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NASA NTRS2022

Deuterium Isotope Fractionation of Polycyclic Aromatic Hydrocarbons in Meteorites as an Indicator of Interstellar/Protosolar Processing History

The stable isotope composition of soluble and insoluble organic compounds in carbonaceous chondrites can be used to determine the provenance of organic molecules in space. Deuterium enrichment in meteoritic organics could be a residual signal of synthetic reactions occurring in the cold interstellar medium or an indicator of hydrothermal parent-body reactions. δD values have been measured in grains and bulk samples for a wide range of meteorites; however, these reservoirs are highly variable and may have experienced fractionation during thermal and/or aqueous alteration. Among the plethora of organic compounds in meteorites are polycyclic aromatic hydrocarbons (PAHs), which are stable and abundant in carbonaceous chondrites, and their δD ratio may preserve evidence about their formation environment as well as the influence of parent-body processes. This study tests hypotheses about the potential links between PAHs-deuteration concentrations and their formation conditions by examining the δD ratio of PAHs in three CM carbonaceous chondrites representing an aqueous alteration gradient. We use deuterium enrichments in soluble 2–5-ring PAHs as an indicator of either photon-driven deuteration due to unimolecular photodissociation in warm regions of space, gas-phase ion–molecule reactions in cold interstellar regions of space, or UV photolysis in ices. We also test hypothesized reaction pathways during parent-body processing that differ between partially and fully aromatized PAHs. New methodological approaches were developed to extract small, volatile PAHs without fractionation. Our results suggest that meteoritic PAHs could have formed through reactions in cold regions, with possible overprinting of deuterium enrichment during aqueous parent-body alteration, but the data could not rule out PAH alteration in icy mantles as well.

H V Graham↗
DOE OSTI2019

Calibrating Interstellar Abundances Using Supernova Remnant Radiative Shocks

Using integral field data we extract the optical spectra of shocked interstellar clouds in Kepler’s supernova remnant located in the inner regions of our Galaxy, as well as in the Large Magellanic Cloud, the Small Magellanic Cloud, NGC 6822, and IC 1613. Using self-consistent shock modeling, we make a new determination of the chemical composition of the interstellar medium in N, O, Ne, S, Cl, and Ar in these galaxies and obtain accurate estimates of the fraction of refractory grains destroyed in the shock. By comparing our derived abundances with those obtained in recent works using observations of B-stars, F supergiant stars, and H ii regions, we provide a new calibration for abundance scaling in the range of 7.9≲12+log O/H≲9.1.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS2012

Hydrocarbons on Saturns Satellites: Relationship to Interstellar Dust and the Solar Nebula

To understand the origin and evolution of our Solar System, and the basic components that led to life on Earth, we study interstellar and planetary spectroscopic signatures. The possible relationship of organic material detected in carbonaceous meteorites, interplanetary dust particles (IDPs), comets and the interstellar medium have been the source of speculation over the years as the composition and processes that governed the early solar nebula have been explored to understand the extent to which primitive material survived or became processed. The Cassini VIMS has provided new data relevant to this problem. Three of Saturn's satellites, Phoebe, Iapetus, and Hyperion, are found to have aromatic and aliphatic hydrocarbons on their surfaces. The aromatic hydrocarbon signature (C-H stretching mode at 3.28 micrometers) is proportionally significantly stronger (relative to the aliphatic bands) than that seen in other Solar System bodies (e.g., comets) and materials (Stardust samples, IDPs, meteorites) and the distinctive sub-features of the 3.4 micrometer aliphatic band (CH2 and CH3 groups) are reminiscent of those widely detected throughout the diffuse ISM. Phoebe may be a captured object that originated in the region beyond the present orbit of Neptune, where the solar nebula contained a large fraction of original interstellar ice and dust that was less processed than material closer to the Sun. Debris from Phoebe now resident on Iapetus and Hyperion, as well as o Phoebe itself, thus presents a unique blend of hydrocarbons, amenable to comparisons with interstellar hydrocarbons and other Solar System materials. The dust ring surrounding Saturn, in which Phoebe is embedded, probably originated from a collision with Phoebe. Dust ring particles are the likely source of the organic-bearing materials, and perhaps the recently identified small particles of Fe detected on Saturn's satellites. Lab measurements of the absolute band strengths of representative aliphatic and aromatic molecules, together with measurements from the VIMS data, allow us to calculate the number of C atoms to find the relative abundances of C atoms in the two kinds of organic molecules. The strength of the prominent aromatic C-H stretch band relative to the aliphatic band complex in Phoebe and Iapetus indicates that the relative abundance of aromatic to aliphatic carbon is very large (greater than 200). In contract, the aromatic band is nearly imperceptible in spectra of interplanetary dust particles (IDP), returned samples from comet 91P/Wild 2, insoluable carbonaceous material in most meteorites, and the diffuse interstellar dust (DISM) (although aromatics are known in all these materials-here we consider only the spectroscopic signature)

Cruikshank, D. P.↗
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 NTRS2017

Deep K-Band Observations of TMC-1 with the Green Bank Telescope: Detection of HC7O, Nondetection of HC11N, and a Search for New Organic Molecules

The 100-meter Robert C. Byrd Green Bank Telescope K-band (KFPA) receiver was used to perform a high-sensitivity search for rotational emission lines from complex organic molecules in the cold interstellar medium toward TMC-1 (Taurus Molecular Cloud - cyanopolyyne peak), focussing on the identification of new carbon-chain-bearing species as well as molecules of possible prebiotic relevance. We report a detection of the carbon-chain oxide species HC7O and derive a column density of (7.8 plus or minus 0.9) times 10 (sup 11) per square centimeter. This species is theorized to form as a result of associative electron detachment reactions between oxygen atoms and C7H minus, and/or reaction of C6H2 plus with CO (followed by dissociative electron recombination). Upper limits are given for the related HC6O, C6O, and C7O molecules. In addition, we obtained the first detections of emission from individual (sup 13) C isotopologues of HC7N, and derive abundance ratios HC7N/HCCCC (sup 13) CCCN equal to 110 plus or minus 16 and HC7N/HCCCC (sup 13) CCCN equal to 96 plus or minus 11, indicative of significant (sup 13) C depletion in this species relative to the local interstellar elemental (sup 12) C divided by (sup 13) C ratio of 60-70. The observed spectral region covered two transitions of HC11N, but emission from this species was not detected, and the corresponding column density upper limit is 7.4 times 10 (sup 10) per square centimeter (at 95 percent confidence). This is significantly lower than the value of 2.8 times 10 (sup 11) per square centimeter previously claimed by Bell et al. and confirms the recent nondetection of HC11N in TMC-1 by Loomis et al. Upper limits were also obtained for the column densities of malononitrile and the nitrogen heterocycles quinoline, isoquinoline, and pyrimidine.

ISM: abundances, ISM: lines and bands, ISM: molecu↗
NASA NTRS2001

Chemical and Isotopic Analysis of Trace Organic Matter on Meteorites and Interstellar Dust Using a Laser Microprobe Instrument

Polycyclic Aromatic Hydrocarbons (PAHs) are of considerable interest today because they are ubiquitous on Earth and in the interstellar medium (ISM). In fact, about 20% of cosmic carbon in the galaxy is estimated to be in the form of PAHs. Investigation of these species has obvious uses for determining the cosmochemistry of the solar system. Work in this laboratory has focused on four main areas: 1) Mapping the spatial distribution of PAHs in a variety of meteoritic samples and comparing this distribution with mineralogical features of the meteorite to determine whether a correlation exists between the two. 2) Developing a method for detection of fullerenes in extraterrestrial samples using microprobe Laser Desorption Ionization Mass Spectroscopy and utilizing this technique to investigate fullerene presence, while exploring the possibility of spatially mapping the fullerene distribution in these samples through in situ detection. 3) Investigating a possible formation pathway for meteoritic and ancient terrestrial kerogen involving the photochemical reactions of PAHs with alkanes under prebiotic and astrophysically relevant conditions. 4) Studying reaction pathways and identifying the photoproducts generated during the photochemical evolution of PAH-containing interstellar ice analogs as part of an ongoing collaboration with researchers at the Astrochemistry Lab at NASA Ames. All areas involve elucidation of the solar system formation and chemistry using microprobe Laser Desorption Laser Ionization Mass Spectrometry. A brief description of microprobe Laser Desorption Ionization Mass Spectroscopy, which allows selective investigation of subattomole levels of organic species on the surface of a sample at 10-40 micrometer spatial resolution, is given.

Zare, Richard N.↗
DOE OSTI2016

The galactic census of high- and medium-mass protostars. III. {sup 12}Co maps and physical properties of dense clump envelopes and their embedding gmcs

We report the second complete molecular line data release from the Census of High- and Medium-mass Protostars (CHaMP), a large-scale, unbiased, uniform mapping survey at sub-parsec resolution, of millimeter-wave line emission from 303 massive, dense molecular clumps in the Milky Way. This release is for all {sup 12}CO J = 1 → 0 emission associated with the dense gas, the first from Phase II of the survey, which includes {sup 12}CO, {sup 13}CO, and C{sup 18}O. The observed clump emission traced by both {sup 12}CO and HCO{sup +} (from Phase I) shows very similar morphology, indicating that, for dense molecular clouds and complexes of all sizes, parsec-scale clumps contain Ξ ∼ 75% of the mass, while only 25% of the mass lies in extended (≳10 pc) or “low density” components in these same areas. The mass fraction of all gas above a density of 10{sup 9} m{sup −3} is ξ{sub 9} ≳ 50%. This suggests that parsec-scale clumps may be the basic building blocks of the molecular interstellar medium, rather than the standard GMC concept. Using {sup 12}CO emission, we derive physical properties of these clumps in their entirety, and compare them to properties from HCO{sup +}, tracing their denser interiors. We compare the standard X-factor converting I{sub {sup 1}{sup 2}CO} to N{sub H{sub 2}} with alternative conversions, and show that only the latter give whole-clump properties that are physically consistent with those of their interiors. We infer that the clump population is systematically closer to virial equilibrium than when considering only their interiors, with perhaps half being long-lived (10s of Myr), pressure-confined entities that only terminally engage in vigorous massive star formation, supporting other evidence along these lines that was previously published.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Galaxy And Mass Assembly (GAMA): Gas Fueling of Spiral Galaxies in the Local Universe. I. The Effect of the Group Environment on Star Formation in Spiral Galaxies

We quantify the effect of the galaxy group environment (for group masses of 10{sup 12.5}–10{sup 14.0} M {sub ⊙}) on the current star formation rate (SFR) of a pure, morphologically selected sample of disk-dominated (i.e., late-type spiral) galaxies with redshift ≤0.13. The sample embraces a full representation of quiescent and star-forming disks with stellar mass M {sub *} ≥ 10{sup 9.5} M {sub ⊙}. We focus on the effects on SFR of interactions between grouped galaxies and the putative intrahalo medium (IHM) of their host group dark matter halos, isolating these effects from those induced through galaxy–galaxy interactions, and utilizing a radiation transfer analysis to remove the inclination dependence of derived SFRs. The dependence of SFR on M {sub *} is controlled for by measuring offsets Δlog(ψ {sub *}) of grouped galaxies about a single power-law relation in specific SFR, ψ{sub ∗}∝M{sub ∗}{sup −0.45±0.01}, exhibited by non-grouped “field” galaxies in the sample. While a small minority of the group satellites are strongly quenched, the group centrals and a large majority of satellites exhibit levels of ψ {sub *} statistically indistinguishable from their field counterparts, for all M {sub *}, albeit with a higher scatter of 0.44 dex about the field reference relation (versus 0.27 dex for the field). Modeling the distributions in Δlog(ψ {sub *}), we find that (i) after infall into groups, disk-dominated galaxies continue to be characterized by a similar rapid cycling of gas into and out of their interstellar medium shown prior to infall, with inflows and outflows of ∼1.5–5 x SFR and ∼1–4 x SFR, respectively; and (ii) the independence of the continuity of these gas flow cycles on M {sub *} appears inconsistent with the required fueling being sourced from gas in the circumgalactic medium on scales of ∼100 kpc. Instead, our data favor ongoing fueling of satellites from the IHM of the host group halo on ∼Mpc scales, i.e., from gas not initially associated with the galaxies upon infall. Consequently, the color–density relation of the galaxy population as a whole would appear to be primarily due to a change in the mix of disk- and spheroid-dominated morphologies in the denser group environment compared to the field, rather than to a reduced propensity of the IHM in higher-mass structures to cool and accrete onto galaxies. We also suggest that the required substantial accretion of IHM gas by satellite disk-dominated galaxies will lead to a progressive reduction in the specific angular momentum of these systems, thereby representing an efficient secular mechanism to transform morphology from star-forming disk-dominated types to more passive spheroid-dominated types.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

Structure of Energetic Particle Mediated Shocks Revisited

The structure of collisionless shock waves is often modified by the presence of energetic particles that are not equilibrated with the thermal plasma (such as pickup ions [PUIs] and solar energetic particles [SEPs]). This is relevant to the inner and outer heliosphere and the Very Local Interstellar Medium (VLISM), where observations of shock waves (e.g., in the inner heliosphere) show that both the magnetic field and thermal gas pressure are less than the energetic particle component pressures. Voyager 2 observations revealed that the heliospheric termination shock (HTS) is very broad and mediated by energetic particles. PUIs and SEPs contribute both a collisionless heat flux and a higher-order viscosity. We show that the incorporation of both effects can completely determine the structure of collisionless shocks mediated by energetic ions. Since the reduced form of the PUI-mediated plasma model is structurally identical to the classical cosmic ray two-fluid model, we note that the presence of viscosity, at least formally, eliminates the need for a gas sub-shock in the classical two-fluid model, including in that regime where three are possible. By considering parameters upstream of the HTS, we show that the thermal gas remains relatively cold and the shock is mediated by PUIs. We determine the structure of the weak interstellar shock observed by Voyager 1 . We consider the inclusion of the thermal heat flux and viscosity to address the most general form of an energetic particle-thermal plasma two-fluid model.

79 ASTRONOMY AND ASTROPHYSICS↗
NASA NTRS1996

Assessment of the polycyclic aromatic hydrocarbon-diffuse interstellar band proposal

The potential link between neutral and/or ionized polycyclic aromatic hydrocarbons (PAHs) and the diffuse interstellar band (DIB) carriers is examined. Based on the study of the general physical and chemical properties of PAHs, an assessment is made of their possible contribution to the DIB carriers. It is found that, under the conditions reigning in the diffuse interstellar medium, PAHs can be present in the form of neutral molecules as well as positive and/or negative ions. The charge distribution of small PAHs is dominated, however, by two charge states at one time with compact PAHs present only in the neutral and cationic forms. Each PAH has a distinct spectral signature depending on its charge state. Moreover, the spectra of ionized PAHs are always clearly dominated by a single band in the DIB spectral range. In the case of compact PAH ions, the strongest absorption band is of type A (i.e., the band is broad, falls in the high-energy range of the spectrum, and possesses a large oscillator strength), and seems to correlate with strong and broad DIBs. For noncompact PAH ions, the strongest absorption band is of type I (i.e., the band is narrow, falls in the low-energy range of the spectrum, and possesses a small oscillator strength), and seems to correlate with weak and narrow DIBs. Potential molecular size and structure constraints for interstellar PAHs are derived by comparing known DIB characteristics to the spectroscopic properties of PAHs. It is found that (i) only neutral PAHs larger than about 30 carbon atoms could, if present, contribute to the DIBs. (ii) For compact PAHs, only ions with less than about 250 carbon atoms could, if present, contribute to the DIBs. (iii) The observed distribution of the DIBs between strong/moderate and broad bands on the one hand and weak and narow bands on the other can easily be interpreted in the context of the PAH proposal by a distribution of compact and noncompact PAH ions, respectively. A plausible correlation between PAH charge states and DIB "families" is thus provided by the PAH-DIB proposal. Following this proposal, DIB families would reflect conditions within a cloud which locally determine the relative importance of cations, anions, and neutral species, rather than tracers of a specific species. Observational predictions are given to establish the viability of the PAH hypothesis. It is concluded that small PAH ions are very promising candidates as DIB carriers provided their population is dominated by a finite number (100-200) of species. A key test for the PAH proposal, consisting of laboratory and astronomical investigations in the ultraviolet range, is called for.

NASA Discipline Number 52-10↗
NASA NTRS1995

Assessment of the Polycyclic Aromatic Hydrocarbon-Diffuse Interstellar Band Proposal

The potential link between neutral and/or ionized polycyclic aromatic hydrocarbons (PAHs) and the diffuse interstellar band (DIB) carriers is examined. Based on the study of the general physical and chemical properties of PAHs, an assessment is made of their possible contribution to the DIB carriers. It is found that, under the conditions reigning in the diffuse interstellar medium, PAHs can be present in the form of neutral molecules as well as positive and/or negative ions. The charge distribution of small PAHs is dominated, however, by two charge states at one time with compact PAHs present only in the neutral and cationic forms. Each PAH has a distinct spectral signature depending on its charge state. Moreover, the spectra of ionized PAHs are always clearly dominated by a single band in the DIB spectral range. In the case of compact PAH ions, the strongest absorption band is of type A (i.e., the band is broad, falls in the high energy range of the spectrum, and possess a large oscillator strength), and seems to correlate with strong and broad DIBs. In the case of non-compact PAH ions, the strongest absorption band is of type I (i.e., the band is narrow, falls in the low energy range of the spectrum, and possess a small oscillator strength), and seems to correlate with weak and narrow DIBs. Potential molecular size and structure constraints for interstellar PAHs are derived by comparing known DIB characteristics to the spectroscopic properties of PAHs. It is found that: (i) Only neutral PAHs larger than about 30 carbon atoms could, if present, contribute to the DIBs. (ii) For compact PAHs, only ions with less than about 250 carbon atoms could, if present, contribute to the DIBs. (iii) The observed distribution of the DIBs between strong/moderate and broad bands on the one hand and weak and narrow bands on the other hand can easily be interpreted in the context of the PAH proposal by a distribution between compact and non-compact PAH ions, respectively. A plausible correlation between PAH charge states and DIB "families" is thus provided by the PAH-DIB proposal. Following this proposal, DIB families would provide tracers of conditions within a cloud which globally determine the relative importance of cations, anions, and neutral species, rather than tracers of a specific species. Observational predictions are given to establish the viability of the PAH hypothesis. It is concluded that small PAH ions are very promising candidates as DIB carriers provided their population is dominated by a finite number (100-200) of species. A key test for the PAH proposal, consisting of laboratory and astronomical investigations of the ultraviolet range, is called for.

Salama, Farid↗
DOE OSTI2016

Complex hydrocarbon chemistry in interstellar and solar system ices revealed: a combined infrared spectroscopy and reflectron time-of-flight mass spectrometry analysis of ethane (C{sub 2}H{sub 6}) and D6-ethane (C{sub 2}D{sub 6}) ices exposed to ionizing radiation

The irradiation of pure ethane (C{sub 2}H{sub 6}/C{sub 2}D{sub 6}) ices at 5.5 K, under ultrahigh vacuum conditions was conducted to investigate the formation of complex hydrocarbons via interaction with energetic electrons simulating the secondary electrons produced in the track of galactic cosmic rays. The chemical modifications of the ices were monitored in situ using Fourier transform infrared spectroscopy (FTIR) and during temperature-programmed desorption via mass spectrometry exploiting a quadrupole mass spectrometer with electron impact ionization (EI-QMS) as well as a reflectron time-of-flight mass spectrometer coupled to a photoionization source (PI-ReTOF-MS). FTIR confirmed previous ethane studies by detecting six molecules: methane (CH{sub 4}), acetylene (C{sub 2}H{sub 2}), ethylene (C{sub 2}H{sub 4}), the ethyl radical (C{sub 2}H{sub 5}), 1-butene (C{sub 4}H{sub 8}), and n-butane (C{sub 4}H{sub 10}). However, the TPD phase, along with EI-QMS, and most importantly, PI-ReTOF-MS, revealed the formation of at least 23 hydrocarbons, many for the first time in ethane ice, which can be arranged in four groups with an increasing carbon-to-hydrogen ratio: C{sub n}H{sub 2n+2} (n = 3, 4, 6, 8, 10), C{sub n}H{sub 2n} (n = 3–10), C{sub n}H{sub 2n−2} (n = 3–10), and C{sub n}H{sub 2n−4} (n = 4–6). The processing of simple ethane ices is relevant to the hydrocarbon chemistry in the interstellar medium, as ethane has been shown to be a major product of methane, as well as in the outer solar system. These data reveal that the processing of ethane ices can synthesize several key hydrocarbons such as C{sub 3}H{sub 4} and C{sub 4}H{sub 6} isomers, which have been found to synthesize polycyclic aromatic hydrocarbons like indene (C{sub 9}H{sub 8}) and naphthalene (C{sub 10}H{sub 8}) in the ISM and in hydrocarbon-rich atmospheres of planets and their moons such as Titan.

37 INORGANIC, ORGANIC, PHYSICAL, AND ANALYTICAL CH↗
NASA NTRS1986

The abundance of interstellar sulphur and zinc in high density sight-lines

On the basis of early absorption line studies of individual lines of sight with the Copernicus satellite, chlorine, sulphur and zinc were classed together as elements which showed little or no depletion, relative to hydrogen, in the interstellar medium. The abundances of other less volatile elements, such as Fe and Mg were found to vary widely from one sight-line to another with gas-phase abundances in some cases being orders of magnitude below their solar counterparts. Detailed studies are reported of the depletion/density behavior of two other volatile elements which were previously considered to be virtually undepleted, S and Zn, using equivalent width data from both Copernicus and IUE observations. The results provide further evidence that the established dependence of depletion on n bar (H) extends to volatile elements and show that their use as tracers of metallicity, or for estimating hydrogen column densities, may lead to large errors in sight-lines through dense regions. It now appears that such elements may take part in the surface chemistry of grains and be important constituents of grain mantle material, although they probably do not contribute significantly to the bulk mass of grains. Due to the very similar atomic masses and ionization potentials of sulphur and phosphorous, the thermal velocity distributions of the singly ionized species of these elements in interstellar clouds should be very similar. However, a comparison of Doppler widths (b-values) derived for SIT and PIT in the same sight-lines from the Bohlin et al Copernicus equivalent width measurements has revealed an unexpected systematic discrepancy of a factor of approx. 1.7. This Discrepancy indicates that the normally adopted oscillators strengths of the PII lambda lambda 1153 and 1302 A lines may require revision.

Harris, A. W.↗
DOE OSTI2018

Chandra Detection of an Evolved Population of Young Stars in Serpens South

We present a Chandra study of the deeply embedded Serpens South star-forming region, examining cluster structure and disk properties at the earliest stages. In total, 152 X-ray sources are detected. Combined with Spitzer and 2MASS photometry, 66 X-ray sources are reliably matched to an IR counterpart. We identify 21 class I, 6 flat spectrum, 16 class II, and 18 class III young stars; 5 were unclassified. Eighteen sources were variable in X-rays, 8 exhibiting flare-like emission and one source being periodic. The cluster’s X-ray luminosity distance was estimated: the best match was to the nearer distance of 260 pc for the front of the Aquila Rift complex. The ratio of N {sub H} to A {sub K} is found to be ∼0.68 × 10{sup 22}, similar to that measured in other young low-mass regions, but lower than that measured in the interstellar medium and high-mass clusters (∼(1.6–2) × 10{sup 22}). We find that the spatial distribution closely follows that of the dense filament from which the stars have formed, with the class II population still strongly associated with the filament. There are four subclusters in the field, with three forming knots in the filament, and a fourth to the west, which may not be associated but may be contributing to the distributed class III population. A high percentage of diskless class IIIs (upper limit 30% of classified X-ray sources) in such a young cluster could indicate that processing of disks is influenced by the cluster environment and is not solely dependent on timescale.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2023

Lyman continuum leaker candidates among highly ionised, low-redshift dwarf galaxies selected from He II

Contemporary research suggests that the reionisation of the intergalactic medium (IGM) in the early Universe was predominantly realised by star-forming (proto-)galaxies (SFGs). Due to observational constraints, our knowledge on the origins of sufficient amounts of ionising Lyman continuum (LyC) photons and the mechanisms facilitating their transport into the IGM remains sparse. Recent efforts have thus focussed on the study of local analogues to these high-redshift objects. We aim to acquire a set of very low-redshift SFGs that exhibit signs of a hard radiation field being present. A subsequent analysis of their emission line properties is intended to shed light on how the conditions prevalent in these objects compare to those predicted to be present in early SFGs that are thought to be LyC emitters (LCEs). We used archival spectroscopic SDSS DR12 data to select a sample of low-redshift He II 4686 emitters and restricted it to a set of SFGs with an emission line diagnostic sensitive to the presence of an active galactic nucleus, which serves as our only selection criterion. We performed a population spectral synthesis with FADO to reconstruct these galaxies’ star-formation histories (SFHs). Utilising the spectroscopic information at hand, we constrained the predominant ionisation mechanisms in these galaxies and inferred information on interstellar medium (ISM) conditions relevant for the escape of LyC radiation. Our final sample consists of eighteen ionised, metal-poor galaxies (IMPs). These low-mass (6.2 ≤ log(M * /M ⊙ ) ≤ 8.8), low-metallicity (7.54 ≤log(O/H) + 12 ≤ 8.13) dwarf galaxies appear to be predominantly ionised by stellar sources. We find large [OIII] 5007/[OII] 3727 ratios and [SII] 6717,6731/Hα deficiencies, which provide strong indications for these galaxies to be LCEs. At least 40% of these objects are candidates for featuring cosmologically significant LyC escape fractions ≳10%. The IMPs’ SFHs exhibit strong similarities and almost all galaxies appear to contain an old (> 1 Gyr) stellar component, while also harbouring a young, two-stage (~10 Myr and < 1 Myr) starburst, which we speculate might be related to LyC escape. The properties of the compact emission line galaxies presented here align well with those observed in many local LCEs. In fact, our sample may prove as an extension to the rather small catalogue of local LCEs, as the extreme ISM conditions we find are assumed to facilitate LyC leakage. Notably, all of our eighteen candidates are significantly closer (z < 0.1) than most established LCEs. If the inferred LyC photon loss is genuine, this demonstrates that selecting SFGs from He II 4686 is a powerful selection criterion in the search for LCEs.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2017

PRESSURE PULSES AT VOYAGER 2 : DRIVERS OF INTERSTELLAR TRANSIENTS?

Voyager 1 ( V1 ) crossed the heliopause into the local interstellar medium (LISM) in 2012. The LISM is a dynamic region periodically disturbed by solar transients with outward-propagating shocks, cosmic-ray intensity changes and anisotropies, and plasma wave oscillations. Voyager 2 ( V2 ) trails V1 and thus may observe the solar transients that are later observed at V1. V2 crossed the termination shock in 2007 and is now in the heliosheath. Starting in 2012, when solar maximum conditions reached V2 , five possible merged interaction regions (MIRs) have been observed by V2 in the heliosheath. The timing is consistent with these MIRs driving the transients observed by V1 in the LISM. The largest heliosheath MIR was observed by V2 in late 2015 and should reach V1 in 2018.

79 ASTRONOMY AND ASTROPHYSICS↗
DOE OSTI2024

Hydrogen ejection from hydrocarbons: Characterization and relevance in soot formation and interstellar chemistry

Polycyclic aromatic hydrocarbons (PAHs) play a major role in the chemistry of combustion, pyrolysis, and the interstellar medium. Production (or activation) of radical PAHs and propagation of their resulting reactions require efficient dehydrogenation, but the preferred method of hydrogen loss is not well understood. Unimolecular hydrogen ejection (i.e., direct C─H bond fission) and bimolecular radical abstraction are two main candidate pathways. We performed a computational study to characterize the role of H ejection, particularly as a driver for radical-centric hydrocarbon-growth mechanisms and particle formation. Electronic structure calculations establish that C─H bond strengths span a broad range of energies, which can be weaker than 30 kcal/mol in some C 9 and C 13 PAH radicals. At T > 1200 K, calculated thermal rates for hydrogen ejection from weak C─H bonds at zigzag sites on PAH radicals are significantly larger than typical H-abstraction rates. These results are highly relevant in the context of chain reactions of radical species and soot inception under fuel-rich combustion conditions. Furthermore, calculated microcanonical rates that include the additional internal energy released by bond formation (e.g., ring closure to yield C 9 H 9 ) yield significantly higher rates than those associated with full thermalization. These microcanonical considerations are relevant to the astrochemical processes associated with hydrocarbon growth and processing in the low-density interstellar environment.

08 HYDROGEN↗
NASA NTRS2001

Evolution of the Earth and Origin of Life: The Role of Gas/Fluid Interactions with Rocks

The work under the Cooperative Agreement will be centered on questions of the evolution of Life on the early Earth and possibly on Mars. It is still hotly debated whether the essential organic molecules were delivered to the early Earth from space (by comets, meteorites or interplanetary dust particles) or were generated in situ on Earth. Prior work that has shown that the matrix of igneous minerals is a medium in which progenitors of organic molecules assemble from H2O, C02 and N2 incorporated as minority "impurities" in minerals of igneous rocks during crystallization from H2O/CO2/N2-laden magmas. The underlying processes involve a redox. conversion whereby C, H, and N become chemically reduced, while 0 becomes oxidized to the peroxy state. During Year 02 the work will be divided into three tasks. Task 1: After carboxylic (fatty) acids and N-bearing compounds have been identified, other extractable organic molecules including lipids, oily substances and amino acids will be studied. Dedicated lipid analysis will be combined with gas chromatographic-mass spectroscopic (GCMS) analysis of organic compounds extracted from minerals and rocks. Task 2: Using infrared (IR) spectroscopy, C-H entities that are indicators for the organic progenitors in mineral matrices will be studied. A preliminary heating experiment with MgO single crystals has shown that the C-H entities can be pyrolyzed, causing the IR bands to disappear, but at room temperature the IR bands reappear in a matter of days to weeks. This work will be expanded, both by studying synthetic MgO crystals and olivine crystals from the Earth's upper mantle. The C-H bands will be compared to the published "organic" IR feature of dust in the interstellar medium (ISM) and interplanetary dust particles (IDP). Task 3: A paradox marks the evolution of early Life: Oxygen is highly toxic to primitive life, yet early organisms "learned" to detoxify reactive oxygen species, to utilize oxygen, and even produce it. Why would organisms on the early anaerobic Earth be under evolutionary pressure to evolve defenses against reactive oxygen species? Minerals in igneous rocks are now known to contain peroxy. When such minerals weather, the peroxy hydrolyzes to H2O2. The hypothesis will be tested whether organisms living in intimate contact with rock surfaces are subjected to a constant trickle of H202 and thus under stress to develop strategies to either detoxify the reactive oxygen species or repair the molecular damage that they cause. Understanding these processes is central to the Astrobiology mission. It opens new avenues toward understanding the evolution of early life on Earth, and the potential for aerobic life elsewhere. This Cooperative Agreement also has a strong educational and public outreach component involving high school, undergraduate students, and high school teachers.

Freund, Friedemann↗