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All sourcesNASA NTRSDOE OSTI
NASA NTRS1995

Observational aspects of polycyclic aromatic hydrocarbon charging in the Interstellar Medium

We have investigated the charging processes which affect small carbonaceous dust grains and polycyclic aromatic hydrocarbons (PAH's). Because of their high abundance, interstellar PAH molecules can dominate the charge balance of the interstellar medium (ISM), which controls the heating and cooling interstellar gas and interstellar chemistry. We present the results of our model, which compare well with observations and suggest further applications to both laboratory measurements and data obtainable from the KAO.

Bakes, E. L. O.↗
DOE OSTI2022

A Laboratory-driven Multiscale Investigation of X-Ray Induced Mass Loss and Photochemical Evolution in Cosmic Carbon and Silicate Dust

We present the results of an integrated laboratory and modeling investigation into the impact of stellar X-rays on cosmic dust. Carbonaceous grains were prepared in a cooled (< 200 K) supersonic expansion from aromatic molecular precursors, and were later irradiated with 970 eV X-rays. Silicate (enstatite) grains were prepared via laser ablation, thermally annealed, and later irradiated with 500 eV X-rays. Infrared spectra of the 3.4 μm band of the carbon sample prepared with benzene revealed 84% ± 5% band area loss for an X-ray dose of 5.2 x 10 23 eV.cm -2 . Infrared spectra of the 8 -12 μm Si-O band of the silicate sample revealed band area loss up to 63% ± 5% for doses of 2.3 x 10 23 eV.cm -2 . A hybrid Monte Carlo particle trajectory approach was used to model the impact of X-rays and ensuing photoelectrons, Auger and collisionally ionized electrons through the bulk. As a result of x-ray ionization and ensuing Coulomb explosions on surface molecules, the calculated mass loss is 60% for the carbonaceous sample and 46% for the silicate sample, within a factor of 2 of the IR band loss, supporting an X-ray induced mass-loss mechanism. We apply the laboratory X-ray destruction rates to estimate the lifetimes of dust grains in protoplanetary disks surrounding 1 M ⊙ and 0.1 M ⊙ G and M stars. In both cases, X-ray destruction timescales are short (a few million years) at the disk surface, but are found to be much longer than typical disk lifetimes (≳10 Myr) over the disk bulk.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Rate coefficients for rotational state-to-state transitions in H 2 O + H 2 O collisions for cometary and planetary applications, as predicted by mixed quantum-classical theory

We present new calculations of collision cross sections for state-to-state transitions between the rotational states in an H 2 O + H 2 O system, which are used to generate a new database of collisional rate coefficients for cometary and planetary applications. Calculations were carried out using a mixed quantum-classical theory approach that is implemented in the code MQCT. The large basis set of rotational states used in these calculations permits us to predict thermally averaged cross sections for 441 transitions in para- and ortho-H 2 O in a broad range of temperatures. It is found that all state-to-state transitions in the H 2 O + H 2 O system split into two well-defined groups, one with higher cross-section values and lower energy transfer, which corresponds to the dipole-dipole driven processes. The other group has smaller cross sections and higher energy transfer, driven by higher-order interaction terms. We present a detailed analysis of the theoretical error bars, and we symmetrized the state-to-state transition matrixes to ensure that excitation and quenching processes for each transition satisfy the principle of microscopic reversibility. We also compare our results with other data available from the literature for H 2 O + H 2 O collisions.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1996

GAuche-Ethyl Alcohol: Laboratory Assignments and Interstellar Identification

Ethyl alcohol (ethanol) is known to possess a pair of closely-spaced excited torsional substates (gauche +, gauche -) at an energy of approximately 57 K above the ground (trans) torsional substrate. We report an extended analysis of some gauch- - gauch+ Q branch (delta J=0) transitions witha three-substrate fixed frame axis method (FFAM) Hamilton.

ISM↗
DOE OSTI2022

State-to-state rate coefficients for HCS+ in rotationally inelastic collisions with H2 at low temperatures

ABSTRACT HCS+ ions have been detected in several regions of the interstellar medium (ISM), but an accurate determination of the chemical-physical conditions in the molecular clouds where this molecule is observed requires detailed knowledge of the collisional rate coefficients with the most common colliders in those environments. In this work, we study the dynamics of rotationally inelastic collisions of HCS+ + H2 at low temperature, and report, for the first time, a set of rate coefficients for this system. We used a recently developed potential energy surface for the HCS+–H2 van der Waals complex and computed state-to-state rotational rate coefficients for the lower rotational states of HCS+ in collision with both para- and ortho-H2, analysing the influence of the computed rate coefficients on the determination of critical densities. Additionally, the computed rate coefficients are compared with those obtained by scaling the ones from HCS+ in collision with He (an approximation that is sometimes used when data is lacking), and large differences are found. Furthermore, the approximation of using the rates for the HCO+ + H2 collision as a rough approximation for those of the HCS+ + H2 system is also evaluated. Finally, the complete set of de-excitation rate coefficients for the lowest 30 rotational states of HCS+ by collision with H2 is reported from 5 to 100 K.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2013

Interstellar and Planetary Analogs in the Laboratory

We present and discuss the unique capabilities of the laboratory facility, COSmIC, that was developed at NASA Ames to investigate the interaction of ionizing radiation (UV, charged particles) with molecular species (neutral molecules, radicals and ions) and carbonaceous grains in the Solar System and in the Interstellar Medium (ISM). COSmIC stands for Cosmic Simulation Chamber, a laboratory chamber where interstellar and planetary analogs are generated, processed and analyzed. It is composed of a pulsed discharge nozzle (PDN) expansion that generates a free jet supersonic expansion in a plasma cavity coupled to two ultrahigh-sensitivity, complementary in situ diagnostics: a cavity ring down spectroscopy (CRDS) system for photonic detection and a Reflectron time-of-flight mass spectrometer (ReTOF-MS) for mass detection. This setup allows the study of molecules, ions and solids under the low temperature and high vacuum conditions that are required to simulate some interstellar, circumstellar and planetary physical environments providing new fundamental insights on the molecular level into the processes that are critical to the chemistry in the ISM, circumstellar and planet forming regions, and on icy objects in the Solar System. Recent laboratory results that were obtained using COSmIC will be discussed, in particular the progress that have been achieved in monitoring in the laboratory the formation of solid particles from their gas-phase molecular precursors in environments as varied as circumstellar outflow and planetary atmospheres.

Salama, Farid↗
NASA NTRS2019

Cool, Evolved Stars: Results, Challenges, and Promises for the Next Decade

Cool, evolved stars are the main source of chemical enrichment of the interstellar medium (ISM), and understanding their mass loss and structure offers a unique opportunity to study the cycle of matter in the Universe. Pulsation, convection, and other dynamic processes in cool evolved stars create an atmosphere where molecules and dust can form, including those necessary to the formation of life (e.g. Carbon-bearing molecules). Understanding the structure and composition of these stars is thus vital to several aspects of stellar astrophysics, ranging from ISM studies to modeling young galaxies and to exoplanet research. Recent modeling efforts and increasingly precise observations now reveal that our understanding of cool stars photospheric, chromospheric, and atmospheric structures is limited by inadequate knowledge of the dynamic and chemical processes at work. Here we outline promising scientific opportunities for the next decade that can provide essential constraints on stellar photospheres, chromospheres, and circumstellar envelopes (CSE), and tie together analyses of the spectra and interferometric and imaging observations of evolved stars. We identify and discuss the following main opportunities: (1) identify and model the physical processes that must be included in current 1D and 3D atmosphere models of cool, evolved stars; (2) refine our understanding of photospheric, chromospheric, and outer atmospheric regions of cool evolved stars, their properties and parameters, through high-resolution spectroscopic observations, and interferometric observations at high angular resolution; (3) include the neglected role of chromospheric activity in the mass loss process of red giant branch (RGB) and red super giant (RSG) stars and understand the role played by their magnetic fields; (4) identify the important shaping mechanisms for planetary nebulae (PNe) and their relation with the parent Asymptotic Giant Branch (AGB) stars.

Gioia Rau↗
DOE OSTI2022

Formation of phosphorus monoxide through the $$\mathbf {P}(^{4}S)+\mathbf {O}_{\mathbf {2}}(^{3}\Sigma ^{-})\rightarrow \mathbf {O}(^{3}P)+\mathbf {PO}(^{2}\Pi )$$ reaction

Phosphorus is a key and vital element for a diverse set of important biological molecules, being indispensable for life as we know. A deeper comprehension of its role in astrochemistry and atmospheric chemistry may aid in finding answers to how this element became available on Earth. The PO molecule is one of the main reservoirs of phosphorus in the interstellar medium (ISM), and a better understanding of the mechanisms and rate coefficients for its formation in the ISM is important for modelling its abundances. Here in this work, we perform multireference configuration interaction calculations on the formation of PO via the P( 4 S) + O 2 (3 Σ - ) reaction, analyzing its potential energy surface and rate coefficients for the global reaction on both doublet and quartet states. We also perform DFT (M06-2X) and CCSD(T) calculations, in order to compare the results. We found that the OPO system possesses a high multiconfigurational character, making DFT and CCSD methodologies not suitable for its potential energy landscape calculation. The rate coefficients have been calculated using the master equation system solver (MESS) package, and the results compared to recent experimental data. It is shown that the quartet state contributes for temperatures higher than 700K. The computed rate coefficient can be described by a modified Arrhenius equation [α(T/300) β exp (-γ/T)] with α = 1.44 x 10 -12 cm 3 s -1 , β= -1.66 and γ= 704 K.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1996

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

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.

Sandford, Scott A.↗
NASA NTRS1993

Cosmic evolution of extragalactic C 1, C 2, and CO luminosity

Carbon is the fourth most abundant element in the Galaxy with an abundance of approximately 4 x 10(exp -4) relative to hydrogen. Of all abundant metals it is the easiest to observe in the interstellar medium (ISM). Carbon can be found in four dominant forms: dust grains, C 2, C 1, and CO. The latter is the most abundant molecule (next to H2) in molecular clouds. All three gas-phase forms produce strong sub-mm wavelength emission lines and are the principal tracers of the warm and dense neutral phases of the ISM. We calculate the gas-phase abundances of neutral carbon (C 1), ionized carbon (C 2), and carbon monoxide (CO) as a function of cosmic time or redshift z in an idealized scenario of galactic evolution.

Bally, John↗
NASA NTRS2007

Molecules and Dust in the Humunculus: Ejecta of Eta Carinae

In the 18401s, Eta Carinae ejected massive amounts of nitrogen-rich, carbon- and oxygen-poor material which we see as the hourglass-shaped Homunculus. With the Hubble Space Telescope Imaging Spectrograph, we detected multiple shells in line of sight, the most massive and intriguing being at -513 km/s. Numerous lines of Fe I, Fe II, Ni II, Cr II, Sc II, Sr II, Ti II, V II, etc are identified as well as nearly a thousand H2 lines. The metals have energy level populations consistent with 760K and excited by photons < 8.5eV. We have now identified CH, CH+, OH, and NH at the same velocity, but at 60K, suggesting stratification in the outer ejecta. Analysis of the interior, photoionized emission hourglass structure, known as the Little Homunculus, indicates He, N overabundances and C, 0 underabundances (approximately 1/80 solar). A skirt of neutral and partially ionized gas lies between the lobes of the hourglasses. A portion is seen as the Strontium Filament, a metal- ionized, neutral hydrogen structure. Relative abundances of TiNi are 1/80 solar, CrNi are 1/20 solar. This complex of ejecta appears to have been ejected by a massive star(s) at the end of the hydrogen-burning phase when convection led to overproduction of nitrogen at the expense of carbon and oxygen. Given the underabundances of carbon and oxygen, the chemistry of this system is quite different to the normal ISM, leading to a nitrogen- dominated chemistry. What little C and 0 that is formed is immediately taken up by SiO and Al0 molecules leading to a very different gas/dust ratio than the normal ISM. Dust in this ejecta is abundance, but known to be very grey in character. Observations with HST/STIS and VLT/UVES will be presented along with simple physical models and CLOUD modeling. Insight by the participants will be solicited.

Gull, T.↗
DOE OSTI2023

Dynamics in Star-forming Cores (DiSCo): project overview and the first look towards the B1 and NGC 1333 regions in Perseus

The internal velocity structure within dense gaseous cores plays a crucial role in providing the initial conditions for star formation in molecular clouds. However, the kinematic properties of dense gas at core scales (~0.01–0.1 pc) has not been extensively characterized because of instrument limitations until the unique capabilities of GBT-Argus became available. The ongoing GBT-Argus Large Program, Dynamics in Star-forming Cores (DiSCo) thus aims to investigate the origin and distribution of angular momentum of star-forming cores. DiSCo will survey all starless cores and Class 0 protostellar cores in the Perseus molecular complex down to ~0.01 pc scales with <0.05 km s –1 velocity resolution using the dense gas tracer N 2 H + . Here, we present the first data sets from DiSCo towards the B1 and NGC 1333 regions in Perseus. Our results suggest that a dense core’s internal velocity structure has little correlation with other core-scale properties, indicating these gas motions may be originated externally from cloud-scale turbulence. These first data sets also reaffirm the ability of GBT-Argus for studying dense core velocity structure and provided an empirical basis for future studies that address the angular momentum problem with a statistically broad sample.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2000

Interstellar PAHs

Tremendous strides have been made in our understanding of interstellar material over the past twenty years thanks to significant, parallel developments in two closely related areas: observational astronomy and laboratory astrophysics. Twenty years ago the composition of interstellar dust was largely guessed at and the notion of abundant, gas phase, polycyclic aromatic hydrocarbons (PAHs) anywhere in the interstellar medium (ISM) considered impossible. Today the dust composition of the diffuse and dense ISM is reasonably well constrained and the spectroscopic case for interstellar PAHs, shockingly large molecules by early interstellar chemistry standards, is very strong.

Allamandola, Louis J.↗
NASA NTRS2005

Processing and Synthesis of Pre-Biotic Chemicals in Hypervelocity Impacts

Hypervelocity impacts (HVIs) may have played a significant role in establishing the initial organic inventory for pre-biotic chemistry on the Earth and other planetary bodies. In addition to the delivery of organic compounds intact to planetary surfaces, generally at velocities below approx.20 km/s, HVIs also enable synthesis of new molecules. The cooling post-impact plasma plumes of HVIs in the interstellar medium (ISM), the protosolar nebula (PSN), and the early solar system comprise pervasive conditions for organic synthesis. Such plasma synthesis (PS) can operate over many length scales (from nm-scale dust to planets) and energy scales (from molecular rearrangement to atomization and recondensation). HVI experiments with the flexibility to probe the highest velocities and distinguish synthetic routes are a high priority to understand the relevance of PS to exobiology. We describe here recent studies of PS at small spatial scales and extremely high velocities with pulsed laser ablation (PLA). PLA can simulate the extreme plasma conditions generated in impacts of dust particles at speeds of up to 100 km/s or more. When applied to carbonaceous solids, new and pre-biotically relevant molecular species are formed with high efficiency [1,2].

Brickerhoff, W. B.↗
NASA NTRS2016

The Formation of Nucleobases from the Irradiation of Purine in Astophysical Ices and Comparisons with Meteorites.

N-heterocycles have been identified in meteorites and their extraterrestrial origins are suggested by isotopic ratio measurements. Although small N- heterocycles have not been detected in the interstellar medium (ISM), recent experiments in our lab have shown that the irradiation of the aromatic molecules like benzene (C6H6) and naphthalene (C10H8) in mixed molecular ices leads to the formation of O- and N-heterocyclic molecules. Among the class of N-heterocycles are the nucleobases, which are of astrobiological interest because they are the information bearing units of DNA and RNA. Nucleobases have been detected in meteorites [3-5], with isotopic signatures that are also consistent with an extraterrestrial origin. Three of the biologically relevant nucleobases (uracil, cytosine, and guanine) have a pyrimidine core structure while the remaining two (adenine and guanine) possess a purine core. Previous experiments in our lab have demonstrated that all of the bio-logical nucleobases (and numerous other molecules) with a pyrimidine core structure can be produced by irradiating pyrimidine in mixed molecular ices of several compositions [6-8]. In this work, we study the formation of purine-based molecules, including the nucleobases adenine, and guanine, from the ultraviolet (UV) irradiation of purine in ices consisting mixtures of H2O and NH3 at low temperature. The experiments are designed to simulate the astrophysical conditions under which these species may be formed in dense molecular clouds, protoplanetary disks, or on the surfaces of icy bodies in planetary systems.

Astrochemistry↗
NASA NTRS2022

Accurate Modeling of Lyα Profiles and their Impact on Photolysis of Terrestrial Planet Atmospheres

Accurately measuring and modeling the Lyα (Lyα; λ1215.67 Å) emission line from low-mass stars is vital for our ability to build predictive high energy stellar spectra, yet interstellar medium (ISM) absorption of this line typically prevents model-measurement comparisons. Lyα also controls the photodissociation of important molecules, like water and methane, in exoplanet atmospheres such that any photochemical models assessing potential biosignatures or atmospheric abundances require accurate Lyα host star flux estimates. Recent observations of three early M and K stars (K3, M0, M1) with exceptionally high radial velocities (>100 km s −1 ) reveal the intrinsic profiles of these types of stars as most of their Lyα flux is shifted away from the geocoronal line core and contamination from the ISM. These observations indicate that previous stellar spectra computed with the PHOENIX atmosphere code have underpredicted the core of Lyα in these types of stars. With these observations, we have been able to better understand the microphysics in the upper atmosphere and improve the predictive capabilities of the PHOENIX atmosphere code. Since these wavelengths drive the photolysis of key molecular species, we also present results analyzing the impact of the resulting changes to the synthetic stellar spectra on observable chemistry in terrestrial planet atmospheres.

Sarah Peacock↗
NASA NTRS2010

Synergism of Saturn, Enceladus and Titan and Formation of HCNO Exobiological Molecules

Saturn as a system has two very exotic moons Titan and Enceladus. Titan with energy input from Saturn's magnetosphere, solar UV irradiation, and cosmic rays can make HCN based molecules as discussed in earlier paper by [1]. Space radiation effects at both moons, and as coupled by the Saturn magnetosphere could cause an unexpected series of events leading to the evolution of biological models at Titan composed of HCNO with oxygen as the new ingredient. The "Old Faithful" model by [2] suggests that Enceladus, highly irradiated by Saturn magnetospheric electrons, has episodic ejections of water vapor driven by radiolytic oxidation gas products into Saturn's magnetosphere. At Titan Cassini discovered 1) that keV oxygen ions, evidently from Enceladus, are bombarding Titan's upper atmosphere [3] and 2) the discovery of heavy positive and negative ions within Titan's upper atmosphere [4]. Initial models of heavy ion formation in Titan's upper atmosphere invoked polymerization of aromatics such as Benzenes and their radicals to make PAHs [5], while a more recent model by [6] has raised the possibility of carbon chains forming from the polymerization of acetylene and its radicals to eventually make fullerenes. Laboratory measurements indicate that fullerenes, which are hollow carbon shells, can trap the keV oxygen and with the clustering of fullerenes and possible mixture with PAHs, some with nitrogen molecules, can make the larger aerosols with oxygen within them. Then with further ionizing irradiation from cosmic rays deep in the atmosphere "tholin" molecules are produced with all the molecular components present from which organic molecules can form. Among the molecular components are amino acids, the fundamental building blocks of life as we know it. This process maybe a common chemical pathway, both at the system level and at the molecular level, to form prebiotic and perhaps even biotic molecules. Such processes can be occurring throughout our universe, such as molecular clouds in the ISM.

Sittler, Edward C., Jr.↗
NASA NTRS2012

Photochemistry of Pyrimidine in Astrophysical Ices: Formation of Nucleobases and Other Prebiotic Species

Nucleobases are N-heterocycles that are the informational subunits of DNA and RNA. They are divided into two molecular groups: pyrimidine bases (uracil, cytosine, and thymine) and purine bases (adenine and guanine). Nucleobases have been detected in meteorites, and their extraterrestrial origin confirmed by isotopic measurements. Although no N-heterocycles have ever been observed in the ISM, the positions of the 6.2- m interstellar emission features suggest a population of such molecules is likely to be present. However, laboratory experiments have shown that the ultraviolet (UV) irradiation of pyrimidine in ices of astrophysical relevance such as H2O, NH3, CH3OH, CH4, CO, or combinations of these at low temperature (less than or equal to 20 K) leads to the formation of several pyrimidine derivatives including the nucleobases uracil and cytosine, as well as precursors such as 4(3H)-pyrimidone and 4-aminopyrimidine. Quantum calculations on the formation of 4(3H)-pyrimidone and uracil from the irradiation of pyrimidine in pure H2O ices are in agreement with their experimental formation pathways.10 In those residues, other species of prebiotic interest such as urea as well as the amino acids glycine and alanine could also be identified. However, only very small amounts of pyrimidine derivatives containing CH3 groups could be detected, suggesting that the addition of methyl groups to pyrimidine is not an efficient process. For this reason, the nucleobase thymine was not observed in any of the samples. In this work, we study the formation of nucleobases and other photo-products of prebiotic interest from the UV irradiation of pyrimidine in ices containing H2O, NH3, CH3OH, and CO, mixed in astrophysical proportions.

Nuevo, Michel↗