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NASA NTRS1974

Hot hydrogen atoms reactions of interest in molecular evolution and interstellar chemistry

Hot hydrogen atoms which are photochemically generated initiate reactions among mixtures of methane, ethane, water and ammonia, to produce ethanol, organic amines, organic acids, and amino acids. Both ethanol and ethyl amine can also act as substrates for formation of amino acids. The one carbon substrate methane is sufficient as a carbon source to produce amino acids. Typical quantum yields for formation of amino acids are approximately 0.00002 to 0.00004. In one experiment, 6 protein amino acids were identified and 8 nonprotein amino acids verified utilizing gas chromatography-mass spectroscopy. We propose that hot atoms, especially hydrogen, initiate reactions in the thermodynamic nonequilibrium environment of interstellar space as well as in the atmospheres of planets.

Becker, R. S.↗

NASA NTRS1993

A new way to measure the composition of the interstellar gas surrounding the heliosphere

The composition of neutral gas in the Local Interstellar Medium can be studied by direct, in situ measuring of interstellar neutral atoms penetrating into interplanetary space. A novel experimental approach for in situ atom detection, which has never been used earlier in space, is proposed. The technique is based on the conversion of neutral atoms to negative ions at a specially prepared sensitive surface. Negative ions are subsequently analyzed and detected in an essentially noise-free, multicoincidence mode. It is shown that interstellar hydrogen, deuterium, and oxygen atoms can be measured by the proposed technique. The experiment can be performed from a high-apogee Earth-orbiting satellite or from a deep space probe.

Gruntman, Michael A.↗

NASA NTRS1980

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

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

Hollenbach, D.↗

NASA NTRS1994

Large-scale characteristics of interstellar dust from COBE DIRBE observations

Observations from the COBE Diffuse Infrared Background Experiment of the 140 and 240 micrometer emissions from the Galatic plane region (absolute value of b less than 10 deg) are combined with radio surveys that trace the molecular (H2), neutral atomic (H I), and extended low-density (n(sub e) approximately 10 to 100/cm(exp 3)) ionized (H II) gas phases of the interstellar medium to derive physical conditions such as the dust temperature, dust-to-gas mass ratio, and far-infrared emissivity (1) averaged over these gas phases along each line of sight and (2) within each of these three gas phases. This analysis shows large-scale longitudinal and latitudinal gradients in the dust temperature and a decrease in dust temperature with increasing Galactocentric distance. The derived dust temperatures are significantly different from those derived in similar analyses using the Infrared Astronomical Satellite (IRAS) 60 and 100 micrometer data, suggesting that small (5 A approximately less than radius approximately less than 200 A) transiently heated dust particles contribute significantly o the Galactic 60 micrometer emission. It is found that 60% to 75% of the far-infrared luminosity arises from cold (approximately 17 to 22 K) dust associated with diffuse H I clouds, 15% to 30% from cold (approximately 19 K) dust associated with molecular gas, and less than 10% from warm (approximately 29 K) dust in extended low-density H II regions, consistent with the results of the IRAS analyses of the Galactic 60 and 100 micrometer emission. Within 2 deg of longitude of the Galactic center, the derived gas-to-dust mass ratio along the line of sight, G(sub d), reverses its general trend of decreasing G(sub d) toward the inner Galaxy and increases by a factor of approximately 2 to 3 toward the Galactic center. One possible explanation for this result is that the ratio of H2 column density to (12)CO intensity is lower in the Galactic center region than in the Galactic disk.

Sodroski, T. J.↗

NASA NTRS2012

Primary Volatile Abundances in Comets from Infrared Spectroscopy: Implications for Reactions on Grain Surfaces in the Interstellar/Nebular Environment

Comets retain relatively primitive icy material remaining from the epoch of Solar System formation, however the extent to which they are modified from their initial state remains a key question in cometary science. High-resolution lR spectroscopy has emerged as a powerful tool for measuring vibrational emissions from primary volatiles (i.e., those contained in the nuclei of comets). With modern instrumentation, most notably NIRSPEC at the Keck II 10-m telescope, we can quantify species of astrobiological importance (e.g., H20, C2H2, CH4, C2H6, CO, H2CO, CH30H, HCN, NH3). In space environments, compounds of keen interest to astrobiology could originate from HCN and NH3 (leading to amino acids), H2CO (leading to sugars), or C2H6 and CH4 (suggested precursors of ethyl- and methylamine). Measuring the abundances of these precursor molecules (and their variability among comets) is a feasible task that contributes to understanding their delivery to Earth's early biosphere and to the synthesis of more complex pre biotic compounds. Over 20 comets have now been measured with IR spectroscopy, and this sample reveals significant diversity in primary volatile compositions. From this, a taxonomic classification scheme is emerging, presumably reflecting the diverse conditions experienced by pre-cometary grains in interstellar and subsequent nebular environs. The importance of H-atom addition to C2H2 on the surfaces of interstellar grains to produce C2H6 was validated by the discovery of abundant ethane in comet C/1996 B2 (Hyakutake) with C2H6/CH4 well above that achievable by gas-phase chemistry , and then in irradiation experiments on laboratory ices at 10 - 50 K. The large abundance ratios C2H6/CH4 observed universally in comets establish H-atom addition as an important and likely ubiquitous process, and comparing C2H6/C2H2 among comets can provide information on its efficiency. The IR is uniquely capable since symmetric hydrocarbons (e.g., C2H2, CH4, C2H6) have no electric dipole moment and thus no allowed pure rotational transitions. CO should also be hydrogenated on grain surfaces. Irradiation experiments on interstellar ice analogs show this to require very low temperatures, the resulting yields of H2CO and CH30H being highly dependent on temperature in the range approx 10 - 25 K. The relative abundances of these chemically-related molecules in comets provide one measure of the efficiency of H-atom addition to CO Oxidation of CO is also important on grain mantles, as evidenced by the widespread presence of C02 ice towards interstellar sources observed with ISO and in a survey of 17 comets observed with AKARI. H-atom addition to C2H2 produces the vinyl radical, and through subsequent oxidation1reduction reactions can lead to vinyl alcohol, acetaldehyde, and ethanol This may have implications for interpreting observed abundance ratios CO/C2H2. We will discuss possible implications regarding formation conditions in the context of measured primary volatile compositions, emphasizing recently observed comets and published results. These are continually providing new insights regarding our taxonomic scheme and also delivery of pre-biological material to the young Earth.

DiSanti, M. A.↗

NASA NTRS1990

Current star formation in S0 galaxies: NGC 4710

Elliptical (E) and lenticular (S0) galaxies lack the substantial interstellar medium (ISM) found in the star-forming spiral galaxies. However, significant numbers of E and S0 galaxies are known to contain detectable amounts of interstellar matter (e.g., Jura 1988). Thus, it is worth investigating whether these galaxies are currently able to form stars from their ISM, or whether they should be consigned to the dustbin of inert objects (Thronson and Bally 1987). The results strongly imply that current star formation is responsible for NGC 4710's far infrared and radio continuum properties. If this is indeed the case, then one expects this star formation to be fueled by molecular gas, which is presumably dominated by H2 and can be traced by the CO-12 J=1 to 0 line. Both Kenney and Young (1988) and Sage and Wrobel (1989) have detected such an emission line from NGC 4710, and infer the presence of more than 10(exp 8) solar mass of H2. The origin of the molecular gas in NGC 4710 remains a mystery. The galaxy is very deficient in HI (Kenney and Young, in preparation), suggesting that it originally was a spiral galaxy from which the outer, mainly atomic, gas was stripped by the ram pressure of the Virgo Cluster's intracluster medium, leaving only a central interstellar medium (ISM) rich in molecular gas. Alternatively, the CO may have originated via stellar mass loss with subsequent cooling, cooling flows, or capture from a gas-rich companion. Information on the morphology and kinematics of the CO can be compared with that of the galaxy's other gases and stars to distinguish among these various possible origins for the molecular gas. Major axis CO mapping with single dishes indicate an unresolved source. Thus, a millimeter array is currently being used to image NGC 4710 in CO to provide the needed morphological and kinematical data.

Wrobel, J. M.↗

NASA NTRS1992

Mid-infrared rotational line emission from interstellar molecular hydrogen

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.

Burton, Michael G.↗

NASA NTRS1984

Gamma rays from atomic and molecular gas in the large complex of clouds in Orion and Monoceros

A comparison of COS-B gamma-ray observations of the large complex of interstellar clouds in Orion and Monoceros with the Columbia CO and Berkeley H I surveys of this region reveals a good correlation between gamma-ray emission and total gas distribution. The observed gamma-ray emission is explainable in terms of interactions of cosmic rays that are uniformly distributed in this region with the interstellar gas. The correlation is used as the basis of a calibration of the ratio between H2 column density and the integrated CO line intensity; the value of (2.6 + or - 1.2) X 10 to the 20th mol/sq cm K km s thereby obtained is consistent with the value derived from a similar analysis for the inner galaxy.

Bloemen, J. B. G. M.↗

NASA NTRS1990

The gas/dust ratio in spiral galaxies

IRAS data are used here to calculate warm dust masses, which are then compared with the molecular and atomic gas masses for 58 spiral galaxies in order to constrain the fraction and the phase of the interstellar medium in spiral galaxies that contributes to the emission measured by IRAS. The dispersion in the ratio of dust mass to total gas mass is larger than expected on the basis of measurement errors. The dispersion in the ratio of dust mass to inner disk gas mass is less than the dispersion in the ratio of IR to radio emission. The inner gas to warm dust mass ratio for spiral galaxies is 1080 + or - 70, indicating that 80-90 percent of the dust mass in spiral galaxies is radiating at over 100 microns and has a temperature less than about 30 K. However, the bulk of the dust in spiral galaxies is less than about 15 K regardless of the phase of the ISM. Both H I and H2-associated dust contributes to the warm 30 K emission.

Devereux, Nicholas A.↗

NASA NTRS1976

An analysis of the interstellar material in the line of sight toward Omicron Persei

Chemical abundances and physical conditions in the interstellar gas toward Omicron Per are analyzed using Copernicus UV spectrophotometric data on 69 lines due to ions and atoms as well as 19 lines of H2 and HD. The UV extinction toward the star is determined along with the column densities of atomic and molecular hydrogen, the atomic column densities and temperature in the H I region, and the nature of the ionized regions along the line of sight. It is concluded that: (1) Omicron Per is apparently embedded in the near edge of an extended dense molecular cloud, the outer parts of which give rise to the observed absorption lines; (2) the H I region may consist of more than one cloud; (3) the far-UV extinction rise is unusually steep, indicating a high proportion of small grains; (4) the element depletions are somewhat greater than in Zeta Oph, particularly for sulfur and argon; and (5) the molecular abundances are generally consistent with gas-phase formation models rather than with molecular formation on grain surfaces, unless a substantial amount of small grains is present to increase the available surface area.

Snow, T. P., Jr.↗

NASA NTRS2014

Interaction Between the Broad-Lined Type Ic Supernova 2012ap and Carriers of Diffuse Interstellar Bands

The diffuse interstellar bands (DIBs) are absorption features observed in optical and near-infrared spectra that are thought to be associated with carbon-rich polyatomic molecules in interstellar gas. However, because the central wavelengths of these bands do not correspond with electronic transitions of any known atomic or molecular species, their nature has remained uncertain since their discovery almost a century ago. Here we report on unusually strong DIBs in optical spectra of the broad- lined Type Ic supernova SN2012ap that exhibit changes in equivalent width over short (. 30 days) timescales. The 4428 A and 6283 A DIB features get weaker with time, whereas the 5780 A feature shows a marginal increase. These nonuniform changes suggest that the supernova is interacting with a nearby source of the DIBs and that the DIB carriers possess high ionization potentials, such as small cations or charged fullerenes. We conclude that moderate-resolution spectra of supernovae with DIB absorptions obtained within weeks of outburst could reveal unique information about the mass-loss environment of their progenitor systems and provide new constraints on the properties of DIB carriers.

broad↗

NASA NTRS1990

A model for the infrared emission from an OB star cluster environment

Researchers developed an interactive radiative transfer code that predicts the infrared emission from an HII region containing diffuse ionized and atomic gas and dense molecular clouds. This model complements the investigation of the redistribution of OB star luminosity in the interstellar medium (Leisawitz and Hauser 1988, Ap. J., 332, 954). The model can be used as a diagnostic tool to probe the radiation field and matter density in an HII region, place constraints on the proximity and orientation of an illuminated molecular cloud with respect to the ionizing stars, test for the presence of small, transiently heated dust grains, and determine whether the dust-to-gas ratio is normal. Predictions of the model agree qualitatively and quantitatively with observations of blister-type HII regions ionized by well-studied OB clusters in which the distribution of dense neutral material is known. This is illustrated by a model for Infrared Astronomy Satellite (IRAS) observations of the region around NGC 7380 (S142). Researchers plan to use the model in a survey of regions of massive star formation in the outer Galaxy to study OB stars embedded to various degrees in their parental molecular clouds.

Leisawitz, D.↗

NASA NTRS1990

Physical conditions in photodissociation regions - Application to galactic nuclei

A procedure is outlined which determines the physical characteristics of the neutral interstellar medium in the nuclei of luminous galaxies. The method uses millimeter and IR observations to find the mass and density of the molecular and atomic gas components as well as the UV flux incident on clouds. The area and volume filling factors and approximate number of clouds and cloud radii are also found. For the Galactic center, about 100 clouds of radius about 0.4 pc and density about 100,000/cu cm are found within about 5 pc. The atomic gas temperature is about 700 K and the FUV field on clouds is about 100,000 times the local Galactic FUV field. The flux is consistent with a central source of luminosity of 2-3 x 10 to the 7th solar. Roughly 100,000 clouds of radius roughly 0.4 pc are found within the roughly 330 pc nuclear region of M82. The large number of clouds produces a projected area filling factor approaching unity. Cloud heating may be dominated by an intense interstellar UV flux.

Wolfire, Mark G.↗

NASA NTRS2002

Interstellar Antifreeze: Ethylene Glycol

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.

Hollis, J. M.↗

NASA NTRS1989

Observations of molecular and atomic gas in photodissociation regions

Dense gas at the ionized/neutral boundaries of molecular clouds illuminated by far-UV photons plays an important role in the appearance of the neutral interstellar medium. It also is a laboratory for the study of UV-photochemistry and of a number of heating and cooling phenomena not seen elsewhere. Fine structure lines of neutral and low ionization potential species dominate the cooling in the outer part of the photodissociation regions. Observations of these lines show that the regions are dense and highly clumped. Observations of H2 and CO show that heating by UV photons plays a significant role in the excitation of molecular lines near the H II/neutral boundary. Warm CO is more abundant in these regions than predicted by the standard theoretical models. Optical reflection nebulas provide an ideal laboratory for the study of photodissocciation region phenomena.

Jaffe, D. T.↗

NASA NTRS2010

A Comparison of Oxidized Carbon Abundances among Comets

Comets contain relatively well preserved icy material remaining from the epoch of Solar System formation, however the extent to which these ices are modified from their initial state remains a fundamental question in cometary science. As a comet approaches the Sun, sublimation of the ices contained in its nucleus (termed " native ices") releases parent volatiles into the coma, where they can be measured spectroscopically. One means of assessing the degree to which interstellar ices were processed prior to their incorporation into cometary nuclei is to measure the relative abundances of chemically-related parent volatiles. For example, formation of C2H6 by hydrogen atom addition (e.g., to C2H2) on surfaces of ice-mantled grains was proposed to explain the high C2H6 to CH4 abundance observed in C/1996 B2 (Hyakutake) [1]. The large C2H6/CH4 abundance ratios measured universally in comets, compared with those predicted by gas phase production of C2H6, establishes H-atom addition as an important and likely ubiquitous process. CO should also be hydrogenated on grain surfaces. Laboratory irradiation experiments on interstellar ice analogs indicate this to require very low temperatures (T approx. 10-25 K), the resulting yields of H2CO and CH3OH being highly dependent both on hydrogen density (i.e., fluence) and on temperature ([2],[3]). This relatively narrow range in temperature reflects a lack of mobility below 8-10 K on the one hand, and reduced sticking times for H-atoms as grain surfaces are warmed above 20 K on the other. The relative abundances of these three chemically-related molecules in comets provides one measure of the efficiency of H-atom addition to CO on pre-cometary grains (Fig. 1).

DiSanti, M. A.↗

NASA NTRS1994

Highly ionized gas in the Galactic halo

We reexamine the values of electron density n(sub e) and gas pressure P/k in the interstellar medium (ISM) of the Galactic halo, as inferred from C IV emission and absorption lines and using current C IV atomic data. In a homogeneous model with 4.7 less than or equal to log T less than or equal to 5.3, the data are consistent with 0.01 less than or equal to n(sub e) less than or equal to 0.02/cu cm and 2200 less than or equal to P/k less than or equal to 3700/cu cm K, a factor of 2-3 higher than advocated by Martin & Bowyer (1990) and comparable to the thermal pressure in the disk. If some of the C IV absorption arises from nonemitting, photoionized gas, then the inferred density and pressure will increase accordingly. The volume filling factor for homogeneous models ranges from 0.5% to 5%. Because of the constraints arising from filling factor and radiated power, most of the C IV must arise from gas near the peak of the cooling curve, at log t less than or equal to 5.6. We relate both emission-line and absorption-line observations to recent models in which turbulent mixing layers and isobarically cooling supernova remnants (SNRs) provide significant amounts of halo gas at approximately 10(exp 5.3) K and process 20-40 solar mass/yr with a power of approximately 10(exp 41) ergs/sec. Since the observed C IV and N V absorption scale heights have been reported to differ, at 4.9 kpc and 1.6 kpc, respectively, we examine inhomogeneous models with different exponential scale heights of T, P, and SN energy input. The ISM may change its character with distance above the Galactic plane, as superbubbles and mixing layers dominate over isolated SNRs as the source of the C IV. For appropiate scale heights, the midplane pressure is twice the homogeneous values quoted above. The O IV lambda 1034 diffuse emission line, which can be used as a temperature diagnostic of the hot gas, is predicted to be comparable in strength to that of C IV lambda 1549 (approximately 6000 photons/cu cm/s/sr. The ions C IV, N V, and O VI are predicted to show a decreasing trend in vertical extent, with the O IV scale height perhaps as low as approximately 1 kpc.

Shull, J. Michael↗

NASA NTRS2012

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

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

structure↗