges in astrochemistry are discussed and future research directions are proposed. PMID:20554702

Sulfur chemistry in dense interstellar clouds

NASA Technical Reports Server (NTRS)

Prasad, S. S.; Huntress, W. T., Jr.

1982-01-01

A model is presented for the gas phase chemistry of molecules containing sulfur in dense interstellar clouds. The sulfur chemistry is different from that used in previous models as a result of an extensive search of the recent literature and the availability of new laboratory data. The changes have a significant effect on the calculated abundance of sulfur compounds. The linked chemistry of sulfur and oxygen in the present model requires a severe depletion of sulfur and low fractional abundances of both O and O2 in the dense clouds. In contrast, the high abundance of SO and the low abundance of CS relative to SO in the HVS in the KL may indicate an oxygen-rich, high temperature environment compared to OMC-1. The formation of S-H bonds is slow because of the absence of radiative association between S(+) and H2. The present model underestimates the abundance of H2S unless a radiative association reaction between HS(+) and H2 is postulated.

The molecular composition of dense interstellar clouds

NASA Technical Reports Server (NTRS)

Allen, M.; Robinson, G. W.

1977-01-01

Presented in this paper is an ab initio chemical model for dense interstellar clouds that incorporates 598 grain surface reactions, with small grains providing the reaction area. Gas-phase molecules are depleted through collisions with grains. The abundances of 372 chemical species are calculated as a function of time and are found to be of sufficient magnitude to explain most observations. Peak abundances are achieved on time scales of the order of 100,000 to 1 million years, depending on cloud density and kinetic temperature. The reaction rates for ion-molecule chemistry are approximately the same, indicating that surface and gas-phase chemistry may be coupled in certain regions. The composition of grain mantles is shown to be a function of grain radius. In certain grain-size ranges, large molecules containing two or more heavy atoms are more predominant than lighter 'ices' - H2O, NH3, and CH4. It is possible that absorption due to these large molecules in the mantle may contribute to the observed 3-micron band in astronomical spectra.

Experimental evidence for glycolaldehyde and ethylene glycol formation by surface hydrogenation of CO molecules under dense molecular cloud conditions

NASA Astrophysics Data System (ADS)

Fedoseev, G.; Cuppen, H. M.; Ioppolo, S.; Lamberts, T.; Linnartz, H.

2015-04-01

This study focuses on the formation of two molecules of astrobiological importance - glycolaldehyde (HC(O)CH2OH) and ethylene glycol (H2C(OH)CH2OH) - by surface hydrogenation of CO molecules. Our experiments aim at simulating the CO freeze-out stage in interstellar dark cloud regions, well before thermal and energetic processing become dominant. It is shown that along with the formation of H2CO and CH3OH - two well-established products of CO hydrogenation - also molecules with more than one carbon atom form. The key step in this process is believed to be the recombination of two HCO radicals followed by the formation of a C-C bond. The experimentally established reaction pathways are implemented into a continuous-time random-walk Monte Carlo model, previously used to model the formation of CH3OH on astrochemical time-scales, to study their impact on the solid-state abundances in dense interstellar clouds of glycolaldehyde and ethylene glycol.

Search with COPERNICUS for interstellar N/sub 2/ in diffuse clouds

DOE Office of Scientific and Technical Information (OSTI.GOV)

Lutz, B.L.; Owen, T.; Snow, T.P. Jr.

1979-01-01

Multiple Copernicus scans of the rho'/sup 1/..sigma../sub u//sup +/--X/sup 1/..sigma../sub g//sup +/(0--0) and l/sup 1/Pi/sub u/--X/sup 1/..sigma../sub g//sup +/(0--0) band regions of N/sub 2/ in the spectra of delta Sco and epsilon Per result in upper limits of N (N/sub 2/) < or =1.0--3.8 x 10/sup 12/ cm/sup -2/ and N (N/sub 2/) < or =1.2--4.4 x 10/sup 12/ cm/sup -2/, respectively, depending upon the cloud temperature. These limits are consistent with the column densities expected from current chemical models for diffuse interstellar clouds, representing relative abundances with respect to hydrogen nuclei of N (N/sub 2/)/2N (H/sub 2/)+N (H I)more » < or =0.69--2.6 x 10/sup -9/ for delta Sco and < or =0.31--1.1 x 10/sup -8/ for epsilon Per.« less

The size distribution of interstellar grains

NASA Technical Reports Server (NTRS)

Witt, Adolf N.

1987-01-01

Three major areas involving interstellar grains were investigated. First, studies were performed of scattering in reflection nebulae with the goal of deriving scattering characteristics of dust grains such as the albedo and the phase function asymmetry throughout the visible and the ultraviolet. Secondly, studies were performed of the wavelength dependence of interstellar extinction designed to demonstrate the wide range of grain size distributions naturally occurring in individual clouds in different parts of the galaxy. And thirdly, studies were also performed of the ultraviolet powered emission of dust grains in the 0.5 to 1.0 micron wavelength range in reflection nebulae. Findings considered of major importance are highlighted.

KSC-02pd2055

NASA Image and Video Library

2002-11-11

KENNEDY SPACE CENTER, FLA. - The Cosmic Hot Interstellar Plasma Spectrometer, or CHIPSat, undergoes final processing before launch. CHIPSat, a suitcase-size 131-pound satellite, will provide invaluable information into the origin, physical processes and properties of the hot gas contained in the interstellar medium. This can provide important clues about the formation and evolution of galaxies since the interstellar medium literally contains the seeds of future stars. CHIPSat is scheduled for launch, with the Ice, Cloud, and Land Elevation Satellite (ICESat), on a Delta II expendable launch vehicle from Vandenberg Air Force Base, Calif., on Jan. 11, 2003, between 4:45 p.m. - 5:30 p.m. PST.

KSC-02pd2053

NASA Image and Video Library

2002-11-11

KENNEDY SPACE CENTER, FLA. - The Cosmic Hot Interstellar Plasma Spectrometer, or CHIPSat, undergoes final processing before launch. CHIPSat, a suitcase-size 131-pound satellite, will provide invaluable information into the origin, physical processes and properties of the hot gas contained in the interstellar medium. This can provide important clues about the formation and evolution of galaxies since the interstellar medium literally contains the seeds of future stars. CHIPSat is scheduled for launch, with the Ice, Cloud, and Land Elevation Satellite (ICESat), on a Delta II expendable launch vehicle from Vandenberg Air Force Base, Calif., on Jan. 11, 2003, between 4:45 p.m. - 5:30 p.m. PST.

KSC-02pd2056

NASA Image and Video Library

2002-11-11

KENNEDY SPACE CENTER, FLA. - The Cosmic Hot Interstellar Plasma Spectrometer, or CHIPSat, undergoes final processing before launch. CHIPSat, a suitcase-size 131-pound satellite, will provide invaluable information into the origin, physical processes and properties of the hot gas contained in the interstellar medium. This can provide important clues about the formation and evolution of galaxies since the interstellar medium literally contains the seeds of future stars. CHIPSat is scheduled for launch, with the Ice, Cloud, and Land Elevation Satellite (ICESat), on a Delta II expendable launch vehicle from Vandenberg Air Force Base, Calif., on Jan. 11, 2003, between 4:45 p.m. - 5:30 p.m. PST.

KSC-02pd2054

NASA Image and Video Library

2002-11-11

KENNEDY SPACE CENTER, FLA. - The Cosmic Hot Interstellar Plasma Spectrometer, or CHIPSat, undergoes final processing before launch. CHIPSat, a suitcase-size 131-pound satellite, will provide invaluable information into the origin, physical processes and properties of the hot gas contained in the interstellar medium. This can provide important clues about the formation and evolution of galaxies since the interstellar medium literally contains the seeds of future stars. CHIPSat is scheduled for launch, with the Ice, Cloud, and Land Elevation Satellite (ICESat), on a Delta II expendable launch vehicle from Vandenberg Air Force Base, Calif., on Jan. 11, 2003, between 4:45 p.m. - 5:30 p.m. PST.

Organic matter in meteorites and comets - Possible origins

NASA Technical Reports Server (NTRS)

Anders, Edward

1991-01-01

At least six extraterrestrial environments may have contributed organic compounds to meteorites and comets: solar nebula, giant-planet subnebulae, asteroid interiors containing liquid water, carbon star atmospheres, and diffuse or dark interstellar clouds. The record in meteorites is partly obscured by pervasive reheating that transformed much of the organic matter to kerogen; nonetheless, it seems that all six formation sites contributed. For comets, the large abundance of HCHO, HCN, and unsaturated hydrocarbons suggests an interstellar component of 50 percent or more, but the contributions of various interstellar processes, and of a solar-nebula component, are hard to quantify. A research program is outlined that may help reduce these uncertainties.

The origin and evolution of dust in interstellar and circumstellar environments

NASA Technical Reports Server (NTRS)

Whittet, Douglas C. B.; Leung, Chun M.

1993-01-01

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.

Origin of the solar system

NASA Technical Reports Server (NTRS)

Cameron, A. G. W.

1988-01-01

The current status of the classical model of solar-system formation is surveyed, reviewing the results of recent observational and theoretical investigations. Topics addressed include interstellar clouds, the collapse of interstellar gas, the primitive solar nebula, the formation of the sun, planetesimal accumulation, planetary accumulation, major planetary collisions, the development of planetary atmospheres, and comets. The relative merits of conflicting theories on many key problems are indicated, with reference to more detailed reviews in the literature.

The X-ray Halo of GX5-1

NASA Technical Reports Server (NTRS)

Smith, Randall K.; Dame, T. M.; Costantini, Elisa; Predehl, Peter

2006-01-01

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.

Prospects for Studying Interstellar Magnetic Fields with a Far-Infrared Polarimeter for SAFIR

NASA Technical Reports Server (NTRS)

Dowell, C. Darren; Chuss, D. T.; Dotson, J. L.

2008-01-01

Polarimetry at mid-infrared through millimeter wavelengths using airborne and ground-based telescopes has revealed magnetic structures in dense molecular clouds in the interstellar medium, primarily in regions of star formation. Furthermore, spectropolarimetry has offered clues about the composition of the dust grains and the mechanism by which they are aligned with respect to the local magnetic field. The sensitivity of the observations to date has been limited by the emission from the atmosphere and warm telescopes. A factor of 1000 in sensitivity can be gained by using instead a cold space telescope. With 5 arcminute resolution, Planck will make the first submillimeter polarization survey of the full Galaxy early in the next decade. We discuss the science case for and basic design of a far-infrared polarimeter on the SAFIR space telescope, which offers resolution in the few arcsecond range and wavelength selection of cold and warm dust components. Key science themes include the formation and evolution of molecular clouds in nearby spiral galaxies, the magnetic structure of the Galactic center, and interstellar turbulence.

Laboratory Studies of Stabilities of Heterocyclic Aromatic Molecules: Suggested Gas Phase Ion-Molecule Routes to Production in Interstellar Gas Clouds

NASA Technical Reports Server (NTRS)

Adams, Nigel G.; Fondren, L. Dalila; McLain, Jason L.; Jackson, Doug M.

2006-01-01

Several ring compounds have been detected in interstellar gas clouds, ISC, including the aromatic, benzene. Polycyclic aromatic hydrocarbons, PAHs, have been implicated as carriers of diffuse interstellar bands (DIBs) and unidentified infrared (UIR) bands. Heterocyclic aromatic rings of intermediate size containing nitrogen, possibly PreLife molecules, were included in early searches but were not detected and a recent search for Pyrimidine was unsuccessful. Our laboratory investigations of routes to such molecules could establish their existence in ISC and suggest conditions under which their concentrations would be maximized thus aiding the searches. The stability of such ring compounds (C5H5N, C4H4N2, C5H11N and C4H8O2) has been tested in the laboratory using charge transfer excitation in ion-molecule reactions. The fragmentation paths, including production of C4H4(+), C3H3N(+) and HCN, suggest reverse routes to the parent molecules, which are presently under laboratory investigation as production sources.

Deep, Broadband Spectral Line Surveys of Molecule-rich Interstellar Clouds

DOE Office of Scientific and Technical Information (OSTI.GOV)

Widicus Weaver, Susanna L.; Laas, Jacob C.; Zou, Luyao

2017-09-01

Spectral line surveys are an indispensable tool for exploring the physical and chemical evolution of astrophysical environments due to the vast amount of data that can be obtained in a relatively short amount of time. We present deep, broadband spectral line surveys of 30 interstellar clouds using two broadband λ  = 1.3 mm receivers at the Caltech Submillimeter Observatory. This information can be used to probe the influence of physical environment on molecular complexity. We observed a wide variety of sources to examine the relative abundances of organic molecules as they relate to the physical properties of the source (i.e., temperature,more » density, dynamics, etc.). The spectra are highly sensitive, with noise levels ≤25 mK at a velocity resolution of ∼0.35 km s{sup −1}. In the initial analysis presented here, column densities and rotational temperatures have been determined for the molecular species that contribute significantly to the spectral line density in this wavelength regime. We present these results and discuss their implications for complex molecule formation in the interstellar medium.« less

The jet-ISM interactions in IC 5063

NASA Astrophysics Data System (ADS)

Mukherjee, Dipanjan; Wagner, Alexander Y.; Bicknell, Geoffrey V.; Morganti, Raffaella; Oosterloo, Tom; Nesvadba, Nicole; Sutherland, Ralph S.

2018-05-01

The interstellar medium of the radio galaxy IC 5063 is highly perturbed by an AGN jet expanding in the gaseous disc of the galaxy. We model this interaction with relativistic hydrodynamic simulations and multiphase initial conditions for the interstellar medium and compare the results with recent observations. As the jets flood through the intercloud channels of the disc, they ablate, accelerate, and disperse clouds to velocities exceeding 400 km s-1. Clouds are also destroyed or displaced in bulk from the central regions of the galaxy. Our models with jet powers of 1044 and 1045 erg s-1 are capable of reproducing many of the observed features in the position velocity diagram of IC 5063, and confirm the notion that the jet is responsible for the strongly perturbed gas dynamics seen in the ionized, neutral, and molecular gas phases. In our simulations, we also see strong venting of the jet plasma perpendicular to the disc, which entrains clumps and diffuse filaments into the halo of the galaxy. Our simulations are the first 3D hydrodynamic simulations of the jet and interstellar matter of IC 5063.

Interstellar Molecules

ERIC Educational Resources Information Center

Solomon, Philip M.

1973-01-01

Radioastronomy reveals that clouds between the stars, once believed to consist of simple atoms, contain molecules as complex as seven atoms and may be the most massive objects in our Galaxy. (Author/DF)

Evolution of Interstellar Grains

NASA Technical Reports Server (NTRS)

Allamandola, Lou J.; DeVincenzi, Donald L. (Technical Monitor)

1998-01-01

During the past two decades observations combined with laboratory simulations, have revolutionized our understanding of interstellar ice and dust, the raw materials from which planets, comets and stars form. Most interstellar material is concentrated in large molecular clouds where simple molecules are formed by dust-grain and gas-phase reactions. Gaseous species striking the cold (10K) dust stick, forming an icy grain mantle. This accretion, coupled with UV photolysis, produces a complex chemical mixture containing volatile, non-volatile, and isotopically fractionated species. Ices in molecular clouds contain the very simple molecules H2O, CH3OH, CO, CO2, H2, and perhaps some NH3 and H2CO, as well as more complex species. The evidence for these compounds, as well as carbon-rich materials, will be reviewed and the possible connections with comets and meteorites will be presented in the first part of the talk . The second part of the presentation will focus on interstellar/precometary ice photochemical evolution and the species likely to be found in comets. The chemical composition and photochemical evolution of realistic interstellar/pre-cometary ice analogs will be discussed. Ultraviolet photolysis of these ices produces H2, H2CO, CO2, CO, CH4, HCO, and more complex molecules. When ices representative of interstellar grains and comets are exposed to UV radiation at low temperature a series of moderately complex organic molecules are formed in the ice including: CH3CH2OH (ethanol), HC(=O)NH2 (formamide), CH3C(=O)NH2 (acetamide), and R-C=N (nitriles). Several of these are already known to be in the interstellar medium, and their presence indicates the importance of grain processing. After warming to room temperature an organic residue remains. This is composed primarily of hexamethylenetetramine (HMT, C6H12N4), with lesser amounts of polyoxymethylene-related species (POMs), amides, and ketones. This is in sharp contrast to the organic residues produced by irradiating unrealistic interstellar ice analogs or thermally promoted polymerization-type reactions in unirradiated realistic ice mixtures.

Thermal emission from interstellar dust in and near the Pleiades

NASA Technical Reports Server (NTRS)

White, Richard E.

1989-01-01

IRAS survey coadds for a 8.7 deg x 4.3 deg field near the Pleiades provide evidence for dynamical interaction between the cluster and the surrounding interstellar medium. The far-infrared images show large region of faint emission with bright rims east of the cluster, suggestive of a wake. Images of the far-infrared color temperature and 100 micron optical depth reveal temperature maxima and optical depth minima near the bright cluster stars, as well as a strong optical depth peak at the core of the adjacent CO cloud. Models for thermal dust emission near the stars indicate that most of the apparent optical depth minima near stars are illusory, but also provide indirect evidence for small interaction between the stars and the encroaching dust cloud.

Probing the Origin and Evolution of Interstellar and Protoplanetary Biogenic Molecules:A Comprehensive Survey of Interstellar Ices with SPHEREx

NASA Astrophysics Data System (ADS)

Melnick, Gary J.; SPHEREx Science Team

2016-01-01

Many of the most important building blocks of life are locked in interstellar and protoplanetary ices. Examples include H2O, CO, CO2, and CH3OH, among others. There is growing evidence that in some environments, such as within the cores of dense molecular clouds and the mid-plane of protoplanetary disks, the amounts of these species in ices far exceeds that in the gas phase. As a result, collisions between ice-bearing bodies and newly forming planets are thought to be a major means of delivering these key species to young planets. There currently exist fewer than 250 ice absorption spectra toward Galactic molecular clouds, which is insufficient to reliably trace the ice content of clouds through the various stages of collapse to star and planet formation, or assess the effects of their environments and physical conditions, such as cloud density, internal temperature, presence or absence of embedded sources, external UV and X-ray radiation, gas-phase composition, or cosmic-ray ionization rate, on the ice composition for clouds at similar evolutionary stages. Ultimately, our goal is to understand how these findings connect to our own Solar System.SPHEREx, which is a mission in NASA's Small Explorer (SMEX) program that was selected for a Phase A study in July 2015, will be a game changer for the study of interstellar, circumstellar, and protoplanetary disk ices. SPHEREx will obtain spectra over the entire sky in the optical and near-IR, including the 2.5 to 4.8 micron region, which contains the above biogenic ice features. SPHEREx will detect millions of potential background continuum point sources already catalogued by NASA's Wide-field Infrared Survey Explorer (WISE) at 3.4 and 4.6 microns for which there is evidence for intervening gas and dust based on the 2MASS+WISE colors with sufficient sensitivity to yield ice absorption spectra with SNR ≥ 100 per spectral resolution element. The resulting > 100-fold increase in the number of high-quality ice absorption spectra toward a wide variety of regions distributed throughout the Galaxy will reveal correlations between ice content and environment not possible with current spectra. Finally, SPHEREx will provide JWST with an ice source catalog for follow-up.

Recognition and characterization of hierarchical interstellar structure. II - Structure tree statistics

NASA Technical Reports Server (NTRS)

Houlahan, Padraig; Scalo, John

1992-01-01

A new method of image analysis is described, in which images partitioned into 'clouds' are represented by simplified skeleton images, called structure trees, that preserve the spatial relations of the component clouds while disregarding information concerning their sizes and shapes. The method can be used to discriminate between images of projected hierarchical (multiply nested) and random three-dimensional simulated collections of clouds constructed on the basis of observed interstellar properties, and even intermediate systems formed by combining random and hierarchical simulations. For a given structure type, the method can distinguish between different subclasses of models with different parameters and reliably estimate their hierarchical parameters: average number of children per parent, scale reduction factor per level of hierarchy, density contrast, and number of resolved levels. An application to a column density image of the Taurus complex constructed from IRAS data is given. Moderately strong evidence for a hierarchical structural component is found, and parameters of the hierarchy, as well as the average volume filling factor and mass efficiency of fragmentation per level of hierarchy, are estimated. The existence of nested structure contradicts models in which large molecular clouds are supposed to fragment, in a single stage, into roughly stellar-mass cores.

Starry Messages - Searching for Signatures of Interstellar Archaeology

NASA Astrophysics Data System (ADS)

Carrigan, R. A., Jr.

Searching for signatures of cosmic-scale archaeological artefacts such as Dyson spheres or Kardashev civilizations is an interesting alternative to conventional SETI. Uncovering such an artifact does not require the intentional transmission of a signal on the part of the originating civilization. This type of search is called interstellar archaeology or sometimes cosmic archaeology . The detection of intelligence elsewhere in the Universe with interstellar archaeology or SETI would have broad implications for science. For example, the constraints of the anthropic principle would have to be loosened if a different type of intelligence was discovered elsewhere. A variety of interstellar archaeology signatures are discussed including non-natural planetary atmospheric constituents, stellar doping with isotopes of nuclear wastes, Dyson spheres, as well as signatures of stellar and galactic-scale engineering. The concept of a Fermi bubble due to interstellar migration is introduced in the discussion of galactic signatures. These potential interstellar archaeological signatures are classified using the Kardashev scale. A modified Drake equation is used to evaluate the relative challenges of finding various sources. With few exceptions interstellar archaeological signatures are clouded and beyond current technological capabilities. However SETI for so-called cultural transmissions and planetary atmosphere signatures are within reach.

The detection of interstellar C I in the immediate vicinity of the sun

NASA Technical Reports Server (NTRS)

Bruhweiler, F. C.; Kondo, Y.

1982-01-01

Multiple stacked IUE spectra reveal the presence of interstellar C I 1657 in the trough of a corresponding photospheric feature in the nearby star, Alpha PsA (d = 7 pc). This represents the first detection of this neutral atom in the interstellar medium within the immediate vicinity of the sun. It is suggested that C I may be a much better diagnostic tool in studying the local interstellar medium than the neutral species K I and Na I, which are observable at visual wavelengths. Variations in C I column density, coupled with b-values deduced from the Mg II doublet ratio, may prove to be an important means to unravel density and temperature fluctuations in the very local interstellar medium. Comparison of the line of sight toward Alpha PsA with previous Copernicus interstellar Mg II results for that of Alpha Leo tentatively indicates that the distribution of Mg II in the local cloud is not homogeneous about the sun. Rough constraints on the ionization fraction of hydrogen toward Alpha PsA do not conflict with previous data, implying that the very local interstellar medium is significantly ionized.

Starry messages: Searching for signatures of interstellar archaeology

DOE Office of Scientific and Technical Information (OSTI.GOV)

Carrigan, Richard A., Jr.; /Fermilab

2009-12-01

Searching for signatures of cosmic-scale archaeological artifacts such as Dyson spheres or Kardashev civilizations is an interesting alternative to conventional SETI. Uncovering such an artifact does not require the intentional transmission of a signal on the part of the original civilization. This type of search is called interstellar archaeology or sometimes cosmic archaeology. The detection of intelligence elsewhere in the Universe with interstellar archaeology or SETI would have broad implications for science. For example, the constraints of the anthropic principle would have to be loosened if a different type of intelligence was discovered elsewhere. A variety of interstellar archaeology signaturesmore » are discussed including non-natural planetary atmospheric constituents, stellar doping with isotopes of nuclear wastes, Dyson spheres, as well as signatures of stellar and galactic-scale engineering. The concept of a Fermi bubble due to interstellar migration is introduced in the discussion of galactic signatures. These potential interstellar archaeological signatures are classified using the Kardashev scale. A modified Drake equation is used to evaluate the relative challenges of finding various sources. With few exceptions interstellar archaeological signatures are clouded and beyond current technological capabilities. However SETI for so-called cultural transmissions and planetary atmosphere signatures are within reach.« less

Effect of Supernovae on the Local Interstellar Material

NASA Astrophysics Data System (ADS)

Frisch, Priscilla; Dwarkadas, Vikram V.

A range of astronomical data indicates that ancient supernovae created the galactic environment of the Sun and sculpted the physical properties of the interstellar medium near the heliosphere. In this paper, we review the characteristics of the local interstellar medium that have been affected by supernovae. The kinematics, magnetic field, elemental abundances, and configuration of the nearest interstellar material support the view that the Sun is at the edge of the Loop I superbubble, which has merged into the low-density Local Bubble. The energy source for the higher temperature X-ray-emitting plasma pervading the Local Bubble is uncertain. Winds from massive stars and nearby supernovae, perhaps from the Sco-Cen association, may have contributed radioisotopes found in the geologic record and galactic cosmic ray population. Nested supernova shells in the Orion and Sco-Cen regions suggest spatially distinct sites of episodic star formation. The heliosphere properties vary with the pressure of the surrounding interstellar cloud. A nearby supernova would modify this pressure equilibrium and thereby severely disrupt the heliosphere as well as the local interstellar medium.

Adolescent Interstellar Cloud Poised to Make Star-forming Debut

NASA Astrophysics Data System (ADS)

2001-06-01

Astronomers using the National Science Foundation's (NSF) 140-foot radio telescope at the National Radio Astronomy Observatory (NRAO) in Green Bank, W.Va., have discovered a highly unusual, massive interstellar cloud that appears poised to begin a burst of star formation. The cloud may be the first ever to be detected in the transition between atomic and molecular states. NRAO scientists Felix J. Lockman and Anthony H. Minter presented their findings at the American Astronomical Society meeting in Pasadena, Calif. Radio Image of G28.17+0.05 The scientists discovered the cloud, identified as G28.17+0.05, lying along the inner plane of the Milky Way Galaxy, approximately 16,300 light-years from Earth. Observations of the cloud indicate that it is near one of the Galaxy's sweeping spiral arms, which are outlined by young stars and the massive clouds that form them. Lockman and Minter speculate that as the interstellar cloud slams into the Galactic arm, the resulting shock wave may be precipitating the conversion of the neutral hydrogen atoms into heavier molecules, which could herald the onset of star formation. "These may be the first observations of a cloud that is in the transition between the neutral atomic hydrogen and molecular phases," said Lockman. "This provides astronomers a unique opportunity to study the chemistry of very young interstellar clouds, which could give us significant insights into the early stages of star formation and the structure of the Galaxy." Interstellar clouds that contain neutral atomic hydrogen, called HI (H-one) clouds, are thought of as giant, cold blobs of gas. Researchers study these objects because they offer intriguing glimpses of the composition of our Galaxy and the cosmos, and reveal much about how stars and planets are born. Hydrogen atoms in these clouds give off natural signals (at the 21-cm wavelength), which can be detected only by radio telescopes. The scientists discovered that this HI cloud was unusual in many respects. First, it was uncharacteristically massive, about 500 light- years across and containing nearly 100,000 times the mass of the sun in atomic hydrogen. The gas in clouds this large and massive has typically undergone the transition to the molecular phase, and has begun making stars. The size and mass of this cloud indicate that it is gravitationally bound, which means that it should be collapsing and forming new stars. "When you find a cloud that is as massive as the one we detected, and one that is gravitationally bound as this structure indicates, then you would expect to see areas of star formation," said Lockman. The scientists were able to identify a few indicators of star formation, but not at the rate that one would expect. "We think we have caught something in a special state." Lockman said, "It could be one of the missing links in the cycle of star formation." The core of the cloud also gives off radio signals at 1720 MHz from the molecule OH in an unusual state of excitation. Since other astronomers have detected similar signals throughout the Galactic plane, the researchers believe that these emissions may be an indication that this previously undetected type of cloud may turn out to be fairly common. "We suspect that this cloud may be the first example of an object that may be fairly common in the inner Galactic plane," said Lockman, "but has not been recognized. That is, a cloud that is observed while entering a spiral shock and is in the transition between atomic to molecular hydrogen." The NRAO 140-Foot Telescope The scientists caution, however, that additional research is needed to confirm their speculations. "The presence of anomalous OH through the Galactic plane does suggest that other clouds of this nature can be detected," said Lockman, "and it would be particularly valuable if a similar cloud could be detected entering the 'spiral shock' on the opposite side of the Galactic center." The patterns of velocities of atomic and molecular gas should be reversed there, due to the difference in galactic rotation. Such a discovery could help to validate the possible interaction among the spiral shock, atomic hydrogen, and star formation. The NSF's 140-foot radio telescope now is decommissioned after a long and highly productive career. Research will continue on the newly commissioned Robert C. Byrd Green Bank Telescope, which is the world's largest fully steerable radio telescope. "Though the 140-foot telescope enabled us to make remarkable observations," commented Minter, "we anticipate that the new Green Bank Telescope, with its increased sensitivity and better resolution, will enable us to see more clearly the nature of this peculiar object." In addition to Minter and Lockman, other astronomers involved in this research include Glen I. Langston, NRAO; and Jennifer A. Lockman who was a student from the College of Charleston, S.C., at the time the research was conducted. The National Radio Astronomy Observatory is a facility of the National Science Foundation, operated under cooperative agreement by Associated Universities, Inc.

The interstellar chemistry of H2C3O isomers

PubMed Central

Loison, Jean-Christophe; Agúndez, Marcelino; Marcelino, Núria; Wakelam, Valentine; Hickson, Kevin M.; Cernicharo, José; Gerin, Maryvonne; Roueff, Evelyne; Guélin, Michel

2016-01-01

We present the detection of two H2C3O isomers, propynal and cyclopropenone, toward various starless cores and molecular clouds, together with upper limits for the third isomer propadienone. We review the processes controlling the abundances of H2C3O isomers in interstellar media showing that the reactions involved are gas-phase ones. We show that the abundances of these species are controlled by kinetic rather than thermodynamic effects. PMID:27013768

HIGH-ENERGY ELECTRON IRRADIATION OF INTERSTELLAR CARBONACEOUS DUST ANALOGS: COSMIC-RAY EFFECTS ON THE CARRIERS OF THE 3.4 μ m ABSORPTION BAND

DOE Office of Scientific and Technical Information (OSTI.GOV)

Maté, Belén; Molpeceres, Germán; Jiménez-Redondo, Miguel

2016-11-01

The effects of cosmic rays on the carriers of the interstellar 3.4 μ m absorption band have been investigated in the laboratory. This band is attributed to stretching vibrations of CH{sub 3} and CH{sub 2} in carbonaceous dust. It is widely observed in the diffuse interstellar medium, but disappears in dense clouds. Destruction of CH{sub 3} and CH{sub 2} by cosmic rays could become relevant in dense clouds, shielded from the external ultraviolet field. For the simulations, samples of hydrogenated amorphous carbon (a-C:H) have been irradiated with 5 keV electrons. The decay of the band intensity versus electron fluence reflectsmore » a-C:H dehydrogenation, which is well described by a model assuming that H{sub 2} molecules, formed by the recombination of H atoms liberated through CH bond breaking, diffuse out of the sample. The CH bond destruction rates derived from the present experiments are in good accordance with those from previous ion irradiation experiments of HAC. The experimental simplicity of electron bombardment has allowed the use of higher-energy doses than in the ion experiments. The effects of cosmic rays on the aliphatic components of cosmic dust are found to be small. The estimated cosmic-ray destruction times for the 3.4 μ m band carriers lie in the 10{sup 8} yr range and cannot account for the disappearance of this band in dense clouds, which have characteristic lifetimes of 3 × 10{sup 7} yr. The results invite a more detailed investigation of the mechanisms of CH bond formation and breaking in the intermediate region between diffuse and dense clouds.« less

Water in dense molecular clouds

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wannier, P.G.; Kuiper, T.B.H.; Frerking, M.A.

1991-08-01

The G.P. Kuiper Airborne Observatory (KAO) was used to make initial observations of the half-millimeter ground-state transition of water in seven giant molecular clouds and in two late-type stars. No significant detections were made, and the resulting upper limits are significantly below those expected from other, indirect observations and from several theoretical models. The implied interstellar H2O/CO abundance is less than 0.003 in the cores of three giant molecular clouds. This value is less than expected from cloud chemistry models and also than estimates based on HDO and H3O(+) observations. 78 refs.

Rotational spectrum of 13C{2}-methyl formate (HCOO13CH{3}) and detection of the two 13C-methyl formate in Orion

NASA Astrophysics Data System (ADS)

Carvajal, M.; Margulès, L.; Tercero, B.; Demyk, K.; Kleiner, I.; Guillemin, J. C.; Lattanzi, V.; Walters, A.; Demaison, J.; Wlodarczak, G.; Huet, T. R.; Møllendal, H.; Ilyushin, V. V.; Cernicharo, J.

2009-06-01

Context: Laboratory measurements and analysis of the microwave and millimeter-wave spectra of potential interstellar molecules are a prerequisite for their subsequent identification by radioastronomical techniques. The spectral analysis provides spectroscopic parameters that are used in the assignment procedure of the laboratory spectra, and that also predict the frequencies of transitions not measured in the laboratory with a high degree of precision. Aims: An experimental laboratory study and its theoretical analysis is presented for 13C2-methyl formate (HCOO13CH3) allowing a search for this isotopologue in the Orion molecular cloud. The 13C1-methyl formate (H13COOCH3) molecule was also searched for in this interstellar cloud, using previously published spectroscopic data. Methods: The experimental spectra of 13C2-methyl formate were recorded in the microwave and sub-mm energy ranges (4-20 GHz, 8-80 GHz, 150-700 GHz). The spectra were analyzed using the Rho-Axis Method (RAM), which takes the CH3 internal rotation and the coupling between internal rotation and global rotation into account. Results: Twenty-seven spectroscopic constants of 13C2-methyl formate have been obtained from a fit of 936 transitions of the ground torsional state with a standard (unitless) deviation of 1.08. A prediction of line positions and intensities is also produced. This prediction allowed us to identify 230 13C2-methyl formate lines in the Orion interstellar molecular cloud. We refitted all previously published ground state transitions of the 13C1-methyl formate molecule in order to provide a prediction of its ground state spectrum. 234 lines of 13C1-methyl formate were detected in the Orion interstellar cloud using that prediction. Tables A.1-A.5 are only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/500/1109

Astrochemistry

NASA Astrophysics Data System (ADS)

Millar, T. J.

2015-08-01

In the last 40 years a wide range of molecules, including neutrals, cations and anions, containing up to 13 atoms—in addition to detections of {{\\text{C}}60} and {{\\text{C}}70} —have been found in the harsh environment of the interstellar medium. The exquisite sensitivity and very high spectral and, more recently, spatial resolution, of modern telescopes has enabled the physics of star formation to be probed through rotational line emission. In this article, I review the basic properties of interstellar clouds and the processes that initiate the chemistry and generate chemical complexity, particularly in regions of star and planet formation. Our understanding of astrochemistry has evolved over the years. Before 1990, the general consensus was that molecules were formed in binary, gas-phase, or volume, reactions, most importantly ion-neutral reactions despite the very low ionization in clouds. Since then, observations have indicated unambiguously that there is also a contribution from surface processes, particularly on the icy mantles that form around refractory grain cores in cold, dense gas. The balance between these two processes depends on particular physical conditions and can vary during the life cycle of a particular volume of interstellar cloud. The complex chemistry that occurs in space is driven mostly through interaction of the gas with cosmic ray protons, a source of ionization that enables a rich ion-neutral chemistry. In addition, I show that the interaction between the gas and the dust in cold, dense regions also leads to additional chemical complexity through reactions that take place in ices at only a few tens of degrees above absolute zero. Although densities are low compared to those in terrestrial environments, the extremely long life times of interstellar clouds and their enormous sizes, enable complex molecules to be synthesised and detected. I show that in some instances, particularly in reactions involving deuterium, the rotational populations of reactants, together with spin-selection rules, can determine the detailed abundances. Although the review is mainly focused on regions associated with star formation, I also consider chemistry in other interesting astronomical regions—in the early Universe and in the envelopes formed by mass loss during the final stages of stellar evolution.

Radio emission from supernova remnants in a cloudy interstellar medium

NASA Technical Reports Server (NTRS)

Blandford, R. D.; Cowie, L. L.

1982-01-01

The van der Laan (1962) theory of SNR radio emission is modified in light of the inhomogeneity of the interstellar medium, and in order to allow for particle acceleration in shock fronts. It is proposed that most of the radio emission in 10-20 pc radius SNRs originates in cold interstellar clouds that have been crushed by the high pressure hot gas within the expanding remnant. Under these circumstances, simple reacceleration of ambient interstellar cosmic ray electrons can account for the surface brightness-diameter distribution of observed remnants, with the additional, relativistic particle energy compensating for the decreased filling factor of the radio-emitting regions. Warm interstellar gas, at about 8000 K, may also be compressed within very large SNRs (of radius of 30-100 pc) and account for both the giant radio loops, when these SNRs are seen individually, and the anomalously bright galactic nonthermal radio background, which may be the superposition of a number of such features.

Infrared emission spectra of candidate interstellar aromatic molecules

NASA Technical Reports Server (NTRS)

Schlemmer, S.; Balucani, N.; Wagner, D. R.; Steiner, B.; Saykally, R. J.

1996-01-01

Interstellar dust is responsible, through surface reactions, for the creation of molecular hydrogen, the main component of the interstellar clouds in which new stars form. Intermediate between small, gas-phase molecules and dust are the polycyclic aromatic hydrocarbons (PAHs). Such molecules could account for 2-30% of the carbon in the Galaxy, and may provide nucleation sites for the formation of carbonaceous dust. Although PAHs have been proposed as the sources of the unidentified infrared emission bands that are observed in the spectra of a variety of interstellar sources, the emission characteristics of such molecules are still poorly understood. Here we report laboratory emission spectra of several representative PAHs, obtained in conditions approximating those of the interstellar medium, and measured over the entire spectral region spanned by the unidentified infrared bands. We find that neutral PAHs of small and moderate size can at best make only a minor contribution to these emission bands. Cations of these molecules, as well as much larger PAHs and their cations, remain viable candidates for the sources of these bands.

Far Ultraviolet Spectroscopy of the Intergalactic and Interstellar Absorption Toward 3C 273

NASA Technical Reports Server (NTRS)

Sembach, Kenneth R.; Howk, J. Christopher; Savage, Blair D.; Shull, J. Michael; Oegerle, William R.; Fisher, Richard R. (Technical Monitor)

2001-01-01

We present Far Ultraviolet Spectroscopic Explorer observations of the molecular, neutral atomic, weakly ionized, and highly ionized components of the interstellar and intergalactic material toward the quasar 3C273. We identify Ly-beta absorption in eight of the known intergalactic Ly-alpha absorbers along the sight line with the rest-frame equivalent widths W(sub r)(Ly-alpha) > 50 micro-angstroms. Refined estimates of the H(I) column densities and Doppler parameters (b) of the clouds are presented. We find a range of b = 16-46 km/s. We detect multiple H(I) lines (Ly-beta - Ly-theta) in the 1590 km/s Virgo absorber and estimate logN(H(I)) = 15.85 +/- 0.10, ten times more H(I) than all of the other absorbers along the sight line combined. The Doppler width of this absorber, b = 16 km/s, implies T < 15,000 K. We detect O(VI) absorption at 1015 km/s at the 2-3(sigma) level that may be associated with hot, X-ray emitting gas in the Virgo Cluster. We detect weak C(III) and O(VI) absorption in the IGM at z=0.12007; this absorber is predominantly ionized and has N(H+)/N(H(I)) > 4000/Z, where Z is the metallicity. Strong Galactic interstellar O(VI) is present between -100 and +100 km/s with an additional high-velocity wing containing about 13% of the total O(VI) between +100 and +240 km/s. The Galactic O(VI), N(V), and C(IV) lines have similar shapes, with roughly constant ratios across the -100 to +100 km/s velocity range. The high velocity O(VI) wing is not detected in other species. Much of the interstellar high ion absorption probably occurs within a highly fragmented medium within the Loop IV remnant or in the outer cavity walls of the remnant. Multiple hot gas production mechanisms are required. The broad O(VI) absorption wing likely traces the expulsion of hot gas out of the Galactic disk into the halo. A flux limit of 5.4 x 10(epx -16) erg/sq cm/s on the amount of diffuse O(VI) emission present = 3.5' off the 3C273 sight line combined with the observed O(VI) column density toward 3C273, logN O(VI) = 14.73 +/- 0.04, implies n(sub e) < 0.02/cubic cm and P/k < 11,500/cubic cm for an assumed temperature of 3 x 10(exp 5) K. The elemental abundances in the neutral and weakly-ionized interstellar clouds are similar to those found for other halo clouds. The warm neutral and warm ionized clouds along the sight line have similar dust-phase abundances, implying that the properties of the dust grains in the two types of clouds are similar. Interstellar H2 absorption is present at positive velocities at a level of logN(H2) = 15.71, but is very weak at the velocities of the main column density concentration along the sight line observed in H(I) 21 cm emission.

Deuterium Abundance in the Local ISM and Possible Spatial Variations

NASA Technical Reports Server (NTRS)

Linsky, Jeffrey L.

1998-01-01

Excellent HST/GHRS spectra of interstellar hydrogen and deuterium Lyman-(alpha) absorption toward nearby stars allow us to identify systematic errors that have plagued earlier work and to measure accurate values of the D/H ratio in local interstellar gas. Analysis of 12 sightlines through the Local Interstellar Cloud leads to a mean value of D/H = (1.50 +/- 0.10) x 10(exp -5) with all data points lying within +/- l(delta) of the mean. Whether or not the D/H ratio has different values elsewhere in the Galaxy and beyond is a very important open question that will be one of the major objectives of the Far Ultraviolet Spectroscopic Explorer (FUSE) mission.

Processing Mechanisms for Interstellar Ices: Connections to the Solar System

NASA Technical Reports Server (NTRS)

Pendleton, Y. J.; Cuzzi, Jeffrey N. (Technical Monitor)

1995-01-01

The organic component of the interstellar medium, which has revealed itself through the ubiquitous 3.4 micrometers hydrocarbon absorption feature, is widespread throughout the disk of our galaxy and has been attributed to dust grains residing in the diffuse interstellar medium. The absorption band positions near 3.4 micrometers are characteristic of C-H stretching vibrations in the -CH3 and -CH2- groups of saturated aliphatic hydrocarbons associated with perturbing chemical groups. The production of complex molecules is thought to occur within dense molecular clouds when ice-mantled grains are processed by various energetic mechanisms. Studies of the processing of interstellar ices and the subsequent production of organic residues have relevance to studies of ices in the solar system, because primitive, icy solar system bodies such as those in the Kuiper belt are likely reservoirs of organic material, either preserved from the interstellar medium or produced in situ. Connections between the interstellar medium and the early solar nebula have long been a source of interest. A comparison of the interstellar organics and the Murchison meteorite illustrates the importance of probing the interstellar connection to the solar system, because although the carbonaceous meteorites are undoubtedly highly processed, they do retain specific interstellar signatures (such as diamonds, SiC grains, graphite and enriched D/H). The organic component, while not proven interstellar, has a remarkable similarity to the interstellar organics observed in over a dozen sightlines through our galaxy. This paper compares spectra from laboratory organics produced through the processing of interstellar ice analog materials with the high resolution infrared observations of the interstellar medium in order to investigate the mechanisms (such as ion bombardment, plasma processing, and UV photolysis) that may be producing the organics in the ISM.

Properties of interstellar wind leading to shape morphology of the dust surrounding HD 61005

NASA Astrophysics Data System (ADS)

Pástor, P.

2017-08-01

Aims: A structure formed by dust particles ejected from the debris ring around HD 61005 is observed in the scattered light. The main aim here is to constrain interstellar wind parameters that lead to shape morphology in the vicinity of HD 61005 using currently available observational data for the debris ring. Methods: Equation of motion of 2 × 105 dust particles ejected from the debris ring under the action of the electromagnetic radiation, stellar wind, and interstellar wind is solved. A two-dimensional (2D) grid is placed in a given direction for accumulation of the light scattered on the dust particles in order to determine the shape morphology. The interaction of the interstellar wind and the stellar wind is considered. Results: Groups of unknown properties of the interstellar wind that create the observed morphology are determined. A relation between number densities of gas components in the interstellar wind and its relative velocity is found. Variations of the shape morphology caused by the interaction with the interstellar clouds of various temperatures are studied. When the interstellar wind velocity is tilted from debris ring axis a simple relation between the properties of the interstellar wind and an angle between the line of sight and the interstellar wind velocity exists. Dust particles that are most significantly influenced by stellar radiation move on the boundary of observed structure. Conclusions: Observed structure at HD 61005 can be explained as a result of dust particles moving under the action of the interstellar wind. Required number densities or velocities of the interstellar wind are much higher than that of the interstellar wind entering the solar system.

High-latitude dust clouds LDN 183 and LDN 169: distances and extinctions

NASA Astrophysics Data System (ADS)

Straižys, V.; Boyle, R. P.; Zdanavičius, J.; Janusz, R.; Corbally, C. J.; Munari, U.; Andersson, B.-G.; Zdanavičius, K.; Kazlauskas, A.; Maskoliūnas, M.; Černis, K.; Macijauskas, M.

2018-03-01

Interstellar extinction is investigated in a 2°× 2° area containing the dust and molecular clouds LDN 183 (MBM 37) and LDN 169, which are located at RA = 15h 54m, Dec = - 3°. The study is based on a photometric classification in spectral and luminosity classes of 782 stars selected from the catalogs of 1299 stars down to V = 20 mag observed in the Vilnius seven-color system. For control, the MK types for the 18 brightest stars with V between 8.5 and 12.8 mag were determined spectroscopically. For 14 stars, located closer than 200 pc, distances were calculated from trigonometric parallaxes taken from the Gaia Data Release 1. For about 70% of the observed stars, two-dimensional spectral types, interstellar extinctions AV, and distances were determined. Using 57 stars closer than 200 pc, we estimate that the front edge of the clouds begins at 105 ± 8 pc. The extinction layer in the vicinities of the clouds can be about 20 pc thick. In the outer parts of the clouds and between the clouds, the extinction is 0.5-2.0 mag. Behind the Serpens/Libra clouds, the extinction range does not increase; this means that the dust layer at 105 pc is a single extinction source. Full Tables 1 and 2 are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/611/A9

Interstellar gas and X-rays toward the Young supernova remnant RCW 86; pursuit of the origin of the thermal and non-thermal X-ray

NASA Astrophysics Data System (ADS)

Sano, H.; Reynoso, E. M.; Mitsuishi, I.; Nakamura, K.; Furukawa, N.; Mruganka, K.; Fukuda, T.; Yoshiike, S.; Nishimura, A.; Ohama, A.; Torii, K.; Kuwahara, T.; Okuda, T.; Yamamoto, H.; Tachihara, K.; Fukui, Y.

2017-09-01

We have analyzed the atomic and molecular gas using the 21 cm HI and 2.6/1.3 mm CO emissions toward the young supernova remnant (SNR) RCW 86 in order to identify the interstellar medium with which the shock waves of the SNR interact. We have found an HI intensity depression in the velocity range between -46 and - 28 kms-1 toward the SNR, suggesting a cavity in the interstellar medium. The HI cavity coincides with the thermal and non-thermal emitting X-ray shell. The thermal X-rays are coincident with the edge of the HI distribution, which indicates a strong density gradient, while the non-thermal X-rays are found toward the less dense, inner part of the HI cavity. The most significant non-thermal X-rays are seen toward the southwestern part of the shell where the HI gas traces the dense and cold component. We also identified CO clouds which are likely interacting with the SNR shock waves in the same velocity range as the HI, although the CO clouds are distributed only in a limited part of the SNR shell. The most massive cloud is located in the southeastern part of the shell, showing detailed correspondence with the thermal X-rays. These CO clouds show an enhanced CO J = 2- 1 / 1- 0 intensity ratio, suggesting heating/compression by the shock front. We interpret that the shock-cloud interaction enhances non-thermal X-rays in the southwest and the thermal X-rays are emitted by the shock-heated gas of density 10-100 cm-3. Moreover, we can clearly see an HI envelope around the CO cloud, suggesting that the progenitor had a weaker wind than the massive progenitor of the core-collapse SNR RX J1713.7-3949. It seems likely that the progenitor of RCW 86 was a system consisting of a white dwarf and a low-mass star with low-velocity accretion winds.

Odin observations of H2O and O2 in comets and interstellar clouds

NASA Astrophysics Data System (ADS)

Hjalmarson, Åke; Odin Team

2002-11-01

We here report on results from single-position observations, and in some cases also mapping, of the 557 GHz ortho-H2O line in several comets and in many interstellar molecular clouds by the Odin sub-millimetre wave spectroscopy satellite. The H2O production rates have been accurately determined in four comets, C/2001 A2 (LINEAR), 19P/Borrelly, C/2000 WM1 (LINEAR), and 153P/2002 C1 (Ikeya-Zhang). In comet Ikeya-Zhang our detection at a low level of the corresponding H218O emission line verifies the H2O production rate (which depends upon the assumed radiative and collisional excitation and also upon radiative transfer modelling) and is consistent with a nearly terrestrial 16O/18O-isotope ratio. In an astrobiological context, the cometary H2O production rates are especially important as reference levels for comparison with abundances of other molecules simultaneously observed with ground-based telescopes. In interstellar clouds the observed gas-phase H2O abundances (vs H2) range from 5×10-4 in the Orion KL outflow/shock region (where essentially all oxygen is locked up in H2O) to circa 10-8 in quiescent cloud regions (where H2O) is just one of many trace molecules). From an astrobiological point of view, the molecular abundances in star forming clouds are important in terms of initial conditions for the chemistry in proto-planetary disks ("proto-solar nebulae"), the formation sites of new planetary systems. In simultaneous observations, Odin has also detected the 572 GHz ortho-NH3 line in cold and warm clouds as well as in the Orion outflow and Bar/PDR regions (an area of increased ionisation caused by the intense UV flux from newly born massive stars). In other simultaneous observations, we have performed sensitive searches for O2 at 119 GHz. Although no detection can be reported as yet, the resulting very low abundance limits (<10-7) are very intriguing when they are compared with current "standard" model expectations, which fall in the range 10-5-10-4.

Cold and warm atomic gas around the Perseus molecular cloud. I. Basic properties

DOE Office of Scientific and Technical Information (OSTI.GOV)

Stanimirović, Snežana; Murray, Claire E.; Miller, Jesse

2014-10-01

Using the Arecibo Observatory, we have obtained neutral hydrogen (HI) absorption and emission spectral pairs in the direction of 26 background radio continuum sources in the vicinity of the Perseus molecular cloud. Strong absorption lines were detected in all cases, allowing us to estimate spin temperature (T{sub s} ) and optical depth for 107 individual Gaussian components along these lines of sight. Basic properties of individual H I clouds (spin temperature, optical depth, and the column density of the cold and warm neutral medium (CNM and WNM), respectively) in and around Perseus are very similar to those found for randommore » interstellar lines of sight sampled by the Millennium H I survey. This suggests that the neutral gas found in and around molecular clouds is not atypical. However, lines of sight in the vicinity of Perseus have, on average, a higher total H I column density and the CNM fraction, suggesting an enhanced amount of cold H I relative to an average interstellar field. Our estimated optical depth and spin temperature are in stark contrast with the recent attempt at using Planck data to estimate properties of the optically thick H I. Only ∼15% of lines of sight in our study have a column density weighted average spin temperature lower than 50 K, in comparison with ≳ 85% of Planck's sky coverage. The observed CNM fraction is inversely proportional to the optical depth weighted average spin temperature, in excellent agreement with the recent numerical simulations by Kim et al. While the CNM fraction is, on average, higher around Perseus relative to a random interstellar field, it is generally low, between 10%-50%. This suggests that extended WNM envelopes around molecular clouds and/or significant mixing of CNM and WNM throughout molecular clouds are present and should be considered in the models of molecule and star formation. Our detailed comparison of H I absorption with CO emission spectra shows that only 3 of the 26 directions are clear candidates for probing the CO-dark gas as they have N(H I)>10{sup 21} cm{sup –2} yet no detectable CO emission.« less

Toward Measuring Galactic Dense Molecular Gas Properties and 3D Distribution with Hi-GAL

NASA Astrophysics Data System (ADS)

Zetterlund, Erika; Glenn, Jason; Maloney, Phil

2016-01-01

The Herschel Space Observatory's submillimeter dust continuum survey Hi-GAL provides a powerful new dataset for characterizing the structure of the dense interstellar medium of the Milky Way. Hi-GAL observed a 2° wide strip covering the entire 360° of the Galactic plane in broad bands centered at 70, 160, 250, 350, and 500 μm, with angular resolution ranging from 10 to 40 arcseconds. We are adapting a molecular cloud clump-finding algorithm and a distance probability density function distance-determination method developed for the Bolocam Galactic Plane Survey (BGPS) to the Hi-GAL data. Using these methods we expect to generate a database of 105 cloud clumps, derive distance information for roughly half the clumps, and derive precise distances for approximately 20% of them. With five-color photometry and distances, we will measure the cloud clump properties, such as luminosities, physical sizes, and masses, and construct a three-dimensional map of the Milky Way's dense molecular gas distribution.The cloud clump properties and the dense gas distribution will provide critical ground truths for comparison to theoretical models of molecular cloud structure formation and galaxy evolution models that seek to emulate spiral galaxies. For example, such models cannot resolve star formation and use prescriptive recipes, such as converting a fixed fraction of interstellar gas to stars at a specified interstellar medium density threshold. The models should be compared to observed dense molecular gas properties and galactic distributions.As a pilot survey to refine the clump-finding and distance measurement algorithms developed for BGPS, we have identified molecular cloud clumps in six 2° × 2° patches of the Galactic plane, including one in the inner Galaxy along the line of sight through the Molecular Ring and the termination of the Galactic bar and one toward the outer Galaxy. Distances have been derived for the inner Galaxy clumps and compared to Bolocam Galactic Plane Survey results. We present the pilot survey clump catalog, distances, clump properties, and a comparison to BGPS.

Composition, structure, and chemistry of interstellar dust

NASA Technical Reports Server (NTRS)

Tielens, A. G. G. M.; Allamandola, L. J.

1987-01-01

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.

Life from the stars?. [extraterrestrial sources contributing to chemical evolution on Earth

NASA Technical Reports Server (NTRS)

Pendleton, Yvonne J.; Cruikshank, Dale P.

1994-01-01

Scientists are now seriously considering the possibility that organic matter from interstellar space could have influenced, or even spurred, the origin of life on Earth. Various aspects of chemical evolution are discussed along with possible extraterrestrial sources responsible for contributing to Earth's life-producing, chemical composition. Specific topics covered include the following: interstellar matter, molecular clouds, asteroid dust, organic molecules in our solar system, interplanetary dust and comets, meteoritic composition, and organic-rich solar-system bodies.

Working Group on Circumstellar/Interstellar Relationships

NASA Technical Reports Server (NTRS)

Glassgold, A. E.

1986-01-01

Stars of various types are believed to be the main source of interstellar (IS) dust grans. The most important confirmed source is evolved giant and supergiant stars. Supernovae also contribute to the mass loss. The differences between circumstellar (CS) and IS dust were reviewed using the following topics: alteration of CS dust grains, size distribution, space observation of CS and IS dust, comparison of infrared spectra, isotopic signatures, Magellanic clouds and nearby galaxies, life cycles of dust grains, and physical and chemical data.

An infrared measurement of chemical desorption from interstellar ice analogues

NASA Astrophysics Data System (ADS)

Oba, Y.; Tomaru, T.; Lamberts, T.; Kouchi, A.; Watanabe, N.

2018-03-01

In molecular clouds at temperatures as low as 10 K, all species except hydrogen and helium should be locked in the heterogeneous ice on dust grain surfaces. Nevertheless, astronomical observations have detected over 150 different species in the gas phase in these clouds. The mechanism by which molecules are released from the dust surface below thermal desorption temperatures to be detectable in the gas phase is crucial for understanding the chemical evolution in such cold clouds. Chemical desorption, caused by the excess energy of an exothermic reaction, was first proposed as a key molecular release mechanism almost 50 years ago1. Chemical desorption can, in principle, take place at any temperature, even below the thermal desorption temperature. Therefore, astrochemical network models commonly include this process2,3. Although there have been a few previous experimental efforts4-6, no infrared measurement of the surface (which has a strong advantage to quantify chemical desorption) has been performed. Here, we report the first infrared in situ measurement of chemical desorption during the reactions H + H2S → HS + H2 (reaction 1) and HS + H → H2S (reaction 2), which are key to interstellar sulphur chemistry2,3. The present study clearly demonstrates that chemical desorption is a more efficient process for releasing H2S into the gas phase than was previously believed. The obtained effective cross-section for chemical desorption indicates that the chemical desorption rate exceeds the photodesorption rate in typical interstellar environments.

GOT C+: Galactic Plane Survey of the 1.9 THz [CII] Line

NASA Astrophysics Data System (ADS)

Langer, William

2012-01-01

The ionized carbon [CII] 1.9 THz fine structure line is a major gas coolant in the interstellar medium (ISM) and controls the thermal conditions in diffuse gas clouds and Photodissociation Regions (PDRs). The [CII] line is also an important tracer of the atomic gas and atomic to molecular transition in diffuse clouds throughout the Galaxy. I will review some of the results from the recently completed Galactic Observations of Terahertz C+ (GOT C+) survey. This Herschel Open Time Key Project is a sparse, but uniform volume sample survey of [CII] line emission throughout the Galactic disk using the HIFI heterodyne receiver. HIFI observations, with their high spectral resolution, isolate and locate individual clouds in the Galaxy and provide excitation information on the gas. I will present [CII] position-velocity maps that reveal the distribution and motion of the clouds in the inner Galaxy and discuss results on the physical properties of the gas using spectral observations of [CII] and ancillary HI and 12CO, 13CO, and C18O J=1-0 data. The [CII] emission is also a useful tracer of the "Dark H2 Gas", and I will discuss its distribution in a sample of interstellar clouds. This research was conducted at the Jet Propulsion Laboratory, California Institute of Technology under contract with the National Aeronautics and Space Administration.

Interstellar absorption in the Mg II resonance line k2 and h2 emissions

NASA Technical Reports Server (NTRS)

Boehm-Vitense, E.

1981-01-01

High-resolution (0.2 A) IUE spectra for the long wavelength range (1800-3000 A) have been studied. It is shown that narrow interstellar Mg II lines are seen in the center of the k2 and h2 emissions from nearby stars with large rotational velocities. For all observed stars, the radial velocity of the central k3 absorption component in the rest system of the star is strongly correlated with the mirror image of the radial velocity of the stars; this shows that a major fraction if not all of the k3 absorption is due to interstellar absorption in the solar neighborhood. The violet to red asymmetry of the k2 emission also correlates with the radial velocities of the star; this shows that the shift of k3 is due to the velocity shift of the local interstellar cloud with respect to the star.

The Atacama Large Millimeter/submillimeter Array (alma): Early Results

NASA Astrophysics Data System (ADS)

Wootten, Alwyn

2012-06-01

New radioastronomical instruments, such as ALMA or the Jansky VLA, have increased spectral throughput by orders of magnitude over previously available capabilities. ALMA brings orders of magnitude increases in spectral sensitivity and spatial resolution over what has previously been available. These increased capabilities open new possibilities for studies of complex molecules in the interstellar medium. Complex interstellar molecules may form on the surfaces of interstellar grains, after which they may be liberated into the gas phase by shocks, radiation, or other external influences. Emission from complex molecules may be diluted owing to the large number of transitions large molecules may undergo, particularly in warm regions of interstellar clouds. High sensitivity and spatial resolution are necessary to explore the distributions and relationships of these molecules. Of particular interest are the distributions of large organic molecules. Observations which establish the relationships between various large molecules are now emerging from these new instruments and will be discussed.

Infrared Spectroscopy of Carbonaceous-chondrite Inclusions in the Kapoeta Meteorite: Discovery of Nanodiamonds with New Spectral Features and Astrophysical Implications

NASA Astrophysics Data System (ADS)

Abdu, Yassir A.; Hawthorne, Frank C.; Varela, Maria E.

2018-03-01

We report the finding of nanodiamonds, coexisting with amorphous carbon, in carbonaceous-chondrite (CC) material from the Kapoeta achondritic meteorite by Fourier-transform infrared (FTIR) spectroscopy and micro-Raman spectroscopy. In the C–H stretching region (3100–2600 cm‑1), the FTIR spectrum of the Kapoeta CC material (KBr pellet) shows bands attributable to aliphatic CH2 and CH3 groups, and is very similar to IR spectra of organic matter in carbonaceous chondrites and the diffuse interstellar medium. Nanodiamonds, as evidenced by micro-Raman spectroscopy, were found in a dark region (∼400 μm in size) in the KBr pellet. Micro-FTIR spectra collected from this region are dramatically different from the KBr-pellet spectrum, and their C–H stretching region is dominated by a strong and broad absorption band centered at ∼2886 cm‑1 (3.47 μm), very similar to that observed in IR absorption spectra of hydrocarbon dust in dense interstellar clouds. Micro-FTIR spectroscopy also indicates the presence of an aldehyde and a nitrile, and both of the molecules are ubiquitous in dense interstellar clouds. In addition, IR peaks in the 1500–800 cm‑1 region are also observed, which may be attributed to different levels of nitrogen aggregation in diamonds. This is the first evidence for the presence of the 3.47 μm interstellar IR band in meteorites. Our results further support the assignment of this band to tertiary CH groups on the surfaces of nanodiamonds. The presence of the above interstellar bands and the absence of shock features in the Kapoeta nanodiamonds, as indicated by Raman spectroscopy, suggest formation by a nebular-condensation process similar to chemical-vapor deposition.

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bzowski, M.; Kubiak, M. A.; Sokol, J. M.

Because of its high ionization potential and weak interaction with hydrogen, neutral interstellar helium (NISHe) is almost unaffected at the heliospheric interface with the interstellar medium and freely enters the solar system. This second most abundant species provides some of the best information on the characteristics of the interstellar gas in the local interstellar cloud. The Interstellar Boundary Explorer (IBEX) is the second mission to directly detect NISHe. We present a comparison between recent IBEX NISHe observations and simulations carried out using a well-tested quantitative simulation code. Simulation and observation results compare well for times when measured fluxes are dominatedmore » by NISHe (and contributions from other species are small). Differences between simulations and observations indicate a previously undetected secondary population of neutral helium, likely produced by interaction of interstellar helium with plasma in the outer heliosheath. Interstellar neutral parameters are statistically different from previous in situ results obtained mostly from the GAS/Ulysses experiment, but they do agree with the local interstellar flow vector obtained from studies of interstellar absorption: the newly established flow direction is ecliptic longitude 79.{sup 0}2, latitude -5.{sup 0}1, the velocity is {approx}22.8 km s{sup -1}, and the temperature is 6200 K. These new results imply a markedly lower absolute velocity of the gas and thus significantly lower dynamic pressure on the boundaries of the heliosphere and different orientation of the Hydrogen Deflection Plane compared to prior results from Ulysses. A different orientation of this plane also suggests a new geometry of the interstellar magnetic field, and the lower dynamic pressure calls for a compensation by other components of the pressure balance, most likely a higher density of interstellar plasma and strength of interstellar magnetic field.« less

Millimeter-wave Absorption Studies of Molecules in Diffuse Clouds

NASA Astrophysics Data System (ADS)

Lucas, Robert; Liszt, Harvey S.

1999-10-01

With IRAM instruments in the last few years, we have been using compact extragalactic millimeter wave radio sources as background objects to study the absorption spectrum of diffuse interstellar gas at millimeter wavelengths. The molecular content of interstellar gas has turned out to be unexpectedly rich. Simple polyatomic molecules such as HCO+, C2H are quite ubiquitous near the Galactic plane (beta < 15o), and many species are detected in some directions (CO, HCO+, H2CO, HCN, HNC, CN, C2H, C3H2, H2S, CS, HCS+, SO, SiO). Remarkable proportionality relations are found between related species such as HCO+ and OH, or CN, HCN and HNC. The high abundance of some species is still a challenge for current models of diffuse cloud chemistry. A factor of 10 increase in the sensitivity will make such studies achievable in denser clouds, where the chemistry is still more active and where abundances are nowadays only available by emission measurements, and thus subject to uncertainties due to sometimes poorly understood line formation and excitation conditions.

Into the Darkness: Interstellar Extinction Near the Cepheus OB3 Molecular Cloud

NASA Astrophysics Data System (ADS)

Fitzpatrick, Edward L.; Jacklin, S.; Massa, D.

2014-01-01

We present the results of a followup investigation to a study performed by Massa and Savage (1984, ApJ, 279, 310) of the properties of UV interstellar extinction in the region of the Cepheus OB3 molecular cloud. That study was performed using UV photometry and spectro-photometry from the ANS and IUE satellites. We have extended this study into the IR, utilizing the uniform database of IR photometry available from the 2MASS project. This is a part of a larger program whose goal is to study the properties of extinction in localized regions, where we hope to find clues to dust grain growth and destruction processes through spatial correlations of extinction with distinct environmental properties. Similarly to Massa and Savage’s UV results, we find that the IR extinction properties on the Cepheus OB3 region vary systematically with the apparent proximity of the target stars to the molecular cloud. We also find that the UV extinction and the IR extinction are crudely correlated. The methodology leading to these results and their implications are discussed.

Structures Of Magnetically-Supported Filaments And Their Appearance In The Linear Polarization

NASA Astrophysics Data System (ADS)

Tomisaka, Kohji

2017-10-01

Dust thermal emissions observed with Herschel have revealed that interstellar molecular clouds consist of many filaments. Polarization observation of interstellar extinctions in the optical and near IR wavelengths shows that the dense filaments are extending perpendicular to the interstellar magnetic field. Magnetohydrostatic structures of such filaments are studied. It is well known that a hydrostatic filament without magnetic field has a maximum line mass of ¥lambda_max=2c_s^2/G (c_s:the isothermal sound speed and G: the gravitational constant). On the other hand, the magnetically-supported maximum line mass increases in proportion to the magnetic flux per unit length threading the filament (¥phi), as ¥lambda_max 2c_s^2/G + ¥phi/(2¥pi G^1/2). Comparison is made with 3D clouds. Stability of these magnetized filaments is studied using time-dependent 3D MHD simulations to discuss star formation in the filaments. Polarization pattern expected for the magnetically subcritical filaments is calculated. The distribution function of the angle between B-field and the axis of the filament, which is obtained with Planck Satellite, is compared with this mock observation.

Formation of Benzene in the Interstellar Medium

NASA Technical Reports Server (NTRS)

Jones, Brant M.; Zhang, Fangtong; Kaiser, Ralf I.; Jamal, Adeel; Mebel, Alexander M.; Cordiner, Martin A.; Charnley, Steven B.; Crim, F. Fleming (Editor)

2010-01-01

Polycyclic aromatic hydrocarbons and related species have been suggested to play a key role in the astrochemical evolution of the interstellar medium, but the formation mechanism of even their simplest building block-the aromatic benzene molecule-has remained elusive for decades. Here we demonstrate in crossed molecular beam experiments combined with electronic structure and statistical calculations that benzene (C6H6) can be synthesized via the barrierless, exoergic reaction of the ethynyl radical and 1,3- butadiene, C2H + H2CCHCHCH2 --> C6H6, + H, under single collision conditions. This reaction portrays the simplest representative of a reaction class in which aromatic molecules with a benzene core can be formed from acyclic precursors via barrierless reactions of ethynyl radicals with substituted 1,3-butadlene molecules. Unique gas-grain astrochemical models imply that this low-temperature route controls the synthesis of the very first aromatic ring from acyclic precursors in cold molecular clouds, such as in the Taurus Molecular Cloud. Rapid, subsequent barrierless reactions of benzene with ethynyl radicals can lead to naphthalene-like structures thus effectively propagating the ethynyl-radical mediated formation of aromatic molecules in the interstellar medium.

Formation of benzene in the interstellar medium

PubMed Central

Jones, Brant M.; Zhang, Fangtong; Kaiser, Ralf I.; Jamal, Adeel; Mebel, Alexander M.; Cordiner, Martin A.; Charnley, Steven B.

2011-01-01

Polycyclic aromatic hydrocarbons and related species have been suggested to play a key role in the astrochemical evolution of the interstellar medium, but the formation mechanism of even their simplest building block—the aromatic benzene molecule—has remained elusive for decades. Here we demonstrate in crossed molecular beam experiments combined with electronic structure and statistical calculations that benzene (C6H6) can be synthesized via the barrierless, exoergic reaction of the ethynyl radical and 1,3-butadiene, C2H + H2CCHCHCH2 → C6H6 + H, under single collision conditions. This reaction portrays the simplest representative of a reaction class in which aromatic molecules with a benzene core can be formed from acyclic precursors via barrierless reactions of ethynyl radicals with substituted 1,3-butadiene molecules. Unique gas-grain astrochemical models imply that this low-temperature route controls the synthesis of the very first aromatic ring from acyclic precursors in cold molecular clouds, such as in the Taurus Molecular Cloud. Rapid, subsequent barrierless reactions of benzene with ethynyl radicals can lead to naphthalene-like structures thus effectively propagating the ethynyl-radical mediated formation of aromatic molecules in the interstellar medium. PMID:21187430

Observations of Carbon Isotopic Fractionation in Interstellar Formaldehyde

NASA Technical Reports Server (NTRS)

Wirstrom, E. S.; Charnley, S. B.; Geppert, W. D.; Persson, C. M.

2012-01-01

Primitive Solar System materials (e.g. chondrites. IDPs, the Stardust sample) show large variations in isotopic composition of the major volatiles (H, C, N, and O ) even within samples, witnessing to various degrees of processing in the protosolar nebula. For ex ample. the very pronounced D enhancements observed in IDPs [I] . are only generated in the cold. dense component of the interstellar medium (ISM), or protoplanetary disks, through ion-molecule reactions in the presence of interstellar dust. If this isotopic anomaly has an interstellar origin, this leaves open the possibility for preservation of other isotopic signatures throughout the form ation of the Solar System. The most common form of carbon in the ISM is CO molecules, and there are two potential sources of C-13 fractionation in this reservoir: low temperature chemistry and selective photodissociation. While gas-phase chemistry in cold interstellar clouds preferentially incorporates C-13 into CO [2], the effect of self-shielding in the presence of UV radiation instead leads to a relative enhancement of the more abundant isotopologue, 12CO. Solar System organic material exhibit rather small fluctuations in delta C-13 as compared to delta N-15 and delta D [3][1], the reason for which is still unclear. However, the fact that both C-13 depleted and enhanced material exists could indicate an interstellar origin where the two fractionation processes have both played a part. Formaldehyde (H2CO) is observed in the gas-phase in a wide range of interstellar environments, as well as in cometary comae. It is proposed as an important reactant in the formation of more complex organic molecules in the heated environments around young stars, and formaldehyde polymers have been suggested as the common origin of chondritic insoluable organic matter (IOM) and cometary refractory organic solids [4]. The relatively high gas-phase abundance of H2CO observed in molecular clouds (10(exp- 9) - 10(exp- 8) relative to H2) makes it feasible to observe its less common isotopologues. As a step in our investigation of C-13 fractionation patterns in the ISM, we here present comparisons between observations of the C-13 fraction in formaldehyde, and chemical fractionation models.

Interstellar Gas Flow Vector and Temperature Determination over 5 Years of IBEX Observations

NASA Astrophysics Data System (ADS)

Möbius, E.; Bzowski, M.; Fuselier, S. A.; Heirtzler, D.; Kubiak, M. A.; Kucharek, H.; Lee, M. A.; Leonard, T.; McComas, D. J.; Schwadron, N.; Sokół, J. M.; Wurz, P.

2015-01-01

The Interstellar Boundary Explorer (IBEX) observes the interstellar neutral gas flow trajectories at their perihelion in Earth's orbit every year from December through early April, when the Earth's orbital motion is into the oncoming flow. These observations have defined a narrow region of possible, but very tightly coupled interstellar neutral flow parameters, with inflow speed, latitude, and temperature as well-defined functions of inflow longitude. The best- fit flow vector is different by ≈ 3° and lower by ≈ 3 km/s than obtained previously with Ulysses GAS, but the temperature is comparable. The possible coupled parameter space reaches to the previous flow vector, but only for a substantially higher temperature (by ≈ 2000 K). Along with recent pickup ion observations and including historical observations of the interstellar gas, these findings have led to a discussion, whether the interstellar gas flow into the solar system has been stable or variable over time. These intriguing possibilities call for more detailed analysis and a longer database. IBEX has accumulated observations over six interstellar flow seasons. We review key observations and refinements in the analysis, in particular, towards narrowing the uncertainties in the temperature determination. We also address ongoing attempts to optimize the flow vector determination through varying the IBEX spacecraft pointing and discuss related implications for the local interstellar cloud and its interaction with the heliosphere.

CO near the Pleiades: Encounter of a star cluster with a small molecular cloud

NASA Technical Reports Server (NTRS)

Bally, J.; White, R. E.

1986-01-01

Although there is a large amount of interstellar matter near the Pleiades star cluster, the observed dust and gas is not a remnant of the placental molecular cloud from which the star cluster was formed. Carbon monoxide (CO) associated with the visible reflection nebulae was discovered by Cohen (1975). Its radial velocity differs from that of the cluster by many times the cluster escape velocity, which implies that the cloud-cluster association is the result of a chance encounter. This circumstance and the proximity of the Pleiades to the sun creates an unique opportunity for study of interstellar processes at high spatial resolution. To study the molecular component of the gas, a 1.7 square degree field was mapped with the AT&T Bell Laboratories 7-meter antenna (1.7' beam) on a 1' grid in the J=1.0 C(12)O line, obtaining over 6,000 spectra with 50 kHz resolution. The cloud core was mapped in the J=1-0 line of C(13)O. Further observations include an unsuccessful search for CS (J=2-1) at AT&T BL, and some C(12)O J=2-1 spectra obtained at the Millimeter Wave Observatory of the University of Texas.

PAHs molecules and heating of the interstellar gas

NASA Technical Reports Server (NTRS)

Verstraete, Laurent; Leger, Alain; Dhendecourt, Louis B.; Dutuit, O.; Defourneau, D.

1989-01-01

Until now it has remained difficult to account for the rather high temperatures seen in many diffuse interstellar clouds. Various heating mechanisms have been considered: photoionization of minor species, ionization of H by cosmic rays, and photoelectric effect on small grains. Yet all these processes are either too weak or efficient under too restricting conditions to balance the observed cooling rates. A major heat source is thus still missing in the thermal balance of the diffuse gas. Using photoionization cross sections measured in the lab, it was shown that in order to balance the observed cooling rates in cold diffuse clouds (T approx. 80 K) the PAHs would have to contain 15 percent of the cosmic abundance of carbon. This value does not contradict the former estimation of 6 percent deduced from the IR emission bands since this latter is to be taken as a lower limit. Further, it was estimated that the contribution to the heating rate due to PAH's in a warm HI cloud, assuming the same PAH abundance as for a cold HI cloud, would represent a significant fraction of the value required to keep the medium in thermal balance. Thus, photoionization of PAHs might well be a major heat source for the cold and warm HI media.

Terahertz Spectroscopy of Deuterated Methylene Bi-Radical CD_2

NASA Astrophysics Data System (ADS)

Ozeki, Hiroyuki; Bailleux, Stephane

2015-06-01

Methylene, the parent of the carbene compounds, plays a crucial role in many chemical reactions. This bi-radical is a known interstellar molecule that has been detected towards hot cores in dense interstellar clouds. CH_2 is also thought to be present in cometary atmospheres. In the gas phase chemical models of both dense and diffuse molecular clouds, CH_2 is a key intermediate in interstellar carbon chemistry which is produced primarily by dissociative recombination of the methyl ion, CH^+_3. Recently tentative detection of the mono-deuterated methyl ion, CH_2D^+ has been reported toward an infrared source in the vicinity of Orion. Deuterated methylene CHD and CD_2 can be produced from this ion or its counterpart CHD^+_2 by dissociative recombination with an electron: CH2D+ + e- → CHD+ H or {CH2 + D}, CHD2+ ~+ e- → CHD+ D or CD2 + H. Thus, both CHD and CD_2 can be observed in warm interstellar clouds, where the deuterium fractionation process is important. Precise laboratory reference data are desirable for radioastronomical observation of these molecules. Here we report on our high-resolution spectroscopic investigation on the deuterated methylene radical, CD_2 (X ^3B_1) up to 1.45 THz. At present time, eleven out of the twelve fine-structure components of four pure rotational transitions have been newly recorded, and these measurements double the number of previously observed transitions. CD_2 was generated in a discharge in CD_2CO which was obtained from the flash pyrolysis of acetic anhydride-d6 ((CD_3CO)_2O). Effort is currently made to measure the astronomically important 111 - 000 transition whose fine-structure components are predicted to occur at 1.224,1.228 and 1.234 THz. D. C. Lis, P. F. Goldsmith, E. A. Bergin et al. 2009, in Submillimeter Astrophysics and Technology, ASP Conf. Ser., 417, 23. H. Ozeki and S. Saito J. Chem. Phys. 1996, 104, 2167.

Molecular Diagnostics of the Interstellar Medium and Star Forming Regions

NASA Astrophysics Data System (ADS)

Hartquist, T. W.; Dalgarno, A.

1996-03-01

Selected examples of the use of observationally inferred molecular level populations and chemical compositions in the diagnosis of interstellar sources and processes important in them (and in other diffuse astrophysical sources) are given. The sources considered include the interclump medium of a giant molecular cloud, dark cores which are the progenitors of star formation, material responding to recent star formation and which may form further stars, and stellar ejecta (including those of supernovae) about to merge with the interstellar medium. The measurement of the microwave background, mixing of material between different nuclear burning zones in evolved stars and turbulent boundary layers (which are present in and influence the structures and evolution of all diffuse astrophysical sources) are treated.

Comment on "The shape and composition of interstellar silicate grains"

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bradley, J P; Ishii, H

2007-09-27

In the paper entitled 'The shape and composition of interstellar silicate grains' (A & A, 462, 667-676 (2007)), Min et al. explore non-spherical grain shape and composition in modeling the interstellar 10 and 20 {micro}m extinction features. This progression towards more realistic models is vitally important to enabling valid comparisons between dust observations and laboratory measurements. Min et al. proceed to compare their model results with GEMS (glass with embedded metals and sulfides) from IDPs (interplanetary dust particles) and to discuss the nature and origin of GEMS. Specifically, they evaluate the hypothesis of Bradley (1994) that GEMS are interstellar (IS)more » amorphous silicates. From a comparison of the mineralogy, chemical compositions, and infrared (IR) spectral properties of GEMS with their modeling results, Min et al. conclude: 'GEMS are, in general, not unprocessed leftovers from the diffuse ISM'. This conclusion is based, however, on erroneous and incomplete GEMS data. It is important to clarify first that Bradley (1994) never proposed that GEMS are unprocessed leftovers from the diffuse ISM, nor did he suggest that individual subnanogram mass GEMS are a representative sampling of the enormous mass of silicates in the diffuse ISM. Bradley (1994) simply showed that GEMS properties are consistent with those of IS amorphous silicates. It is widely accepted that circumstellar outflows are important sources of IS silicates, and whether GEMS are processed or not, the circumstellar heritage of some has been rigorously confirmed through measurements of non-solar oxygen (O) isotope abundances (Messenger et al., 2003; Floss et al., 2006). Keller et al. (2000) assert that even GEMS without detectable O isotope anomalies are probably also extrasolar IS silicates because they are embedded in carbonaceous material with non-solar D/H isotopic composition. (Much of the silicate dust in the ISM may be isotopically homogenized (Zhukovska et al., 2007)). Recent measurements show that the elemental compositions of GEMS with non-solar isotopic compositions are 'remarkably similar' to those with solar isotopic compositions (Keller & Messenger, 2007). About 80% of all isotopically anomalous IS silicates identified to date are GEMS with detectable and variable O isotopic memories of a circumstellar ancestry (Messenger, 2007). Bradley (1999) proposed that GEMS are IS silicates from 'a presolar interstellar molecular cloud, presumably the local molecular cloud from which the solar system formed'. Although based on incorrect data (detailed below), Min et al. propose that most GEMS actually formed in the presolar molecular cloud, and they further propose that none of them are IS silicates. IS silicate sources include molecular clouds, circumstellar outflows, supernovae, and even recently discovered black hole winds (Molster & Waters; 2003; Jones, 2005; Zhukovska et al. 2007; Markwick-Kemper et al. 2007). The average IS 10 {micro}m extinction feature observed along lines of sight towards the galactic center (modeled by Min et al.) presumably provides a good average for IS silicates, but it cannot distinguish amorphous silicates originating in the presolar molecular cloud from amorphous silicates originating in other interstellar molecular clouds or indeed other sources of amorphous IS silicates. Even if most GEMS accreted in the presolar molecular cloud, then they must also be representatives of some portion of the IS amorphous silicate population. Laboratory heating experiments indicate it is highly unlikely that GEMS were modified in a protoplanetary accretion disk environment (Brownlee et al. 2005). In summary, Min et al. conclude from their modeling of the shape and composition of IS silicates that the properties of GEMS are generally inconsistent with those of IS silicates. First, it has been rigorously confirmed via ion microprobe measurements that some GEMS are indeed presolar IS silicates. Second, regardless of whether GEMS, or components of GEMS, originated in presolar circumstellar outflows or a presolar molecular cloud they are all IS silicates. Third, key GEMS data reported in Min et al. are inaccurate. Had complete isotopic, chemical, mineralogical and infrared (IR) spectral properties of GEMS been considered, Min et al. may have concluded that the properties of GEMS, although not an exact match, are generally consistent with those of amorphous silicates in the ISM.« less

Star-Studded Strings around Cocoon Nebula

NASA Image and Video Library

2011-04-13

Dense filaments of gas in the IC5146 interstellar cloud can be seen clearly in this image taken in infrared light by the Herschel space observatory. The blue region is a stellar nursery known as the Cocoon nebula.

Dark Reflections in the Southern Cross

NASA Image and Video Library

2010-10-27

NASA Wide-field Infrared Survey Explorer captured this colorful image of the reflection nebula IRAS 12116-6001. This cloud of interstellar dust cannot be seen directly in visible light, but WISE detectors observed the nebula at infrared wavelengths.

Desorption from interstellar grains

NASA Technical Reports Server (NTRS)

Leger, A.; Jura, M.; Omont, A.

1985-01-01

Different desorption mechanisms from interstellar grains are considered to resolve the conflict between the observed presence of gaseous species in molecular clouds and their expected depletion onto grains. The physics of desorption is discussed with particular reference to the process of grain heating and the specific heat of the dust material. Impulsive heating by X-rays and cosmic rays is addressed. Spot heating of the grains by cosmic rays and how this can lead to desorption of mantles from very large grains is considered. It is concluded that CO depletion on grains will be small in regions with A(V) less than five from the cloud surface and n(H) less than 10,000, in agreement with observations and in contrast to expectations from pure thermal equilibrium. Even in very dense and obscured regions and in the absence of internal ultraviolet sources, the classical evaporation of CO or N2 and O2-rich mantles by cosmic rays is important.

REVISITING THE CHLORINE ABUNDANCE IN DIFFUSE INTERSTELLAR CLOUDS FROM MEASUREMENTS WITH THE COPERNICUS SATELLITE

DOE Office of Scientific and Technical Information (OSTI.GOV)

Moomey, Daniel; Federman, S. R.; Sheffer, Y., E-mail: steven.federman@utoledo.edu, E-mail: ysheffer@astro.umd.edu

2012-01-10

We reanalyzed interstellar Cl I and Cl II spectra acquired with the Copernicus satellite. The directions for this study come from those of Crenny and Federman and sample the transition from atomic to molecular-rich clouds where the unique chemistry leading to molecules containing chlorine is initiated. Our profile syntheses relied on up-to-date laboratory oscillator strengths and component structures derived from published high-resolution measurements of K I absorption that were supplemented with Ca II and Na I D results. We obtain self-consistent results for the Cl I lines at 1088, 1097, and 1347 A from which precise column densities are derived.more » The improved set of results reveals clearer correspondences with H{sub 2} and total hydrogen column densities. These linear relationships arise from rapid conversion of Cl{sup +} to Cl{sup 0} in regions where H{sub 2} is present.« less

Turbulence in space plasmas and beyond

NASA Astrophysics Data System (ADS)

Galtier, S.

2018-07-01

Most of the visible matter in the Universe is in the form of highly turbulent plasmas. For a long time the turbulent character of astrophysical fluids has been neglected and not well understood. One reason for this is the extremely complicated physics involved in astrophysical processes ranging from the machinery of stars, solar and stellar winds, accretion disks to interstellar clouds and galaxies. The other reason is that turbulence constitutes in itself a difficult subject where most of the fundamental results belongs to the incompressible hydrodynamics. Nevertheless, significant theoretical progress has been made during the last years to incorporate some ingredients like compressibility or small-scale plasma physics which are fundamental in astrophysics. This paper reviews some of these results with a strong focus on space plasmas (solar wind, solar corona). Turbulence in interstellar clouds (supersonic flows) and cosmology (space-time fluctuations) are also briefly mentioned.

Low-frequency Carbon Radio Recombination Lines. II. The Diffuse Interstellar Medium

DOE Office of Scientific and Technical Information (OSTI.GOV)

Salgado, F.; Morabito, L. K.; Oonk, J. B. R.

In the second paper of the series, we have modeled low-frequency carbon radio recombination lines (CRRLs) from the interstellar medium. Anticipating the Low Frequency Array survey of Galactic CRRLs, we focus our study on the physical conditions of the diffuse, cold neutral medium. We have used the improved departure coefficients computed in the first paper of the series to calculate line-to-continuum ratios. The results show that the line width and integrated optical depths of CRRLs are sensitive probes of the electron density, gas temperature, and emission measure of the cloud. Furthermore, the ratio of CRRL to the [C ii] atmore » the 158 μ m line is a strong function of the temperature and density of diffuse clouds. Guided by our calculations, we analyze CRRL observations and illustrate their use with data from the literature.« less

Chemical energy in cold-cloud aggregates - The origin of meteoritic chondrules

NASA Technical Reports Server (NTRS)

Clayton, D. D.

1980-01-01

If interstellar particles and molecules accumulate into larger particles during the collapse of a cold cloud, the resulting aggregates contain a large store of internal chemical energy. It is here proposed that subsequent warming of these accumulates leads to a thermal runaway when exothermic chemical reactions begin within the aggregate. These, after cooling, are the crystalline chondrules found so abundantly within chondritic meteorites. Chemical energy can also heat meteoritic parent bodies of any size, and both thermal metamorphism and certain molten meteorites are proposed to have occurred in this way. If this new theory is correct, (1) the model of chemical condensation in a hot gaseous solar system is eliminated, and (2) a new way of studying the chemical evolution of the interstellar medium has been found. A simple dust experiment on a comet flyby is proposed to test some features of this controversy.

Environmental Catastrophes in the Earth's History Due to Solar Systems Encounters with Giant Molecular Clouds

NASA Technical Reports Server (NTRS)

Pavlov, Alexander A.

2011-01-01

In its motion through the Milky Way galaxy, the solar system encounters an average density (>=330 H atoms/cubic cm) giant molecular cloud (GMC) approximately every 108 years, a dense (approx 2 x 103 H atoms/cubic cm) GMC every approx 109 years and will inevitably encounter them in the future. However, there have been no studies linking such events with severe (snowball) glaciations in Earth history. Here we show that dramatic climate change can be caused by interstellar dust accumulating in Earth's atmosphere during the solar system's immersion into a dense (approx ,2 x 103 H atoms/cubic cm) GMC. The stratospheric dust layer from such interstellar particles could provide enough radiative forcing to trigger the runaway ice-albedo feedback that results in global snowball glaciations. We also demonstrate that more frequent collisions with less dense GMCs could cause moderate ice ages.

Dust That's Worth Keeping

DOE Office of Scientific and Technical Information (OSTI.GOV)

Hazi, A

2006-01-25

Images taken of interstellar space often display a colorful canvas of portions of the electromagnetic spectrum. Dispersed throughout the images are interstellar clouds of dust and gas--remnants ejected from stars and supernovae over billions and billions of years. For more than 40 years, astronomers have observed that interstellar dust exhibits a consistent effect at a spectral wavelength of 2,175 angstroms, the equivalent of 5.7 electronvolts in energy on the electromagnetic spectrum. At this wavelength, light from stars is absorbed by dust in the interstellar medium, blocking the stars light from reaching Earth. The 2,175-angstrom feature, which looks like a bumpmore » on spectra, is the strongest ultraviolet-visible light spectral signature of interstellar dust and is visible along nearly every observational line of sight. Scientists have sought to solve the mystery of what causes the 2,175-angstrom feature by reproducing the effect in the laboratory. They speculated a number of possibilities, including fullerenes (buckyballs), nanodiamonds, and even interstellar organisms. However, none of these materials fits the data for the unique spectral feature. Limitations in the energy and spatial resolution achievable with electron microscopes and ion microprobes--the two main instruments used to study samples of dust--have also prevented scientists from finding the answer. A collaborative effort led by Livermore physicist John Bradley and funded by the National Aeronautics and Space Administration (NASA) has used a new-generation transmission electron microscope (TEM) and nanoscale ion microprobe to unlock the mystery. The Livermore group includes physicists Zu Rong Dai, Ian Hutcheon, Peter Weber, and Sasa Bajt and postdoctoral researchers Hope Ishii, Giles Graham, and Julie Smith. They collaborated with the University of California at Davis (UCD), Lawrence Berkeley National Laboratory, Washington University's Laboratory for Space Sciences in St. Louis, and NASA's Ames Research Center for their discovery. The team analyzed micrometer-size interplanetary dust particles (IDPs), each about one-tenth the diameter of a human hair. Within the particles, they found carriers of the 2,175-angstrom feature: organic carbon mixed with amorphous silicates (glass with embedded metals and sulfides, GEMS), two of the most common materials in interstellar space. Ishii says, ''Organic carbon and amorphous silicates are abundant in interstellar dust clouds, and abundant carriers are needed to account for the frequent astronomical observation of the 2,175-angstrom feature. It makes sense that this ubiquitous feature would come from common materials in interstellar space''. The group's results increase scientific understanding of the starting materials for the formation of the Sun, solar system, and life on Earth.« less

Dust That's Worth Keeping

NASA Technical Reports Server (NTRS)

Hazi, A.

2006-01-01

Images taken of interstellar space often display a colorful canvas of portions of the electromagnetic spectrum. Dispersed throughout the images are interstellar clouds of dust and gas--remnants ejected from stars and supernovae over billions and billions of years. For more than 40 years, astronomers have observed that interstellar dust exhibits a consistent effect at a spectral wavelength of 2,175 angstroms, the equivalent of 5.7 electronvolts in energy on the electromagnetic spectrum. At this wavelength, light from stars is absorbed by dust in the interstellar medium, blocking the stars light from reaching Earth. The 2,175-angstrom feature, which looks like a bump on spectra, is the strongest ultraviolet-visible light spectral signature of interstellar dust and is visible along nearly every observational line of sight. Scientists have sought to solve the mystery of what causes the 2,175-angstrom feature by reproducing the effect in the laboratory. They speculated a number of possibilities, including fullerenes (buckyballs), nanodiamonds, and even interstellar organisms. However, none of these materials fits the data for the unique spectral feature. Limitations in the energy and spatial resolution achievable with electron microscopes and ion microprobes--the two main instruments used to study samples of dust--have also prevented scientists from finding the answer. A collaborative effort led by Livermore physicist John Bradley and funded by the National Aeronautics and Space Administration (NASA) has used a new-generation transmission electron microscope (TEM) and nanoscale ion microprobe to unlock the mystery. The Livermore group includes physicists Zu Rong Dai, Ian Hutcheon, Peter Weber, and Sasa Bajt and postdoctoral researchers Hope Ishii, Giles Graham, and Julie Smith. They collaborated with the University of California at Davis (UCD), Lawrence Berkeley National Laboratory, Washington University's Laboratory for Space Sciences in St. Louis, and NASA's Ames Research Center for their discovery. The team analyzed micrometer-size interplanetary dust particles (IDPs), each about one-tenth the diameter of a human hair. Within the particles, they found carriers of the 2,175-angstrom feature: organic carbon mixed with amorphous silicates (glass with embedded metals and sulfides, GEMS), two of the most common materials in interstellar space. Ishii says, 'Organic carbon and amorphous silicates are abundant in interstellar dust clouds, and abundant carriers are needed to account for the frequent astronomical observation of the 2,175-angstrom feature. It makes sense that this ubiquitous feature would come from common materials in interstellar space'. The group's results increase scientific understanding of the starting materials for the formation of the Sun, solar system, and life on Earth.

A Combined Experimental and Theoretical Study on the Formation of Interstellar Propylene Oxide (CH3CHCH2O)—A Chiral Molecule

NASA Astrophysics Data System (ADS)

Bergantini, Alexandre; Abplanalp, Matthew J.; Pokhilko, Pavel; Krylov, Anna I.; Shingledecker, Christopher N.; Herbst, Eric; Kaiser, Ralf I.

2018-06-01

This work reveals via a combined experimental, computational, and astrochemical modeling study that racemic propylene oxide (c-C3H6O)—the first chiral molecule detected outside Earth toward the high-mass star-forming region Sagittarius B2(N)—can be synthesized by non-equilibrium reactions initiated by the effects of secondary electrons generated in the track of cosmic rays interacting with ice-coated interstellar grains through excited-state and spin-forbidden reaction pathways operating within low-temperature interstellar ices at 10 K. Our findings confront traditional hypotheses that thermal chemistries followed by processing of interstellar grains dictate the formation of complex organic molecules (COMs) in molecular clouds. Instead, we reveal a hitherto poorly quantified reaction class involving excited-state and spin-forbidden chemistry leading to racemic mixtures of COMs inside interstellar ices prior to their sublimation in star-forming regions. This fundamental production mechanism is of essential consequence in aiding our understanding of the origin and evolution of chiral molecules in the universe.

Grain Growth and Silicates in Dense Clouds

NASA Technical Reports Server (NTRS)

Pendeleton, Yvonne J.; Chiar, J. E.; Ennico, K.; Boogert, A.; Greene, T.; Knez, C.; Lada, C.; Roellig, T.; Tielens, A.; Werner, M.;

The Oort cloud and the Galaxy - Dynamical interactions

NASA Technical Reports Server (NTRS)

Weissman, Paul R.

1986-01-01

The results of recent dynamical studies of the Oort cloud and its interaction with the Galaxy are discussed. Various studies which used Monte Carlo simulations to investigate the evolution of comets in the Oort cloud and the manner in which they are injected into the planetary region are reviewed. Work done on perturbation of cometary orbits by stars, interstellar clouds, and the Galaxy is examined. The growing consensus that there is a massive inner Oort cloud with a population up to 100 times that of the dynamically active outer cloud is addressed. Variations on the Oort hypothesis are discussed. It is argued that speculations about the existence of a small unseen solar companion star or a tenth planet causing periodic comet showers from the inner Oort cloud are not supported by dynamical studies or analyses of the terrestrial and lunar cratering record. Evidence for Oort clouds around other stars is summarized.

Infrared emission from hydrogenated amorphous carbon and amorphous carbon grains in the interstellar medium

NASA Technical Reports Server (NTRS)

Duley, W. W.; Jones, A. P.; Taylor, S. D.; Williams, D. A.

1993-01-01

The correlations deduced by Boulanger et al. (1990) from IRAS maps of the Chamaeleon, Taurus and Ursa Major molecular cloud complexes are interpreted in terms of the evolutionary hydrogenated amorphous carbon model of interstellar dust. In particular, regions of relatively strong 12-micron emission may be regions where recently accreted carbon is being converted by ambient UV to small PAHs in situ. Regions of weak 12-micron emission are probably quiescent regions where carbon has been annealed to amorphous carbon. Observational consequences of these inferences are briefly described.

Chemical evolution of primitive solar system bodies

NASA Technical Reports Server (NTRS)

Oro, J.; Mills, T.

1989-01-01

Observations on organic molecules and compounds containing biogenic elements in the interstellar medium and in the primitive bodies of the solar system are reviewed. The discovery of phosphorus molecular species in dense interstellar clouds, the existence of organic ions in the dust and gas phase of the comas of Comet Halley, and the presence of presolar, deuterium-hydrogen ratios in the amino acids of carbonaceous chondrites are discussed. The relationships between comets, dark asteroids, and carbonaceous chondrites are examined. Also, consideration is given to the chemical evolution of Titan, the primitive earth, and early Mars.

Calculations concerning the HCO(+)/HOC(+) abundance ratio in dense interstellar clouds

NASA Technical Reports Server (NTRS)

Defrees, D. J.; Mclean, A. D.; Herbst, E.

1984-01-01

Calculations have been performed to determine the rate coefficients of several reactions involved in both the formation and depletion of interstellar HCO(+) and HOC(+). The abundance of HOC(+) deduced from these calculations is consistent with the tentative identification of HOC(+) in Sgr B2 by Woods et al. (1983). The large HCO(+)/HOC(+) abundance ratio observed by Woods et al. is due at least in part to a more rapid formation rate for HCO(+) and probably due as well to a more rapid depletion rate for HOC(+).

Extinct radioactivities - A three-phase mixing model. [for early solar system abundances

NASA Technical Reports Server (NTRS)

Clayton, D. D.

1983-01-01

A new class of models is advanced for interpreting the relationship of radioactive abundances in the early solar system to their average concentration in the interstellar medium. The model assumes that fresh radioactivities are ejected from supernovae into the hot interstellar medium, and that the time scales for changes of phase into molecular clouds determine how much survives for formation therein of the solar system. A more realistic and physically motivated understanding of the low observed concentrations of I-129, Pu-244, and Pd-107 may result.

Prebiotic chemical evolution in the astrophysical context.

PubMed

Ziurys, L M; Adande, G R; Edwards, J L; Schmidt, D R; Halfen, D T; Woolf, N J

2015-06-01

An ever increasing amount of molecular material is being discovered in the interstellar medium, associated with the birth and death of stars and planetary systems. Radio and millimeter-wave astronomical observations, made possible by high-resolution laboratory spectroscopy, uniquely trace the history of gas-phase molecules with biogenic elements. Using a combination of both disciplines, the full extent of the cycling of molecular matter, from circumstellar ejecta of dying stars - objects which expel large amounts of carbon - to nascent solar systems, has been investigated. Such stellar ejecta have been found to exhibit a rich and varied chemical content. Observations demonstrate that this molecular material is passed onto planetary nebulae, the final phase of stellar evolution. Here the star sheds almost its entire original mass, becoming an ultraviolet-emitting white dwarf. Molecules such as H2CO, HCN, HCO(+), and CCH are present in significant concentrations across the entire age span of such nebulae. These data suggest that gas-phase polyatomic, carbon-containing molecules survive the planetary nebula phase and subsequently are transported into the interstellar medium, seeding the chemistry of diffuse and then dense clouds. The extent of the chemical complexity in dense clouds is unknown, hindered by the high spectral line density. Organic species such as acetamide and methyl amine are present in such objects, and NH2CHO has a wide Galactic distribution. However, organophosphorus compounds have not yet been detected in dense clouds. Based on carbon and nitrogen isotope ratios, molecular material from the ISM appears to become incorporated into solar system planetesimals. It is therefore likely that interstellar synthesis influences prebiotic chemistry on planet surfaces.

An IFU-view of Planetary Nebulae: Exploring NGC 6720 (Ring Nebula) with KCWI

NASA Astrophysics Data System (ADS)

Hoadley, Keri; Matuszewski, Matt; Hamden, Erika; Martin, Christopher; Neill, Don; Kyne, Gillian

2018-01-01

Studying the interaction between the ejected stellar material and interstellar clouds is important for understanding how stellar deaths influences the pollution of matter that will later form other stars. Planetary nebulae provide ideal laboratories to study such interactions. I will present on a case study of one close-by planetary nebula, the Ring Nebula (M 57, NGC 6720), to infer the abundances, temperatures, structures, and dynamics of important atomic and ionic species in two distinct regions of the nebula using a newly-commissioned integral field spectrograph (IFS) on Keck: the Keck Cosmic Web Imager (KCWI). The advantage of an IFS over traditional filter-imaging techniques is the ability to simultaneously observe the spectrum of any given pixel in the imaging area, which provides crucial information about the dynamics of the observed region. This technique is powerful for diffuse or extended astrophysical objects, and I will demonstrate the different imaging and spectral modes of KCWI used to observe the Ring Nebula.KCWI observations of the Ring Nebula focused mainly on the innermost region of the nebula, with a little coverage of the Inner Ring. We also observed the length of the Ring in one set of observations, for which we will estimate the elemental abundances, temperatures, and dynamics of the region. KCWI observations also capture an inner arc and blob that have distinctly difference characteristics than the Ring itself and may be a direct observation of either the planetary nebula ramming into an interstellar cloud projected onto the sightline or a dense interstellar cloud being illuminated by the stellar continuum from the hot central white dwarf.

GUSTO: Gal/Xgal U/LDB Spectroscopic-Stratospheric TeraHertz Observatory

NASA Astrophysics Data System (ADS)

Kidd Walker, Christopher; Kulesa, Craig; Goldsmith, Paul; Groppi, Christopher; Helmich, Frank; Hollenbach, David; Kawamura, Jonathan; Langer, William; Melnick, Gary; Neufeld, David; Pineda, Jorge; Stacey, Gordon; Stark, Antony; Tielens, Alexander; Wolfire, Mark; Yorke, Harold; Young, Erick

2018-01-01

GUSTO is a recently selected NASA Explorer mission that will map in unprecedented detail the structure, dynamics, energy balance, and evolution of the interstellar medium within the Milky Way and Large Magellanic Cloud. GUSTO is a balloon-borne, 0.85-m on-axis telescope that will observe in three important interstellar lines: [CII], [OI], and [NII] at 158, 63, and 205 microns, respectively. With its 60" angular resolution, high-velocity resolution, and efficient “On-The-Fly” mapping strategy, GUSTO will address key unanswered questions about the stellar life cycle and provide new insights into the birth and evolution of stars and galaxies. From its Ultra-Long-Duration Balloon (ULDB) platform at an altitude of 33 km, GUSTO will survey ~100 deg2 of the Milky Way and 24 deg2 of the LMC at 60" angular resolution using three 8-pixel heterodyne array receivers. The GUSTO receivers provide sub-km/s velocity resolution and bandwidths sufficiently wide to track all clouds orbiting in the Milky Way and LMC. GUSTO will detect and locate in three dimensions every important interstellar cloud (AV > 0.5–1) in the surveyed regions. The baseline mission of 100 days can be completed in one ULDB Antarctic balloon flight, and an extended mission of up to 169 days is possible. GUSTO’s observing campaign comprises three distinct surveys: GPS: A Galactic Plane Survey (42 days); LMCS: An LMC Survey (36 days); TDS: Targeted Deep Surveys of selected regions in the Galaxy and LMC (18 days). In our presentation we will discuss both the science goals of GUSTO and the mission implementation.

The re-flight of the Colorado high-resolution Echelle stellar spectrograph (CHESS): improvements, calibrations, and post-flight results

NASA Astrophysics Data System (ADS)

Hoadley, Keri; France, Kevin; Kruczek, Nicholas; Fleming, Brian; Nell, Nicholas; Kane, Robert; Swanson, Jack; Green, James; Erickson, Nicholas; Wilson, Jacob

2016-07-01

In this proceeding, we describe the scientific motivation and technical development of the Colorado High- resolution Echelle Stellar Spectrograph (CHESS), focusing on the hardware advancements and testing supporting the second flight of the payload (CHESS-2). CHESS is a far ultraviolet (FUV) rocket-borne instrument designed to study the atomic-to-molecular transitions within translucent cloud regions in the interstellar medium (ISM). CHESS is an objective f/12.4 echelle spectrograph with resolving power > 100,000 over the band pass 1000 - 1600 Å. The spectrograph was designed to employ an R2 echelle grating with "low" line density. We compare the FUV performance of experimental echelle etching processes (lithographically by LightSmyth, Inc. and etching via electron-beam technology by JPL Microdevices Laboratory) with traditional, mechanically-ruled gratings (Bach Research, Inc. and Richardson Gratings). The cross-dispersing grating, developed and ruled by Horiba Jobin-Yvon, is a holographically-ruled, "low" line density, powered optic with a toroidal surface curvature. Both gratings were coated with aluminum and lithium fluoride (Al+LiF) at Goddard Space Flight Center (GSFC). Results from final efficiency and reflectivity measurements for the optical components of CHESS-2 are presented. CHESS-2 utilizes a 40mm-diameter cross-strip anode readout microchannel plate (MCP) detector fabricated by Sensor Sciences, Inc., to achieve high spatial resolution with high count rate capabilities (global rates 1 MHz). We present pre-flight laboratory spectra and calibration results. CHESS-2 launched on 21 February 2016 aboard NASA/CU sounding rocket mission 36.297 UG. We observed the intervening ISM material along the sightline to epsilon Per and present initial characterization of the column densities, temperature, and kinematics of atomic and molecular species in the observation.

The third flight of the Colorado high-resolution echelle stellar spectrograph (CHESS): improvements, calibrations, and preliminary results

NASA Astrophysics Data System (ADS)

Kruczek, Nicholas; Nell, Nicholas; France, Kevin; Hoadley, Keri; Fleming, Brian; Kane, Robert; Ulrich, Stefan; Egan, Arika; Beatty, Dawson

2017-08-01

In this proceeding, we describe the scientific motivation and technical development of the Colorado HighResolution Echelle Stellar Spectrograph (CHESS), focusing on the hardware advancements and testing of components for the third launch of the payload (CHESS-3). CHESS is a far ultraviolet rocket-borne instrument designed to study the atomic-to-molecular transitions within translucent cloud regions in the interstellar medium. CHESS is an objective echelle spectrograph, which uses a mechanically-ruled echelle and a powered (f/12.4) crossdispersing grating, and is designed to achieve a resolving power R > 100,000 over the bandpass λλ 1000-1600 Å. Results from final efficiency and reflectivity measurements for the optical components of CHESS-3 are presented. An important role of sounding rocket experiments is the testing and verification of the space flight capabilities of experimental technologies. CHESS-3 utilizes a 40mm-diameter cross-strip anode microchannel plate detector fabricated by Sensor Sciences LLC, capable of achieving high spatial resolution and a high global count rate (˜1 MHz). We present pre-flight laboratory spectra and calibration results, including wavelength solution and resolving power of the instrument. The fourth launch of CHESS (CHESS-4) will demonstrate a δ-doped CCD, assembled in collaboration with the Microdevices Laboratory at JPL and Arizona State University. In support of CHESS-4, the CHESS-3 payload included a photomultiplier tube, used as a secondary confirmation of the optical alignment of the payload during flight. CHESS-3 launched on 26 June 2017 aboard NASA/CU sounding rocket mission 36.323 UG. We present initial flight results for the CHESS-3 observation of the β1 Scorpii sightline.

Radiative heating of interstellar grains falling toward the solar nebula: 1-D diffusion calculations

NASA Technical Reports Server (NTRS)

Simonelli, D. P.; Pollack, J. B.; McKay, C. P.

1997-01-01

As the dense molecular cloud that was the precursor of our Solar System was collapsing to form a protosun and the surrounding solar-nebula accretion disk, infalling interstellar grains were heated much more effectively by radiation from the forming protosun than by radiation from the disk's accretion shock. Accordingly, we have estimated the temperatures experienced by these infalling grains using radiative diffusion calculations whose sole energy source is radiation from the protosun. Although the calculations are 1-dimensional, they make use of 2-D, cylindrically symmetric models of the density structure of a collapsing, rotating cloud. The temperature calculations also utilize recent models for the composition and radiative properties of interstellar grains (Pollack et al. 1994. Astrophys. J. 421, 615-639), thereby allowing us to estimate which grain species might have survived, intact, to the disk accretion shock and what accretion rates and molecular-cloud rotation rates aid that survival. Not surprisingly, we find that the large uncertainties in the free parameter values allow a wide range of grain-survival results: (1) For physically plausible high accretion rates or low rotation rates (which produce small accretion disks), all of the infalling grain species, even the refractory silicates and iron, will vaporize in the protosun's radiation field before reaching the disk accretion shock. (2) For equally plausible low accretion rates or high rotation rates (which produce large accretion disks), all non-ice species, even volatile organics, will survive intact to the disk accretion shock. These grain-survival conclusions are subject to several limitations which need to be addressed by future, more sophisticated radiative-transfer models. Nevertheless, our results can serve as useful inputs to models of the processing that interstellar grains undergo at the solar nebula's accretion shock, and thus help address the broader question of interstellar inheritance in the solar nebula and present Solar System. These results may also help constrain the size of the accretion disk: for example, if we require that the calculations produce partial survival of organic grains into the solar nebula, we infer that some material entered the disk intact at distances comparable to or greater than a few AU. Intriguingly, this is comparable to the heliocentric distance that separates the C-rich outer parts of the current Solar System from the C-poor inner regions.

Radiative heating of interstellar grains falling toward the solar nebula: 1-D diffusion calculations.

PubMed

Simonelli, D P; Pollack, J B; McKay, C P

1997-02-01

As the dense molecular cloud that was the precursor of our Solar System was collapsing to form a protosun and the surrounding solar-nebula accretion disk, infalling interstellar grains were heated much more effectively by radiation from the forming protosun than by radiation from the disk's accretion shock. Accordingly, we have estimated the temperatures experienced by these infalling grains using radiative diffusion calculations whose sole energy source is radiation from the protosun. Although the calculations are 1-dimensional, they make use of 2-D, cylindrically symmetric models of the density structure of a collapsing, rotating cloud. The temperature calculations also utilize recent models for the composition and radiative properties of interstellar grains (Pollack et al. 1994. Astrophys. J. 421, 615-639), thereby allowing us to estimate which grain species might have survived, intact, to the disk accretion shock and what accretion rates and molecular-cloud rotation rates aid that survival. Not surprisingly, we find that the large uncertainties in the free parameter values allow a wide range of grain-survival results: (1) For physically plausible high accretion rates or low rotation rates (which produce small accretion disks), all of the infalling grain species, even the refractory silicates and iron, will vaporize in the protosun's radiation field before reaching the disk accretion shock. (2) For equally plausible low accretion rates or high rotation rates (which produce large accretion disks), all non-ice species, even volatile organics, will survive intact to the disk accretion shock. These grain-survival conclusions are subject to several limitations which need to be addressed by future, more sophisticated radiative-transfer models. Nevertheless, our results can serve as useful inputs to models of the processing that interstellar grains undergo at the solar nebula's accretion shock, and thus help address the broader question of interstellar inheritance in the solar nebula and present Solar System. These results may also help constrain the size of the accretion disk: for example, if we require that the calculations produce partial survival of organic grains into the solar nebula, we infer that some material entered the disk intact at distances comparable to or greater than a few AU. Intriguingly, this is comparable to the heliocentric distance that separates the C-rich outer parts of the current Solar System from the C-poor inner regions.

THE MID-INFRARED EXTINCTION LAW AND ITS VARIATION IN THE COALSACK NEBULA

DOE Office of Scientific and Technical Information (OSTI.GOV)

Wang Shu; Gao Jian; Jiang, B. W.

In recent years, the wavelength dependence of interstellar extinction from the ultraviolet (UV) to the near- and mid-infrared (IR) has been studied extensively. Although it is well established that the UV/optical extinction law varies significantly among the different lines of sight, it is not clear how IR extinction varies among various environments. In this work, using the color-excess method and taking red giants as the extinction tracer, we determine interstellar extinction A{sub {lambda}} in the four Spitzer/IRAC bands in [3.6], [4.5], [5.8], [8.0] {mu}m (relative to A{sub K{sub s}}, extinction in the Two Micron All Sky Survey (2MASS) K{sub s}more » band at 2.16 {mu}m) of the Coalsack nebula, a nearby starless dark cloud, based on the data obtained from the 2MASS and Spitzer/GLIMPSE surveys. We select five individual regions across the nebula that span a wide variety of physical conditions ranging from diffuse and translucent to dense environments, as traced by the visual extinction, the Spitzer/MIPS 24 {mu}m emission, and CO emission. We find that A{sub {lambda}}/A{sub K{sub s}}, mid-IR extinction relative to A{sub K{sub s}}, decreases from diffuse to dense environments, which may be explained in terms of ineffective dust growth in dense regions. The mean extinction (relative to A{sub K{sub s}}) is calculated for the four IRAC bands as well and exhibits a flat mid-IR extinction law consistent with previous determinations for other regions. Extinction in the IRAC 4.5 {mu}m band is anomalously high, much higher than that of the other three IRAC bands, and cannot be explained in terms of CO and CO{sub 2} ice. Mid-IR extinction in the four IRAC bands has also been derived for four representative regions in the Coalsack Globule 2, which respectively exhibit strong ice absorption, moderate or weak ice absorption, and very weak or no ice absorption. The derived mid-IR extinction curves are all flat, with A{sub {lambda}}/A{sub K{sub s}} increasing with the decrease of the 3.1 {mu}m H{sub 2}O ice absorption optical depth {tau}{sub ice}.« less

Diagnosing the Neutral Interstellar Gas Flow at 1 AU with IBEX-Lo

NASA Astrophysics Data System (ADS)

Möbius, E.; Kucharek, H.; Clark, G.; O'Neill, M.; Petersen, L.; Bzowski, M.; Saul, L.; Wurz, P.; Fuselier, S. A.; Izmodenov, V. V.; McComas, D. J.; Müller, H. R.; Alexashov, D. B.

2009-08-01

Every year in fall and spring the Interstellar Boundary Explorer (IBEX) will observe directly the interstellar gas flow at 1 AU over periods of several months. The IBEX-Lo sensor employs a powerful triple time-of-flight mass spectrometer. It can distinguish and image the O and He flow distributions in the northern fall and spring, making use of sensor viewing perpendicular to the Sun-pointing spin axis. To effectively image the narrow flow distributions IBEX-Lo has a high angular resolution quadrant in its collimator. This quadrant is employed selectively for the interstellar gas flow viewing in the spring by electrostatically shutting off the remainder of the aperture. The operational scenarios, the expected data, and the necessary modeling to extract the interstellar parameters and the conditions in the heliospheric boundary are described. The combination of two key interstellar species will facilitate a direct comparison of the pristine interstellar flow, represented by He, which has not been altered in the heliospheric boundary region, with a flow that is processed in the outer heliosheath, represented by O. The O flow distribution consists of a depleted pristine component and decelerated and heated neutrals. Extracting the latter so-called secondary component of interstellar neutrals will provide quantitative constraints for several important parameters of the heliosheath interaction in current global heliospheric models. Finding the fraction and width of the secondary component yields an independent value for the global filtration factor of species, such as O and H. Thus far filtration can only be inferred, barring observations in the local interstellar cloud proper. The direction of the secondary component will provide independent information on the interstellar magnetic field strength and orientation, which has been inferred from SOHO SWAN Ly- α backscattering observations and the two Voyager crossings of the termination shock.

Multicolor optical polarimetry of reddened stars in the small Magellanic cloud

NASA Technical Reports Server (NTRS)

Magalhaes, Antonio M.; Coyne, G. V.; Piirola, Valero; Rodrigues, C. V.

1989-01-01

First results of an on-going program to determine the wavelength dependence of the interstellar optical polarization of reddened stars in the Small Magellanic Cloud (SMC) are presented. IUE observations of reddened stars in the SMC (Bouchet et al. 1985) generally show marked differences in the extinction law as compared to both the Galaxy and the Large Megallanic Cloud. The aim here is to determine the wavelength dependence of the optical linear polarization in the direction of several such stars in the SMC in order to further constrain the dust composition and size distribution in that galaxy.

Optical Polarization as a Probe of the Local Interstellar Medium

NASA Technical Reports Server (NTRS)

Tinbergen, J.

1984-01-01

The use of interstellar polarization as a tool for measuring interstellar dust is discussed. Problems resulting from dust and magnetic field configurations becoming mixed up are discussed, as is the availability of sufficiently bright stars to obtain the photons needed for precision measurements. It is proposed that: (1) on the scale of several hundred parsec, there is a preferential magnetic field direction, as evidenced by observations at the Galactic poles and selected longitudes in the Galactic plane; (2) the local (r 50 pc) region is devoid of dust, as evidenced by the mean square degree of polarization as a function of distance; and, less certainly, that (3) at a distance of less than 5 pc, there is a patch of dust which may be of interest in connection with cloud models.

IRAS Colors of the Pleiades

NASA Technical Reports Server (NTRS)

Carey, Sean J.; Shipman, R. F.; Clark, F. O.

1996-01-01

We present large scale images of the infrared emission of the region around the Pleiades using the ISSA data product from the IRAS mission. Residual Zodiacal background and a discontinuity in the image due to the scanning strategy of the satellite necessitated special background subtraction methods. The 60/100 color image clearly shows the heating of the ambient interstellar medium by the cluster. The 12/100 and 25/100 images peak on the cluster as expected for exposure of small dust grains to an enhanced UV radiation field; however, the 25/100 color declines to below the average interstellar value at the periphery of the cluster. Potential causes of the color deficit are discussed. A new method of identifying dense molecular material through infrared emission properties is presented. The difference between the 100 micron flux density and the 60 micron flux density scaled by the average interstellar 60/100 color ratio (Delta I(sub 100) is a sensitive diagnostic of material with embedded heating sources (Delta I(sub 100) less than 0) and cold, dense cores (Delta I(sub 100) greater than 0). The dense cores of the Taurus cloud complex as well as Lynds 1457 are clearly identified by this method, while the IR bright but diffuse Pleiades molecular cloud is virtually indistinguishable from the nearby infrared cirrus.

Copernicus studies of interstellar material in the Perseus II complex. III - The line of sight to Zeta Persei

NASA Technical Reports Server (NTRS)

Snow, T. P., Jr.

1977-01-01

Ultraviolet spectrophotometric data obtained with Copernicus are used to analyze the distribution, composition, density, temperature, and kinematics of the interstellar material along the line of sight to Zeta Persei. The far-UV extinction curve for the star is evaluated along with the kinematics of the interstellar gas, observations of atomic and molecular hydrogen, curves of growth for neutral and ionized species, atomic abundances and depletions, ionization equilibria, and observations of CO and OH lines. The results show that there are apparently three clouds along the line of sight to Zeta Persei: a main cloud at approximately +13 km/s which contains most of the material and forms all the neutral and molecular lines as well as most of the ionic lines, a second component at +22 km/s which must contribute to the strong UV lines of most ions, and a third component at roughly +2 km/s which gives rise to a strong Si III line at 1206 A. It is also found that the UV extinction curve has a somewhat steep far-UV rise, indicating the presence of a substantial number of small grains, and that about 30% of the hydrogen nuclei over the entire line of sight are in molecular form.

A relativistic neutron fireball from a supernova explosion as a possible source of chiral influence.

PubMed

Gusev, G A; Saito, T; Tsarev, V A; Uryson, A V

2007-06-01

We elaborate on a previously proposed idea that polarized electrons produced from neutrons, released in a supernova (SN) explosion, can cause chiral dissymmetry of molecules in interstellar gas-dust clouds. A specific physical mechanism of a relativistic neutron fireball with Lorentz factor of the order of 100 is assumed for propelling a great number of free neutrons outside the dense SN shell. A relativistic chiral electron-proton plasma, produced from neutron decays, is slowed down owing to collective effects in the interstellar plasma. As collective effects do not involve the particle spin, the electrons can carry their helicities to the cloud. The estimates show high chiral efficiency of such electrons. In addition to this mechanism, production of circularly polarized ultraviolet photons through polarized-electron bremsstrahlung at an early stage of the fireball evolution is considered. It is shown that these photons can escape from the fireball plasma. However, for an average density of neutrals in the interstellar medium of the order of 0.2 cm(-3) and at distances of the order of 10 pc from the SN, these photons will be absorbed with a factor of about 10(-7) due to the photoeffect. In this case, their chiral efficiency will be about five orders of magnitude less than that for polarized electrons.

Disintegration of Dust Aggregates in Interstellar Shocks and the Lifetime of Dust Grains in the ISM

NASA Technical Reports Server (NTRS)

Dominik, C.; Jones, A. P.; Tielens, A. G. G. M.; Cuzzi, Jeff (Technical Monitor)

1994-01-01

Interstellar grains are destroyed by shock waves moving through the ISM. In fact, the destruction of grains may be so effective that it is difficult to explain the observed abundance of dust in the ISM as a steady state between input of grains from stellar sources and destruction of grains in shocks. This is especially a problem for the larger grains. Therefore, the dust grains must be protected in some way. Jones et al. have already considered coatings and the increased post-shock drag effects for low density grains. In molecular clouds and dense clouds, coagulation of grains is an important process, and the largest interstellar grains may indeed be aggregates of smaller grains rather than homogeneous particles. This may provide a means to protect the larger grains, in that, in moderate velocity grain-grain collisions in a shock the aggregates may disintegrate rather than be vaporized. The released small particles are more resilient to shock destruction (except in fast shocks) and may reform larger grains later, recovering the observed size distribution. We have developed a model for the binding forces in grain aggregates and apply this model to the collisions between an aggregate and fast small grains. We discuss the results in the light of statistical collision probabilities and grain life times.

Fragmentation of interstellar clouds and star formation

NASA Technical Reports Server (NTRS)

Silk, J.

1982-01-01

The principal issues are addressed: the fragmentation of molecular clouds into units of stellar mass and the impact of star formation on molecular clouds. The observational evidence for fragmentation is summarized, and the gravitational instability described of a uniform spherical cloud collapsing from rest. The implications are considered of a finite pressure for the minimum fragment mass that is attainable in opacity-limited fragmentation. The role of magnetic fields is discussed in resolving the angular momentum problem and in making the collapse anisotropic, with notable consequences for fragmentation theory. Interactions between fragments are described, with emphasis on the effect of protostellar winds on the ambient cloud matter and on inhibiting further star formation. Such interactions are likely to have profound consequences for regulating the rate of star formation and on the energetics and dynamics of molecular clouds.

Observation of interstellar lithium in the low-metallicity Small Magellanic Cloud.

PubMed

Howk, J Christopher; Lehner, Nicolas; Fields, Brian D; Mathews, Grant J

2012-09-06

The primordial abundances of light elements produced in the standard theory of Big Bang nucleosynthesis (BBN) depend only on the cosmic ratio of baryons to photons, a quantity inferred from observations of the microwave background. The predicted primordial (7)Li abundance is four times that measured in the atmospheres of Galactic halo stars. This discrepancy could be caused by modification of surface lithium abundances during the stars' lifetimes or by physics beyond the Standard Model that affects early nucleosynthesis. The lithium abundance of low-metallicity gas provides an alternative constraint on the primordial abundance and cosmic evolution of lithium that is not susceptible to the in situ modifications that may affect stellar atmospheres. Here we report observations of interstellar (7)Li in the low-metallicity gas of the Small Magellanic Cloud, a nearby galaxy with a quarter the Sun's metallicity. The present-day (7)Li abundance of the Small Magellanic Cloud is nearly equal to the BBN predictions, severely constraining the amount of possible subsequent enrichment of the gas by stellar and cosmic-ray nucleosynthesis. Our measurements can be reconciled with standard BBN with an extremely fine-tuned depletion of stellar Li with metallicity. They are also consistent with non-standard BBN.

The Interstellar Medium

NASA Technical Reports Server (NTRS)

Tielens, Alexander G. G. M.

1995-01-01

The Interstellar Medium (ISM) forms an integral part of the lifecycle of stars and the galaxy. Stars are formed by gravitational contraction of interstellar clouds. Over their life, stars return much of their mass to the ISM through winds and supernova explosions, resulting in a slow enrichment in heavy elements. Understanding the origin and evolution of the ISM is a key problem within astrophysics. The KAO has made many important contributions to studies of the interstellar medium both on the macro and on the micro scale. In this overview, I will concentrate on two breakthroughs in the last decade in which KAO observations have played a major role: (1) the importance of large Polycyclic Aromatic Hydrocarbon (PAH) molecules for the ISM (section 3) and (2) the study of Photodissociation Regions (PDRs) as an analog for the diffuse ISM at large (section 4). Appropriately, the micro and macro problem are intricately interwoven in these problems. Finally, section 5 reviews the origin of the (CII) emission observed by COBE.

Extreme ultraviolet observations of G191-B2B and the local interstellar medium with the Hopkins Ultraviolet Telescope

NASA Technical Reports Server (NTRS)

Kimble, Randy A.; Davidsen, Arthur F.; Blair, William P.; Bowers, Charles W.; Van Dyke Dixon, W.; Durrance, Samuel T.; Feldman, Paul D.; Ferguson, Henry C.; Henry, Richard C.; Kriss, Gerard A.

1993-01-01

During the Astro-l mission in 1990 December, the Hopkins Ultraviolet Telescope (HUT) was used to observe the extreme ultraviolet spectrum (415-912 A) of the hot DA white dwarf GI91-B2B. Absorption by neutral helium shortward of the 504 A He I absorption edge is clearly detected in the raw spectrum. Model fits to the observed spectrum require interstellar neutral helium and neutral hydrogen column densities of 1.45 +/- 0.065 x 10 exp 17/sq cm and 1.69 +/- 0.12 x 10 exp 18/sq cm, respectively. Comparison of the neutral columns yields a direct assessment of the ionization state of the local interstellar cloud surrounding the Sun. The neutral hydrogen to helium ratio of 11.6 +/- 1.0 observed by HUT strongly contradicts the widespread view that hydrogen is much more ionized than helium in the local interstellar medium, a view which has motivated some exotic theoretical explanations for the supposed high ionization.

Self-Assembling Amphiphilic Molecules: A Possible Relationship Between Interstellar Chemistry and Meteoritic Organics

NASA Technical Reports Server (NTRS)

Sandford, Scott A.; Dworkin, Jason P.; Deamer, David W.; Allamandola, Louis J.; DeVincenzi, Donald (Technical Monitor)

2001-01-01

Interstellar gas and dust comprise the primary material from which the solar system formed. Evidence that some of this material was organic in nature and survived incorporation into the protosolar nebula is provided by the presence of deuterium-enriched organics in meteorites and interplanetary dust particles. Once the inner planets had sufficiently cooled, late accretionary infall of meteoroids and cosmic dust must have seeded them with some of these complex organic compounds. Delivery of such extraterrestrial compounds may have contributed to the organic inventory necessary for the origin of life. Interstellar ices, the building blocks of comets, tie up a large fraction of the biogenic elements available in molecular clouds. In our efforts to understand their synthesis, chemical composition, and physical properties, we report here that a complex mixture of molecules is produced by ultraviolet (UV) photolysis of realistic, interstellar ice analogs, and that some of the components have properties relevant to the origin of life, including the ability to self-assemble into vesicular structures.

TIME VARIATION OF AV AND RV FOR TYPE Ia SUPERNOVAE BEHIND INTERSTELLAR DUST

NASA Astrophysics Data System (ADS)

Huang, Xiaosheng; Biederman, M.; Herger, B.; Aldering, G. S.

2014-01-01

TIME VARIATION OF AV AND RV FOR TYPE Ia SUPERNOVAE BEHIND NON-UNIFORM INTERSTELLAR DUST ABSTRACT We investigate the time variation of the visual extinction, AV, and the total-to-selective extinction ratio, RV, resulting from interstellar dust in front of an expanding photospheric disk of a type Ia supernova (SN Ia). We simulate interstellar dust clouds according to a power law power spectrum and produce extinction maps that either follow a pseudo-Gaussian distribution or a lognormal distribution. The RV maps are produced through a correlation between AV and RV. With maps of AV and RV generated in each case (pseudo-Gaussian and lognormal), we then compute the effective AV and RV for a SN as its photospheric disk expands behind the dust screen. We find for a small percentage of SNe the AV and RV values can vary by a large factor from day to day in the first 40 days after explosion.

Ice Chemistry in Starless Molecular Cores

NASA Astrophysics Data System (ADS)

Kalvāns, J.

2015-06-01

Starless molecular cores are natural laboratories for interstellar molecular chemistry research. The chemistry of ices in such objects was investigated with a three-phase (gas, surface, and mantle) model. We considered the center part of five starless cores, with their physical conditions derived from observations. The ice chemistry of oxygen, nitrogen, sulfur, and complex organic molecules (COMs) was analyzed. We found that an ice-depth dimension, measured, e.g., in monolayers, is essential for modeling of chemistry in interstellar ices. Particularly, the H2O:CO:CO2:N2:NH3 ice abundance ratio regulates the production and destruction of minor species. It is suggested that photodesorption during the core-collapse period is responsible for the high abundance of interstellar H2O2 and O2H and other species synthesized on the surface. The calculated abundances of COMs in ice were compared to observed gas-phase values. Smaller activation barriers for CO and H2CO hydrogenation may help explain the production of a number of COMs. The observed abundance of methyl formate HCOOCH3 could be reproduced with a 1 kyr, 20 K temperature spike. Possible desorption mechanisms, relevant for COMs, are gas turbulence (ice exposure to interstellar photons) or a weak shock within the cloud core (grain collisions). To reproduce the observed COM abundances with the present 0D model, 1%-10% of ice mass needs to be sublimated. We estimate that the lifetime for starless cores likely does not exceed 1 Myr. Taurus cores are likely to be younger than their counterparts in most other clouds.

ARC-1983-AC83-0768-2

NASA Image and Video Library

1983-11-09

Milky way - The real shape of our galaxy is revealed in this infrared image obtained by IRAS. Infrared light penetrates the dust clouds and shows that the galaxy appears as a thin disk, just like the edge-on spiral galaxies we see throughtout the cosmos. The bulge in the band is the center of the galaxy. The yellow and green knots and blobs scattered along the band are giant clouds of interstellar gas and dust heated by nearby stars.

Cyanide and isocyanide abundances in the cold, dark cloud TMC-1

NASA Technical Reports Server (NTRS)

Irvine, W. M.; Schloerb, F. P.

1984-01-01

Cold, dark molecular clouds are particularly useful for the study of interstellar chemistry because their physical parameters are better understood than those of heterogeneous, complex giant molecular clouds. Another advantage is their relatively small distance from the solar system. The present investigaation has the objective to provide accurate abundance ratios for several cyanides and isocyanides in order to constrain models of dark cloud chemistry. The relative abundances of such related species can be particularly useful for the study of chemical processes. The cloud TMC-1 considered in the current study has a remarkably high abundance of acetylene and polyacetylene derivatives. Data at 3 mm, corresponding to the J = 1 to 0 transitions of HCN, H(C-13)N, HN(C-13), HC(N-15), and H(N-15)C were obtained.

Identification Code of Interstellar Cloud within IRAF

NASA Astrophysics Data System (ADS)

Lee, Youngung; Jung, Jae Hoon; Kim, Hyun-Goo

1997-12-01

We present a code which identifies individual clouds in crowded region using IMFORT interface within Image Reduction and Analysis Facility(IRAF). We define a cloud as an object composed of all pixels in longitude, latitude, and velocity that are simply connected and that lie above some threshold temperature. The code searches the whole pixels of the data cube in efficient way to isolate individual clouds. Along with identification of clouds it is designed to estimate their mean values of longitudes, latitudes, and velocities. In addition, a function of generating individual images(or cube data) of identified clouds is added up. We also present identified individual clouds using a 12CO survey data cube of Galactic Anticenter Region(Lee et al. 1997) as a test example. We used a threshold temperature of 5 sigma rms noise level of the data. With a higher threshold temperature, we isolated subclouds of a huge cloud identified originally. As the most important parameter to identify clouds is the threshold value, its effect to the size and velocity dispersion is discussed rigorously.

THE SIZE AND EXTENT OF THE INTERSTELLAR GAS CLOUD SURROUNDING THE SUN

NASA Technical Reports Server (NTRS)

2002-01-01

Two astronomical observatories in orbit around the Earth, the Hubble Space Telescope and the Extreme Ultraviolet Explorer, have been used to investigate the structure of the interstellar gas cloud in which the Sun resides. A compilation and interpretation of the results of these studies is being presented today at the American Astronomical Society meeting in Madison, WI by Drs. Jeffrey L. Linsky, Nikolai Piskunov, and Brian E. Wood of JILA and the University of Colorado. Measuring the properties of the local interstellar medium (LISM) is important for understanding how the LISM interacts with the solar wind and for investigating the possibility that changes in the LISM in the immediate vicinity of the Sun could have an effect on the Earth's atmosphere and climate. When astronomers observe the ultraviolet light emitted by nearby stars, it is always found that some of the light is absorbed by atoms and ions in the LISM. The properties of the LISM can therefore be investigated by measuring the amount of absorption observed. Using ultraviolet spectra taken by the Hubble Space Telescope and the Extreme Ultraviolet Explorer, the total amount of local interstellar matter has been measured for many lines of sight through the LISM. These studies suggest that the interstellar gas surrounding the Sun has a temperature of about 7000 K (about 12,000 degrees Fahrenheit) and has a velocity of about 16 miles per second relative to the Sun. Measurements made for very short (less than 10 light years) lines of sight toward very nearby stars suggest an average density of about 0.1 hydrogen atoms per cubic centimeter for the local interstellar gas, hydrogen being by far the most abundant atom in the LISM. Measurements made for lines of sight toward more distant stars suggest lower average densities. This suggests that parts of the lines of sight to these more distant stars are in regions of space with extremely low densities. We believe this means that these stars lie outside the local gas cloud. We can estimate a distance from the Sun to the edge of the local cloud for each line of sight to a star that lies outside the cloud. Combining these measurements, we can then crudely map out the shape of the local cloud. The results are shown as orange ellipsoids in the accompanying illustration. According to this model, the cloud is about 60 light years across along its largest axis, and the Sun is only about 4 light years from the edge. Note that we have very little data toward the Galactic Center. Other data not presented here suggest that the cloud actually extends further in that direction than our crude model suggests. The space outside the local gas cloud is not completely empty, but is instead believed to be occupied by extremely hot (about 1,000,000 K) gas with very low densities (about 0.001 particles per cubic centimeter). The local cloud is in fact thought to reside in a region of space called the 'Local Bubble', in which most of the volume of the bubble consists of this very hot gas. The Local Bubble, which is roughly 300 light years in diameter, may have been created by a supernova explosion. The gas cloud surrounding the Sun is not completely uniform. Different parts of the cloud, which we can perhaps call 'cloudlets', are moving at slightly different speeds and in slightly different directions. The purple ellipsoids in the accompanying illustration indicate our estimate for the size and shape of the cloudlet in which the Sun resides. The crudeness of these models is indicated by the fact that part of the purple ellipsoid actually lies outside the yellow ellipsoid. The purple cloudlet in the illustration is about 30 light years across along its largest axis and the Sun is only about 0.1 light years from the edge of the cloudlet. Furthermore, the Sun's velocity relative to this cloudlet should carry the Sun outside the cloudlet in a few thousand years. Another example of the inhomogeneity of the local cloud is that we have found that the abundance of magnesium varies greatly within the cloud. For one line of sight we find that the abundance of magnesium is consistent with the magnesium abundance found in the Sun (about 39 magnesium atoms per million hydrogen atoms), but for other lines of sight the magnesium abundance is as much as 40 times lower than the solar abundance. For these lines of sight, perhaps most of the magnesium is locked up in dust grains and is therefore undetectable. This work is supported by grants from NASA. For further information, contact: Dr. Jeffrey L. Linsky (303) 492-7838 jlinsky@jila.colorado.edu Dr. Nikolai Piskunov +46-18-51-4490 piskunov@astro.uu.se Brian E. Wood (303) 492-5122 woodb@marmot.colorado.edu FIGURE CAPTION: A model of the gas cloud surrounding the Sun (orange ellipsoid), as seen from three different viewpoints: from the Galactic Center (GC), from the North Galactic Pole (NGP), and from a Galactic longitude of 270 degrees. This model is based on estimates of the location of the edge of the cloud (blue asterisks) made for 15 lines-of-sight to nearby stars. The yellow asterisks mark the locations of 3 very nearby stars that we believe lie inside the local cloud. The Sun's location is indicated by a small black circle. The names of the 18 stars used to probe the shape of the local cloud are given in the figure. The sizes of the asterisks indicate their distances from the point-of-view, with the larger asterisks being in the foreground and the smaller asterisks being in the background. Different parts of the cloud, which we can perhaps call 'cloudlets', are moving at slightly different speeds and in slightly different directions. The purple ellipsoid indicates our estimate for the size and shape of the cloudlet in which the Sun resides.

The Exobiological Role of Interstellar Polycyclic Aromatic Hydrocarbons and Ices

NASA Technical Reports Server (NTRS)

Hudgins, Douglas M.; DeVincenzi, Donald (Technical Monitor)

2002-01-01

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. Before this time, the composition of interstellar dust was largely guessed-at, the presence of ices in interstellar clouds ignored, and the notion that large, gas phase, carbon rich molecules might be abundant and widespread throughout the interstellar medium (ISM) considered impossible. Today, the composition of dust in the 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. Shrouded within the protective confines of cold, opaque molecular clouds--the birthplace of stars and planets--these cold dust particles secrete mantles of mixed molecular ices whose compositions are also well constrained. Finally, amidst the molecular inventory of these ice mantles are likely to be found polycyclic aromatic hydrocarbons (PAHs), shockingly large molecules by the standards of interstellar chemistry, the telltale infrared spectral signature of which is now recognized throughout the Universe. In the first part of this talk, we will review the spectroscopic evidence that forms the basis for the currently accepted abundance and ubiquity of PANs in the ISM. We will then look at a few specific examples which illustrate how experimental and theoretical data can be applied to interpret the interstellar spectra and track how the PAN population evolves as it passes from its formation site in the circumstellar outflows of dying stars, through the various phases of the ISM, and into forniing planetary systems. Nevertheless, despite the fact that PANs likely represent the single largest molecular reservoir of organic carbon in evolving planetary systems, they are not what would be considered "biogenic" molecules. Although interesting from a chemical and astrophysical standpoint, in the absence of a mechanism by which this population can be dislodged from the precipitous thermodynamic well afforded by their extensive aromatic networks, they are of little Astrobiological significance. Consequently, for the remainder of the talk, we will consider the photochemical evolution of PANS under conditions similar to those found in the ISM and in proto-planetary systems with an eye toward means by which this rich repository of pre-biotic organic "ore" might be converted into materials of greater importance to Astrobiology.

Deuterium and the Local Interstellar Medium: Properties for the Procyon and Capella Lines of Sight

NASA Technical Reports Server (NTRS)

Linsky, Jeffrey L.; Diplas, Athanassios; Wood, Brian E.; Brown, Alexander; Ayres, Thomas R.; Savage, Blair D.

1995-01-01

We present Goddard High-Resolution Spectrograph observations of the interstellar H I and D I Ly-alpha lines and the Mg II and Fe II resonance lines formed along the lines of sight toward the nearby stars Procyon (3.5 pc, l = 214 deg, b = 13 deg) and Capella (12.5 pc, l = 163 deg, b = 5 deg). New observations of Capella were obtained at orbital phase 0.80, when the radial velocities of the intrinsic Ly-alpha emission lines of each star were nearly reversed from those of the previous observations at phase 0.26. Since the intrinsic Ly-alpha line of the Capella system (the 'continuum' against which the interstellar absorption is measured) has different shapes at phases 0.26 and 0.80, we can derive both the intrinsic stellar profiles and the interstellar absorption lines more precisely by jointly analyzing the two data sets. For the analysis of the Procyon line of sight, we first assumed that the intrinsic Ly-alpha line profile is a broadened solar profile, but this assumption does not lead to a good fit to the observed D I line profile for any value of D/H. We then assumed that (D/H)(sub LISM) = 1.6 x 10(exp -5), the same value as for the Capella line of sight, and we modified the broadened solar profile to achieve agreement between the simulated and observed line profiles. The resulting asymmetric intrinsic stellar line profile is consistent with the shapes of the scaled Mg II line profiles. We believe therefore that the Procyon data are consistent with (D/H)(sub LISM) = 1.6 x 10(exp -5), but the uncertainty in the intrinsic Ly-alpha emission-line profile does not permit us to conclude that the D/H ratio is constant in the local interstellar medium (LISM). The temperature and turbulence in the Procyon line of sight are T = 6900 +/- 80 (+/- 300 systematic error) K and zeta = 1.21 +/- 0.27 km/s. These properties are similar to those of Capella, except that the gas toward Procyon is divided into two velocity components separated by 2.6 km/s and the Procyon line of sight has a mean neutral hydrogen density that is a factor of 2.4 larger than that of the Capella line of sight. This suggests that the first 5.3 pc along the Capella line of sight lies within the local cloud and the remaining 7.2 pc lies in the hot gas surrounding the local cloud. We propose that n(H I) = 0.1065 +/- 0.0028 cm(exp -2) be adopted for the neutral hydrogen density within the local cloud and that zeta = 1.21 +/- 0.27 km/s be adopted for the nonthermal motions. The existence of different second velocity components toward the nearby stars Procyon and Sirius provides the first glimpse of a turbulent cloudlet boundary layer between the local cloud and the surrounding hot interstellar gas.

Dark clouds in the vicinity of the emission nebula Sh2-205: interstellar extinction and distances

NASA Astrophysics Data System (ADS)

Straižys, V.; Čepas, V.; Boyle, R. P.; Zdanavičius, J.; Maskoliūnas, M.; Kazlauskas, A.; Zdanavičius, K.; Černis, K.

2016-05-01

Results of CCD photometry in the seven-colour Vilnius system for 922 stars down to V = 16-17 mag and for 302 stars down to 19.5 mag are used to investigate the interstellar extinction in an area of 1.5 square degrees in the direction of the P7 and P8 clumps of the dark cloud TGU H942, which lies in the vicinity of the emission nebula Sh2-205. In addition, we used 662 red clump giants that were identified by combining the 2MASS and WISE infrared surveys. The resulting plots of extinction versus distance were compared with previous results of the distribution and radial velocities of CO clouds and with dust maps in different passbands of the IRAS and WISE orbiting observatories. A possible distance of the front edge of the nearest cloud layer at 130 ± 10 pc was found. This dust layer probably covers all the investigated area, which results in extinction of up to 1.8 mag in some directions. A second rise of the extinction seems to be present at 500-600 pc. Within this layer, the clumps P7 and P8 of the dust cloud TGU H942, the Sh2-205 emission nebula, and the infrared cluster FSR 655 are probably located. In the direction of these clouds, we identified 88 young stellar objects and a new infrared cluster. Full Tables 1 and 2 are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/590/A21

Extinction and Star Formation Study in Molecular Clouds with DENIS infrared data and USNO optical data

NASA Astrophysics Data System (ADS)

Cambrésy, Laurent

1999-11-01

This thesis consists in a study of molecular clouds, essentially of the point of view of the interstellar environment, but also of the one of the star formation. The original method to estimate extinction presented here is based on adaptive star counts as well as on a wavelet decomposition. For the first time, an extinction map of the whole sky is proposed (USNO-PMM optical data). Access to very large field maps offers the opportunity to analyze the interstellar matter distribution in various environments. A first result is that the contained mass in regions for which AV > 1 would not exceed half of the total cloud mass. Using DENIS data, it becomes possible to probe dense regions of clouds. For instance, star counts in the Chamaeleon complex show cores which were not resolved before. Moreover, the selection of stars with a strong infrared excess yields about fifty T Tauri candidates. From their luminosity function, I derived the average lifetime of circumstellar disc of low--mass stars: ~4cdot 106 years. It is difficult to understand the relation between extinction and molecular emission, but it appears clearly that molecular emission is a bad estimator of the column density for low extinction area. Actually, thresholds exist in the CO detection and I conclude that photodissociation, density and cloud geometry have important consequences on the CO emission when AV < 2. Investigation of the relation between extinction and far--infrared emission in Polaris leads to a four times larger emissivity in cold areas than in hot areas. This result explains the low temperatures in this cloud and implies severe restrictions concerning the use of far--infrared fluxes as an extinction estimator.

Efficient radiative transfer methods for continuum and line transfer in large three-dimensional models

NASA Astrophysics Data System (ADS)

Juvela, Mika J.

The relationship between physical conditions of an interstellar cloud and the observed radiation is defined by the radiative transfer problem. Radiative transfer calculations are needed if, e.g., one wants to disentangle abundance variations from excitation effects or wants to model variations of dust properties inside an interstellar cloud. New observational facilities (e.g., ALMA and Herschel) will bring improved accuracy both in terms of intensity and spatial resolution. This will enable detailed studies of the densest sub-structures of interstellar clouds and star forming regions. Such observations must be interpreted with accurate radiative transfer methods and realistic source models. In many cases this will mean modelling in three dimensions. High optical depths and observed wide range of linear scales are, however, challenging for radiative transfer modelling. A large range of linear scales can be accessed only with hierarchical models. Figure 1 shows an example of the use of a hierarchical grid for radiative transfer calculations when the original model cloud (L=10 pc, =500 cm-3) was based a MHD simulation carried out on a regular grid (Juvela & Padoan, 2005). For computed line intensities an accuracy of 10% was still reached when the number of individual cells (and the run time) was reduced by a factor of ten. This illustrates how, as long as cloud is not extremely optically thick, most of the emission comes from a small sub-volume. It is also worth noting that while errors are ~10% for any given point they are much smaller when compared with intensity variations. In particular, calculations on hierarchical grid recovered the spatial power spectrum of line emission with very good accuracy. Monte Carlo codes are used widely in both continuum and line transfer calculations. Like any lambda iteration schemes these suffer from slow convergence when models are optically thick. In line transfer Accelerated Monte Carlo methods (AMC) present a partial solution to this problem (Juvela & Padoan, 2000; Hogerheijde & van der Tak, 2000). AMC methods can be used similarly in continuum calculations to speed up the computation of dust temperatures (Juvela, 2005). The sampling problems associated with high optical depths can be solved with weighted sampling and the handling of models with τV ~ 1000 is perfectly feasible. Transiently heated small dust grains pose another problem because the calculation of their temperature distribution is very time consuming. However, a 3D model will contain thousands of cells at very similar conditions. If dust temperature distributions are calculated only once for such a set an approximate solution can be found in a much shorter time time. (Juvela & Padoan, 2003; see Figure 2a). MHD simulations with Automatic Mesh Refinement (AMR) techniques present an exciting development for the modelling of interstellar clouds. Cloud models consist of a hierarchy of grids with different grid steps and the ratio between the cloud size and the smallest resolution elements can be 106 or even larger. We are currently working on radiative transfer codes (line and continuum) that could be used efficiently on such grids (see Figure 2b). The radiative transfer problem can be solved relatively independently on each of the sub-grids. This means that the use of convergence acceleration methods can be limited to those sub-grids where they are needed and, on the other hand, parallelization of the code is straightforward.

Radiofrequency recombination lines as diagnostics of the cool interstellar medium.

NASA Technical Reports Server (NTRS)

Dupree, A. K.

1971-01-01

Quantitative details are given of a new diagnostic technique for the carbon and hydrogen (H I) recombination lines. Theoretical results are presented for conditions expected in H I clouds, and are compared with available observations for Orion A and NGC 2024.

Extraterrestrial organic matter: a review

NASA Technical Reports Server (NTRS)

Irvine, W. M.

1998-01-01

We review the nature of the widespread organic material present in the Milky Way Galaxy and in the Solar System. Attention is given to the links between these environments and between primitive Solar System objects and the early Earth, indicating the preservation of organic material as an interstellar cloud collapsed to form the Solar System and as the Earth accreted such material from asteroids, comets and interplanetary dust particles. In the interstellar medium of the Milky Way Galaxy more than 100 molecular species, the bulk of them organic, have been securely identified, primarily through spectroscopy at the highest radio frequencies. There is considerable evidence for significantly heavier organic molecules, particularly polycyclic aromatics, although precise identification of individual species has not yet been obtained. The so-called diffuse interstellar bands are probably important in this context. The low temperature kinetics in interstellar clouds leads to very large isotopic fractionation, particularly for hydrogen, and this signature is present in organic components preserved in carbonaceous chondritic meteorites. Outer belt asteroids are the probable parent bodies of the carbonaceous chondrites, which may contain as much as 5% organic material, including a rich variety of amino acids, purines, pyrimidines, and other species of potential prebiotic interest. Richer in volatiles and hence less thermally processed are the comets, whose organic matter is abundant and poorly characterized. Cometary volatiles, observed after sublimation into the coma, include many species also present in the interstellar medium. There is evidence that most of the Earth's volatiles may have been supplied by a 'late' bombardment of comets and carbonaceous meteorites, scattered into the inner Solar System following the formation of the giant planets. How much in the way of intact organic molecules of potential prebiotic interest survived delivery to the Earth has become an increasingly debated topic over the last several years. The principal source for such intact organics was probably accretion of interplanetary dust particles of cometary origin.

A Rigorous Attempt to Verify Interstellar Glycine

NASA Technical Reports Server (NTRS)

Snyder, L. E.; Lovas, F. J.; Hollis, J. M.; Friedel, D. N.; Jewell, P. R.; Remijan, A.; Ilyushin, V. V.; Alekseev, E. A.; Dyubko, S. F.

2004-01-01

In 2003, Kuan, Charnley, and co-workers reported the detection of interstellar glycine (NH2CH2COOH) based on observations of 27 lines in 19 different spectral bands in one or more of the sources Sgr BP(N-LMH), Orion KL, and W51 e1/e2. They supported their detection report with rotational temperature diagrams for all three sources. In this paper, we present essential criteria which can be used in a straightforward analysis technique to confirm the identity of an interstellar asymmetric rotor such as glycine. We use new laboratory measurements of glycine as a basis for applying this analysis technique, both to our previously unpublished 12 m telescope data and to the previously published SEST data of Nummelin and colleagues. We conclude that key lines necessary for an interstellar glycine identification have not yet been found. We identify several common molecular candidates that should be examined further as more likely carriers of the lines reported as glycine. Finally, we illustrate that rotational temperature diagrams used without the support of correct spectroscopic assignments are not a reliable tool for the identification of interstellar molecules. Subject headings: ISM: abundances - ISM: clouds - ISM: individual (Sagittarius B2[N-

The interstellar abundances of tin and four other heavy elements

NASA Technical Reports Server (NTRS)

Hobbs, L. M.; Welty, D. E.; Morton, D. C.; Spitzer, L.; York, D. G.

1993-01-01

Spectra recorded at 1150-1600 A with an instrumental resolution near 16 km/s were obtained with the Goddard High-Resolution Spectrograph on board the HST. The gaseous interstellar abundances of five heavy elements along the light paths to 23 Ori, 15 Mon, 1 Sco, Pi Sco, and Pi Aqr were determined from the observations. The 1400.450 A line of Sn II was detected and identified toward three stars; at Z = 50, tin is the first element from the fifth row of the periodic table to be identified in the interstellar medium. One spectral line of each of Cu II (Z = 29) and Ga II (Z = 31), three lines of Ge II (Z = 32), and two lines of Kr I (Z = 36) were also detected toward some or all of the five stars. The depletions of these five heavy elements generally decrease monotonically with increasing atomic number toward each of the six stars, and tin is generally undepleted within the observational errors. The depletions of 26 elements from the interstellar gas in an average dense interstellar cloud appear to correlate with the elemental 'nebular' condensation temperatures more closely than with the first ionization potentials.

MIRIS observation of near-infrared diffuse Galactic light

NASA Astrophysics Data System (ADS)

Onishi, Yosuke; Sano, Kei; Matsuura, Shuji; Jeong, Woong-Seob; Pyo, Jeonghyun; Kim, Il-Jong; Seo, Hyun Jong; Han, Wonyong; Lee, DaeHee; Moon, Bongkon; Park, Wonkee; Park, Younsik; Kim, MinGyu; Matsumoto, Toshio; Matsuhara, Hideo; Nakagawa, Takao; Tsumura, Kohji; Shirahata, Mai; Arai, Toshiaki; Ienaka, Nobuyuki

2018-06-01

We report near-infrared (IR) observations of high Galactic latitude clouds to investigate diffuse Galactic light (DGL), which is starlight scattered by interstellar dust grains. The observations were performed at 1.1 and 1.6 μm with a wide-field camera instrument, the Multi-purpose Infra-Red Imaging System (MIRIS) onboard the Korean satellite STSAT-3. The DGL brightness is measured by correlating the near-IR images with a far-IR 100 μm map of interstellar dust thermal emission. The wide-field observation of DGL provides the most accurate DGL measurement achieved to-date. We also find a linear correlation between optical and near-IR DGL in the MBM32 field. To study interstellar dust properties in MBM32, we adopt recent dust models with and without μm-sized very large grains and predict the DGL spectra, taking into account the reddening effect of the interstellar radiation field. The result shows that the observed color of the near-IR DGL is closer to the model spectra without very large grains. This may imply that dust growth in the observed MBM32 field is not active owing to the low density of its interstellar medium.

Translucency changes of direct esthetic restorative materials after curing, aging and treatment

PubMed Central

2016-01-01

The purpose of this article was to review the changes in translucency of direct esthetic restorative materials after curing, aging and treatment. As a criterion for the evaluation of clinical translucency changes, visual perceptibility threshold in translucency parameter difference (ΔTP) of 2 was used. Translucency changes after curing were perceivable depending on experimental methods and products (largest ΔTP in resin composites = 15.9). Translucency changes after aging were reported as either relatively stable or showed perceivable changes by aging protocols (largest ΔTP in resin composites = -3.8). Translucency changes after curing, aging and treatment were perceivable in several products and experimental methods. Therefore, shade matching of direct esthetic materials should be performed considering these instabilities of translucency in direct esthetic materials. PMID:27847744

Organic compounds in the Murchison meteorite.

NASA Technical Reports Server (NTRS)

Ponnamperuma, C.

1972-01-01

Impressive supporting evidence for the concept of the chemical evolution of life has appeared in the discovery of biologically important compounds in extraterrestrial samples. The approaches pursued to detect extraterrestrial organic compounds include the study of interstellar space by radioastronomy, the evaluation of the Apollo lunar samples, and the analysis of meteorites, both ancient and recent. It has been found that the clouds of gas in the interstellar medium contain a wide variety of molecules, most of which are organic in nature. The carbonaceous chondrites contain polymeric organic matter. Amino acids have been detected in the Murchison meteorite.

The Origin of Cosmic Rays: What can GLAST Say?

NASA Technical Reports Server (NTRS)

Ormes, Jonathan F.; Digel, Seith; Moskalenko, Igor V.; Moiseev, Alexander; Williamson, Roger

2000-01-01

Gamma rays in the band from 30 MeV to 300 GeV, used in combination with direct measurements and with data from radio and X-ray bands, provide a powerful tool for studying the origin of Galactic cosmic rays. Gamma-ray Large Area Space Telescope (GLAST) with its fine 10-20 arcmin angular resolution will be able to map the sites of acceleration of cosmic rays and their interactions with interstellar matter, It will provide information that is necessary to study the acceleration of energetic particles in supernova shocks, their transport in the interstellar medium and penetration into molecular clouds.

Sulfur and Hydrogen Isotope Anomalies in Meteorite Sulfonic Acids

NASA Technical Reports Server (NTRS)

Cooper, George W.; Thiemens, Mark H.; Jackson, Teresa L.; Chang, Sherwood

1997-01-01

Intramolecular carbon, hydrogen, and sulfur isotope ratios were measured on a homologous series of organic sulfonic acids discovered in the Murchison meteorite. Mass-independent sulfur isotope fractionations were observed along with high deuterium/hydrogen ratios. The deuterium enrichments indicate formation of the hydrocarbon portion of these compounds in a low-temperature environment that is consistent with that of interstellar clouds. Sulfur-33 enrichments observed in methanesulfonic acid could have resulted from gas-phase ultraviolet irradiation of a precursor, carbon disulfide. The source of the sulfonic acid precursors may have been the reactive interstellar molecule carbon monosulfide.

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

NASA Technical Reports Server (NTRS)

Grasdalen, Gary L.; Sloan, Gregory; Stout, Natalie; Strom, Stephen E.; Welty, Alan D.

1989-01-01

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.

Cosmic-ray ionisation of dense molecular clouds

NASA Astrophysics Data System (ADS)

Vaupre, Solenn

2015-07-01

Cosmic rays (CR) are of tremendous importance in the dynamical and chemical evolution of interstellar molecular clouds, where stars and planets form. CRs are likely accelerated in the shells of supernova remnants (SNR), thus molecular clouds nearby can be irradiated by intense fluxes of CRs. CR protons have two major effects on dense molecular clouds: 1) when they encounter the dense medium, high-energy protons (>280 MeV) create pions that decay into gamma-rays. This process makes SNR-molecular cloud associations intense GeV and/or TeV sources whose spectra mimic the CR spectrum. 2) at lower energies, CRs penetrate the cloud and ionise the gas, leading to the formation of molecular species characteristic of the presence of CRs, called tracers of the ionisation. Studying these tracers gives information on low-energy CRs that are unaccessible to any other observations. I studied the CR ionisation of molecular clouds next to three SNRs: W28, W51C and W44. These SNRs are known to be interacting with the nearby clouds, from the presence of shocked gas, OH masers and pion-decay induced gamma-ray emission. My work includes millimeter observations and chemical modeling of tracers of the ionisation in these dense molecular clouds. In these three regions, we determined an enhanced CR ionisation rate, supporting the hypothesis of an origin of the CRs in the SNR nearby. The evolution of the CR ionisation rate with the distance to the SNR brings valuable constraints on the propagation properties of low-energy CRs. The method used relies on observations of the molecular ions HCO+ and DCO+, which shows crucial limitations at high ionisation. Therefore, I investigated, both through modeling and observations, the chemical abundances of several other species to try and identity alternative tracers of the ionisation. In particular, in the W44 region, observations of N2H+ bring additional constraints on the physical conditions, volatile abundances in the cloud, and the ionisation state. This research brought valuable insight in to the CR induced chemistry in the interstellar medium. It also brought new perspectives of interdisciplinary research towards the understanding of CRs, from millimeter to gamma-ray observations.

Stormy Clouds of Star Birth

NASA Technical Reports Server (NTRS)

2004-01-01

Hidden behind a shroud of dust in the constellation Cygnus is an exceptionally bright source of radio emission called DR21. Visible light images reveal no trace of what is happening in this region because of heavy dust obscuration. In fact, visible light is attenuated in DR21 by a factor of more than 10,000,000,000,000,000,000,000,000,000, 000,000,000,000 (ten thousand trillion heptillion).

New images from NASA's Spitzer Space Telescope allow us to peek behind the cosmic veil and pinpoint one of the most massive natal stars yet seen in our Milky Way galaxy. The never-before-seen star is 100,000 times as bright as the Sun. Also revealed for the first time is a powerful outflow of hot gas emanating from this star and bursting through a giant molecular cloud.

This image shows a 24-micron image mosaic, obtained with the Multiband Imaging Photometer aboard Spitzer (MIPS). This image maps the cooler infrared emission from interstellar dust found throughout the interstellar medium. The DR21 complex is clearly seen near the center of the strip, which covers about twice the area of the IRAC image.

Perhaps the most fascinating feature in this image is a long and shadowy linear filament extending towards the 10 o'clock position of DR21. This jet of cold and dense gas, nearly 50 light-years in extent, appears in silhouette against a warmer background. This filament is too long and massive to be a stellar jet and may have formed from a pre-existing molecular cloud core sculpted by DR21's strong winds. Regardless of its true nature, this jet and the numerous other arcs and wisps of cool dust signify the interstellar turbulence normally unseen by the human eye.

The Effects of Grain Size and Temperature Distributions on the Formation of Interstellar Ice Mantles

NASA Astrophysics Data System (ADS)

Pauly, Tyler; Garrod, Robin T.

2016-02-01

Computational models of interstellar gas-grain chemistry have historically adopted a single dust-grain size of 0.1 micron, assumed to be representative of the size distribution present in the interstellar medium. Here, we investigate the effects of a broad grain-size distribution on the chemistry of dust-grain surfaces and the subsequent build-up of molecular ices on the grains, using a three-phase gas-grain chemical model of a quiescent dark cloud. We include an explicit treatment of the grain temperatures, governed both by the visual extinction of the cloud and the size of each individual grain-size population. We find that the temperature difference plays a significant role in determining the total bulk ice composition across the grain-size distribution, while the effects of geometrical differences between size populations appear marginal. We also consider collapse from a diffuse to a dark cloud, allowing dust temperatures to fall. Under the initial diffuse conditions, small grains are too warm to promote grain-mantle build-up, with most ices forming on the mid-sized grains. As collapse proceeds, the more abundant, smallest grains cool and become the dominant ice carriers; the large population of small grains means that this ice is distributed across many grains, with perhaps no more than 40 monolayers of ice each (versus several hundred assuming a single grain size). This effect may be important for the subsequent processing and desorption of the ice during the hot-core phase of star formation, exposing a significant proportion of the ice to the gas phase, increasing the importance of ice-surface chemistry and surface-gas interactions.

Observational discrimination between modes of shock propagation in interstellar clouds: Predictions of CH+ and SH+ column densities in diffuse clouds

NASA Technical Reports Server (NTRS)

Flower, D. R.; Desforets, G. P.; Roueff, E.; Hartquist, T. W.

1986-01-01

Considerable effort in recent years has been devoted to the study of shocks in the diffuse interstellar medium. This work has been motivated partly by the observations of rotationally excited states of H2, and partly by the realization that species such as CH(+), OH and H2O might be formed preferentially in hot, post-shock gas. The problem of CH(+) and the difficulties encountered when trying to explain the high column densities, observed along lines of sight to certain hot stars, have been reviewed earlier. The importance of a transverse magnetic field on the structure of an interstellar shock was also demonstrated earlier. Transverse magnetic fields above a critical strength give rise to an acceleration zone or precursor, in which the parameters on the flow vary continuously. Chemical reactions, which change the degree of ionization of the gas, also modify the structure of the shock considerably. Recent work has shown that large column densities of CH(+) can be produced in magnetohydrodynamic shock models. Shock speeds U sub s approx. = 10 km/s and initial magnetic field strengths of a few micro G are sufficient to produce ion-neutral drift velocities which can drive the endothermic C(+)(H2,H)CH(+) reaction. It was also shown that single-fluid hydrodynamic models do not generate sufficiently large column densities of CH(+) unless unacceptably high shock velocities (u sub s approx. 20 km/s) are assumed in the models. Thus, the observed column densities of CH(+) provide a constraint on the mode of shock propagation in diffuse clouds. More precisely, they determine a lower limit to the ion-neutral drift velocity.

Mapping low-frequency carbon radio recombination lines towards Cassiopeia A at 340, 148, 54, and 43 MHz

NASA Astrophysics Data System (ADS)

Salas, P.; Oonk, J. B. R.; van Weeren, R. J.; Wolfire, M. G.; Emig, K. L.; Toribio, M. C.; Röttgering, H. J. A.; Tielens, A. G. G. M.

2018-04-01

Quantitative understanding of the interstellar medium requires knowledge of its physical conditions. Low-frequency carbon radio recombination lines (CRRLs) trace cold interstellar gas and can be used to determine its physical conditions (e.g. electron temperature and density). In this work, we present spatially resolved observations of the low-frequency (≤390 MHz) CRRLs centred around C268α, C357α, C494α, and C539α towards Cassiopeia A on scales of ≤1.2 pc. We compare the spatial distribution of CRRLs with other interstellar medium tracers. This comparison reveals a spatial offset between the peak of the CRRLs and other tracers, which is very characteristic for photodissociation regions and that we take as evidence for CRRLs being preferentially detected from the surfaces of molecular clouds. Using the CRRLs, we constrain the gas electron temperature and density. These constraints on the gas conditions suggest variations of less than a factor of 2 in pressure over ˜1 pc scales, and an average hydrogen density of 200-470 cm-3. From the electron temperature and density maps, we also constrain the ionized carbon emission measure, column density, and path length. Based on these, the hydrogen column density is larger than 1022 cm-2, with a peak of ˜4 × 1022 cm-2 towards the south of Cassiopeia A. Towards the southern peak, the line-of-sight length is ˜40 pc over a ˜2 pc wide structure, which implies that the gas is a thin surface layer on a large (molecular) cloud that is only partially intersected by Cassiopeia A. These observations highlight the utility of CRRLs as tracers of low-density extended H I and CO-dark gas halo's around molecular clouds.

Galactic Observations of Terahertz C+ (GOT C+): Inner Galaxy Survey

NASA Astrophysics Data System (ADS)

Yorke, Harold; Langer, William; Velusamy, T.; Pineda, J. L.; Goldsmith, P. F.; Li, D.

To understand the lifecycle of the interstellar gas and star formation we need detailed information about the diffuse atomic and diffuse molecular gas cloud properties. The ionized carbon [CII] 1.9 THz fine structure line is an important tracer of the atomic gas in the diffuse regions and the interface regions of atomic gas to molecular clouds. Furthermore, C+ is a major ISM coolant and among the Galaxy's strongest far-IR emission lines, and thus controls the thermal conditions throughout large parts of the Galaxy. Until now our knowledge of interstellar gas has been limited to the diffuse atomic phase traced by HI and to the dense molecular H2 phase traced by CO. However, we are missing an important phase of the ISM, called "dark gas" in which there is no or little, HI, and mostly molecular hydrogen but with insufficient shielding of UV to allow CO to form. C+ emission and absorption lines at 1.9 THz have the potential to trace such cloud transitions and evolution. Galactic Observations of the Terahertz C+ Line (GOT C+) is a Herschel Space Observatory Open Time Key Program to study the diffuse interstellar medium by sampling [CII] 1.9 THz line emission throughout the Galactic disk. We discuss the broader perspective of this survey and the first results of GOT C+ obtained during the Science Demonstration Phase (SDP) and Priority Science Phase (PSP) of HIFI, which focus on approximately 100 lines of sight in the inner galaxy. These observations are being carried out with the Herschel Space Observatory, which is an ESA cornerstone mission, with contributions from NASA. This research was conducted at the Jet Propulsion Laboratory, California Institute of Technology under contract with the National Aeronautics and Space Administration. JLP is a Caltech-JPL Postdoctoral Associate.

On the impact of the magnitude of interstellar pressure on physical properties of molecular cloud

NASA Astrophysics Data System (ADS)

Anathpindika, S.; Burkert, A.; Kuiper, R.

2017-04-01

Recently reported variations in the typical physical properties of Galactic and extra-Galactic molecular clouds (MCs), and, in their star-forming ability, have been attributed to local variations in the magnitude of interstellar pressure. Inferences from these surveys have called into question two long-standing beliefs that: (1) MCs are virialized and (2) they obey the Larson's third law. Here we invoked the framework of cloud formation via collision between warm gas-flows to examine if these latest observational inferences can be reconciled. To this end, we traced the temporal evolution of the gas surface density, the fraction of dense gas, the distribution of gas column density (N-PDF) and the virial nature of the assembled clouds. We conclude that these physical properties exhibit temporal variation and their respective peak magnitude also increases in proportion with the magnitude of external pressure, Pext. The velocity dispersion in assembled clouds appears to follow the power law, σ _{gas}∝ P_{ext}^{0.23}. The power-law tail of the N-PDFs at higher densities becomes shallower with increasing magnitude of external pressure for Pext/kB ≲ 107 K cm-3; at higher magnitudes such as those typically found in the Galactic Central Molecular Zone (Pext/kB > 107 K cm-3), the power-law shows significant steepening. While our results are broadly consistent with inferences from various recent observational surveys, it appears that MCs do not exhibit a unique set of properties, but rather a wide variety that can be reconciled with a range of magnitudes of pressure between 104 and 108 K cm-3.

Optical observations related to the molecular chemistry in diffuse interstellar clouds

NASA Technical Reports Server (NTRS)

Federman, S. R.

1987-01-01

Observations, which have been published since 1979, of molecular species in diffuse clouds are discussed. Particular attention is given to the ultraviolet measurements of CO with the Copernicus and IUE satellites and to ground-based optical measurements of CH, CH(+), CN, and 02. These data encompass large enough samples to test the chemical schemes expected to occur in diffuse clouds. Upper limits for other species (e.g., H2O, H2O(+), and C3) place restrictions on the pathways for molecular production. Moreover, analysis of the rotational distribution of the C2 molecule results in the determination of the physical conditions of the cloud. These parameters, including density, temperature, and the intensity of the radiation field, are necessary for modeling the chemistry.

Molecules as diagnostic tools in the interstellar medium

NASA Astrophysics Data System (ADS)

Spielfiedel, A.; Feautrier, N.; Balança, C.; Dayou, F.; Lique, F.; Senent, M.-L.

Analysis of light emission from different regions of the interstellar medium and circumstellar environments provides crucial information about the chemical composition and the physical conditions in these regions. Interpretation of the observed spectra requires the knowledge of collisional excitation rates as well as radiative rates participating to the line formation. In the first part, the paper focuses on collisional excitation rates of molecules relevant to the interstellar medium. It discusses currently available data and outlines new work carried out by the authors. Due to the use of accurate ab initio potential energy surfaces, the new rate coefficients differ significantly from previously published ones. In the second part, it is analysed from two examples how the use of the new rate coefficients could lead to important changes in the interpretation of molecular emission emerging from molecular clouds.

Micro-ion Traps for Detection of (Pre)-Biotic Organic Compounds on Comets

NASA Technical Reports Server (NTRS)

vanAmerom, Friso H. W.; Chaudhary, A.; Short, R. T.; Brinkerhoff, William; Glavin, Daniel; Mahaffy, Paul R.; Roman, Patrick A.

2013-01-01

Comets are currently believed to be a mixture of interstellar and nebular material. Many of the volatiles in comets are attributed to interstellar chemistry, because the same species of carbonaceous compounds are also observed in ices in interstellar molecular (ISM) clouds. Comets are thus likely to be a relatively pristine reservoir of primitive material and carbonaceous compounds in our solar system. They could be a major contributor to the delivery of prebiotic organic compounds, from which life emerged through impacts on early Earth. Mass spectrometers are very powerful tools to identify unknown chemicals, and much progress bas been made in miniaturizing mas spectrometers for space applications. Most miniatu rized mass spectrometers developed to date, however, are still relatively large, power hungry, complicated to assemble, and would have significant impact on space flight vehicle total payload and resource allocations.

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

NASA Technical Reports Server (NTRS)

Velusamy, T.; Langer, W. D.; Willacy, K.; Pineda, J. L.; Goldsmith, P. F.

2012-01-01

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.

TWO-STAGE FRAGMENTATION FOR CLUSTER FORMATION: ANALYTICAL MODEL AND OBSERVATIONAL CONSIDERATIONS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Bailey, Nicole D.; Basu, Shantanu, E-mail: nwityk@uwo.ca, E-mail: basu@uwo.ca

2012-12-10

Linear analysis of the formation of protostellar cores in planar magnetic interstellar clouds shows that molecular clouds exhibit a preferred length scale for collapse that depends on the mass-to-flux ratio and neutral-ion collision time within the cloud. We extend this linear analysis to the context of clustered star formation. By combining the results of the linear analysis with a realistic ionization profile for the cloud, we find that a molecular cloud may evolve through two fragmentation events in the evolution toward the formation of stars. Our model suggests that the initial fragmentation into clumps occurs for a transcritical cloud onmore » parsec scales while the second fragmentation can occur for transcritical and supercritical cores on subparsec scales. Comparison of our results with several star-forming regions (Perseus, Taurus, Pipe Nebula) shows support for a two-stage fragmentation model.« less

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

NASA Astrophysics Data System (ADS)

Velusamy, T.; Langer, W. D.; Willacy, K.; Pineda, J. L.; Goldsmith, P. F.

2013-03-01

We present the results of the distribution of CO-dark H2 gas in a sample of 2223 interstellar clouds in the inner Galaxy (l=-90° to +57°) 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, 12CO and 13CO 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 intensity and star formation rate in the Galactic disk.

A Model for Atomic and Molecular Interstellar Gas: The Meudon PDR Code

NASA Astrophysics Data System (ADS)

Le Petit, Franck; Nehmé, Cyrine; Le Bourlot, Jacques; Roueff, Evelyne

2006-06-01

We present the revised ``Meudon'' model of photon-dominated region (PDR) code, available on the Web under the GNU Public License. General organization of the code is described down to a level that should allow most observers to use it as an interpretation tool with minimal help from our part. Two grids of models, one for low-excitation diffuse clouds and one for dense highly illuminated clouds, are discussed, and some new results on PDR modelization highlighted.

Measurement and Modeling of Electromagnetic Scattering by Particles and Particle Groups. Chapter 3

NASA Technical Reports Server (NTRS)

Mishchenko, Michael I.

2015-01-01

Small particles forming clouds of interstellar and circumstellar dust, regolith surfaces of many solar system bodies, and cometary atmospheres have a strong and often controlling effect on many ambient physical and chemical processes. Similarly, aerosol and cloud particles exert a strong influence on the regional and global climates of the Earth, other planets of the solar system, and exoplanets. Therefore, detailed and accurate knowledge of physical and chemical characteristics of such particles has the utmost scientific importance.

Nonequilibrium chemistry in shocked molecular clouds. [interstellar gases

NASA Technical Reports Server (NTRS)

Iglesias, E. R.; Silk, J.

1978-01-01

The gas-phase chemistry is studied behind a 10-km/s shock propagating into a dense molecular cloud. The principal conclusions are that: the concentrations of certain molecules (CO, NH3, HCN, N2) are unperturbed by the shock; other molecules (H2CO, CN, HCO(+)) are greatly decreased in abundance; and substantial amounts of H2O, HCO, and CH4 are produced. Approximately 1 million yr (independent of the density) must elapse after shock passage before chemical equilibrium is attained.

Analytic solutions for single and multiple cylinders of gravitating polytropes in magnetostatic equilibrium

NASA Technical Reports Server (NTRS)

Lerche, I.; Low, B. C.

1980-01-01

Exact analytic solutions for the static equilibrium of a gravitating plasma polytrope in the presence of magnetic fields are presented. The means of generating various equilibrium configurations to illustrate directly the complex physical relationships between pressure, magnetic fields, and gravity in self-gravitating systems is demonstrated. One of the solutions is used to model interstellar clouds suspended by magnetic fields against the galactic gravity such as may be formed by the Parker (1966) instability. It is concluded that the pinching effect of closed loops of magnetic fields in the clouds may be a dominant agent in further collapsing the clouds following their formation.

Ortho- and para-hydrogen in dense clouds, protoplanets, and planetary atmospheres

NASA Technical Reports Server (NTRS)

Decampli, W. M.; Cameron, A. G. W.; Bodenheimer, P.; Black, D. C.

1978-01-01

If ortho- and para-hydrogen achieve a thermal ratio on dynamical time scales in a molecular hydrogen cloud, then the specific heat is high enough in the temperature range 35-70 K to possibly induce hydrodynamic collapse. The ortho-para ratio in many interstellar cloud fragments is expected to meet this condition. The same may have been true for the primitive solar nebula. Detailed hydrodynamic and hydrostatic calculations are presented that show the effects of the assumed ortho-para ratio on the evolution of Jupiter during its protoplanetary phase. Some possible consequences of a thermalized ortho-para ratio in the atmospheres of the giant planets are also discussed.

Infrared emission from isolated dust clouds in the presence of very small dust grains

NASA Technical Reports Server (NTRS)

Lis, Dariusz C.; Leung, Chun M.

1991-01-01

Models of the effects of small grain-generated temperature fluctuations on the IR spectrum and surface brightness of externally heated interstellar dust clouds are presently constructed on the basis of a continuum radiation transport computer code which encompasses the transient heating of small dust grains. The models assume a constant fractional abundance of large and small grains throughout the given cloud. A comparison of model results with IRAS observations indicates that the observed 12-25 micron band emissions are associated with about 10-A radius grains, while the 60-100 micron emission is primarily due to large grains which are heated under the equilibrium conditions.

Detection of organic matter in interstellar grains.

PubMed

Pendleton, Y J

1997-06-01

Star formation and the subsequent evolution of planetary systems occurs in dense molecular clouds, which are comprised, in part, of interstellar dust grains gathered from the diffuse interstellar medium (DISM). Radio observations of the interstellar medium reveal the presence of organic molecules in the gas phase and infrared observational studies provide details concerning the solid-state features in dust grains. In particular, a series of absorption bands have been observed near 3.4 microns (approximately 2940 cm-1) towards bright infrared objects which are seen through large column densities of interstellar dust. Comparisons of organic residues, produced under a variety of laboratory conditions, to the diffuse interstellar medium observations have shown that aliphatic hydrocarbon grains are responsible for the spectral absorption features observed near 3.4 microns (approximately 2940 cm-1). These hydrocarbons appear to carry the -CH2- and -CH3 functional groups in the abundance ratio CH2/CH3 approximately 2.5, and the amount of carbon tied up in this component is greater than 4% of the cosmic carbon available. On a galactic scale, the strength of the 3.4 microns band does not scale linearly with visual extinction, but instead increases more rapidly for objects near the Galactic Center. A similar trend is noted in the strength of the Si-O absorption band near 9.7 microns. The similar behavior of the C-H and Si-O stretching bands suggests that these two components may be coupled, perhaps in the form of grains with silicate cores and refractory organic mantles. The ubiquity of the hydrocarbon features seen in the near infrared near 3.4 microns throughout out Galaxy and in other galaxies demonstrates the widespread availability of such material for incorporation into the many newly forming planetary systems. The similarity of the 3.4 microns features in any organic material with aliphatic hydrocarbons underscores the need for complete astronomical observational coverage in the 2-30 microns region, of lines of sight which sample dust in both dense and diffuse interstellar clouds, in order to uniquely specify the composition of interstellar organics. This paper reviews the information available from ground-based observations, although currently the Infrared Satellite Observatory is adding to our body of knowledge on this subject by providing more extensive wavelength coverage. The Murchison carbonaceous meteorite has also been used as an analog to the interstellar observations and has revealed a striking similarity between the light hydrocarbons in the meteorite and the ISM; therefore this review includes comparisons with the meteoritic analog as well as with relevant laboratory residues. Fundamental to the evolution of the biogenic molecules, to the process of planetary system formation, and perhaps to the origin of life, is the connection between the organic material found in the interstellar medium and that incorporated in the most primitive solar system bodies.

Zeta Ophiuchi -- Runaway Star Plowing through Space Dust

NASA Image and Video Library

2011-01-24

The blue star near the center of this image is Zeta Ophiuchi. Zeta Ophiuchi is actually a very massive, hot, bright blue star plowing its way through a large cloud of interstellar dust and gas in this image from NASA Wide-field Infrared Survey Explorer.

A high-resolution X-ray image of Puppis A - Inhomogeneities in the interstellar medium

NASA Technical Reports Server (NTRS)

Petre, R.; Kriss, G. A.; Winkler, P. F.; Canizares, C. R.

1982-01-01

Eleven HRI exposures from the Einstein Observatory are assembled into an 0.1-4 keV image of the Puppis A supernova remnant which displays a complex morphology that may reflect the structure of the shocked interstellar medium. In addition to showing a density gradient of a factor greater than four across the approximately 30 pc diameter of the remnant perpendicular to the galactic plane, a shell of X-ray emission is seen surrounding the northern half of Puppis A, coincident with the radio shell, whose edge brightness profile indicates direct hot plasma heating by the blast wave rather than evaporation from clouds. The interior structure of the supernova remnant suggests inhomogeneities whose sizes range over 0.1-5 pc, but with moderate density contrast. Although isolated clouds of 10-30/cu cm density are responsible for the two brightest X-ray features, they represent only a small fraction of the Puppis A mass.

Evidence for a Neutral Iron Line Generated by MeV Protons from Supernova Remnants Interacting with Molecular Clouds

NASA Astrophysics Data System (ADS)

Nobukawa, Kumiko K.; Nobukawa, Masayoshi; Koyama, Katsuji; Yamauchi, Shigeo; Uchiyama, Hideki; Okon, Hiromichi; Tanaka, Takaaki; Uchida, Hiroyuki; Tsuru, Takeshi G.

2018-02-01

Supernova remnants (SNRs) have been prime candidates for Galactic cosmic-ray accelerators. When low-energy cosmic-ray protons (LECRp) collide with interstellar gas, they ionize neutral iron atoms and emit the neutral iron line (Fe I Kα) at 6.40 keV. We search for the iron K-shell line in seven SNRs from the Suzaku archive data of the Galactic plane in the 6^\\circ ≲ l≲ 40^\\circ ,| b| < 1^\\circ region. All of these SNRs interact with molecular clouds. We discover Fe I Kα line emissions from five SNRs (W28, Kes 67, Kes 69, Kes 78, and W44). The spectra and morphologies suggest that the Fe I Kα line is produced by interactions between LECRp and the adjacent cold gas. The proton energy density is estimated to be ≳10–100 eV cm‑3, which is more than 10 times higher than that in the ambient interstellar medium.

The Evolution of Interstellar Gas: Massive Stars and the Dispersal of Neutral Material

NASA Technical Reports Server (NTRS)

Federman, Steven R.

2003-01-01

We studied the effects of newly formed O and B stars on their surrounding interstellar material through a combination of observations and theoretical modeling. The observational data came from measurements of absorption seen in the spectra of background, newly formed stars. Particular attention was given to stellar radiation which converts molecular to atomic material. Laboratory data on absorption cross sections relevant to the analysis and interpretation of carbon monoxide formed part of the effort. The grant supported Postdoctoral Fellows, Drs. Min Yan and Yaron Sheffer, and a laboratory technician. Though the students themselves were not supported. one M.S. Thesis and two Ph.D. dissertations from the University of Toledo were based on the research done under the grant. The research accomplished under this grant led directly to other funded programs. An observing proposal to study the chemistry of diffuse molecular clouds in the Large and Small Magellanic Clouds with ESO s Very Large Telescope was another example of a successful outcome of my LTSA program.

KSC-03pd0069

NASA Image and Video Library

2003-01-12

VANDENBERG AFB, Calif. -- A Boeing Delta II rocket soars above the clouds here today at Vandenberg AFB, Calif. The NASA payloads aboard the rocket are the ICESat, an Ice Cloud and land Elevation Satellite, and CHIPSat, a Cosmic Hot Interstellar Plasma Spectrometer. ICESat, a 661-pound satellite, is a benchmark satellite for the Earth Observing System that will help scientists determine if the global sea level is rising or falling. It will observe the ice sheets that blanket the Earth’s poles to determine if they are growing or shrinking. It will assist in developing an understanding of how changes in the Earth’s atmosphere and climate affect polar ice masses and global sea level. The Geoscience Laser Altimeter System is the sole instrument on the satellite. CHIPSat, a suitcase-size 131-pound satellite, will provide information about the origin, physical processes and properties of the hot gas contained in the interstellar medium. This launch marks the first Delta from Vandenberg this year. (USAF photo by: SSgt. Lee A Osberry Jr.)

KSC-03pd0068

NASA Image and Video Library

2003-01-12

VANDENBERG AFB, Calif. -- A Boeing Delta II rocket soars above the clouds here today at Vandenberg AFB, Calif. The NASA payload aboard the rocket are the ICESat, an Ice Cloud and land Elevation Satellite, and CHIPSat, a Cosmic Hot Interstellar Plasma Spectrometer. ICESat, a 661-pound satellite, is a benchmark satellite for the Earth Observing System that will help scientists determine if the global sea level is rising or falling. It will observe the ice sheets that blanket the Earth’s poles to determine if they are growing or shrinking. It will assist in developing an understanding of how changes in the Earth’s atmosphere and climate affect polar ice masses and global sea level. The Geoscience Laser Altimeter System is the sole instrument on the satellite. CHIPSat, a suitcase-size 131-pound satellite, will provide information about the origin, physical processes and properties of the hot gas contained in the interstellar medium. This launch marks the first Delta from Vandenberg this year. (USAF photo by: SSgt Lee A Osberry Jr.)

Translucency of human teeth and dental restorative materials and its clinical relevance

NASA Astrophysics Data System (ADS)

Lee, Yong-Keun

2015-04-01

The purpose was to review the translucency of human teeth and related dental materials that should be considered for the development of esthetic restorative materials. Translucency is the relative amount of light transmission or diffuse reflection from a substrate surface through a turbid medium. Translucency influences the masking ability, color blending effect, and the degree of light curing through these materials. Regarding the translucency indices, transmission coefficient, translucency parameter, and contrast ratio have been used, and correlations among these indices were confirmed. Translucency of human enamel and dentine increases in direct proportion to the wavelength of incident light in the visible light range. As for the translucency changes by aging, limited differences were reported in human dentine, while those for enamel proved to increase. There have been studies for the adjustment of translucency in dental esthetic restorative materials; the size and amount of filler and the kind of resin matrix were modified in resin composites, and the kind of ingredient and the degree of crystallization were modified in ceramics. Based on the translucency properties of human enamel and dentine, those of replacing restorative materials should be optimized for successful esthetic rehabilitation. Biomimetic simulation of the natural tooth microstructure might be a promising method.

Detection of nitric oxide in the dark cloud L134N

NASA Technical Reports Server (NTRS)

Mcgonagle, D.; Irvine, W. M.; Minh, Y. C.; Ziurys, L. M.

1990-01-01

The first detection of interstellar nitric oxide (NO) in a cold dark cloud, L134N is reported. Nitric oxide was observed by means of its two 2 Pi 1/2, J = 3/2 - 1/2, rotational transitions at 150.2 and 150.5 GHz, which occur because of Lambda-doubling. The inferred column density for L134N is about 5 x 10 to the 14th/sq cm toward the SO peak in that cloud. This value corresponds to a fractional abundance relative to molecular hydrogen of about 6 x 10 to the -8th and is in good agreement with predictions of quiescent cloud ion-molecule chemistry. NO was not detected toward the dark cloud TMC-1 at an upper limit of 3 x 10 to the -8th or less.

Point-spread imaging for measurement of skin translucency and scattering.

PubMed

Jiang, Zhi-xing; Kaplan, Peter D

2008-08-01

The translucency of skin has long been identified as an important cue for healthy and youthful looking skin. There is currently no universal definition for skin translucency let alone a measurement method. We propose that skin translucency is the light scattering beneath skin surface. We demonstrate the use of polarization gated point spreading imaging for non-invasive, in vivo measurement of the translucency and the reduced scattering coefficient m's of skin. We developed a polarization-gated point-spread imaging system to measure the spread of the incident pencil-thin laser beam on the skin. Skin translucency was calculated as the spread of the laser beam. From the measurement of the shift of the light diffuse center from the light injection point, the reduced scattering coefficient m's of the skin was calculated. We validated the measurement technique with milk as an in vitro model for skin. The measured m's of milk solution was found to be linearly proportional to the milk concentration, in agreement with Beer's law. The calculated translucency decreased as the milk concentration increased or as the reduced scattering coefficient m's increased. It was also found that the translucency decreased as the absorption coefficient of the milk solution increased. The measured translucency of a set of custom made clay tiles correlated well with the consumer perception of the incremental ranking of the translucency. In vivo measurement of skin translucency and the reduced scattering coefficient m's were carried out on several volunteers. The measured reduced scattering coefficient m's was in agreement with those in the literature. The measured skin translucency for different skin ethnicities of Caucasian, North Asian, South Asian and African American were in line with the expectation that skin translucency decreases as the skin color gets darker.

Changes in the morphology of interstellar ice analogues after hydrogen atom exposure.

PubMed

Accolla, Mario; Congiu, Emanuele; Dulieu, François; Manicò, Giulio; Chaabouni, Henda; Matar, Elie; Mokrane, Hakima; Lemaire, Jean Louis; Pirronello, Valerio

2011-05-07

The morphology of water ice in the interstellar medium is still an open question. Although accretion of gaseous water could not be the only possible origin of the observed icy mantles covering dust grains in cold molecular clouds, it is well known that water accreted from the gas phase on surfaces kept at 10 K forms ice films that exhibit a very high porosity. It is also known that in the dark clouds H(2) formation occurs on the icy surface of dust grains and that part of the energy (4.48 eV) released when adsorbed atoms react to form H(2) is deposited in the ice. The experimental study described in the present work focuses on how relevant changes of the ice morphology result from atomic hydrogen exposure and subsequent recombination. Using the temperature-programmed desorption (TPD) technique and a method of inversion analysis of TPD spectra, we show that there is an exponential decrease in the porosity of the amorphous water ice sample following D-atom irradiation. This decrease is inversely proportional to the thickness of the ice and has a value of ϕ(0) = 2 × 10(16) D-atoms cm(-2) per layer of H(2)O. We also use a model which confirms that the binding sites on the porous ice are destroyed regardless of their energy depth, and that the reduction of the porosity corresponds in fact to a reduction of the effective area. This reduction appears to be compatible with the fraction of D(2) formation energy transferred to the porous ice network. Under interstellar conditions, this effect is likely to be efficient and, together with other compaction processes, provides a good argument to believe that interstellar ice is amorphous and non-porous. This journal is © the Owner Societies 2011

Modelling ultraviolet-line diagnostics of stars, the ionized and the neutral interstellar medium in star-forming galaxies

NASA Astrophysics Data System (ADS)

Vidal-García, A.; Charlot, S.; Bruzual, G.; Hubeny, I.

2017-09-01

We combine state-of-the-art models for the production of stellar radiation and its transfer through the interstellar medium (ISM) to investigate ultraviolet-line diagnostics of stars, the ionized and the neutral ISM in star-forming galaxies. We start by assessing the reliability of our stellar population synthesis modelling by fitting absorption-line indices in the ISM-free ultraviolet spectra of 10 Large Magellanic Cloud clusters. In doing so, we find that neglecting stochastic sampling of the stellar initial mass function in these young (∼10-100 Myr), low-mass clusters affects negligibly ultraviolet-based age and metallicity estimates but can lead to significant overestimates of stellar mass. Then, we proceed and develop a simple approach, based on an idealized description of the main features of the ISM, to compute in a physically consistent way the combined influence of nebular emission and interstellar absorption on ultraviolet spectra of star-forming galaxies. Our model accounts for the transfer of radiation through the ionized interiors and outer neutral envelopes of short-lived stellar birth clouds, as well as for radiative transfer through a diffuse intercloud medium. We use this approach to explore the entangled signatures of stars, the ionized and the neutral ISM in ultraviolet spectra of star-forming galaxies. We find that, aside from a few notable exceptions, most standard ultraviolet indices defined in the spectra of ISM-free stellar populations are prone to significant contamination by the ISM, which increases with metallicity. We also identify several nebular-emission and interstellar-absorption features, which stand out as particularly clean tracers of the different phases of the ISM.

Recent Advances in Organic Cosmochemistry

NASA Technical Reports Server (NTRS)

Sandford, Scott A.; Witteborn, Fred C. (Technical Monitor)

1994-01-01

The Astrochemistry Laboratory at NASA's Ames Research Center pursues a variety of activities, most of which center around the use of spectroscopy (ultraviolet to far-infrared) for the interpretation of astronomical and meteoritic data. One of our key activities is the study of the chemical and physical properties of cometary, interstellar, and planetary ice analogs and matrix-isolated molecules of astrophysical interest. As a result of these studies it is now known that a significant fraction of the carbon in the interstellar medium (ISM) is in reasonably complex forms, some of which are clearly of interest for exobiology. Examples of compounds known or suspected to be present in space include polycyclic aromatic hydrocarbons (PAHs), microdiamonds, an aliphatic-rich component found in the diffuse interstellar medium, and a variety of molecular species produced by the irradiation of mixed molecular ices in dense clouds. A number of the species produced by irradiation contain nitrogen and appear to offer an additional means of producing some of the amino acids found in meteorites. I will review these complex carbonaceous materials and discuss how they are connected with each other and the organic materials that ultimately ended up as part of our own Solar System. Specific points that will probably be covered include: (1) the composition of the ices in interstellar dense molecular clouds; (2) the more complex organic compounds produced when these ices are irradiated and/or warmed; (3) the detection of microdiamonds in space; (4) the discovery that aliphatic materials may constitute as much as 15% of all the carbon in the diffuse ISM, appears to be present everywhere in the galaxy, and yet seems to be present everywhere in the galaxy, and yet seems to be significantly concentrated towards the center of the galaxy.

Ionization in the local interstellar and intergalactic media

DOE Office of Scientific and Technical Information (OSTI.GOV)

Cheng, K.

1990-01-01

Detailed photoionization calculations for the local interstellar medium (LISM) and the intergalactic medium (IGM) are presented. Constraints in the LISM are imposed by H I column density derived from IUE and Copernicus data toward nearby B stars and hot white dwarfs. The EUV radiation field is modeled including contributions from discrete stellar sources and from a thermal bremsstrahlung-radiative recombination spectrum emitted from the surrounding 10(exp 6) K coronal substrate. Lower limits to the fractional ionization of hydrogen and helium of 0.17 and 0.30 respectively are established. The derived limits have important implications for the interpretation of the H I andmore » He I backscattering results. The high He ionization fraction results primarily from very strong line emission below 500 A originating in the surrounding coronal substrate while the H ionization is dominated by the EUV radiation from the discrete stellar sources. The dual effects of thermal conduction and the EUV spectrum of the 10(exp 6) K plasma on ionization in the cloud skin are explored. The EUV radiation field and Auger ionization have insignificant effects on the resulting ionic column densities of Si IV, C IV, N V and O VI through the cloud skin. Calculations show that the abundances of these species are dominated by collisional ionization in the thermal conduction front. Because of a low charge exchange rate with hydrogen, the ionic column density ratios of N(C III)/N(C II) and N(N II)/N(N I) are dominated by the EUV radiation field in the local interstellar medium. These ratios should be important diagnostics for the EUV radiation field and serve as surrogate indicators of the interstellar He and H ionization fraction respectively. The same photoionization model is applied to the intergalactic medium.« less

Isotopic Anomalies in Primitive Solar System Matter: Spin-State-Dependent Fractionation of Nitrogen and Deuterium in Interstellar Clouds

NASA Technical Reports Server (NTRS)

Wirstrom, Eva S.; Charnley, Steven B.; Cordiner, Martin A.; Milam, Stefanie N.

2012-01-01

Organic material found in meteorites and interplanetary dust particles is enriched in D and N-15. This is consistent with the idea that the functional groups carrying these isotopic anomalies, nitriles and amines, were formed by ion-molecule chemistry in the protosolar nebula, Theoretical models of interstellar fractionation at low temperatures predict large enrichments in both D and N-15 and can account for the largest isotopic enrichments measured in carbonaceous meteorites. However, more recent measurements have shown that, in some primitive samples, a large N-15 enrichment does not correlate with one in D, and that some D-enriched primitive material displays little, if any, N-15 enrichment. By considering the spin-state dependence in ion-molecule reactions involving the ortho and para forms of H2, we show that ammonia and related molecules can exhibit such a wide range of fractionation for both N-15 and D in dense cloud cores. We also show that while the nitriles, HCN and HNC, contain the greatest N=15 enrichment, this is not expected to correlate with extreme D enrichment. These calculations therefore support the view that solar system N-15 and D isotopic anomalies have an interstellar heritage. We also compare our results to existing astronomical observations and briefly discuss future tests of this model.

Isotopic Anomalies in Primitive Solar System Matter: Spin-State Dependent Fractionation of Nitrogen and Deuterium in Interstellar Clouds

NASA Technical Reports Server (NTRS)

Wirstrom, Eva S.; Charnley, Steven B.; Cordiner, Martin A.; Milan, Stefanie N.

2012-01-01

Organic material found in meteorites and interplanetary dust particles is enriched in D and N-15, This is consistent with the idea that the functional groups carrying these isotopic anomalies, nitriles and amines, were formed by ion-molecule chemistry in the protosolar core. Theoretical models of interstellar fractionation at low temperatures predict large enrichments in both D and N-15 and can account for the largest isotop c enrichments measured in carbonaceous meteorites, However, more recent measurements have shown that, in some primitive samples, a large N-15 enrichment does not correlate with one in D, and that some D-enriched primitive material displays little, if any, N-15 enrichment. By considering the spin-state dependence in ion-molecule reactions involving the ortho and para forms of H2, we show that ammonia and related molecules can exhibit such a wide range of fractionation for both N-15 and D in dense cloud cores, We also show that while the nitriles, HCN and HNC, contain the greatest N-15 enrichment, this is not expected to correlate with extreme D emichment. These calculations therefore support the view that Solar System N-15 and D isotopic anomalies have an interstellar heritage, We also compare our results to existing astronomical observations and briefly discuss future tests of this model.

Spectral Study of A 1Π–X 1Σ+ Transitions of CO Relevant to Interstellar Clouds

NASA Astrophysics Data System (ADS)

Cheng, Junxia; Zhang, Hong; Cheng, Xinlu

2018-05-01

Highly correlated ab initio calculations were performed for an accurate determination of the A 1Π–X 1Σ+ system of the CO molecule. A highly accurate multi-reference configuration interaction approach was used to investigate the potential energy curves (PECs) and the transition dipole moment curve (TDMC). The resultant PECs and TDMC found by using the aug-cc-pV5Z (aV5Z) basis set and 5330 active spaces are in good agreement with the experimental data. Moreover, the Einstein A coefficients, lifetimes, ro-vibrational intensities, absorption oscillator strengths, and integrated cross sections are calculated so that the vibrational bands include v″ = 0–39 \\to v‧ = 0–23. For applications in the atmosphere and interstellar clouds, we studied the transition lineshapes to Gaussian and Lorentzian profiles at different temperatures and pressures. The intensities were calculated at high temperature that was used to satisfy some astrophysical applications, such as in planetary atmospheres. The results are potentially useful for important SAO/NASA Astrophysics Data System and databases such as HITRAN, HITEMP, and the National Institute of Standards and Technology. Because the results from many laboratory techniques and our calculations now agree, analyses of interstellar CO based on absorption from A 1Π–X 1Σ+ are no longer hindered by present spectral parameters.

Cloud fluid models of gas dynamics and star formation in galaxies

NASA Technical Reports Server (NTRS)

Struck-Marcell, Curtis; Scalo, John M.; Appleton, P. N.

1987-01-01

The large dynamic range of star formation in galaxies, and the apparently complex environmental influences involved in triggering or suppressing star formation, challenges the understanding. The key to this understanding may be the detailed study of simple physical models for the dominant nonlinear interactions in interstellar cloud systems. One such model is described, a generalized Oort model cloud fluid, and two simple applications of it are explored. The first of these is the relaxation of an isolated volume of cloud fluid following a disturbance. Though very idealized, this closed box study suggests a physical mechanism for starbursts, which is based on the approximate commensurability of massive cloud lifetimes and cloud collisional growth times. The second application is to the modeling of colliding ring galaxies. In this case, the driving processes operating on a dynamical timescale interact with the local cloud processes operating on the above timescale. The results is a variety of interesting nonequilibrium behaviors, including spatial variations of star formation that do not depend monotonically on gas density.

Effect of Ambipolar Diffusion on Ion Abundances in Contracting Protostellar Cores

NASA Astrophysics Data System (ADS)

Ciolek, Glenn E.; Mouschovias, Telemachos Ch.

1998-09-01

Numerical simulations and analytical solutions have established that ambipolar diffusion can reduce the dust-to-gas ratio in magnetically and thermally supercritical cores during the epoch of core formation. We study the effect that this has on the ion chemistry in contracting protostellar cores and present a simplified analytical method that allows one to calculate the ion power-law exponent k (≡d ln ni/d ln nn, where ni and nn are the ion and neutral densities, respectively) as a function of core density. We find that, as in earlier numerical simulations, no single value of k can adequately describe the ion abundance for nn <~ 109 cm-3, a result that is contrary to the ``canonical'' value of k = 1/2 found in previous static equilibrium chemistry calculations and often used to study the effect of ambipolar diffusion in interstellar clouds. For typical cloud and grain parameters, reduction of the abundance of grains results in k > 1/2 during the core formation epoch (densities <~105 cm-3). As a consequence, observations of the degree of ionization in cores could be used, in principle, to determine whether ambipolar diffusion is responsible for core formation in interstellar molecular clouds. For densities >>105 cm-3, k is generally <<1/2.

Racemic amino acids from the ultraviolet photolysis of interstellar ice analogues.

PubMed

Bernstein, Max P; Dworkin, Jason P; Sandford, Scott A; Cooper, George W; Allamandola, Louis J

2002-03-28

The delivery of extraterrestrial organic molecules to Earth by meteorites may have been important for the origin and early evolution of life. Indigenous amino acids have been found in meteorites-over 70 in the Murchison meteorite alone. Although it has been generally accepted that the meteoritic amino acids formed in liquid water on a parent body, the water in the Murchison meteorite is depleted in deuterium relative to the indigenous organic acids. Moreover, the meteoritical evidence for an excess of laevo-rotatory amino acids is hard to understand in the context of liquid-water reactions on meteorite parent bodies. Here we report a laboratory demonstration that glycine, alanine and serine naturally form from ultraviolet photolysis of the analogues of icy interstellar grains. Such amino acids would naturally have a deuterium excess similar to that seen in interstellar molecular clouds, and the formation process could also result in enantiomeric excesses if the incident radiation is circularly polarized. These results suggest that at least some meteoritic amino acids are the result of interstellar photochemistry, rather than formation in liquid water on an early Solar System body.

Radical formation, chemical processing, and explosion of interstellar grains

NASA Technical Reports Server (NTRS)

Greenberg, J. M.

1976-01-01

The ultraviolet radiation in interstellar space is shown to create a sufficient steady-state density of free radicals in the grain mantle material consisting of oxygen, carbon, nitrogen, and hydrogen to satisfy the critical condition for initiation of chain reactions. The criterion for minimum critical particle size for maintaining the chain reaction is of the order of the larger grain sizes in a distribution satisfying the average extinction and polarization measures. The triggering of the explosion of interstellar grains leading to the ejection of complex interstellar molecules is shown to be most probable where the grains are largest and where radiation is suddenly introduced; i.e., in regions of new star formation. Similar conditions prevail at the boundaries between very dark clouds and H II regions. When the energy released by the chemical activity of the free radicals is inadequate to explode the grain, the resulting mantle material must consist of extremely large organic molecules which are much more resistant to the hostile environment of H II regions than the classical dirty-ice mantles made up of water, methane, and ammonia.

H2-rich interstellar grain mantles: An equilibrium description

NASA Technical Reports Server (NTRS)

Dissly, Richard W.; Allen, Mark; Anicich, Vincent G.

1994-01-01

Experiments simulating the codeposition of molecular hydrogen and water ice on interstellar grains demonstrate that amorphous water ice at 12 K can incorporate a substantial amount of H2, up to a mole ratio of H2/H2O = 0.53. We find that the physical behavior of approximately 80% of the hydrogen can be explained satisfactorily in terms of an equilibrium population, thermodynamically governed by a wide distribution of binding site energies. Such a description predicts that gas phase accretion could lead to mole fractions of H2 in interstellar grain mantles of nearly 0.3; for the probable conditions of WL5 in the rho Ophiuchi cloud, an H2 mole fraction of between 0.05 and 0.3 is predicted, in possible agreement with the observed abundance reported by Sandford, Allamandola, & Geballe. Accretion of gas phase H2 onto grain mantles, rather than photochemical production of H2 within the ice, could be a general explanation for frozen H2 in interstellar ices. We speculate on the implications of such a composition for grain mantle chemistry and physics.

Racemic amino acids from the ultraviolet photolysis of interstellar ice analogues

NASA Technical Reports Server (NTRS)

Bernstein, Max P.; Dworkin, Jason P.; Sandford, Scott A.; Cooper, George W.; Allamandola, Louis J.

2002-01-01

The delivery of extraterrestrial organic molecules to Earth by meteorites may have been important for the origin and early evolution of life. Indigenous amino acids have been found in meteorites-over 70 in the Murchison meteorite alone. Although it has been generally accepted that the meteoritic amino acids formed in liquid water on a parent body, the water in the Murchison meteorite is depleted in deuterium relative to the indigenous organic acids. Moreover, the meteoritical evidence for an excess of laevo-rotatory amino acids is hard to understand in the context of liquid-water reactions on meteorite parent bodies. Here we report a laboratory demonstration that glycine, alanine and serine naturally form from ultraviolet photolysis of the analogues of icy interstellar grains. Such amino acids would naturally have a deuterium excess similar to that seen in interstellar molecular clouds, and the formation process could also result in enantiomeric excesses if the incident radiation is circularly polarized. These results suggest that at least some meteoritic amino acids are the result of interstellar photochemistry, rather than formation in liquid water on an early Solar System body.

Evolutionary Models of Cold, Magnetized, Interstellar Clouds

NASA Technical Reports Server (NTRS)

Gammie, Charles F.; Ostriker, Eve; Stone, James M.

2004-01-01

We modeled the long-term and small-scale evolution of molecular clouds using direct 2D and 3D magnetohydrodynamic (MHD) simulations. This work followed up on previous research by our group under auspices of the ATP in which we studied the energetics of turbulent, magnetized clouds and their internal structure on intermediate scales. Our new work focused on both global and smallscale aspects of the evolution of turbulent, magnetized clouds, and in particular studied the response of turbulent proto-cloud material to passage through the Galactic spiral potential, and the dynamical collapse of turbulent, magnetized (supercritical) clouds into fragments to initiate the formation of a stellar cluster. Technical advances under this program include developing an adaptive-mesh MHD code as a successor to ZEUS (ATHENA) in order to follow cloud fragmentation, developing a shearing-sheet MHD code which includes self-gravity and externally-imposed gravity to follow the evolution of clouds in the Galactic potential, and developing radiative transfer models to evaluate the internal ionization of clumpy clouds exposed to external photoionizing UV and CR radiation. Gammie's work at UIUC focused on the radiative transfer aspects of this program.

Chemical Evolution of Interstellar Dust into Planetary Materials

NASA Technical Reports Server (NTRS)

Fomenkova, M. N.; Chang, S.; DeVincenzi, Donald L. (Technical Monitor)

1995-01-01

Comets are believed to retain some interstellar materials, stored in fairly pristine conditions since-their formation. The composition and properties of cometary dust grains should reflect those of grains in the outer part of the protosolar nebula which, at least in part, were inherited from the presolar molecular cloud. However, infrared emission features in comets differ from their interstellar counterparts. These differences imply processing of interstellar material on its way to incorporation in comets, but C and N appear to be retained. Overall dust evolution from the interstellar medium (ISM) to planetary materials is accompanied by an increase in proportion of complex organics and a decrease in pure carbon phases. The composition of cometary dust grains was measured in situ during fly-by missions to comet Halley in 1986. The mass spectra of about 5000 cometary dust grains with masses of 5 x 10(exp -17) - 5 x 10(exp -12) g provide data about the presence and relative abundances of the major elements H, C, N, O,Na, Mg, Al, Si, S, Cl, K, Ca, Ti, Cr, Fe, Ni. The bulk abundances of major rock-forming elements integrated over all spectra were found to be solar within a factor of 2, while the volatile elements H, C, N, O in dust are depleted in respect to their total cosmic abundances. The abundances of C and N in comet dust are much closer to interstellar than to meteoritic and are higher than those of dust in the diffuse ISM. In dense molecular clouds dust grains are covered by icy mantles, the average composition of which is estimated to be H:C:N:O = 96:14:1:34. Up to 40% of elemental C and O may be sequestered in mantles. If we use this upper limit to add H, C, N and O as icy mantle material to the abundances residing in dust in the diffuse ISM, then the resulting values for H. C, and N match cometary abundances. Thus, ice mantles undergoing chemical evolution on grains in the dense ISM appear to have been transformed into less volatile and more complex organic residues wherein the H, C and N are largely retained and ultimately accreted in cometary dust. The abundance of O is about the same for cometary dust, meteorites and interstellar dust. In all these samples, most of O in a solid phase is bonded to silicates. In dense molecular clouds, the abundance of O in dust+mantles is significantly higher then in cometary dust. This difference may reflect the greater lability of oxygenated species toward astrophysical processing. Laboratory studies show that O-bearing functional groups in organic compounds tend to be relatively easily removed by heating and/or UV and particle irradiation . In Halley's coma, O-containing organic grains, being unstable, were located closest to the nucleus. The decomposition of the organic grain component in the coma provided a significant extended source contribution to O-containing gaseous species such as CO and H2CO.

Modeling Dust in the Magellanic Clouds

NASA Astrophysics Data System (ADS)

Zonca, Alberto; Casu, Silvia; Mulas, Giacomo; Aresu, Giambattista; Cecchi-Pestellini, Cesare

2015-09-01

We model the extinction profiles observed in the Small and Large Magellanic clouds with a synthetic population of dust grains consisting of core-mantle particles and a collection of free-flying polycyclic aromatic hydrocarbons (PAHs). All different flavors of the extinction curves observed in the Magellanic Clouds (MCs) can be described by the present model, which has been previously (successfully) applied to a large sample of diffuse and translucent lines of sight in the Milky Way. We find that in the MCs the extinction produced by classical grains is generally larger than absorption by PAHs. Within this model, the nonlinear far-UV rise is accounted for by PAHs, whose presence in turn is always associated with a gap in the size distribution of classical particles. This hints either at a physical connection between (e.g., a common cause for) PAHs and the absence of middle-sized dust particles or the need for an additional component in the model that can account for the nonlinear far-UV rise without contributing to the UV bump at ∼217 nm such as, e.g., nanodiamonds.

Probing the Origin and Evolution of Interstellar and Protoplanetary Biogenic Ices with SPHEREx

NASA Astrophysics Data System (ADS)

Melnick, Gary; SPHEREx Science Team

2018-01-01

Many of the most important building blocks of life are locked in interstellar and protoplanetary ices. Examples include H2O, CO, CO2, and CH3OH, among others. There is growing evidence that within the cores of dense molecular clouds and the mid-plane of protoplanetary disks the abundance of these species in ices far exceeds that in the gas phase. As a result, collisions between ice-bearing bodies and newly forming planets are thought to be a major means of delivering these key species to young planets. There currently exist fewer than 250 ice absorption spectra toward Galactic molecular clouds, which is insufficient to reliably trace the ice content of clouds through the various evolutionary stages of collapse to form stars and planets. Likewise, the current number of spectra is inadequate to assess the effects of environment, such as cloud density and temperature, presence or absence of embedded sources, external FUV and X-ray radiation, gas-phase composition, or cosmic-ray ionization rate, on the ice composition of clouds at similar stages of evolution. Ultimately, our goal is to understand how these findings connect to our own Solar System.SPHEREx will be a game changer for the study of interstellar, circumstellar, and protoplanetary disk ices. SPHEREx will obtain spectra over the entire sky in the optical and near-IR, including the 2.5 to 5.0 micron region, which contains the above biogenic ice features. SPHEREx will detect millions of potential background continuum point sources already catalogued by NASA’s Wide-field Infrared Survey Explorer (WISE) at 3.4 and 4.6 microns for which there is evidence for intervening gas and dust based on the 2MASS+WISE colors with sufficient sensitivity to yield ice absorption spectra with SNR ≥ 100 per spectral resolution element. The resulting > 100-fold increase in the number of high-quality ice absorption spectra toward a wide variety of regions distributed throughout the Galaxy will reveal correlations between ice content and environment not possible with current spectra or the limited number that will be obtained with JWST. Finally, because SPHEREx and JWST will overlap beyond 2022, SPHEREx will provide JWST with a complete ice source catalog for follow-up.

Is interstellar detection of higher members of the linear radicals CnCH and CnN feasible?

NASA Technical Reports Server (NTRS)

Pauzat, F.; Ellinger, Y.; Mclean, A. D.

1991-01-01

Rotational constants and dipole moments for linear-chain radicals CnCH and CnN are estimated using a combinatiaon of ab initio molecular orbital calculations and observed data on the starting members of the series. CnCH with n = 0-5 have been observed by radioastronomy in carbon-rich interstellar clouds; higher members of the series have 2Pi ground states with large dipole moments and are strong candidates for observation. CN and C3N have also been observed by radioastronomy; higher members of the series, with the possible exception of C5N, have 2Pi ground states with near-zero dipole moments making their interstellar detection hopeless under present observational conditions. C5N can be a strong candidate only if it has a 2Sigma ground state, and best computations so far indicate that this is not the case.

Is interstellar detection of higher members of the linear radicals CnCH and CnN feasible?

PubMed

Pauzat, F; Ellinger, Y; McLean, A D

1991-03-01

Rotational constants and dipole moments for linear-chain radicals CnCH and CnN are estimated using a combination of ab initio molecular orbital calculations and observed data on the starting members of the series. CnCH with n = 0-5 have been observed by radioastronomy in carbon-rich interstellar clouds; higher members of the series have 2 pi ground states with large dipole moments and are strong candidates for observation. CN and C3N have also been observed by radioastronomy; higher members of the series, with the possible exception of C5N, have 2 pi ground states with near-zero dipole moments making their interstellar detection hopeless under present observational conditions. C5N can be a strong candidate only if it has a 2 sigma ground state, and our best computations so far indicate that this is not the case.

Is interstellar detection of higher members of the linear radicals CnCH and CnN feasible

DOE Office of Scientific and Technical Information (OSTI.GOV)

Pauzat, F.; Ellinger, Y.; Mclean, A.D.

1991-03-01

Rotational constants and dipole moments for linear-chain radicals CnCH and CnN are estimated using a combinatiaon of ab initio molecular orbital calculations and observed data on the starting members of the series. CnCH with n = 0-5 have been observed by radioastronomy in carbon-rich interstellar clouds; higher members of the series have 2Pi ground states with large dipole moments and are strong candidates for observation. CN and C3N have also been observed by radioastronomy; higher members of the series, with the possible exception of C5N, have 2Pi ground states with near-zero dipole moments making their interstellar detection hopeless under presentmore » observational conditions. C5N can be a strong candidate only if it has a 2Sigma ground state, and best computations so far indicate that this is not the case. 20 refs.« less

Organic Chemistry in Space

NASA Technical Reports Server (NTRS)

Charnley, Steven

2009-01-01

Astronomical observations, theoretical modeling, laboratory simulation and analysis of extraterrestrial material have enhanced our knowledge of the inventory of organic matter in the interstellar medium (ISM) and on small bodies such as comets and asteroids (Ehrenfreund & Charnley 2000). Comets, asteroids and their fragments, meteorites and interplanetary dust particles (IDPs), contributed significant amounts of extraterrestrial organic matter to the young Earth. This material degraded and reacted in a terrestrial prebiotic chemistry to form organic structures that may have served as building blocks for life on the early Earth. In this talk I will summarize our current understanding of the organic composition and chemistry of interstellar clouds. Molecules of astrobiological relevance include the building blocks of our genetic material: nucleic acids, composed of subunits such as N-heterocycles (purines and pyrimidines), sugars and amino acids. Signatures indicative of inheritance of pristine and modified interstellar material in comets and meteorites will also be discussed.

Polycyclic Aromatic Hydrocarbons and the Diffuse Interstellar Bands: a Survey

NASA Technical Reports Server (NTRS)

Salama, F.; Galazutdinov, G. A.; Krelowski, J.; Allamandola, L. J.; Musaev, F. A.; DeVincenzi, Donald L. (Technical Monitor)

1999-01-01

We discuss the proposal relating the origin of some of the diffuse interstellar bands (DIBs) to neutral and ionized polycyclic aromatic hydrocarbons (PAHs) present in interstellar clouds. Laboratory spectra of several PAHs, isolated at low temperature in inert gas matrices, are compared with an extensive set of astronomical spectra of reddened, early type stars. From this comparison, it is concluded that PAN ions are good candidates to explain some of the DIBs. Unambiguous assignments are difficult, however, due to the shift in wavelengths and the band broadening induced in the laboratory spectra by the solid matrix. Definitive band assignments and, ultimately, the test of the of the proposal that PAH ions carry some of the DIB must await the availability of gas-phase measurements in the laboratory. The present assessment offers a guideline for future laboratory experiments by allowing the preselection of promising PAH molecules to be studied in jet expansions.

GAS PHASE SYNTHESIS OF (ISO)QUINOLINE AND ITS ROLE IN THE FORMATION OF NUCLEOBASES IN THE INTERSTELLAR MEDIUM

DOE Office of Scientific and Technical Information (OSTI.GOV)

Parker, Dorian S. N.; Kaiser, Ralf I.; Kostko, Oleg

Nitrogen-substituted polycyclic aromatic hydrocarbons (NPAHs) have been proposed to play a key role in the astrochemical evolution of the interstellar medium, yet the formation mechanisms of even their simplest prototypes—quinoline and isoquinoline—remain elusive. Here, we reveal a novel concept that under high temperature conditions representing circumstellar envelopes of carbon stars, (iso)quinoline can be synthesized via the reaction of pyridyl radicals with two acetylene molecules. The facile gas phase formation of (iso)quinoline in circumstellar envelopes defines a hitherto elusive reaction class synthesizing aromatic structures with embedded nitrogen atoms that are essential building blocks in contemporary biological-structural motifs. Once ejected from circumstellarmore » shells and incorporated into icy interstellar grains in cold molecular clouds, these NPAHs can be functionalized by photo processing forming nucleobase-type structures as sampled in the Murchison meteorite.« less

C60+ - looking for the bucky-ball in interstellar space

NASA Astrophysics Data System (ADS)

Galazutdinov, G. A.; Shimansky, V. V.; Bondar, A.; Valyavin, G.; Krełowski, J.

2017-03-01

The laboratory gas-phase spectrum recently published by Campbell et al. has reinvigorated attempts to confirm the presence of the C_{60}^+ cation in the interstellar medium, through an analysis of the spectra of hot, reddened stars. This search is hindered by at least two issues that need to be addressed: (I) the wavelength range of interest is severely polluted by strong water-vapour lines coming from the Earth's atmosphere; (II) one of the major bands attributed to C_{60}^+, at 9633 Å, is blended with the stellar Mg II line, which is susceptible to non-local thermodynamic equilibrium effects in hot stellar atmospheres. Both these issues are carefully considered here for the first time, based on high-resolution and high signal-to-noise ratio echellé spectra for 19 lines of sight. The result is that the presence of C_{60}^+ in interstellar clouds is brought into question.

Spallation processes and nuclear interaction products of cosmic rays.

PubMed

Silberberg, R; Tsao, C H

1990-08-01

Most cosmic-ray nuclei heavier than helium have suffered nuclear collisions in the interstellar gas, with transformation of nuclear composition. The isotopic and elemental composition at the sources has to be inferred from the observed composition near the Earth. The source composition permits tests of current ideas on sites of origin, nucleosynthesis in stars, evolution of stars, the mixing and composition of the interstellar medium and injection processes prior to acceleration. The effects of nuclear spallation, production of radioactive nuclides and the time dependence of their decay provide valuable information on the acceleration and propagation of cosmic rays, their nuclear transformations, and their confinement time in the Galaxy. The formation of spallation products that only decay by electron capture and are relatively long-lived permits an investigation of the nature and density fluctuations (like clouds) of the interstellar medium. Since nuclear collisions yield positrons, antiprotons, gamma rays and neutrinos, we shall discuss these topics briefly.

The interstellar medium in the starburst regions of NGC 253 and NGC 3256

NASA Astrophysics Data System (ADS)

Carral, P.; Hollenbach, D. J.; Lord, S. D.; Colgan, S. W. J.; Haas, Michael R.; Rubin, R. H.; Erickson, E. F.

1994-03-01

We discuss observations of the (C II) 158 micrometers, (O I) 63 micrometers, (Si II) 35 micrometers, (O III) 52,88 micrometers, and (S III) 33 micrometers fine-structure transitions toward the central 45 seconds of the starburst galaxies NGC 253 and NGC 3256. The (C II) and (O I) emission probably originates in photodissociated gas at the surfaces of molecular clouds, although a small (less than or approximately 30%) contribution to the (C II) flux from H II regions cannot be ruled out. The (O III) and (S III) lines originate in H II regions and the (Si II) flux is best explained as originating in H II regions with some contribution from photodissociation regions (PDRs). The gas phase silicon abundance is nearly solar in NGC 253, which we interpret as evidence for grain destruction in the starburst region. We find that the photodissociated atomic gas has densities approximately 104/cu cm and temperature 200-300 K. About 2% of the gas is in this phase. The thermal gas pressure in the PDRs, P(PDR)/k approximately 1-3 x 106 K/cu cm, might represent the 'typical' interstellar gas pressure in starburst systems. The Far Ultraviolet (FUV) radiation fields illuminating the clouds are 103-104 stronger than the local Galactic FUV field and come from the contribution of many closely packed O and B stars. For the central 250 pc of NGC 253, we find that the H II gas has an average density ne is approximately 400/cu cm. This corresponds to a thermal pressure P(H II)/k approximately 7 x 106 K/cu cm which is approximately P(PDR)/k, suggesting that the ionized gas is in pressure equilibrium with the photodissociated gas at the surfaces of molecular clouds. The H II gas fills a significant fraction, approximately 0.01-0.3, of the volume between the clouds. The effective temperature of the ionizing stars in NGC 253 is greater than or approximately 34,500 K; 2 x 105 O7.5 stars would produce the observed Lyman continuum photon luminosity. The average separation between the stars is approximately 3 pc. Applying the simple model for the interstellar medium in galactic nuclei of Wolfire, Tielens, & Hollenbach (1990), we find the molecular gas in the central regions of NGC 253 and NGC 3256 to be distributed in a large number (5 x 103 to 5 x 105) of small (0.5-2 pc), dense (approximately 104/cu cm) clouds (or alternatively 'thin-flattened' structures) with volume filling factors 10-3 to 10-2, very different from the local Interstellar Medium (ISM) of the Galaxy. We suggest a self-consistent scenario for the ISM in NGC 253 in which clouds and H II gas are in pressure balance with a supernova-shocked, hot 1-3 x 106 K, low-density (approximately 104/cu cm), all pervasive medium. A feedback mechanism may be indicated in which the pressure generated by the supernovae compresses the molecular clouds and triggers further massive star formation. The similarity of ISM parameters deduced for NGC 253, NGC 3256, and M82 (Lord et al. 1993) suggests that the ISM properties are independent of the luminosity of the starburst or the triggering mechanism, but are rather endemic to starburst systems. The starburst in NGC 3256 appears to be a scaled-up version of the NGC 253 and M82 starbursts.

Far-infrared Spectroscopy of Interstellar Gas

NASA Technical Reports Server (NTRS)

Phillips, T. G.

1984-01-01

Research results of far-infrared spectroscopy with the Kuiper Airborne Observatory are discussed. Both high and intermediate resolution have been successfully employed in the detection of many new molecular and atomic lines including rotational transition of hydrides such as OH, H2O, NH3 and HCl; high J rotational transitions of CO; and the ground state fine structure transitions of atomic carbon, oxygen, singly ionized carbon and doubly ionized oxygen and nitrogen. These transitions have been used to study the physics and chemistry of clouds throughout the galaxy, in the galactic center region and in neighboring galaxies. This discussion is limited to spectroscopic studies of interstellar gas.

Organic Chemistry of Meteorites

NASA Technical Reports Server (NTRS)

Chang, S.; Morrison, David (Technical Monitor)

1994-01-01

Studies of the molecular structures and C,N,H-isotopic compositions of organic matter in meteorites reveal a complex history beginning in the parent interstellar cloud which spawned the solar system. Incorporation of interstellar dust and gas in the protosolar nebula followed by further thermal and aqueous processing on primordial parent bodies of carbonaceous, meteorites have produced an inventory of diverse organic compounds including classes now utilized in biochemistry. This inventory represents one possible set of reactants for chemical models for the origin of living systems on the early Earth. Evidence bearing on the history of meteoritic organic matter from astronomical observations and laboratory investigations will be reviewed and future research directions discussed.

The formation of stellar systems from interstellar molecular clouds

NASA Technical Reports Server (NTRS)

Gehrz, R. D.; Black, D. C.; Solomon, P.M.

1984-01-01

The observational and theoretical study of regions of continuing star formation promises greater insight into the physical conditions and events associated with the formation of the solar system, and elucidates the role played by star formation in the evolutionary cycle which seems to dominate interstellar material's processing by successive generations of stars in the spiral galaxies. Novel astronomical methods incorporated by the new facilities scheduled for development in the 1980s may yield substantial advancements in star formation process theory; most significant among these efforts will be the identification and examination of the elusive protostellar collapse phase of both star and planetary system formation.

Helium in supernova remnants

NASA Astrophysics Data System (ADS)

Danziger, I. J.; Leibowitz, E.

1983-02-01

Observational data for supernova remnants (SNR) which exhibit He abundances exceeding the normal value of 0.10 are examingd. Attention is given to the Crab, Cas A, and Puppis A nebular O, N, and He abundances, sizes, and ages. The overabundances of He and N detected in the Crab and Cas A nebulae are suggested to have arisen from swept-up interstellar matter. Therefore, the He abundances in interstellar matter may be calculated from data on old SNR. However, the Larger and Smaller Magellanic clouds display lowered abundances, which may be due to stratification caused by ionization. The method is useful for SNRs located within 5 mpc of the local group of galaxies.

Stellar Ontogeny: From Dust...

ERIC Educational Resources Information Center

MOSAIC, 1978

1978-01-01

Discusses the process of star formation. Infrared and radio astronomy, particularly microwave astronomy is used to provide information on different stages of stellar formation. The role of dust and gas which swirl through the interstellar regions of a galaxy and the collapse of a cloud in star formation are also presented. (HM)

Carbon chain abundance in the diffuse interstellar medium

NASA Technical Reports Server (NTRS)

Allamandola, L. J.; Hudgins, D. M.; Bauschlicher, C. W. Jr; Langhoff, S. R.

1999-01-01

Thanks to the mid-IR sensitivities of the ISO and IRTS orbiting spectrometers it is now possible to search the diffuse interstellar medium for heretofore inaccessible molecular emission. In view of the recent strong case for the presence of C(7-) (Kirkwood et al. 1998, Tulej et al. 1998),and the fact that carbon chains possess prominent infrared active modes in a very clean portion of the interstellar spectrum, we have analyzed the IRTS spectrum of the diffuse interstellar medium for the infrared signatures of these species. Theoretical and experimental infrared band frequencies and absolute intensities of many different carbon chain species are presented. These include cyanopolyynes, neutral and anionic linear carbon molecules, and neutral and ionized, even-numbered, hydrogenated carbon chains. We show that--as a family--these species have abundances in the diffuse ISM on the order of 10(-10) with respect to hydrogen, values consistent with their abundances in dense molecular clouds. Assuming an average length of 10 C atoms per C-chain implies that roughly a millionth of the cosmically available carbon is in the form of carbon chains and that carbon chains can account for a few percent of the visible to near-IR diffuse interstellar band (DIB) total equivalent width (not DIB number).

Velocity-resolved [{\\rm{C}}\\,{\\rm{II}}] Emission from Cold Diffuse Clouds in the Interstellar Medium

NASA Astrophysics Data System (ADS)

Goldsmith, Paul F.; Pineda, Jorge L.; Neufeld, David A.; Wolfire, Mark G.; Risacher, Christophe; Simon, Robert

2018-04-01

We have combined emission from the 158 μm fine structure transition of C+ observed with the GREAT and upGREAT instruments on SOFIA with 21 cm absorption spectra and visual extinction to characterize the diffuse interstellar clouds found along the lines of sight. The weak [C II] emission is consistent in velocity and line width with the strongest H I component produced by the cold neutral medium. The H I column density and kinetic temperature are known from the 21 cm data and, assuming a fractional abundance of ionized carbon, we calculate the volume density and thermal pressure of each source, which vary considerably, with 27 {cm}}-3≤slant n({{{H}}}0) ≤slant 210 cm‑3 considering only the atomic hydrogen along the lines of sight to be responsible for the C+, while 13 {cm}}-3≤slant n({{{H}}}0+{{{H}}}2)≤slant 190 cm‑3 including the hydrogen in both forms. The thermal pressure varies widely with 1970 cm‑3 K ≤slant {P}th}/k≤slant 10,440 cm‑3 K for H0 alone and 750 cm‑3 K ≤ P th/k ≤ 9360 cm‑3 K including both H0 and H2. The molecular hydrogen fraction varies between 0.10 and 0.67. Photoelectric heating is the dominant heating source, supplemented by a moderately enhanced cosmic ray ionization rate, constrained by the relatively low 45 K to 73 K gas temperatures of the clouds. The resulting thermal balance for the two lower-density clouds is satisfactory, but for the two higher-density clouds, the combined heating rate is insufficient to balance the observed C+ cooling.

THERMAL PRESSURES IN THE INTERSTELLAR MEDIUM OF THE MAGELLANIC CLOUDS

DOE Office of Scientific and Technical Information (OSTI.GOV)

Welty, Daniel E.; York, Donald G.; Lauroesch, James T.

2016-04-20

We discuss the thermal pressures ( n {sub H} T ) in predominantly cold, neutral interstellar gas in the Magellanic Clouds, derived from analyses of the fine-structure excitation of neutral carbon, as seen in high-resolution Hubble Space Telescope /Space Telescope Imaging Spectrograph spectra of seven diverse sight lines in the LMC and SMC. Detailed fits to the line profiles of the absorption from C i, C i*, and C i** yield consistent column densities for the three to six C i multiplets detected in each sight line. In the LMC and SMC, N (C i{sub tot}) is consistent with Galacticmore » trends versus N (Na i) and N (CH), but is slightly lower versus N (K i) and N (H{sub 2}). As for N (Na i) and N (K i), N (C i{sub tot}) is generally significantly lower, for a given N (H{sub tot}), in the LMC and (especially) in the SMC, compared to the local Galactic relationship. For the LMC and SMC components with well-determined column densities for C i, C i*, and C i**, the derived thermal pressures are typically factors of a few higher than the values found for most cold, neutral clouds in the Galactic ISM. Such differences are consistent with the predictions of models for clouds in systems (like the LMC and SMC) that are characterized by lower metallicities, lower dust-to-gas ratios, and enhanced radiation fields—where higher pressures are required for stable cold, neutral clouds. The pressures may be further enhanced by energetic activity (e.g., due to stellar winds, star formation, and/or supernova remnants) in several of the regions probed by these sight lines. Comparisons are made with the C i observed in some quasar absorption-line systems.« less

Directed Panspermia. 3. strategies and Motivation for Seeding Star-Forming Clouds

NASA Astrophysics Data System (ADS)

Mautner, Michael N.

1997-11-01

Microbial swarms aimed at star-forming regions of interstellar clouds can seed stellar associations of 10 - 100 young planetary systems. Swarms of millimeter size, milligram packets can be launched by 35 cm solar sails at 5E-4 c, to penetrate interstellar clouds. Selective capture in high-density planetary accretion zones of densities > 1E-17 kg m-3 is achieved by viscous drag. Strategies are evaluated to seed dense cloud cores, or individual protostellar condensations, accretion disks or young planets therein. Targeting the Ophiuchus cloud is described as a model system. The biological content, dispersed in 30 μm, 1E-10 kg capsules of 1E6 freeze-dried microorganisms each, may be captured by new planets or delivered to planets after incorporation first into carbonaceous asteroids and comets. These objects, as modeled by meteorite materials, contain biologically available organic and mineral nutrients that are shown to sustain microbial growth. The program may be driven by panbiotic ethics, predicated on: 1. The unique position of complex organic life amongst the structures of Nature; 2. Self-propagation as the basic propensity of the living pattern; 3. The biophysical unity humans with of the organic, DNA/protein family of life; and 4. Consequently, the primary human purpose to safeguard and propagate our organic life form. To promote this purpose, panspermia missions with diverse biological payloads will maximize survival at the targets and induce evolutionary pressures. In particular, eukaryotes and simple multicellular organisms in the payload will accelerate higher evolution. Based on the geometries and masses of star-forming regions, the 1E24 kg carbon resources of one solar system, applied during its 5E9 yr lifespan, can seed all newly forming planetary systems in the galaxy.

Laboratory and observational study of the interrelation of the carbonaceous component of interstellar dust and solar system materials

NASA Technical Reports Server (NTRS)

Allamandola, L. J.; Sanford, S. A.; Schutte, W. A.; Tielens, A. G. G. M.

1991-01-01

By studying the chemical and isotopic composition of interstellar ice and dust, one gains insight into the composition and chemical evolution of the solid bodies in the solar nebula and the nature of the material subsequently brought into the inner part of the solar system by comets and meteorites. It is now possible to spectroscopically probe the composition of interstellar ice and dust in the mid-infrared, the spectral range which is most diagnostic of fundamental molecular vibrations. We can compare these spectra of various astronomical objects (including the diffuse and dense interstellar medium, comets, and the icy outer planets and their satellites) with the spectra of analogs we produce in the laboratory under conditions which mimic those in these different objects. In this way one can determine the composition and abundances of the major constituents of the various ices and place general constraints on the types of organics coating the grains in the diffuse interstellar medium. In particular we have shown the ices in the dense clouds contain H2O, CH3OH, CO, perhaps some NH3 and H2CO, we well as nitriles and ketones or esters. Furthermore, by studying the photochemistry of these ice analogs in the laboratory, one gains insight into the chemistry which takes place in interstellar/precometary ices. Chemical and spectroscopic studies of photolyzed analogs (including deuterated species) are now underway. The results of some of these studies will be presented and implications for the evolution of the biogenic elements in interstellar dust and comets will be discussed.

Large Interstellar Polarisation Survey (LIPS). I. FORS2 spectropolarimetry in the Southern Hemisphere

NASA Astrophysics Data System (ADS)

Bagnulo, Stefano; Cox, Nick L. J.; Cikota, Aleksandar; Siebenmorgen, Ralf; Voshchinnikov, Nikolai V.; Patat, Ferdinando; Smith, Keith T.; Smoker, Jonathan V.; Taubenberger, Stefan; Kaper, Lex; Cami, Jan; LIPS Collaboration

2017-12-01

Polarimetric studies of light transmitted through interstellar clouds may give constraints on the properties of the interstellar dust grains. Traditionally, broadband linear polarisation (BBLP) measurements have been considered an important diagnostic tool for the study of the interstellar dust, while comparatively less attention has been paid to spectropolarimetric measurements. However, spectropolarimetry may offer stronger constraints than BBLP, for example by revealing narrowband features, and by allowing us to distinguish the contribution of dust from the contribution of interstellar gas. Therefore, we have decided to carry out a Large Interstellar Polarisation Survey (LIPS) using spectropolarimetric facilities in both hemispheres. Here we present the results obtained in the Southern Hemisphere with the FORS2 instrument of the ESO Very Large Telescope. Our spectra cover the wavelength range 380-950 nm at a spectral resolving power of about 880. We have produced a publicly available catalogue of 127 linear polarisation spectra of 101 targets. We also provide the Serkowski-curve parameters, as well as the wavelength gradient of the polarisation position angle for the interstellar polarisation along 76 different lines of sight. In agreement with previous literature, we found that the best-fit parameters of the Serkowski-curve are not independent of each other. However, the relationships that we obtained are not always consistent with what has been found in previous studies. Table 2 and reduced data are available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/608/A146

Abundances and Excitation of H2, H3+ & CO in Star-Forming Regions

NASA Astrophysics Data System (ADS)

Kulesa, Craig A.

Although most of the 123 reported interstellar molecules to date have been detected through millimeter-wave emission-line spectroscopy, this technique is inapplicable to non-polar molecules like H2 and H3+, which are central to our understanding of interstellar chemistry. Thus high resolution infrared absorption-line spectroscopy bears an important role in interstellar studies: chemically important non-polar molecules can be observed, and their abundances and excitation conditions can be referred to the same ``pencil beam'' absorbing column. In particular, through a weak quadrupole absorption line spectrum at near-infrared wavelengths, the abundance of cold H2 in dark molecular clouds and star forming regions can now be accurately measured and compared along the same ``pencil beam'' line of sight with the abundance of its most commonly cited surrogate, CO, and its rare isotopomers. Also detected via infrared line absorption is the pivotal molecular ion H3+, whose abundance provides the most direct measurement of the cosmic ray ionization rate in dark molecular clouds, a process that initiates the formation of many other observed molecules there. Our growing sample of H2 and CO detections now includes detailed multi-beam studies of the ρ Ophiuchi molecular cloud and NGC 2024 in Orion. We explore the excitation and degree of ortho- and para-H2 thermalization in dark clouds, variation of the CO abundance over a cloud, and the relation of H2 column density to infrared extinction mapping, far-infrared/submillimeter dust continuum emission, and large scale submillimeter CO, [C I] and HCO+ line emission -- all commonly invoked to indirectly trace H2 during the past 30+ years. For each of the distinct velocity components seen toward some embedded young stellar objects, we are also able to determine the temperature, density, and a CO/H2 abundance ratio, thus unraveling some of the internal structure of a star-forming cloud. H2 and H3+ continue to surprise and delight us with more mysteries. We present imaging and spectroscopy of excited H2 line emission from two Crab Nebula filaments, leading to intriguing questions -- such as the rapid formation, excitation, and continued survival of hydrogen molecules in such a hostile environment. Similarly, we depict the recent detection of CO and H3+ emission from the circumstellar disks of nearby Herbig AeBe stars, providing an outstanding diagnostic of energetic pre-planetary environments and a valuable study of the non-thermal excitation of H3+ in its own right. These studies spotlight the role of molecules as regulators and probes of physical processes in molecular clouds and star- & planet-forming regions. See: http://loke.as.arizona.edu/˜ckulesa/research/ for preprints & more information

Deflection of jets induced by jet-cloud and jet-galaxy interactions

NASA Astrophysics Data System (ADS)

Mendoza, S.; Longair, M. S.

2001-06-01

The model first introduced by Raga & Cantó in which astrophysical jets are deflected on passing through an isothermal high-density region is generalized by taking into account gravitational effects on the motion of the jet as it crosses the high-density cloud. The problem is also generalized for relativistic jets in which gravitational effects induced by the cloud are neglected. Two further cases, classical and relativistic, are discussed for the cases in which the jet is deflected on passing through the interstellar gas of a galaxy in which a dark matter halo dominates the gravitational potential. The criteria for the stability of jets due to the formation of internal shocks are also discussed.

The peculiar extinction of Herschel 36

NASA Technical Reports Server (NTRS)

Donn, B.; Hecht, J. H.; Helfer, H. L.; Wolf, J.; Pipher, J. L.

1982-01-01

The extinction of Herschel 36 was measured and found to be peculiar in the same sense as that observed in Orion. Following the treatment of Mathis and Wallenhorst, this can be explained by the presence of large silicate and graphite grains than are normally found in the interstellar medium. Correcting the stellar flux for foreground extinction results in a residual extinction curve for the associated dust cloud, with an unusually small normalized extinction (less than 1.0) at 1500 A. This low UV extinction may be due to the effects of scattering by the dust cloud material.

H2 spectroscopy as an agent for extinction determinations The near-infrared curve for the Orion molecular cloud

NASA Technical Reports Server (NTRS)

Davis, D. S.; Larson, H. P.; Hofmann, R.

1986-01-01

A near-infrared (1.8 to 3.5) microns extinction curve for the Orion molecular cloud is presented. The curve is derived from high-resolution spectra of the Orion H2 source recorded from the Kuiper Airborne Observatory. The data reveal that the Orion extinction law is indistinguishable from a 1/lambda form in the near-infrared, except for strongly enhanced extinction near a wavelength of about 3 microns. The implications of these results, in the context of current interstellar grain models, are discussed.

The cosmic-ray and gas content of the Cygnus region as measured in γ -rays by the Fermi Large Area Telescope

DOE PAGES

Ackermann, M.

2012-02-01

Context. The Cygnus region hosts a giant molecular-cloud complex that actively forms massive stars. Interactions of cosmic rays with interstellar gas and radiation fields make it shine at γ-ray energies. Several γ-ray pulsars and other energetic sources are seen in this direction. Aims. In this paper we analyze the γ-ray emission measured by the Fermi Large Area Telescope in the energy range from 100 MeV to 100 GeV in order to probe the gas and cosmic-ray content on the scale of the whole Cygnus complex. The γ-ray emission on the scale of the central massive stellar clusters and from individualmore » sources is addressed elsewhere. Methods. The signal from bright pulsars is greatly reduced by selecting photons in their off-pulse phase intervals. We compare the diffuse γ-ray emission with interstellar gas maps derived from radio/mm-wave lines and visual extinction data. A general model of the region, including other pulsars and γ-ray sources, is sought. Results. The integral Hi emissivity above 100 MeV averaged over the whole Cygnus complex amounts to [2.06 ± 0.11 (stat.) +0.15 -0.84 (syst.)] × 10 -26 photons s -1 sr -1 H-atom -1, where the systematic error is dominated by the uncertainty on the Hi opacity to calculate its column densities. The integral emissivity and its spectral energy distribution are both consistent within the systematics with LAT measurements in the interstellar space near the solar system. The average XCO = N(H2)/WCO ratio is found to be [1.68 ± 0.05 (stat.) +0.87 -0.10 (Hi opacity)] × 1020 molecules cm -2 (K km s -1) -1, consistent with other LAT measurements in the Local Arm. We detect significant γ-ray emission from dark neutral gas for a mass corresponding to ~ 40% of what is traced by CO. The total interstellar mass in the Cygnus complex inferred from its γ-ray emission amounts to 8 +5 -1 × 106M⊙ at a distance of 1.4 kpc. Conclusions. Despite the conspicuous star formation activity and high masses of the interstellar clouds, the cosmic-ray population in the Cygnus complex averaged over a few hundred parsecs is similar to that of the local interstellar space.« less

The Cosmic-Ray and Gas Content of the Cygnus Region as Measured in Gamma Rays by the Fermi Large Area Telescope

NASA Technical Reports Server (NTRS)

Ackermann, M.; Ajello, M.; Allafort, A.; Baldini, L.; Ballet, J.; Barbiellini, G.; Bastieri, D.; Belfiore, A.; Bellazzini, R.; Berenji, B.;

ISO finds a very steamy cloud in interstellar space

NASA Astrophysics Data System (ADS)

1998-04-01

The discovery, which may provide an important clue to the origin of water in the Solar System, is reported in an article in Astrophysical Journal Letters, to be published on 20 April. This Information Note is distributed simultaneously with a Press Release on the same subject from Cornell University, Johns Hopkins University, the Harvard/Smithsonian Center for Astrophysics, and NASA's Ames Research Center. The water vapour is in the Orion Molecular Cloud, a giant interstellar mass composed primarily of hydrogen molecules. The observations were carried out in October 1997 with the Long Wavelength Spectrometer, devised by a British-led team as one of four instruments on board ISO. Looking at long infrared wavelengths, the astronomers saw the characteristic signature of emission by water vapour. "The interstellar gas cloud that we observed is being pummelled by shock waves that compress and heat the gas," says Martin Harwit of Cornell University, an ISO mission scientist and lead author on the article reporting the finding. "These shock waves are the result of the violent early stages of starbirth, in which a young star spews out gas that slams into its surroundings at high speed. The heated water vapour that we observed is the result of that collision." Harwit also suggests that shock waves are a cause of starbirth as well as a result. "In the future," he says, "they may also trigger the formation of additional stars and planets as they compress the gas cloud that we observed, but only if surplus heat can be radiated away. Even though the interstellar gas is composed primarily of hydrogen molecules, water vapour is a particularly efficient radiator at far-infrared wavelengths and plays a critical role in cooling the gas and facilitating the star formation process. Because the Earth's own wet atmosphere is completely opaque at the wavelengths of interest, the observations that we are reporting today are possible only from space with the use of the ISO satellite." A correct prediction The concentration of water vapour measured by the US team was roughly one part in 2000 by volume, far larger than any measured previously in interstellar space. But the strength of the water radiation detected from Orion was in perfect agreement with theoretical predictions in the PhD thesis of team member Michael Kaufman, a former Johns Hopkins graduate student now at NASA's Ames Research Center. "An enhanced concentration of water is precisely what we expected in this gas cloud," comments team member Gary Melnick of the Harvard-Smithsonian Center for Astrophysics. "We are looking at a region of interstellar space where shock waves have made the gas abnormally warm. For the past 25 years, astrophysicists have been predicting that whenever the temperature exceeds about 100 °C, chemical reactions will convert most of the oxygen atoms in the interstellar gas into water. And that's exactly what we've observed in Orion." The source of the water we live by The high concentration of water measured in Orion may have also had implications for the origin of water in the Solar System and on the Earth itself, according to team member David Neufeld of Johns Hopkins University. "The interstellar gas cloud that we observed in Orion seems to be a huge chemical factory," Neufeld says, "generating enough water molecules in a single day to fill the Earth's oceans sixty times over. Eventually that water vapour will cool and freeze, turning into small solid particles of ice. Similar ice particles were presumably present within the gas cloud from which the Solar System originally formed. It seems quite plausible that much of the water in the Solar System was originally produced in a giant water-vapour factory like the one we have observed in Orion." A footnote about ISO ISO was put into orbit in November 1995, by an Ariane 44PP launcher at the European space base at Kourou in French Guiana. As an unprecedented observatory for infrared astronomy, able to examine cool and hidden places in the Universe, ISO has observed more than 26,000 individual objects. A supply of liquid helium, used to cool the telescope and instruments close to the absolute zero of temperature, has lasted much longer than expected, but ran out on 8 April 1998 (see ESA Press Information Note N°11-98 of 9 April). Further information is available from: ESA Public Relations Division Tel: +33(0)1.53.69.71.55 Fax : +33(0)1.53.69.76.90 ESA ISO: Dr Martin Kessler at +34.(9)1.813.12.53 or mkessler@iso.vilspa.esa.es And from US team members: Prof. Martin Harwit at +1 202-479-6877 or mharwit@ibm.net Prof. David Neufeld at +1 410-516-8582 or neufeld@pha.jhu.edu Dr Gary Melnick at +1 617-495-7388 or gmelnick@cfa.harvard.edu Dr Michael Kaufman at +1 650-604-0320 or kaufman@warped.arc.nasa.gov ISO on the Internet For more details about ISO, and a picture gallery, visit the website: http://isowww.estec.esa.nl Panels showing two examples of measurements carried out on board the ISO, together with an image of the Orion nebula taken with the Wide Field Planetary Camera 2 on NASA's Hubble Space Telescope can be seen on the World Wide Web at http://www.pha.jhu.edu/~neufeld/orionwater.html.

Is life the rule or the exception? The answer may be in the interstellar clouds

NASA Astrophysics Data System (ADS)

2002-05-01

Credits: ESA 2002. Illustration by Medialab Did the main ingredients for life come from outer space? In addition to forming in comets and asteroids, amino acids, the 'building blocks' of life, may form in dust grains in the space between the stars Rosetta artist view hi-res Size hi-res: 397 kb Credits: ESA Rosetta’s mission to a comet An artist's impression of the Rosetta spacecraft, its target Comet 67P/Churyumov-Gerasimenko, and the Philae lander being delivered onto its surface. Rosetta’s 11-year expedition began in March 2004, with an Ariane 5 launch from Kourou in French Guiana, and the spacecraft was then sent towards the outer Solar System. The long journey includes three gravity assists at Earth (2004, 2007, 2009), one at Mars (2007), and two asteroid encounters: (2867) Steins (2008) and (21) Lutetia (2010). Rosetta will reach Comet 67/P Churyumov-Gerasimenko in 2014, and will be the first mission ever to orbit a comet’s nucleus and to deliver a lander, called Philae, on its surface. Artist's Impression of the Herschel Spacecraft hi-res Size hi-res: 138 Kb Artist's Impression of the Herschel Spacecraft Herschel is the only space facility dedicated to the submillimetre and far infrared part of the spectrum. Its vantage point in space provides several decisive advantages, including a low and stable background and full access to this part of the spectrum. Herschel has the potential of discovering the earliest epoch proto-galaxies, revealing the cosmologically evolving AGN-starburst symbiosis, and unraveling the mechanisms involved in the formation of stars and planetary system bodies. The key science objectives emphasise specifically the formation of stars and galaxies, and the interrelation between the two, but also includes the physics of the interstellar medium, astrochemistry, and solar system studies. Herschel will carry a 3.5 metre diameter passively cooled telescope. The science payload complement - two cameras/medium resolution spectrometers (PACS and SPIRE) and a very high resolution heterodyne spectrometer (HIFI) - will be housed in a superfluid helium cryostat. Herschel will be placed in a transfer trajectory towards its operational orbit around the Earth-Sun L2 point by an Ariane 5 (shared with Planck) in early 2007. Once operational FIRST will offer a minimum of 3 years of routine observations; roughly 2/3 of the available observing time is open to the general astronomical community through a standard competitive proposal procedure. This result is consistent with (although of course does not prove) the theory that the main ingredients for life came from outer space, and therefore that chemical processes leading to life are likely to have occurred elsewhere. This reinforces the interest in an already 'hot' research field, astrochemistry. ESA's forthcoming missions Rosetta and Herschel will provide a wealth of new information for this topic. Amino acids are the 'bricks' of the proteins, and proteins are a type of compound present in all living organisms. Amino acids have been found in meteorites that have landed on Earth, but never in space. In meteorites amino acids are generally thought to have been produced soon after the formation of the Solar System, by the action of aqueous fluids on comets and asteroids - objects whose fragments became today's meteorites. However, new results published recently in Nature by two independent groups show evidence that amino acids can also form in space. Between stars there are huge clouds of gas and dust, the dust consisting of tiny grains typically smaller than a millionth of a millimetre. The teams reporting the new results, led by a United States group and a European group, reproduced the physical steps leading to the formation of these grains in the interstellar clouds in their laboratories, and found that amino acids formed spontaneously in the resulting artificial grains. The researchers started with water and a variety of simple molecules that are known to exist in the 'real' clouds, such as carbon monoxide, carbon dioxide, ammonia and hydrogen cyanide. Although these initial ingredients were not exactly the same in each experiment, both groups 'cooked' them in a similar way. In specific chambers in the laboratory they reproduced the common conditions of temperature and pressure known to exist in interstellar clouds, which is, by the way, quite different from our 'normal' conditions. Interstellar clouds have a temperature of 260 °C below zero, and the pressure is also very low (almost zero). Great care was taken to exclude contamination. As a result, grains analogous to those in the clouds were formed. The researchers illuminated the artificial grains with ultraviolet radiation, a process that typically triggers chemical reactions between molecules and that also happens naturally in the real clouds. When they analysed the chemical composition of the grains, they found that amino acids had formed. The United States team detected glycine, alanine and serine, while the European team listed up to 16 amino acids. The differences are not considered relevant since they can be attributed to differences in the initial ingredients. According to the authors, what is relevant is the demonstration that amino acids can indeed form in space, as a by-product of chemical processes that take place naturally in the interstellar clouds of gas and dust. Max P. Bernstein from the United States team points out that the gas and dust in the interstellar clouds serve as 'raw material' to build stars and planetary systems such as our own. These clouds "are thousands of light years across; they are vast, ubiquitous, chemical reactors. As the materials from which all stellar systems are made pass through such clouds, amino acids should have been incorporated into all other planetary systems, and thus been available for the origin of life." The view of life as a common event would therefore be favoured by these results. However, many doubts remain. For example, can these results really be a clue to what happened about four billion years ago on the early Earth? Can researchers be truly confident that the conditions they recreate are those in the interstellar space? Guillermo M. Muñoz Caro from the European team writes "several parameters still need to be better constrained (...) before a reliable estimation on the extraterrestrial delivery of amino acids to the early Earth can be made. To this end, in situ analysis of cometary material will be performed in the near future by space probes such as Rosetta ..." The intention for ESA's spacecraft Rosetta is to provide key data for this question. Rosetta, to be launched next year, will be the first mission ever to orbit and land on a comet, namely Comet 46P/Wirtanen. Starting in 2011, Rosetta will have two years to examine in deep detail the chemical composition of the comet. As Rosetta's project scientist Gerhard Schwehm has stated, "Rosetta will carry sophisticated payloads that will study the composition of the dust and gas released from the comet's nucleus and help to answer the question: did comets bring water and organics to Earth?" If amino acids can also form in the space amid the stars, as the new evidence suggests, research should also focus on the chemistry in the interstellar space. This is exactly one of the main goals of the astronomers preparing for ESA's space telescope Herschel. Herschel, with its impressive mirror of 3.5 metres in diameter (the largest of any imaging space telescope) is due to be launched in 2007. One of its strengths is that it will 'see' a kind of radiation that has never been detected before. This radiation is far-infrared and submillimetre light, precisely what you need to detect if you are searching for complex chemical compounds such as the organic molecules.

Summary of Research/Publications

NASA Technical Reports Server (NTRS)

1997-01-01

Summary of research/publications include:(1) Comment on broadening of water microwave lines by collisions with helium atoms; (2) Calculations of ion-molecule deuterium fractionation reactions involving HD; (3) Ab initio predictions on the rotational spectra of carbon-chain carbene molecules; (4) Theoretical IR spectra of ionized naphthalene; (5) Improved collisional excitation rates for interstellar water; (6) Calculations on the competition between association and reaction for C3H+ + H2; (7) Theoretical infrared spectra of some model polycyclic aromatic hydrocarbons: effect of ionization; (8) Calculations concerning interstellar isomeric abundance ratios for C3H and C3H2; (9) New calculations on the ion-molecule processes C2H2+ + H2 C2H3+ + H and C2H2+ + H2 C2H4+; (10) Anisotropic rigid rotor potential energy function for H2O-H2; (11) A correlated ab initio study of linear carbon-chain radicals CnH (n=2-7); (12) Ab initio characterization of MgCCH, MgCCH+, and MgC2 and pathways to their formation in the interstellar medium; (13) Why HOC+ is detectable in interstellar clouds: The rate of the reaction between HOC+ and H2; (14) A correlated ab initio study of the X 2A 1 and A 2E states of MgCH3; (15) On the stability of interstellar carbon clusters: The rate of the reaction between C3 and O; and (16) The rate of the reaction between CN and C2H2 at interstellar temperatures.

First-trimester translucency: aneuploidy, sonographic findings, and maternal age.

PubMed

Mahieu-Caputo, D; Dommergues, M; Morichon-Delvallez, N; Aubry, M C; Wekemans, M; Dumez, Y

1996-01-01

The positive predictive value of 1st-trimester nuchal translucency for the diagnosis of fetal aneuploidy is reported to range from 19 to 72% in retrospective series and from 2.8 to 4.8% in prospective studies. In order to analyze the biases accounting for such discrepancies, we retrospectively analyzed a consecutive series of 66 cases of translucency and studied the rate of aneuploidy as a function of sonographic findings and maternal age. The aneuploidy rate was significantly higher in the 23 cases in whom translucency involved the fetal trunk (65%) than in the 43 cases in whom translucency was confined to the nuchal area (16%). This rate was not significantly higher in septated translucencies. These trends persisted after adjustment for maternal age, but the rate of chromosomal anomalies was twice lower in younger mothers. Among the 46 cases in whom the maternal age was < 35 years, 10% of the 30 fetuses with localized nuchal translucency had an abnormal karyotype. In contrast, the rate of aneuploidy was 85% among the 7 cases with diffuse translucency for whom the maternal age was 35 years or above. These findings confirm that both maternal age and size of the translucency should be taken into account to evaluate the risks of aneuploidy.

KPOT_wlanger_1: State of the Diffuse ISM: Galactic Observations of the Terahertz CII Line (GOT CPlus)

NASA Astrophysics Data System (ADS)

Langer, W.

2007-10-01

Star formation activity throughout the Galactic disk depends on the thermal and dynamical state of the interstellar gas, which in turn depends on heating and cooling rates, modulated by the gravitational potential and shock and turbulent pressures. Molecular cloud formation, and thus the star formation, may be regulated by pressures in the interstellar medium (ISM). To understand these processes we need information about the properties of the diffuse atomic and diffuse molecular gas clouds, and Photon Dominated Regions (PDR). An important tracer of these regions is the CII line at 158 microns (1900.5 GHz). We propose a "pencil-beam" survey of CII with HIFI band 7b, based on deep integrations and systematic sparse sampling of the Galactic disk plus selected targets, totaling over 900 lines of sight. We will detect both emission and, against the bright inner Galaxy and selected continuum sources, absorption lines. These spectra will provide the astronomical community with a large rich statistical database of the diffuse cloud properties throughout the Galaxy for understanding the Milky Way ISM and, by extension, other galaxies. It will be extremely valuable for determining the properties of the atomic gas, the role of barometric pressure and turbulence in cloud evolution, and the properties of the interface between the atomic and molecular clouds. The CII line is one of the major ISM cooling lines and is present throughout the Galactic plane. It is the strongest far-IR emission line in the Galaxy, with a total luminosity about a 1000 times that of the CO J=1-0 line. Combined with other data, it can be used to determine density, pressure, and radiation environment in gas clouds, and PDRs, and their dynamics via velocity fields. HSO is the best opportunity over the next several years to probe the ISM in this tracer and will provide a template for large-scale surveys with dedicated small telescopes and future surveys of other important ISM tracers.

Herschel HIFI GOT C+ Survey: CII, HI, and CO Emissions in a Sample of Transition Clouds and Star-Forming regions in the Inner Galaxy

NASA Astrophysics Data System (ADS)

Pineda, Jorge; Velusamy, Thangasamy; Langer, William D.; Goldsmith, Paul; Li, Di; Yorke, Harold

The GOT C+ a HIFI Herschel Key Project, studies the diffuse ISM throughout the Galactic Plane, using C+ as cloud tracer. The C+ line at 1.9 THz traces a so-far poorly studied stage in ISM cloud evolution -the transitional clouds going from atomic HI to molecular H2. This transition cloud phase, which is difficult to observe in HI and CO alone, may be best characterized via CII emission or absorption. The C+ line is also an excellent tracer of the warm diffuse gas and the warm, dense gas in the Photon Dominated Regions (PDRs). We can, therefore, use the CII emission as a probe to understand the effects of star formation on their interstellar environment. We present our first results on the transition between dense and hot gas (traced by CII) and dense and cold gas (traced by 12CO and 13CO) along a few representative lines of sight in the inner Galaxy from longitude 325 degrees to 25 degrees, taken during the HIFI Priority Science Phase. Comparisons of the high spectral resolution ( 1 km/s) HIFI data on C+ with HI, 12CO, and 13CO spectra allow us to separate out the different ISM components along each line of sight. Our results provide detailed information about the transition of diffuse atomic to molecular gas clouds needed to understand star formation and the lifecycle of the interstellar gas. These observations are being carried out with the Herschel Space Observatory, which is an ESA cornerstone mission, with contributions from NASA. This research was conducted at the Jet Propulsion Laboratory, California Institute of Technology under contract with the National Aeronautics and Space Administration. JLP was supported under the NASA Postdoctoral Program at JPL, Caltech, administered by Oak Ridge Associated Universities through a contract with NASA, and is currently supported as a Caltech-JPL Postdoctoral associate.

SDP_wlanger_3: State of the Diffuse ISM: Galactic Observations of the Terahertz CII Line (GOT CPlus)

NASA Astrophysics Data System (ADS)

Langer, W.

2011-09-01

Star formation activity throughout the Galactic disk depends on the thermal and dynamical state of the interstellar gas, which in turn depends on heating and cooling rates, modulated by the gravitational potential and shock and turbulent pressures. Molecular cloud formation, and thus the star formation, may be regulated by pressures in the interstellar medium (ISM). To understand these processes we need information about the properties of the diffuse atomic and diffuse molecular gas clouds, and Photon Dominated Regions (PDR). An important tracer of these regions is the CII line at 158 microns (1900.5 GHz). We propose a "pencil-beam" survey of CII with HIFI band 7b, based on deep integrations and systematic sparse sampling of the Galactic disk plus selected targets, totaling over 900 lines of sight. We will detect both emission and, against the bright inner Galaxy and selected continuum sources, absorption lines. These spectra will provide the astronomical community with a large rich statistical database of the diffuse cloud properties throughout the Galaxy for understanding the Milky Way ISM and, by extension, other galaxies. It will be extremely valuable for determining the properties of the atomic gas, the role of barometric pressure and turbulence in cloud evolution, and the properties of the interface between the atomic and molecular clouds. The CII line is one of the major ISM cooling lines and is present throughout the Galactic plane. It is the strongest far-IR emission line in the Galaxy, with a total luminosity about a 1000 times that of the CO J=1-0 line. Combined with other data, it can be used to determine density, pressure, and radiation environment in gas clouds, and PDRs, and their dynamics via velocity fields. HSO is the best opportunity over the next several years to probe the ISM in this tracer and will provide a template for large-scale surveys with dedicated small telescopes and future surveys of other important ISM tracers.

Heavy water stratification in a low-mass protostar

NASA Astrophysics Data System (ADS)

Coutens, A.; Vastel, C.; Cazaux, S.; Bottinelli, S.; Caux, E.; Ceccarelli, C.; Demyk, K.; Taquet, V.; Wakelam, V.

2013-05-01

Context. Despite the low elemental deuterium abundance in the Galaxy, enhanced molecular deuterium fractionation has been found in the environments of low-mass star-forming regions and, in particular, the Class 0 protostar IRAS 16293-2422. Aims: The key program Chemical HErschel Surveys of Star forming regions (CHESS) aims at studying the molecular complexity of the interstellar medium. The high sensitivity and spectral resolution of the Herschel/HIFI (Heterodyne Instrument for Far-Infrared) instrument provide a unique opportunity to observe the fundamental 11,1-00,0 transition of ortho-D2O at 607 GHz and the higher energy 21,2-10,1 transition of para-D2O at 898 GHz, both of which are inaccessible from the ground. Methods: The ortho-D2O transition at 607 GHz was previously detected. We present in this paper the first tentative detection for the para-D2O transition at 898 GHz. The spherical Monte Carlo radiative transfer code RATRAN was used to reproduce the observed line profiles of D2O with the same method that was used to reproduce the HDO and H218O line profiles in IRAS 16293-2422. Results: As for HDO, the absorption component seen on the D2O lines can only be reproduced by adding an external absorbing layer, possibly created by the photodesorption of the ices at the edges of the molecular cloud. The D2O column density is found to be about 2.5 × 1012 cm-2 in this added layer, leading to a D2O/H2O ratio of about 0.5%. At a 3σ uncertainty, upper limits of 0.03% and 0.2% are obtained for this ratio in the hot corino and the colder envelope of IRAS 16293-2422, respectively. Conclusions: The deuterium fractionation derived in our study suggests that the ices present in IRAS 16293-2422 formed on warm dust grains (~15-20 K) in dense (~104-5 × 104 cm-3) translucent clouds. These results allow us to address the earliest phases of star formation and the conditions in which ices form. Based on Herschel/HIFI observations. Herschel is an ESA space observatory with scientific instruments provided by European-led principal Investigator consortia and with important participation from NASA.

Ceramic materials for porcelain veneers: part II. Effect of material, shade, and thickness on translucency.

PubMed

Barizon, Karine T L; Bergeron, Cathia; Vargas, Marcos A; Qian, Fang; Cobb, Deborah S; Gratton, David G; Geraldeli, Saulo

2014-10-01

Information regarding the differences in translucency among new ceramic systems is lacking. The purpose of this study was to compare the relative translucency of the different types of ceramic systems indicated for porcelain veneers and to evaluate the effect of shade and thickness on translucency. Disk specimens 13 mm in diameter and 0.7-mm thick were fabricated for the following 9 materials (n=5): VITA VM9, IPS Empress Esthetic, VITA PM9, Vitablocks Mark II, Kavo Everest G-Blank, IPS Empress CAD, IPS e.max CAD, IPS e.maxPress, and Lava Zirconia. VITA VM9 served as the positive control and Lava as the negative control. The disks were fabricated with the shade that corresponds to A1. For IPS e.maxPress, additional disks were made with different shades (BL2, BL4, A1, B1, O1, O2, V1, V2, V3), thickness (0.3 mm), and translucencies (high translucency, low translucency). Color coordinates (CIE L∗ a∗ b∗) were measured with a tristimulus colorimeter. The translucency parameter was calculated from the color difference of the material on a black versus a white background. One-way ANOVA, the post hoc Tukey honestly significant difference, and the Ryan-Einot-Gabriel-Welsch multiple range tests were used to analyze the data (α=.05). Statistically significant differences in the translucency parameter were found among porcelains (P<.001) according to the following rank: VM9>PM9, Empress Esthetic>Empress CAD>Mark II, Everest, e.max CAD>e.max Press>Lava. Significant differences also were noted when different shades and thickness were compared (P<.001). Different ceramic systems designed for porcelain veneers present varying degrees of translucency. The thickness and shade of lithium disilicate ceramic affect its translucency. Shade affects translucency parameter less than thickness. Copyright © 2014 Editorial Council for the Journal of Prosthetic Dentistry. Published by Elsevier Inc. All rights reserved.

Molecular Astrophysics from Space: the Physical and Chemical Effects of Star Formation and the Destruction of Planetary Systems around Evolved Stars

NASA Technical Reports Server (NTRS)

Neufeld, David

2005-01-01

The research conducted during the reporting period is grouped into three sections: 1) Warm molecular gas in the interstellar medium (ISM); 2) Absorption line studies of "cold" molecular clouds; 3) Vaporization of comets around the AGB star IRC+10216.

03pd0059

NASA Image and Video Library

2003-01-12

NASA's Ice, Cloud and Land Elevation satellite (ICESat) and Cosmic Hot Interstellar Spectrometer (CHIPS) satellite lifted off from Vandenberg Air Force Base, Calif at 4:45 p.m. PST aboard Boeing's Delta II rocket. ICESat will examine the role that ice plays in global climate change, while CHIPS will explore the composition of our galaxy. Photo Credit: "NASA/Bill Ingalls"

03pd0060

NASA Image and Video Library

2003-01-12

NASA's Ice, Cloud and Land Elevation satellite (ICESat) and Cosmic Hot Interstellar Spectrometer (CHIPS) satellite lifted off from Vandenberg Air Force Base, Calif at 4:45 p.m. PST aboard Boeing's Delta II rocket. ICESat will examine the role that ice plays in global climate change, while CHIPS will explore the composition of our galaxy. Photo Credit: "NASA/Bill Ingalls"

03pd0061

NASA Image and Video Library

2003-01-12

NASA's Ice, Cloud and Land Elevation satellite (ICESat) and Cosmic Hot Interstellar Spectrometer (CHIPS) satellite lifted off from Vandenberg Air Force Base, Calif at 4:45 p.m. PST aboard Boeing's Delta II rocket. ICESat will examine the role that ice plays in global climate change, while CHIPS will explore the composition of our galaxy. Photo Credit: "NASA/Bill Ingalls"

The detection of interstellar methylcyanoacetylene

NASA Technical Reports Server (NTRS)

Broten, N. W.; Macleod, J. M.; Avery, L. W.; Friberg, P.; Hjalmarson, A.; Hoglund, B.; Irvine, W. M.

1984-01-01

A new interstellar molecule, methylcyanoacetylene (CH3C3N), has been detected in the molecular cloud TMC-1. The J = 8 to 7, J = 7 to 6, J = 6 to 5, and J = 5 to 4 transitions have been observed. For the first three of these, both the K = 0 and K = 1 components are present, while for J = 5 to 4, only the K = 0 line has been detected. The observed frequencies were calculated by assuming a value of radial velocity V(LSR) = 5.8 km/s for TMC-1, typical of other molecules in the cloud. All observed frequencies are within 10 kHz of the calculated frequencies, which are based on the 1982 laboratory constants of Moises et al. (1982), so the identification is secure. The lines are broadened by hyperfine splitting, and the J = 5 to 4, K = 0 transition shows incipient resolution into three hyperfine components. The rotational temperature determined from these observations is quite low, with T(rot) in the range from 2.7 to 4 K. The total column density is approximately 5 x 10 to the 12th per sq cm.

Life and the Universe: From Astrochemistry to Astrobiology

NASA Technical Reports Server (NTRS)

Allamandola, Louis J.

2013-01-01

Great strides have been made in our understanding of interstellar material thanks to advances in infrared astronomy and laboratory astrophysics. Ionized polycyclic aromatic hydrocarbons (PAHs), shockingly large molecules by earlier astrochemical standards, are widespread and very abundant throughout much of the cosmos. In cold molecular clouds, the birthplace of planets and stars, interstellar atoms and molecules freeze onto extremely cold dust and ice particles forming mixed molecular ices dominated by simple species such as water, methanol, ammonia, and carbon monoxide. Within these clouds, and especially in the vicinity of star and planet forming regions, these ices and PAHs are processed by ultraviolet light and cosmic rays forming hundreds of far more complex species, some of biogenic interest. Eventually, these are delivered to primordial planets by comets and meteorites. As these materials are the building blocks of comets and related to carbonaceous micrometeorites, they are likely to be important sources of complex organic materials delivered to habitable planets (including the primordial Earth) and their composition may be related to the origin of life. This talk will focus on the chemical evolution of these cosmic materials and their relevance to astrobiology.

Radiative thermal conduction fronts

NASA Technical Reports Server (NTRS)

Borkowski, Kazimierz J.; Balbus, Steven A.; Fristrom, Carl C.

1990-01-01

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.

Radiative thermal conduction fronts

NASA Astrophysics Data System (ADS)

Borkowski, Kazimierz J.; Balbus, Steven A.; Fristrom, Carl C.

1990-07-01

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.

Mid-infrared rotational line emission from interstellar molecular hydrogen

NASA Astrophysics Data System (ADS)

Burton, Michael G.; Hollenbach, D. J.; Tielens, A. G. G.

1992-11-01

The line emission from the v = 0-0 S(0), S(2), and S(3), and the v = 1-0 and v = 2-1 S(1) transitions of molecular hydrogen in clouds exposed to high FUV fluxes and in shocks is modeled. Particular attention is given to the lowest pure rotational H2 transitions at 20 and 17 microns, respectively. It is found that, in photodissociation regions (PDRs), the emission comes from warm (greater than about 100 k) molecular gas, situated at optical depths greater than about 1, beyond the hot atomic surface layer of the clouds. For FUV fields, G0 = 1000 to 100,000 times the average interstellar field densities n = 10 exp 3 - 10 exp 7/cu cm, the typical line intensities are in the range 10 exp -6 to 10 exp -4 ergs/s sq cm sr. The predictions for the line intensities from both C-type and J-type shock models are compared. The results are applied to recent observations of the 0-0 S(1) transition in both the PDR and the shocked gas in Orion.

The interstellar medium and star formation of galactic disks. I. Interstellar medium and giant molecular cloud properties with diffuse far-ultraviolet and cosmic-ray backgrounds

NASA Astrophysics Data System (ADS)

Li, Qi; Tan, Jonathan C.; Christie, Duncan; Bisbas, Thomas G.; Wu, Benjamin

2018-05-01

We present a series of adaptive mesh refinement hydrodynamic simulations of flat rotation curve galactic gas disks, with a detailed treatment of the interstellar medium (ISM) physics of the atomic to molecular phase transition under the influence of diffuse far-ultraviolet (FUV) radiation fields and cosmic-ray backgrounds. We explore the effects of different FUV intensities, including a model with a radial gradient designed to mimic the Milky Way. The effects of cosmic rays, including radial gradients in their heating and ionization rates, are also explored. The final simulations in this series achieve 4 pc resolution across the ˜20 kpc global disk diameter, with heating and cooling followed down to temperatures of ˜10 K. The disks are evolved for 300 Myr, which is enough time for the ISM to achieve a quasi-statistical equilibrium. In particular, the mass fraction of molecular gas is stabilized by ˜200 Myr. Additional global ISM properties are analyzed. Giant molecular clouds (GMCs) are also identified and the statistical properties of their populations are examined. GMCs are tracked as the disks evolve. GMC collisions, which may be a means of triggering star cluster formation, are counted and their rates are compared with analytic models. Relatively frequent GMC collision rates are seen in these simulations, and their implications for understanding GMC properties, including the driving of internal turbulence, are discussed.

Cyanomethanimine Isomers in Cold Interstellar Clouds: Insights from Electronic Structure and Kinetic Calculations

NASA Astrophysics Data System (ADS)

Vazart, Fanny; Latouche, Camille; Skouteris, Dimitrios; Balucani, Nadia; Barone, Vincenzo

2015-09-01

New insights into the formation of interstellar cyanomethanimine, a species of great relevance in prebiotic chemistry, are provided by electronic structure and kinetic calculations for the reaction CN + CH2 = NH. This reaction is a facile formation route of Z,E-C-cyanomethanimine, even under the extreme conditions of density and temperature typical of cold interstellar clouds. E-C-cyanomethanimine has been recently identified in Sgr B2(N) in the Green Bank Telescope (GBT) PRIMOS survey by P. Zaleski et al. and no efficient formation routes have been envisaged so far. The rate coefficient expression for the reaction channel leading to the observed isomer E-C-cyanomethanimine is 3.15 × 10-10 × (T/300)0.152 × e(-0.0948/T). According to the present study, the more stable Z-C-cyanomethanimine isomer is formed with a slightly larger yield (4.59 × 10-10 × (T/300)0.153 × e(-0.0871/T). As the detection of E-isomer is favored due to its larger dipole moment, the missing detection of the Z-isomer can be due to the sensitivity limit of the GBT PRIMOS survey and the detection of the Z-isomer should be attempted with more sensitive instrumentation. The CN + CH2 = NH reaction can also play a role in the chemistry of the upper atmosphere of Titan where the cyanomethanimine products can contribute to the buildup of the observed nitrogen-rich organic aerosols that cover the moon.

THE COMPOSITION OF INTERSTELLAR GRAINS TOWARD ζ OPHIUCHI: CONSTRAINING THE ELEMENTAL BUDGET NEAR THE DIFFUSE-DENSE CLOUD TRANSITION

DOE Office of Scientific and Technical Information (OSTI.GOV)

Poteet, Charles A.; Whittet, Douglas C. B.; Draine, Bruce T., E-mail: charles.poteet@gmail.com

2015-03-10

We investigate the composition of interstellar grains along the line of sight toward ζ Ophiuchi, a well-studied environment near the diffuse-dense cloud transition. A spectral decomposition analysis of the solid-state absorbers is performed using archival spectroscopic observations from the Spitzer Space Telescope and Infrared Space Observatory. We find strong evidence for the presence of sub-micron-sized amorphous silicate grains, principally comprised of olivine-like composition, with no convincing evidence of H{sub 2}O ice mantles. However, tentative evidence for thick H{sub 2}O ice mantles on large (a ≈ 2.8 μm) grains is presented. Solid-state abundances of elemental Mg, Si, Fe, and O aremore » inferred from our analysis and compared to standard reference abundances. We find that nearly all of the Mg and Si atoms along the line of sight reside in amorphous silicate grains, while a substantial fraction of the elemental Fe resides in compounds other than silicates. Moreover, we find that the total abundance of elemental O is largely inconsistent with the adopted reference abundances, indicating that as much as ∼156 ppm of interstellar O is missing along the line of sight. After taking into account additional limits on the abundance of elemental O in other O-bearing solids, we conclude that any missing reservoir of elemental O must reside on large grains that are nearly opaque to infrared radiation.« less

Estimating dust distances to Type Ia supernovae from colour excess time evolution

NASA Astrophysics Data System (ADS)

Bulla, M.; Goobar, A.; Amanullah, R.; Feindt, U.; Ferretti, R.

2018-01-01

We present a new technique to infer dust locations towards reddened Type Ia supernovae and to help discriminate between an interstellar and a circumstellar origin for the observed extinction. Using Monte Carlo simulations, we show that the time evolution of the light-curve shape and especially of the colour excess E(B - V) places strong constraints on the distance between dust and the supernova. We apply our approach to two highly reddened Type Ia supernovae for which dust distance estimates are available in the literature: SN 2006X and SN 2014J. For the former, we obtain a time-variable E(B - V) and from this derive a distance of 27.5^{+9.0}_{-4.9} or 22.1^{+6.0}_{-3.8} pc depending on whether dust properties typical of the Large Magellanic Cloud (LMC) or the Milky Way (MW) are used. For the latter, instead, we obtain a constant E(B - V) consistent with dust at distances larger than ∼50 and 38 pc for LMC- and MW-type dust, respectively. Values thus extracted are in excellent agreement with previous estimates for the two supernovae. Our findings suggest that dust responsible for the extinction towards these supernovae is likely to be located within interstellar clouds. We also discuss how other properties of reddened Type Ia supernovae - such as their peculiar extinction and polarization behaviour and the detection of variable, blue-shifted sodium features in some of these events - might be compatible with dust and gas at interstellar-scale distances.

A survey of interstellar HI from L alpha absorption measurements 2

NASA Technical Reports Server (NTRS)

Bohlin, R. C.; Savage, B. D.; Drake, J. F.

1977-01-01

The Copernicus satellite surveyed the spectral region near L alpha to obtain column densities of interstellar HI toward 100 stars. The distance to 10 stars exceeds 2 kpc and 34 stars lie beyond 1 kpc. Stars with color excess E(B-V) up to 0.5 mag are observed. The value of the mean ratio of total neutral hydrogen to color excess was found to equal 5.8 x 10 to the 21st power atoms per (sq cm x mag). For stars with accurate E(B-V), the deviations from this mean are generally less than a factor of 1.5. A notable exception is the dark cloud star, rho Oph. A reduction in visual reddening efficiency for the grains that are larger than normal in the rho Oph dark cloud probably explains this result. The conversion of atomic hydrogen into molecular form in dense clouds was observed in the gas to E(B-V) correlation plots. The best estimate for the mean total gas density for clouds and the intercloud medium, as a whole, in the solar neighborhood and in the plane of the galaxy is 1.15 atoms per cu. cm; those for the atomic gas and molecular gas alone are 0.86 atoms per cu cm and 0.143 molecules per cu cm respectively. For the intercloud medium, where molecular hydrogen is a negligible fraction of the total gas, atomic gas density was found to equal 0.16 atoms per cu cm with a Gaussian scale height perpendicular to the plane of about 350 pc, as derived from high latitude stars.

Gas, dust, stars, star formation, and their evolution in M 33 at giant molecular cloud scales

NASA Astrophysics Data System (ADS)

Komugi, Shinya; Miura, Rie E.; Kuno, Nario; Tosaki, Tomoka

2018-06-01

We report on a multi-parameter analysis of giant molecular clouds (GMCs) in the nearby spiral galaxy M 33. A catalog of GMCs identifed in 12CO(J = 3-2) was used to compile associated 12CO(J = 1-0), dust, stellar mass, and star formation rate. Each of the 58 GMCs are categorized by their evolutionary stage. Applying the principal component analysis on these parameters, we construct two principal components, PC1 and PC2, which retain 75% of the information from the original data set. PC1 is interpreted as expressing the total interstellar matter content, and PC2 as the total activity of star formation. Young (< 10 Myr) GMCs occupy a distinct region in the PC1-PC2 plane, with lower interstellar medium (ISM) content and star formation activity compared to intermediate-age and older clouds. Comparison of average cloud properties in different evolutionary stages imply that GMCs may be heated or grow denser and more massive via aggregation of diffuse material in their first ˜ 10 Myr. The PCA also objectively identified a set of tight relations between ISM and star formation. The ratio of the two CO lines is nearly constant, but weakly modulated by massive star formation. Dust is more strongly correlated with the star formation rate than the CO lines, supporting recent findings that dust may trace molecular gas better than CO. Stellar mass contributes weakly to the star formation rate, reminiscent of an extended form of the Schmidt-Kennicutt relation with the molecular gas term substituted by dust.

Copernicus observations of C I: pressures and carbon abundances in diffuse interstellar clouds

DOE Office of Scientific and Technical Information (OSTI.GOV)

Jenkins, E.B.; Jura, M.; Loewenstein, M.

1983-07-01

Using the Copernicus satellite, we observed ultraviolet absorption lines of interstellar neutral carbon atoms toward 27 stars. In addition to deriving column densities of C I (both in its ground state and the two excited fine-structure levels), we used our equivalent widths to revise the f-values of some of the C I transitions measured by other investigators. We also observed H/sub 2/ from the J = 4 level so that we could compare the rotational excitation of H/sub 2/ with the fine-structure excitation of C I. From the amount of fine-structure excitation of C I in each case, we havemore » derived information on the thermal gas pressures within the diffuse clouds. Most clouds have p/k between 10/sup 3/ cm/sup -3/ K and 10/sup 4/ cm/sup -3/ K, but we found that at least 6% of the C I-bearing material is at p/k>10/sup 4/ cm/sup -3/ K, and one-third of the gas has upper limits for pressure below 10/sup 3/ cm/sup -3/ K, assuming temperatures are not appreciably below 20 K. An analysis of radial velocities for the absorption lines showed no distinctive trends for the kinematics of high- or low-pressure gas components. From the apparent lack of acceleration of high-pressure clouds, we conclude that it is unlikely that streaming intercloud material is causing significant ram pressurization. We have compared our results with the predictions for pressure fluctuations caused by supernova explosions in the theory of McKee and Ostriker.« less

Gas, dust, stars, star formation, and their evolution in M 33 at giant molecular cloud scales

NASA Astrophysics Data System (ADS)

Komugi, Shinya; Miura, Rie E.; Kuno, Nario; Tosaki, Tomoka

2018-04-01

We report on a multi-parameter analysis of giant molecular clouds (GMCs) in the nearby spiral galaxy M 33. A catalog of GMCs identifed in 12CO(J = 3-2) was used to compile associated 12CO(J = 1-0), dust, stellar mass, and star formation rate. Each of the 58 GMCs are categorized by their evolutionary stage. Applying the principal component analysis on these parameters, we construct two principal components, PC1 and PC2, which retain 75% of the information from the original data set. PC1 is interpreted as expressing the total interstellar matter content, and PC2 as the total activity of star formation. Young (< 10 Myr) GMCs occupy a distinct region in the PC1-PC2 plane, with lower interstellar medium (ISM) content and star formation activity compared to intermediate-age and older clouds. Comparison of average cloud properties in different evolutionary stages imply that GMCs may be heated or grow denser and more massive via aggregation of diffuse material in their first ˜ 10 Myr. The PCA also objectively identified a set of tight relations between ISM and star formation. The ratio of the two CO lines is nearly constant, but weakly modulated by massive star formation. Dust is more strongly correlated with the star formation rate than the CO lines, supporting recent findings that dust may trace molecular gas better than CO. Stellar mass contributes weakly to the star formation rate, reminiscent of an extended form of the Schmidt-Kennicutt relation with the molecular gas term substituted by dust.

Reduction of unsaturated compounds under interstellar conditions: chemoselective reduction of C≡C and C=C bonds over C=O functional group

NASA Astrophysics Data System (ADS)

Jonusas, Mindaugas; Guillemin, Jean-Claude; Krim, Lahouari

2017-07-01

The knowledge of the H-addition reactions on unsaturated organic molecules bearing a triple or a double carbon-carbon bond such as propargyl or allyl alcohols and a CO functional group such as propynal, propenal or propanal may play an important role in the understanding of the chemical complexity of the interstellar medium. Why different aldehydes like methanal, ethanal, propynal and propanal are present in dense molecular clouds while the only alcohol detected in those cold regions is methanol? In addition, ethanol has only been detected in hot molecular cores. Are those saturated and unsaturated aldehyde and alcohol species chemically linked in molecular clouds through solid phase H-addition surface reactions or are they formed through different chemical routes? To answer such questions, we have investigated a hydrogenation study of saturated and unsaturated aldehydes and alcohols at 10 K. We prove through this experimental study that while pure unsaturated alcohol ices bombarded by H atoms lead to the formation of the corresponding fully or partially saturated alcohols, surface H-addition reactions on unsaturated aldehyde ices exclusively lead to the formation of fully saturated aldehyde. Such results show that in addition to a chemoselective reduction of C≡C and C=C bonds over the C=O group, there is no link between aldehydes and their corresponding alcohols in reactions involving H atoms in dense molecular clouds. Consequently, this could be one of the reasons why some aldehydes such as propanal are abundant in dense molecular clouds in contrast to the non-detection of alcohol species larger than methanol.

Laboratory simulation of interstellar grain chemistry and the production of complex organic molecules

NASA Technical Reports Server (NTRS)

Allamandola, L. J.; Sandford, S. A.; Valero, G. J.

1990-01-01

During the past 15 years considerable progress in observational techniques has been achieved in the middle infrared (5000 to 500 cm(-1), 2 to 20 microns m), the spectral region most diagnostic of molecular vibrations. Spectra of many different astronomical infrared sources, some deeply embedded in dark molecular clouds, are now available. These spectra provide a powerful probe, not only for the identification of interstellar molecules in both the gas solid phases, but also of the physical and chemical conditions which prevail in these two very different domains. By comparing these astronomical spectra with the spectra of laboratory ices one can determine the composition and abundance of the icy materials frozen on the cold (10K) dust grains present in the interior of molecular clouds. These grains and their ice mantles may well be the building blocks from which comets are made. As an illustration of the processes which can take place as an ice is irradiated and subsequently warmed, researchers present the infrared spectra of the mixture H2O:CH3OH:CO:NH3:C6H14 (100:50:10:10:10). Apart from the last species, the ratio of these compounds is representative of the simplest ices found in interstellar clouds. The last component was incorporated into this particular experiment as a tracer of the behavior of a non-aromatic hydrocarbon. The change in the composition that results from ultraviolet photolysis of this ice mixture using a UV lamp to simulate the interstellar radiation field is shown. Photolysis produces CO, CO2, CH4, HCO, H2CO, as well as a family of moderately volatile hydrocarbons. Less volatile carbonaceous materials are also produced. The evolution of the infrared spectrum of the ice as the sample is warmed up to room temperature is illustrated. Researchers believe that the changes are similar to those which occur as ice is ejected from a comet and warmed up by solar radiation. The warm-up sequence shows that the nitrile or iso-nitrile bearing compound produced during photolysis evaporates between 200 and 250K, suggesting that it is carried by a small molecular species. These molecules could be similar to the source material in Comet Halley that is ejected in grains into the coma, freed by sublimation, and photolyzed by solar radiation to produce the observed jets.

Synthesis of Large Molecules in Cometary Ice Analogs: Physical Properties Related to Self-Assembly Processes

NASA Technical Reports Server (NTRS)

Dworkin, Jason P.; Sandford, Scott A.; Deamer, David W.; Gillette, J. Seb; Zare, Richard N.; Allamandola, Louis J. (Technical Monitor)

1999-01-01

The combination of realistic laboratory simulations and infrared observations have revolutionized our understanding of interstellar dust and ice-the main component of comets. Since comets and carbonaceous micrometeorites may have been important sources of volatiles and carbon compounds on the early Earth, their organic composition may be related to the origin of life. Ices on grains in molecular clouds contain a variety of simple molecules. The D/H ratios of the comets Hale-Bopp and Hyakutake are consistent with a primarily interstellar ice mixture. Within the cloud and especially in the presolar nebula through the early solar system, these icy grains would have been photoprocessed by the ultraviolet producing more complex species such as hexamethylenetetramine, polyoxymethylenes, and simple keones. We reported at the 1999 Bioastronomy meeting laboratory simulations studied to identify the types of molecules which could have been generated in pre-cometary ices. Experiments were conducted by forming a realistic interstellar mixed-molecular ice (H2O, CH3OH, NH3 and CO) at approximately 10 K under high vacuum irradiated with UV light from a hydrogen plasma lamp. The gas mixture was typically 100:50:1:1, however when different ratios were used material with similar characteristics was still produced. The residue that remained after warming to room temperature was analyzed by HPLC, and by several mass spectrometric methods. This material contains a rich mixture of complex compounds with mass spectral profiles resembling those found in IDPs and meteorites. Surface tension measurements show that an amphiphilic component is also present. These species do not appear in various controls or in unphotolyzed samples. Residues from the simulations were also dispersed in aqueous media for microscopy. The organic material forms 10-40 gm diameter droplets that fluoresce at 300-450 nm under UV excitation. These droplets have a morphology and internal structure which appear strikingly similar to those produced by extracts of the Murchison meteorite. Together, these results suggest a link between organic material photochemically synthesized on the cold grains in dense, interstellar molecular clouds and compounds that may have contributed to the organic inventory of the primitive Earth. For example, the amphiphilic properties of such compounds permit self-assembly into the membranous boundary structures that required for the first forms of cellular life.

Interstellar Aldehydes and their corresponding Reduced Alcohols: Interstellar Propanol?

NASA Astrophysics Data System (ADS)

Etim, Emmanuel; Chakrabarti, Sandip Kumar; Das, Ankan; Gorai, Prasanta; Arunan, Elangannan

2016-07-01

There is a well-defined trend of aldehydes and their corresponding reduced alcohols among the known interstellar molecules; methanal (CH_2O) and methanol (CH_3OH); ethenone (C_2H_2O) and vinyl alcohol (CH_2CHOH); ethanal (C_2H_4O) and ethanol(C_2H_5OH); glycolaldehyde (C_2H_4O_2) and ethylene glycol(C_2H_6O_2). The reduced alcohol of propanal (CH_3CH_2CHO) which is propanol (CH_3CH_2CH_2OH) has not yet been observed but its isomer; ethyl methyl ether (CH_3CH_2OCH_3) is a known interstellar molecule. In this article, different studies are carried out in investigating the trend between aldehydes and their corresponding reduced alcohols and the deviation from the trend. Kinetically and with respect to the formation route, alcohols could have been produced from their corresponding reduced aldehydes via two successive hydrogen additions. This is plausible because of (a) the unquestionable high abundance of hydrogen, (b) presence of energy sources within some of the molecular clouds and (c) the ease at which successive hydrogen addition reaction occurs. In terms of stability, the observed alcohols are thermodynamically favorable as compared to their isomers. Regarding the formation process, the hydrogen addition reactions are believed to proceed on the surface of the interstellar grains which leads to the effect of interstellar hydrogen bonding. From the studies, propanol and propan-2-ol are found to be more strongly attached to the surface of the interstellar dust grains which affects its overall gas phase abundance as compared to its isomer ethyl methyl ether which has been observed.

IRAS and the Boston University Arecibo Galactic H I Survey: A catalog of cloud properties

NASA Technical Reports Server (NTRS)

Bania, Thomas M.

1992-01-01

The Infrared Astronomy Satellite (IRAS) Galactic Plane Surface Brightness Images were used to identify infrared emission associated with cool, diffuse H I clouds detected by the Boston University-Arecibo Galactic H I Survey. These clouds are associated with galactic star clusters, H II regions, and molecular clouds. Using emission-absorption experiments toward galactic H II regions, we determined the H I properties of cool H I clouds seen in absorption against the thermal continuum, including their kinematic distances. Correlations were then made between IRAS sources and these H II regions, thus some of the spatial confusion associated with the IRAS fields near the galactic plane was resolved since the distances to these sources was known. Because we can also correlate the BU-Arecibo clouds with existing CO surveys, these results will allow us to determine the intrinsic properties of the gas (neutral and ionized atomic as well as molecular) and dust for interstellar clouds in the inner galaxy. For the IRAS-identified H II region sample, we have established the far infrared (FIR) luminosities and galactic distribution of these sources.

Organic chemical evolution

NASA Technical Reports Server (NTRS)

Chang, S.

1981-01-01

The course of organic chemical evolution preceding the emergence of life on earth is discussed based on evidence of processes occurring in interstellar space, the solar system and the primitive earth. Following a brief review of the equilibrium condensation model for the origin and evolution of the solar system, consideration is given to the nature and organic chemistry of interstellar clouds, comets, Jupiter, meteorites, Venus and Mars, and the prebiotic earth. Major issues to be resolved in the study of organic chemical evolution on earth are identified regarding condensation and accretion in the solar nebula, early geological evolution, the origin and evolution of the atmosphere, organic production rates, organic-inorganic interactions, environmental fluctuations, phase separation and molecular selectivity.

The Potential Role Played by the Fullerene-Like Structures of Interstellar Carbon Dust in the Formation of Molecular Hydrogen in Space

NASA Astrophysics Data System (ADS)

Cataldo, Franco; Iglesias-Groth, Susana

After a general introduction to the problem of formation of molecular hydrogen from atomic hydrogen in the interstellar medium and in the dense molecular clouds in particular, and after the explanation of the key role played by the surfaces on this process, it is proposed that the most suitable carbon surface for the formation of molecular hydrogen (from the radiative association process of atomic hydrogen) can be represented by carbon black rather than by graphite. Furthermore, it is proposed that the fullerene-like structures present in the carbon black graphene sheets are the reaction sites where molecular hydrogen may be formed.

High Abundance of Ions in Cosmic Ices

NASA Technical Reports Server (NTRS)

Gudipati, Murthy S.; Allamandola, Louis J.; Fonda, Mark (Technical Monitor)

2002-01-01

Water-rich, mixed molecular ices and polycyclic aromatic hydrocarbons (PAHs) are common throughout interstellar molecular clouds and the Solar System. Vacuum ultraviolet (VUV) irradiation and particle bombardment of these abiotic ices produces complex organic species, including important biogenic molecules such as amino acids and functionalized PAHs which may have played a role in the origin of life. This ability of such water-rich, oxygen dominated ices to promote production of complex organic species is surprising and points to an important, unusual, but previously overlooked mechanism at play within the ice. Here we report the nature of this mechanism using electronic spectroscopy. VUV-irradiation of PAH/H2O ices leads to an unprecedented and efficient (greater than 70 %) conversion of the neutral PAHs to their cation form (PAH+). Further, these H2O/PAH+ ices are stabile at temperatures below 50 K, a temperature domain common throughout interstellar clouds and the Solar System. Between 50 and 125 K they react to form the complex organics. In view of this, we conclude that charged PAHs and other molecular ions should be common and abundant in many cosmic ices. The chemical, spectroscopic and physical properties of these ion-rich ices can be of fundamental importance for objects as diverse as comets, planets, and molecular clouds and may account for several poorly understood phenomena associated with each of these object classes.

Mapping the Frozen Void

NASA Astrophysics Data System (ADS)

Suutarinen, Aleksi; Fraser, Helen

2013-07-01

Reactions on the surfaces of dust grains play a vital role in the overall chemistry of interstellar matter. These grains become covered by icy layers, which are the largest molecular reservoir in the interstellar medium. Given this, it is surprising that the effect ice has on the overall chain of reactions is poorly characterized. One step on the path of gaining better understanding here is to develop methods of figuring out how much ice is present in these clouds, the links between ice components, and synergy between the ices and gas phase molecules. We do this by examining the absorption spectra of ices on lines of sight towards several stars behind clouds of interstellar matter. From these we can reconstruct spatial maps of the ice distribution on scales of as little as 1000 AU, as a test of the chemical variation within a cloud. By overlapping the ice data with other maps of the same region (gas emission, temperature, density etc) we create combined maps to reveal the astrochemistry of star-forming regions and pre-stellar cores. In this poster we present the continuing results of our ice mapping programme, using data from the AKARI satellite, specifically in slitless spectroscopy observations in the NIR. In this region the key ice features encompass H2O, CO and CO2. The maps illustrate the power of our dedicated AKARI data reduction pipeline, and the novelty of our observing programme. We also detail the next steps' in our ice mapping research. The method is being expanded to include the full 10'x10' AKARI field of view, taking account of image distortion induced by the dispersing optics. These maps are then combined with exiting gas-phase observations and SCUBA maps. The latest attempts at this are shown here. What is clear already is that it is difficult to predict ice abundances from factors such as extinction or gas density alone, and that ice formation and evolution can vary hugely over even very small astronomical scales.

The molecular universe: from astronomy to laboratory astrophysics and back

NASA Astrophysics Data System (ADS)

van Dishoeck, Ewine

2015-08-01

Molecules are found in a wide range of astronomical environments, fromour Solar System to distant starburst galaxies at the highest redshifts. Thanks to the opening up of the infrared and (sub)millimeter wavelength regime, culminating with Herschel and ALMA, more than 180 different species have now been found throughout the various stages of stellar birth and death: diffuse and dense interstellar clouds, protostars and disks, the envelopes of evolved stars and planetary nebulae, and exo-planetary atmospheres. Molecules and solid-state features are now also routinely detected in the interstellar medium of external galaxies, near and far.There are many motivations for studying this molecular universe. From the chemical perspective, interstellar space provides a unique laboratory to study basic molecular processes under very different conditions from those normally found in a laboratory on Earth. For astronomers, molecules are unique probes of the many environments where they are found, providing information on density, temperature, dynamics, ionization fractions and magnetic fields. Molecules also play an important role in the cooling of clouds allowing them to collapse, including the formation of the very first stars and galaxies. Finally, the molecular composition is sensitive to the history of the material, and ultimately provides critical information on our origins.This talk will summarize a number of recent observational highlights and provide examples of cases where the availability of new laboratory data proved crucial in the analysis. This includes basic data such as spectroscopy and collisional rate coefficients, but also an improved understanding of photoprocesses in the gaseous and solid state. Much of the chemistry in star- and planet-forming regions is now thought to be driven by gas-grain chemistry rather than pure gas-phase chemistry, and a few examples of the close link between models and laboratory experiments will be given. In spite of lingering uncertainties, the future of molecular astrophysics is bright and will allow increased understanding of the journey of gas and solids from clouds to comets and planets.

On the Formation of Interstellar Water Ice: Constraints from a Search for Hydrogen Peroxide Ice in Molecular Clouds

NASA Technical Reports Server (NTRS)

Smith, R. G.; Charnely, S. B.; Pendleton, Y. J.; Wright, C. M.; Maldoni, M. M.; Robinson, G.

2011-01-01

Recent surface chemistry experiments have shown that the hydrogenation of molecular oxygen on interstellar dust grains is a plausible formation mechanism, via hydrogen peroxide (H2O2), for the production of water (H2O) ice mantles in the dense interstellar medium. Theoretical chemistry models also predict the formation of a significant abundance of H2O2 ice in grain mantles by this route. At their upper limits, the predicted and experimental abundances are sufficiently high that H2O2 should be detectable in molecular cloud ice spectra. To investigate this further, laboratory spectra have been obtained for H2O2/H2O ice films between 2.5 and 200 micron, from 10 to 180 K, containing 3%, 30%, and 97% H2O2 ice. Integrated absorbances for all the absorption features in low-temperature H2O2 ice have been derived from these spectra. For identifying H2O2 ice, the key results are the presence of unique features near 3.5, 7.0, and 11.3 micron. Comparing the laboratory spectra with the spectra of a group of 24 protostars and field stars, all of which have strong H2O ice absorption bands, no absorption features are found that can definitely be identified with H2O2 ice. In the absence of definite H2O2 features, the H2O2 abundance is constrained by its possible contribution to the weak absorption feature near 3.47 micron found on the long-wavelength wing of the 3 micron H2O ice band. This gives an average upper limit for H2O2, as a percentage of H2O, of 9% +/- 4%. This is a strong constraint on parameters for surface chemistry experiments and dense cloud chemistry models.

Nuchal translucency in pregnancies conceived after assisted reproduction technology.

PubMed

Hui, Pui Wah; Lee, Chin Peng; Tang, Mary Hoi Yin; Ho, Pak Chung

2006-06-01

Nuchal translucency is one of the important markers in the first trimester during antenatal screening for fetal Down's syndrome. With the observation of alterations in biochemical markers in pregnancies conceived after assisted reproduction, this review presents current information related to the thickness of nuchal translucency in these pregnancies. Early small studies did not demonstrate any discrepancy in the thickness of nuchal translucency in fetuses from assisted reproduction and from spontaneous pregnancies, but there has been recent evidence to suggest an increased level of nuchal translucency in singletons from various modes of assisted-reproduction technology. Nuchal translucency in twins following assisted reproduction did not, however, show a similar increase. Although the effect of chorionicity was not specifically addressed, nuchal translucency thickness in twins born after assisted reproduction was reported to be comparable to that in spontaneous singletons. It is possible that singletons and twins after assisted reproduction exhibit different antenatal behavior and pregnancy courses. Similar to other biochemical markers of fetal Down's syndrome, nuchal translucency is increased in singletons after assisted-reproduction technology. Further studies on twin pregnancies, in particular dichorionic twins, are necessary before conclusive evidence can be drawn for multiple pregnancies.

Self-assembling amphiphilic molecules: Synthesis in simulated interstellar/precometary ices

PubMed Central

Dworkin, Jason P.; Deamer, David W.; Sandford, Scott A.; Allamandola, Louis J.

2001-01-01

Interstellar gas and dust constitute the primary material from which the solar system formed. Near the end of the hot early phase of star and planet formation, volatile, less refractory materials were transported into the inner solar system as comets and interplanetary dust particles. Once the inner planets had sufficiently cooled, late accretionary infall seeded them with complex organic compounds [Oró, J. (1961) Nature (London) 190, 389–390; Delsemme, A. H. (1984) Origins Life 14, 51–60; Anders, E. (1989) Nature (London) 342, 255–257; Chyba, C. F. & Sagan, C. (1992) Nature (London) 355, 125–131]. Delivery of such extraterrestrial compounds may have contributed to the organic inventory necessary for the origin of life. Interstellar ices, the building blocks of comets, tie up a large fraction of the biogenic elements available in molecular clouds. In our efforts to understand their synthesis, chemical composition, and physical properties, we report here that a complex mixture of molecules is produced by UV photolysis of realistic, interstellar ice analogs, and that some of the components have properties relevant to the origin of life, including the ability to self-assemble into vesicular structures. PMID:11158552

The Bending Vibrations of the C_3-ISOTOPOLOGUES in the 1.9 Terahertz Region

NASA Astrophysics Data System (ADS)

Breier, A.; Büchling, Thomas; Lutter, Volker; Schnierer, Rico; Fuchs, Guido W.; Giesen, Thomas

2016-06-01

Short carbon chains are fundamental for the chemistry of stellar and interstellar ambiences. The linear carbon chain molecule C_3 has been found in various interstellar and circumstellar environments, encompassing diffuse interstellar clouds, star forming regions, shells of late type stars, as well as cometary tails. Due to the lack of a permanent dipole moment C_3 can only be detected by electronic transitions in the visible spectral range or by vibrational bands in the mid-and far-infrared region. We performed experiments where C_3 was produced via laser-ablation of a graphite rod with a 3 bar He purge and a subsequent adiabatic expansion into a vaccum resulting in a supersonic jet. We report laboratory measurements of the lowest bending mode transitions of six 13C-isotopologues of the linear C_3 molecule. Fifty-eight transitions have been measured between 1.8-1.9 THz with an accuracy of better than 1 MHz. Molecular parameters have been derived to give accurate line frequency positions of all 13C isotopologues to ease their future interstellar detection. A dedicated observation for singly substituted 13CCC is projected within the SOFIA airborne observatory mission.

Empirical relationships between gas abundances and UV selective extinction

NASA Technical Reports Server (NTRS)

Joseph, Charles L.

1990-01-01

Several studies of gas-phase abundances in lines of sight through the outer edges of dense clouds are summarized. These lines of sight have 0.4 less than E(B-V) less than 1.1 and have inferred spatial densities of a few hundred cm(-3). The primary thrust of these studies has been to compare gaseous abundances in interstellar clouds that have various types of peculiar selective extinction. To date, the most notable result has been an empirical relationship between the CN/Fe I abundance ratio and the depth of the 2200 A extinction bump. It is not clear at the present time, however, whether these two parameters are linearly correlated or the data are organized into two discrete ensembles. Based on 19 samples and assuming the clouds form discrete ensembles, lines of sight that have a CN/Fe I abundance ratio greater than 0.3 (dex) appear to have a shallow 2.57 plus or minus 0.55 bump compared to 3.60 plus or minus 0.36 for other dense clouds and compared to the 3.6 Seaton (1979) average. The difference in the strength of the extinction bump between these two ensembles is 1.03 plus or minus 0.23. Although a high-resolution IUE survey of dense clouds is far from complete, the few lines of sight with shallow extinction bumps all show preferential depletion of certain elements, while those lines of sight with normal 2200 A bumps do not. Ca II, Cr II, and Mn II appear to exhibit the strongest preferential depletion compared to S II, P II, and Mg II. Fe II and Si II depletions also appear to be enhanced somewhat in the shallow-bump lines of sight. It should be noted that Copernicus data suggest all elements, including the so-called nondepletors, deplete in diffuse clouds (Snow and Jenkins 1980, Joseph 1988). Those lines of sight through dense clouds that have normal 2200 A extinction bumps appear to be extensions of the depletions found in the diffuse interstellar medium. That is, the overall level of depletion is enhanced, but the element-to-element abundances are similar to those in diffuse clouds. In a separate study, the abundances of neutral atoms were studied in a dense cloud having a shallow 2200 A bump and in one with a normal strength bump.

Contribution of Massive Stars to the Production of Neutron Capture Elements

NASA Astrophysics Data System (ADS)

Federman, Steven

2010-09-01

Elements beyond the Fe-peak must be synthesized through neutron-capture processes. With the aim of understanding the contribution of massive stars to the synthesis of neutron-capture elements during the current epoch, we propose an archival survey of interstellar arsenic, cadmium, tin, and lead. Nucleosynthesis via the weak slow process and the rapid process are the routes involving massive stars, while the main slow process arises from the evolution of low-mass stars. Ultraviolet lines for the dominant ions for each element will be used to extract interstellar abundances. The survey involves about forty sight lines, many of which are associated with regions of massive star formation shaped by core-collapse supernovae {SNe II}. The sample will increase the number of published determinations by factors of 2 to 5. HST spectra are the only means for determining the elemental abundances for this set of species in diffuse interstellar clouds. The survey contains directions that are both molecule poor and molecule rich, thereby enabling us to examine the overall level of depletion onto grains as a function of gas density. Complementary laboratory determinations of oscillator strengths will place the interstellar measurements on an absolute scale. The results from the proposed study will be combined with published interstellar abundances for other neutron capture elements and the suite of measurements will be compared to results from stars throughout the history of the Galaxy.

Interstellar Matters: Neutral Hydrogen and the Galactic Magnetic Field

NASA Astrophysics Data System (ADS)

Verschuur, Gerrit; Schmelz, Joan T.; Asgari-Targhi asgari-Targhi, M.

2018-01-01

The physics of the interstellar medium was revolutionized by the observations of the Galactic Arecibo L-Band Feed Array (GALFA) HI survey done at the Arecibo Observatory. The high-resolution, high-sensitivity, high-dynamic- range images show complex, tangled, extended filaments, and reveal that the fabric of the neutral interstellar medium is deeply tied to the structure of the ambient magnetic field. This discovery prompts an obvious question – how exactly is the interstellar {\\it neutral} hydrogen being affected by the galactic magnetic field? We look into this question by examining a set of GALFA-HI data in great detail. We have chosen a long, straight filament in the southern galactic sky. This structure is both close by and isolated in velocity space. Gaussian analysis of profiles both along and across the filament reveal internal structure – braided strands that can be traced through the simplest part, but become tangled in more complex segments. These braids do not resemble in any way the old spherical HI clouds and rudimentary pressure balance models that were used to explain the pre-GALFA- HI interstellar medium. It is clear that these structures are created, constrained, and dominated by magnetic fields. Like many subfields of astronomy before it, e.g., physics of the solar coronal, extragalactic radio jets, and pulsar environment, scientists are confronted with observations that simply cannot be explained by simple hydrodynamics and are forced to consider magneto-hydrodynamics.

Fullerenes in Space.

PubMed

Maier, John P; Campbell, Ewen K

2017-04-24

In 1985 the football structure of C 60 , buckminsterfullerene was proposed and subsequently confirmed following its macroscopic synthesis in 1990. From the very beginning the role of C 60 and C 60 + in space was considered, particularly in the context of the enigmatic diffuse interstellar bands. These are absorption features found in the spectra of reddened star light. The first astronomical observations were made around one hundred years ago and despite significant efforts none of the interstellar molecules responsible have been identified. The absorption spectrum of C 60 + was measured in a 5 K neon matrix in 1993 and two prominent bands near 9583 Å and 9645 Å were observed. On the basis of this data the likely wavelength range in which the gas phase C 60 + absorptions should lie was predicted. In 1994 two diffuse interstellar bands were found in this spectral region and proposed to be due to C 60 + . It took over 20 years to measure the absorption spectrum of C 60 + under conditions similar to those prevailing in diffuse clouds. In 2015, sophisticated laboratory experiments led to the confirmation that these two interstellar bands are indeed caused by C 60 + , providing the first answer to this century old puzzle. Here, we describe the experiments, concepts and astronomical observations that led to the detection of C 60 + in interstellar space. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.

Structure formation in a colliding flow: The Herschel view of the Draco nebula

NASA Astrophysics Data System (ADS)

Miville-Deschênes, M.-A.; Salomé, Q.; Martin, P. G.; Joncas, G.; Blagrave, K.; Dassas, K.; Abergel, A.; Beelen, A.; Boulanger, F.; Lagache, G.; Lockman, F. J.; Marshall, D. J.

2017-03-01

Context. The Draco nebula is a high Galactic latitude interstellar cloud observed at velocities corresponding to the intermediate velocity cloud regime. This nebula shows unusually strong CO emission and remarkably high-contrast small-scale structures for such a diffuse high Galactic latitude cloud. The 21 cm emission of the Draco nebula reveals that it is likely to have been formed by the collision of a cloud entering the disk of the Milky Way. Such physical conditions are ideal to study the formation of cold and dense gas in colliding flows of diffuse and warm gas. Aims: The objective of this study is to better understand the process of structure formation in a colliding flow and to describe the effects of matter entering the disk on the interstellar medium. Methods: We conducted Herschel-SPIRE observations of the Draco nebula. The clumpfind algorithm was used to identify and characterize the small-scale structures of the cloud. Results: The high-resolution SPIRE map reveals the fragmented structure of the interface between the infalling cloud and the Galactic layer. This front is characterized by a Rayleigh-Taylor (RT) instability structure. From the determination of the typical length of the periodic structure (2.2 pc) we estimated the gas kinematic viscosity. This allowed us to estimate the dissipation scale of the warm neutral medium (0.1 pc), which was found to be compatible with that expected if ambipolar diffusion were the main mechanism of turbulent energy dissipation. The statistical properties of the small-scale structures identified with clumpfind are found to be typical of that seen in molecular clouds and hydrodynamical turbulence in general. The density of the gas has a log-normal distribution with an average value of 103 cm-3. The typical size of the structures is 0.1-0.2 pc, but this estimate is limited by the resolution of the observations. The mass of these structures ranges from 0.2 to 20 M⊙ and the distribution of the more massive structures follows a power-law dN/ dlog (M) M-1.4. We identify a mass-size relation with the same exponent as that found in molecular clouds (M L2.3). On the other hand, we found that only 15% of the mass of the cloud is in gravitationally bound structures. Conclusions: We conclude that the collision of diffuse gas from the Galactic halo with the diffuse interstellar medium of the outer layer of the disk is an efficient mechanism for producing dense structures. The increase of pressure induced by the collision is strong enough to trigger the formation of cold neutral medium out of the warm gas. It is likely that ambipolar diffusion is the mechanism dominating the turbulent energy dissipation. In that case the cold structures are a few times larger than the energy dissipation scale. The dense structures of Draco are the result of the interplay between magnetohydrodynamical turbulence and thermal instability as self-gravity is not dominating the dynamics. Interestingly they have properties typical of those found in more classical molecular clouds. Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA.The reduced Herschel data (FITS files) are only available at the CDS via anonymous ftp to http://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/599/A109

A general theory for the lifetimes of giant molecular clouds under the influence of galactic dynamics

NASA Astrophysics Data System (ADS)

Jeffreson, Sarah M. R.; Kruijssen, J. M. Diederik

2018-05-01

We propose a simple analytic theory for environmentally dependent molecular cloud lifetimes, based on the large-scale (galactic) dynamics of the interstellar medium. Within this theory, the cloud lifetime is set by the time-scales for gravitational collapse, galactic shear, spiral arm interactions, epicyclic perturbations, and cloud-cloud collisions. It is dependent on five observable quantities, accessible through measurements of the galactic rotation curve, the gas and stellar surface densities, and the gas and stellar velocity dispersions of the host galaxy. We determine how the relative importance of each dynamical mechanism varies throughout the space of observable galactic properties, and conclude that gravitational collapse and galactic shear play the greatest role in setting the cloud lifetime for the considered range of galaxy properties, while cloud-cloud collisions exert a much lesser influence. All five environmental mechanisms are nevertheless required to obtain a complete picture of cloud evolution. We apply our theory to the galaxies M31, M51, M83, and the Milky Way, and find a strong dependence of the cloud lifetime upon galactocentric radius in each case, with a typical cloud lifetime between 10 and 50 Myr. Our theory is ideally suited for systematic observational tests with the Atacama Large Millimetre/submillimetre array.

Synthetic observations of molecular clouds in a galactic centre environment - I. Studying maps of column density and integrated intensity

NASA Astrophysics Data System (ADS)

Bertram, Erik; Glover, Simon C. O.; Clark, Paul C.; Ragan, Sarah E.; Klessen, Ralf S.

2016-02-01

We run numerical simulations of molecular clouds, adopting properties similar to those found in the central molecular zone (CMZ) of the Milky Way. For this, we employ the moving mesh code AREPO and perform simulations which account for a simplified treatment of time-dependent chemistry and the non-isothermal nature of gas and dust. We perform simulations using an initial density of n0 = 103 cm-3 and a mass of 1.3 × 105 M⊙. Furthermore, we vary the virial parameter, defined as the ratio of kinetic and potential energy, α = Ekin/|Epot|, by adjusting the velocity dispersion. We set it to α = 0.5, 2.0 and 8.0, in order to analyse the impact of the kinetic energy on our results. We account for the extreme conditions in the CMZ and increase both the interstellar radiation field (ISRF) and the cosmic ray flux (CRF) by a factor of 1000 compared to the values found in the solar neighbourhood. We use the radiative transfer code RADMC-3D to compute synthetic images in various diagnostic lines. These are [C II] at 158 μm, [O I] (145 μm), [O I] (63 μm), 12CO (J = 1 → 0) and 13CO (J = 1 → 0) at 2600 and 2720 μm, respectively. When α is large, the turbulence disperses much of the gas in the cloud, reducing its mean density and allowing the ISRF to penetrate more deeply into the cloud's interior. This significantly alters the chemical composition of the cloud, leading to the dissociation of a significant amount of the molecular gas. On the other hand, when α is small, the cloud remains compact, allowing more of the molecular gas to survive. We show that in each case the atomic tracers accurately reflect most of the physical properties of both the H2 and the total gas of the cloud and that they provide a useful alternative to molecular lines when studying the interstellar medium in the CMZ.