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NASA NTRS2007

Irradiation Effects in Fosterrite and the Nature of Interstellar Grains: A Coordinated Spectroscopy and Electron Microscopy Study

Crystalline and amorphous silicates condense in the outflows of low mass evolved stars and massive red supergiant stars and are injected into the interstellar medium (ISM) where they are rendered almost completely amorphous by a multitude of destructive processes (e.g. shock, grain-grain collisions, and irradiation). Irradiation effects in particular may have played an important role in the genesis and modification of primitive grains in cometary dust, but unraveling those effects requires controlled experiments under appropriate conditions and with an emphasis on materials relevant to the ISM. Here we report our infrared (IR) microspectroscopy and trans-mission electron microscope (TEM) measurements on forsterite that was amorphized through irradiation by high energy heavy ions.

Keller, Lindsay P.↗
NASA NTRS2006

Modeling Chemical Growth Processes in Titan's Atmosphere: 1. Theoretical Rates for Reactions between Benzene and the Ethynyl (C2H) and Cyano (CN) Radicals at Low Temperature and Pressure

Density functional theory calculations at the B3LYP/6-31+G** level were employed to characterize the critical points for adducts, isomers, products, and intervening transition states for the reactions between benzene and the ethynyl (C2H) or cyano (CN) radicals. Both addition reactions were found to have no barriers in their entrance channels, making them efficient at the low temperature and pressure conditions that prevail in the haze-forming region of Titan's atmosphere as well as in the dense interstellar medium (ISM). The dominant products are ethynylbenzene (C6H5C2H) and cyanobenzene (C6H5CN). Hydrogen abstraction reactions were also characterized but found to be non-competitive. Trajectory calculations based on potentials fit to about 600 points calculated at the ROMP2/6-31+G** level for each interaction surface were used to determine reaction rates. The rates incorporated any necessary corrections for back reactions as ascertained from a multiwell treatment used to determine outcome distributions over the range of temperatures and pressures pertinent to Titan and the ISM and are in good agreement with the limited available experimental data.

Woon, David E.↗
NASA NTRS2010

Modelling Cosmic-Ray Effects in the Protosolar Disk

The role that Galactic cosmic rays (GCRs) and solar energetic particles (SEPs) play in the dynamic evolution of protosolar disks and the origin of our Solar System is a fundamental one. The GCRs are an important component of the interstellar medium (ISM), and even play a role in correcting the age determinations of some irons versus CAIs (calcium-aluminum inclusions) in meteoroids . Because CRs also are one of the energy transport mechanisms in a planetary nebula, the question of modelling their effect upon this broad subject is a serious topic for planetary science. The problem is addressed here.

Wilson, Thomas L.↗
NASA NTRS2010

Irradiation of FeS: Implications for the Lifecycle of Sulfur in the Interstellar Medium and Presolar FeS Grains

Fe(Ni) sulfides are ubiquitous in chondritic meteorites and cometary samples where they are the dominant host of sulfur. Despite their abundance in these early solar system materials, their presence in interstellar and circumstellar environments is poorly understood. Fe-sulfides have been reported from astronomical observations of pre- and post-main sequence stars [1, 2] and occur as inclusions in bonafide circumstellar silicate grains [3, 4]. In cold, dense molecular cloud (MC) environments, sulfur is highly depleted from the gas phase [e.g. 5], yet observations of sulfur-bearing molecules in dense cores find a total abundance that is only a small fraction of the sulfur seen in diffuse regions [6], therefore the bulk of the depletion must reside in an abundant unobserved phase. In stark contrast, sulfur is essentially undepleted from the gas phase in the diffuse interstellar medium (ISM) [7-9], indicating that little sulfur is incorporated into solid grains in this environment. This is a rather puzzling observation unless Fe-sulfides are not produced in significant quantities in stellar outflows, or their lifetime in the ISM is very short due to rapid destruction. The main destruction mechanism is sputtering due to supernova shocks in the warm, diffuse ISM [10]. This process involves the reduction of Fe-sulfide with the production of Fe metal as a by-product and returning S to the gas phase. In order to test this hypothesis, we irradiated FeS and analyzed the resulting material using X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM).

Keller, Lindsay P.↗
NASA NTRS2010

Formation of Nucleobases from the UV Photo-Irradiation of Pyrimidine in Astrophysical Ice Analogs

Astrochemistry laboratory simulations have shown that complex organic molecules including compounds of astrobiological interest can be formed under interstellarl/circumstellar conditions from the vacuum UV irradiation of astrophysical ice analogs containing H2O, CO, CO2, CH3OH, NH13, etc. Of all prebiotic compounds, the formation of amino acids under such experimental conditions has been the most extensively studied. Although the presence of amino acids in the interstellar medium (ISM) has yet to be confirmed, they have been detected in meteorites, indicating that biomolecules and/or their precursors can be formed under extraterrestrial, abiotic conditions. Nucleobases, the building blocks of DNA and RNA, as well as other 1V-heterocycles, have also been detected in meteorites, but like amino acids, they have yet to be observed in the ISM. In this work, we present an experimental study of the formation of pyrimidine-based compounds from the UV photo-irradiation of pyrimidine in ice mixtures containing H2O, NH3, and/or CH3OH at low temperature and pressure.

Milam, S. N.↗
NASA NTRS2010

Complex Role of Secondary Electron Emissions in Dust Grain Charging in Space Environments: Measurements on Apollo 11 and 17 Dust Grains

Dust grains in various astrophysical environments are generally charged electrostatically by photoelectric emissions with radiation from nearby sources, or by electron/ion collisions by sticking or secondary electron emissions. Knowledge of the dust grain charges and equilibrium potentials is important for understanding of a variety of physical and dynamical processes in the interstellar medium (ISM), and heliospheric, interplanetary, planetary, and lunar environments. The high vacuum environment on the lunar surface leads to some unusual physical and dynamical phenomena involving dust grains with high adhesive characteristics, and levitation and transportation over long distances. It has been well recognized that the charging properties of individual micron/submicron size dust grains are expected to be substantially different from the corresponding values for bulk materials and theoretical models. In this paper we present experimental results on charging of individual dust grains selected from Apollo 11 and Apollo 17 dust samples by exposing them to mono-energetic electron beams in the 10- 400 eV energy range. The charging rates of positively and negatively charged particles of approximately 0.2 to 13 microns diameters are discussed in terms of the secondary electron emission (SEE) process, which is found to be a complex charging process at electron energies as low as 10-25 eV, with strong particle size dependence. The measurements indicate substantial differences between dust charging properties of individual small size dust grains and of bulk materials.

Abbas, M. M.↗
NASA NTRS2012

Observations of Nitrogen Fractionation in Prestellar Cores: Nitriles Tracing Interstellar Chemistry

Primitive materials provide important clues on the processes that occurred during the formation and early evolution of the Solar System. Space-based and ground-based observations of cometary comae show that comets appear to contain a mixture of the products of both interstellar and nebular chemistries. Significant 15-nitrogen enrichments have been measured in CN and HCN towards a number of comets and may suggest an origin of interstellar chemical fractionation. Additionally, large N-15 enhancements are found in meteorites and has also led to to the view that the N-15 traces material formed in the interstellar medium (ISM), although multiple sources cannot be excluded. Here, we show the results of observations of the nitrogen and carbon fractionation in prestellar cores for various N-bearing species to decipher the origin of primitive material isotopic enrichments.

Milam, S. N.↗
NASA NTRS2012

High-Resolution and Analytical TEM Investigation of Space Radiation Processing Effects in Primitive Solar System Materials and Airless Planetary Surface Environments

Energetic ions present in the diverse plasma conditions in space play a significant role in the formation and modification of solid phases found in environments ranging from the interstellar medium (ISM) to the surfaces of airless bodies such as asteroids and the Moon. These effects are often referred to as space radiation processing, a term that encompasses changes induced in natural space-exposed materials that may be only structural, such as in radiation-induced amorphization, or may involve ion-induced nanoscale to microscale chemical changes, as occurs in preferential sputtering and ion-beam mixing. Ion sputtering in general may also be responsible for partial or complete erosion of space exposed materials, in some instances possibly bringing about the complete destruction of free-floating solid grains in the ISM or in circumstellar nebular dust clouds. We report here on two examples of the application of high-resolution and analytical transmission electron microscopy (TEM) to problems in space radiation processing. The first problem concerns the role of space radiation processing in controlling the overall fate of Fe sulfides as hosts for sulfur in the ISM. The second problem concerns the known, but as yet poorly quantified, role of space radiation processing in lunar space weathering.

Christoffersen, R.↗
NASA NTRS2012

An In-Depth Study of the Abundance Pattern in the Hot Interstellar Medium in NGC 4649

We present our X-ray imaging spectroscopic analysis of data from deep Suzuku and XMM-Newton Observatory exposures of the Virgo Cluster elliptical galaxy NGC 4649 (M60), focusing on the abundance pattern in the hot interstellar medium (ISM). All measured elements show a radial decline in abundance, with the possible exception of O. We construct steady state solutions to the chemical evolution equations that include infall in addition to stellar mass return and SNIa enrichment, and consider recently published SNIa yields. By adjusting a single model parameter to obtain a match to the global abundance pattern in NGC 4649 we infer that accretion of subsolar metallicity external gas has reduced the overall ISM metallicity and diluted the effectiveness of SNIa to skew the pattern towards low alpha/Fe ratios, and estimate the combination of SNIa rate and level of dilution. Evidently, newly-introduced gas is heated as it is integrated into. and interacts with, the hot gas that is already present. These results indicate a complex flow and enrichment history for NGC 4649, reflecting the continual evolution of elliptical galaxies beyond the formation epoch. The heating and circulation of accreted gas may help reconcile this dynamic history with the mostly passive evolution of elliptical stellar populations. In an appendix we examine the effects of the recent updated atomic database AtomDB in spectral fitting of thermal plasmas with hot ISM temperatures in the elliptical galaxy range.

Lowenstein, Michael↗
NASA NTRS2012

Nucleobases and Prebiotic Molecules in Organic Residues Produced from the Ultraviolet Photo-Irradiation of Pyrimidine in NH3 and H2O+NH3 Ices

Although not yet identified in the interstellar medium (ISM), N-heterocycles including nucleobases the information subunits of DNA and RNA are present in carbonaceous chondrites, which indicates that molecules of biological interest can be formed in non-terrestrial environments via abiotic pathways. Recent laboratory experiments and ab-initio calculations have already shown that the irradiation of pyrimidine in pure H2O ices leads to the formation of a suite of oxidized pyrimidine derivatives, including the nucleobase uracil. In the present work, NH3:pyrimidine and H2O:NH3:pyrimidine ice mixtures with different relative proportions were irradiated with UV photons under astrophysically relevant conditions. Liquid- and gas-chromatography analysis of the resulting organic residues has led to the detection of the nucleobases uracil and cytosine, as well as other species of prebiotic interest such as urea and small amino acids. The presence of these molecules in organic residues formed under abiotic conditions supports scenarios in which extraterrestrial organics that formed in space and were subsequently delivered to telluric planets via comets and meteorites could have contributed to the inventory of molecules that triggered the first biological reactions on their surfaces.

Nuevo, Michel↗
NASA NTRS2012

Submillimeter Measurements of Photolysis Products in Interstellar Ice Analogs: A New Experimental Technique

Over 150 molecular species have been confirmed in space, primarily by their rotational spectra at millimeter/submillimeter wavelengths, which yield an unambiguous identification. Many of the known interstellar organic molecules cannot be explained by gas-phase chemistry. It is now presumed that they are produced by surface reactions of the simple ices and/or grains observed and released into the gas phase by sublimation, sputtering, etc. Additionally, the chemical complexity found in meteorites and samples returned from comets far surpasses that of the remote detections for the interstellar medium (ISM), comets, and planetary atmospheres. Laboratory simulations of interstellar/cometary ices have found, from the analysis of the remnant residue of the warmed laboratory sample, that such molecules are readily formed; however, it has yet to be determined if they are formed during the warm phase or within the ice during processing. Most analysis of the ice during processing reveals molecular changes, though the exact quantities and species formed are highly uncertain with current techniques due to overwhelming features of simple ices. Remote sensing with high resolution spectroscopy is currently the only method to detect trace species in the ISM and the primary method for comets and icy bodies in the Solar System due to limitations of sample return. We have recently designed an experiment to simulate interstellar/cometary/planetary ices and detect trace species employing the same techniques used for remote observations. Preliminary results will be presented.

Milam, Stefanie N.↗
NASA NTRS2012

Near-Infrared Band Strengths of Molecules Diluted in N2 and H20 Ice Mixtures Relevant to Interstellar and Planetary Ices

In order to determine the column density of a component of an ice from its infrared absorption features, the strengths of these features must be known. The peak positions, widths, profiles, and strengths of a certain ice component's infrared absorption features are affected be the overall composition of the ice. Many satellites within the solar system have surfaces that are dominated by H2O or N2 and ices in the interstellar medium (ISM) are primarily composed of H2O. The experiments presented here focus on the near-infrared absorption features of CO, CO2, CH4, and NH3 (nu=10,000-4,000/cm, lambda=1-2.5 microns) and the effects of diluting these molecules in N2 or H2O ice (mixture ratio of 5:1). This is a continuation of previous results published by our research group.

Richey, C. R.↗
NASA NTRS2012

A Suzaku Study of Ejecta Structure and Origin of Hard X-ray Emission in the Supernova Remnant G156.2+5.7

We report an X-ray study of the evolved Galactic supernova remnant (SNR) G1S6.2+S.7 based on six pointing observations with Suzaku. The remnant's large extent (100' in diameter) allows us to investigate its radial structure in the northwestern and eastern directions from the apparent center. The X-ray spectra. were well fit with a two-component non-equilibrium ionization model representing the swept-up interstellar medium (ISM) and the metal-rich ejecta. We found prominent central concentrations of Si, S and Fe from the ejecta component; the lighter elements of O, Ne and Mg were distributed more uniformly. The temperature of the ISM component suggests a slow shock (610-960 km/s), hence the remnant's age is estimated to be 7,000-15,000 yr, assuming its distance to be approx. 1.1 kpc. G1S6.2+5.7 has also been thought to emit hard, non-thermal X-rays, despite being considerably older than any other such remnant. In response to a recent discovery of a background cluster of galaxies (2XMM J045637.2+522411), we carefully excluded its contribution, and reexamined the origin of the hard X-ray emission. We found that the residual hard X-ray emission is consistent with the expected level of the cosmic X-ray background. Thus, no robust evidence for the non-thermal emission was obtained from G156.2+5.7. These results are consistent with the picture of an evolved SNR.

Uchida, Hiroyuki↗
NASA NTRS2012

Laboratory Spectra of CO2 Vibrational Modes in Planetary Ice Analogs

Laboratory spectra have shown that CO2 is a powerful diagnostic tool for analyzing infrared data from remote observations, as it has been detected on icy moons in the outer Solar System as well as dust grain surfaces in the interstellar medium (ISM). IR absorption band profiles of CO2 within ice mixtures containing H2O and CH3OH change with respect to temperature and mixture ratios. In this particular study, the CO2 asymmetric stretching mode near 4.3 m (2350 cm (exp-1)), overtone mode near 1.97 m (5080 cm (exp-1)), and the combination bands near 2.7 m (3700 cm (exp-1)), 2.8 m (3600 cm (exp-1)), and 2.02 m (4960 cm (exp -1)), are systematically observed in different mixtures with H2O and CH3OH in temperature ranges from 15K to 150 K. Additionally, some high-temperature deposits (T greater than 50 K) of H2O, CH3OH, and CO2 ice mixtures were performed. These data may then be used to interpret infrared observational data obtained from icy surfaces in the outer Solar System and beyond.

White, Douglas↗
NASA NTRS2011

Initiating Molecular Growth in the Interstellar Medium via Dimeric Complexes of Observed Ions and Molecules

A feasible initiation step for particle growth in the interstellar medium (ISM) is simulated by means of ab quantum chemistry methods. The systems studied are dimer ions formed by pairing nitrogen containing small molecules known to exist in the ISM with ions of unsaturated hydrocarbons or vice versa. Complexation energies, structures of ensuing complexes and electronic excitation spectra of the encounter complexes are estimated using various quantum chemistry methods. Moller-Plesset perturbation theory (MP2, Z-averaged perturbation theory (ZAP2), coupled cluster singles and doubles with perturbative triples corrections (CCSD(T)), and density functional theory (DFT) methods (B3LYP) were employed along with the correlation consistent cc-pVTZ and aug-cc-pVTZ basis sets. Two types of complexes are predicted. One type of complex has electrostatic binding with moderate (7-20 kcal per mol) binding energies, that are nonetheless significantly stronger than typical van der Waals interactions between molecules of this size. The other type of complex develops strong covalent bonds between the fragments. Cyclic isomers of the nitrogen containing complexes are produced very easily by ion-molecule reactions. Some of these complexes show intense ultraviolet visible spectra for electronic transitions with large oscillator strengths at the B3LYP, omegaB97, and equations of motion coupled cluster (EOM-CCSD) levels. The open shell nitrogen containing carbonaceous complexes especially exhibit a large oscillator strength electronic transition in the visible region of the electromagnetic spectrum.

Bera, Partha P.↗
NASA NTRS2012

An In-Depth Study of the Abundance Pattern in the Hot Interstellar Medium in NGC 4649

We present our X-ray imaging spectroscopic analysis of data from deep Suzaku and XMM-Newton Observatory exposures of the Virgo Cluster elliptical galaxy NGC 4649 (M60), focusing on the abundance pattern in the hot interstellar medium (ISM). All measured elements show a radial decline in abundance, with the possible exception of Oxygen. We construct steady state solutions to the chemical evolution equations that include infall in addition to stellar mass return and Type Ia supernovae (SNIa) enrichment, and consider recently published SNIa yields. By adjusting a single model parameter to obtain a match to the global abundance pattern in NGC 4649 we infer that introduction of subsolar metallicity external gas has reduced the overall ISM metallicity and diluted the effectiveness of SNIa to skew the pattern towards low alpha/Fe ratios, and estimate the combination of SNIa rate and level of dilution. Evidently, newly-introduced gas is heated as it is integrated into, and interacts with, the hot gas that is already present. These results indicate a complex flow and enrichment history for NGC 4649, reflecting the continual evolution of elliptical galaxies beyond the formation epoch. The heating and circulation of accreted gas may help reconcile this dynamic history with the mostly passive evolution of elliptical stellar populations. In an appendix we examine the effects of the recent updated atomic database AtomDB in spectral fitting of thermal plasmas with hot ISM temperatures in the elliptical galaxy range.

Loewenstein, Michael↗
NASA NTRS2013

Photoionization Modeling of Oxygen K Absorption in the Interstellar Medium: The Chandra Grating Spectra of XTE J1817-330

We present detailed analyses of oxygen K absorption in the interstellar medium (ISM) using four high-resolution Chandra spectra towards the X-ray low-mass binary XTE J1817-330. The 11-25 A broadband is described with a simple absorption model that takes into account the pileup effect and results in an estimate of the hydrogen column density. The oxygen K-edge region (21-25 A) is fitted with the physical warmabs model, which is based on a photoionization model grid generated with the XSTAR code with the most up-to-date atomic database. This approach allows a benchmark of the atomic data which involves wavelength shifts of both the K lines and photoionization cross sections in order to fit the observed spectra accurately. As a result we obtain: a column density of N(sub H) = 1.38 +/- 0.01 x 10(exp 21) cm(exp -2); ionization parameter of log xi = .2.70 +/- 0.023; oxygen abundance of A(sub O) = 0.689(exp +0.015./-0.010); and ionization fractions of O I/O = 0.911, O II/O = 0.077, and O III/O = 0.012 that are in good agreement with previous studies. Since the oxygen abundance in warmabs is given relative to the solar standard of Grevesse and Sauval (1998), a rescaling with the revision by Asplund et al. (2009) yields A(sub O) = 0.952(exp +0.020/-0.013, a value close to solar that reinforces the new standard. We identify several atomic absorption lines.K-alpha , K-beta, and K-gamma in O I and O II; and K-alpha in O III, O VI, and O VII--last two probably residing in the neighborhood of the source rather than in the ISM. This is the first firm detection of oxygen K resonances with principal quantum numbers n greater than 2 associated to ISM cold absorption.

Gatuzz, E.↗
NASA NTRS2011

Circumstellar Environments of Luminous Infrared Stellar Objects in the Magellanic Clouds

Young stars are formed out of the interstellar medium (ISM) which is replenished by mass loss rates from evolved stars. Circumstellar matter around young and evolved stellar objects usually emits energy in the infrared (IR) wavelength range as the matter is heated by the central star. Surveys of the Magellanic Clouds with the Spitzer Space Telescope in the 3.6-160 micron range have previously been completed. These surveys have led to catalogs of infrared sources: which include HII regions, young stars, super giants, asymptotic giant branch (AGB) stars, post-asymptotic giant branch (post-AGB) stars, and planetary nebulae. The utility of such surveys can be improved upon by using Hubble Space Telescope (HST) data. HST provides higher angular resolution than Spitzer and has allowed for more detailed investigation of these luminous IR objects. This project used previously obtained HST archival data to examine luminous IR objects at optical wavelengths. This allows for the reclassification of stellar objects previously thought as one type of object or in a particular stage of their stellar evolution. An overall objective of this project included looking for extended nebulosity around evolved stars to better understand the life cycle of such objects and classify these nebulae by shape.

large Magellanic clouds↗