Dissertations / Theses on the topic 'Intersteller hydrogen'

H2 is the most abundant molecule in the interstellar medium and forms on the surface of interstellar dust grains. Laboratory studies have been conducted of HD formation on a dust grain analogue, which is a highly-oriented pyrolytic graphite surface held at 15 K, under ultra-high vacuum. The molecules desorb from the surface in a distribution of ro-vibrational states, which are probed using Resonance Enhanced Multi-Photon Ionization Spectroscopy. HD in a particular ro-vibrational state is ionized using laser photons detected by a time-of-flight mass spectrometer. The HD+ ion yields are then data processed to obtain the relative rotational populations of HD formed within one vibrational level and an average rotational temperature can be found. In this thesis, HD formed in vibrational states v = 3 – 7 have been studied. This carries on from previous studies of HD and H2 in the v = 1 and 2 states. Within each vibrational level, the most populated rotational state was found to be J = 1 or 2. The most populated vibrational state was found to be v = 4. The HD experimental results were extrapolated to give the relative ro-vibrational population distribution of nascent H2, which provides a new model for the formation pumping of H2. This new formation pumping model has been implemented into a radiative transfer code, written by Casu and Cecchi-Pestellini, which takes into account formation, radiative and collisional pumping mechanisms to calculate the total population distribution of H2 in an interstellar cloud and to generate H2 spectra. The sensitivity of the H2 spectra to the physical conditions of interstellar dark clouds, such as cloud density and temperature, has been investigated. H2 spectra generated using the new experimentally-derived formation pumping model has also been compared to H2 spectra generated using other established, theoretically-derived formation pumping models.

This dissertation presents a study of shock-excited molecular hydrogen (H2) in the interstellar medium. The aims of this thesis are to understand the shock-excitation process and to understand the global role of shocks in the interstellar medium. These aims are quantified as the investigation of specific problems. To address the problems, a variety of observing techniques have been applied and several sources studied, with particular reference on supernova remnant IC 443. An analytical model for the cooling flow behind a shock has also been developed. The observations show that extensive regions of low surface brightness H2 line emission are common in shocked molecular sources. Models are presented for three sources; IC 443, CRL 616 and OMC-1. In IC 443 emission comes from a sinuous ridge, about 20 parsecs long ans less than a parsec wide, with over 20 bright emission peaks distributed along it. The total H2 line luminosity is ~1600L, making IC 443 one of the brightest galactic H2 emission line objects yet detected. The spatial distributions of accelerated line emissions from other molecules (CO, HCO+, HCN) and atomic gas (HI) are remarkably similar to that of shocked H2. There is evidence for partial dissociation of molecular gas by the shock, but there can be little ionised gas present. Important cooling mechanisms for the hot gas (and possibly the dominant mechanisms) are H2 line radiation and H2 dissociation, except possibly in the densest clumps where far-IR emission from collisionally heated grains may dominate. H" line profiles were obtained in several sources and show considerable variation between sources. In CRL 618 the line is ~250km/s wide, the largest yet measured for a galactic source, and is composed of several discrete components. the high-velocity line emission is interpreted as being due to the shocking of high-velocity, discrete molecular clumps, embedded in and shocked by a stellar wind. Line polarization measurements in OMC-1 show there are two distinct regions of H2 line emission. In the outflow region the line is dichroically polarized by a slab of alligned grains lying between us and the outflow, with polarization vectors parallel to the outflow axis. The alignment mechanism is possibly due to the agency of a magnetic field, and thus the polarization vectors may trace the magnetic field direction which is therefore aligned with the outflow axis. Outside the core region the polarization vectors show a centro-symmetric pattern characteristic of scattering, centred on the region of peak molecular hydrogen emission. This amounts to the discovery of a molecular hydrogen reflection nebula. Observations of five H2 lines, in four types of sources, show no major differences in relative line ratios between sources. This is dispite different pre-shock conditions being expected in each source. The shocked gas cannot be characterised by a single excitation temperature. An analytical model has been developed for the cooling flow behind a jump-shock into molecular gas, driven by an isobaric thermal pressure. The model predicts that, when the density is larger than the critical density needed to thermalise the level populations of the dominant coolant, and t the post-shock temperature is sufficiently large, then the line ratios only depend on the upper-state level energies of the lines and on the form of the cooling function. For the observed excitation temperatures, the dominant cooling mechanism, consistent with the data, is cooling through the vibrational/ rotational lines of the hydrogen molecule itself. This conclusion applies when the temperature is in the range ~500 - 4000 K and the gas density is >10(5)cm-3

The region of space between stars, the interstellar medium, has been found to contain over 160 chemical species to date. These molecules are contained within regions of gas and dust, measuring several light years across, known as interstellar dust clouds. Many of these molecular species are formed in the gas phase, for example, via the reactions of molecules with ions. However, some critical gas phase processes are often slow due to the low temperatures and pressures found in the interstellar medium and cannot readily account for the abundances of some species. Consequently reactions on the surfaces of interstellar dust grains are often invoked to explain the abundances some molecules. These dust grains represent approximately 1 % of the mass of a typical interstellar dust cloud and typically consist of carbon, silicates or metal oxides. The temperature of these interstellar dust grains is low enough (~ 10 K) that over time icy mantles consisting of simple atomic and molecular species can build up on their surfaces. Whether and how these simple species can be processed to form more complex molecules such as alcohols, simple sugars and potentially amino acids is a key astrochemical problem. One way in which astrophysical ices can be processed to form more complex species is via the reactions of species within the ice with simple free radicals such as H, C, N and O. This thesis therefore presents experimental studies of the reactions of atomic species with some astrophysically relevant molecular ices under interstellar conditions. Since hydrogen and oxygen are the first and third most abundant elements in the interstellar medium respectively, these experiments have specifically focussed upon the reactions of hydrogen and oxygen atoms. In addition to the characterization of surface reactions between key astrochemical species, kinetic parameters for use in astrochemical models are derived from these experiments.

Dissertation--Mathematisch-Naturwissenschaftliche Fakultät--Bonn--Rheinische Friedrich-Wilhelms-Universität, 1999. Titre de soutenance : FUV spectroscopy of interstellar molecular hydrogen towards the Magellanic clouds.
FUV = far ultraviolet. Bibliogr. p. 83-85.

This thesis describes the results of a survey of the HI42ɑ recombination line emission at 2.3 GHz from HII regions in the Southern Milky Way, carried out with the 26 m diameter Hartebeesthoek radio telescope. The Galactic Longitude range covered was 290° to 40°. Single recombination lines were detected from 375 positions. Multiple lines were observed towards 90 positions in the inner Galaxy. No line emission could be detected in 28 positions. Continuum antenna temperatures were estimated from drift scans or radio maps observed for the purpose. LTE electron temperatures and turbulent velocities of the HII regions were calculated where possible. The properties of the sample were compared to those observed in HI09ɑ surveys. The lines observed from over 50 positions were first detections, of which half were associated with optically-identified HII regions. In about 150 cases the lines were only the second to be detected from those HII regions. The processes of the radio emission, detection, and analysis were simulated numerically. The detectability of the emission and the magnitude of non-LTE effects and pressure-broadening in multi-component HII regions was predicted and compared to observations. The radio luminosity function of the HII regions was determined over a range of three orders of magnitude in intrinsic brightness for the first time, using techniques which corrected for different types of incompleteness in the samples. The luminosity function was compared to those in five selected spiral galaxies, and shown to lie between those of M33 and M81. An alternate form of the luminosity function was developed for use with a numerical model of the spiral arm structure of the Milky Way. The physical parameters defining the major spiral arms were established by comparing synthesized diagrams of radial velocity versus Galactic Longitude with those actually observed. The faint, extended HII regions S9 and RCW129 in Scorpius, the Barnard Loop in Orion, and S296 in Canis Major were analyzed, using all available data. All the recombination lines from these HII regions were first detections

L'objectif de cette thèse est de comprendre la formation de l'hydrogène moléculaire dans le milieu interstellaire (MIS) via des expériences de laboratoire et des observations astronomiques. Les expériences ont été réalisées avec FORMOLISM, un montage fonctionnant dans l'ultra-vide pour étudier la formation de molécules dans le MIS. On s'intéresse à la distribution en énergie de molécules d'hydrogène formées sur une surface refroidie par cryogénie (< 10 K). La technique de Resonance Enhanced Multi-Photon Ionization (REMPI 2 + 1) est utilisée pour sonder la population des niveaux rovibrationnels de l'état électronique fondamental de l'hydrogène moléculaire. Nous avons examiné différentes surfaces d'intérêt astrophysique : des silicates amorphes et cristallins, et de la glace d'eau solide amorphe poreuse (p-ASW). Nous avons confirmé l'augmentation du taux de formation de l'hydrogène moléculaire sur une surface recouverte au préalable des molécules d'hydrogène et nous avons quantifié la formation D₂en tant que mécanisme de désorption non-thermique. Nous avons mesuré le rapport ortho-para de l'hydrogène moléculaire nouvellement formée sur la surface de p-ASW, qui correspond à la valeur attendue à l'équilibre statistique à haute température (> 100 K). Nous avons fabriqué au laboratoire de nouvelles surfaces de silicates (forstérite et fayalite) pour examiner l'impact de leur morphologie et de leur composition chimique sur la formation de l'hydrogène moléculaire. On a observé l'abaissement de la température de rotation des molécules d'hydrogène formées (par rapport à la température de rotation du jet moléculaire) émergeant de surfaces cristallines. Nous avons également étudié la conversion de spin nucléaire des molécules d'hydrogène absorbées sur une surface de sillicate. Les prédictions observationnelles qui on été déduites de ces expériences ont été testées par spectroscopie à longue fente dans l'infrarouge proche disponible au VLT et au Keck. Des nébuleuses planétaires présentant simultanément des émissions de H₂ont été détectées sur certains de nos objets. La distribution d'intensité de ces raies est comparée à des modèles théoriques de formation H₂dans l'espace. Une partie de cette thèse traite également de la spectroscopie VUV à haute résolution de CO et de ses isotopes, en utilisant le spectromètre à transformée de Fourier disponible au synchroton SOLEIL. Cela complète le travail sur l'hydrogène dans le contexte plus large de l'astrochimie de petites molécules
The goal of this thesis is to understand the formation of molecular hydrogen in the interstellar medium (ISM) via laboratory experiments and astronomical observations. The experiments are performed with FORMOLISM, an ultra-high vacuum setup to study the formation of molecules in the ISM. We are interested in the energy disposal during the exhothermic recombination of two H atoms on a cryogenically cooled surface (< 10 K). Resonance Enhanced Multi-Photon Ionization ( REMPI 2 + 1) spectroscopy is used to probe the population of rovibrational levels in the ground electronic state of molecular hydrogen after formation. We have tested different surfaces of astrophysical relevance : amorphous and crystalline silicates, porous amorphous solid water, and a bare silicate pre-dosed with hydrogen molecules. We have confirmed the formation enhancement of molecular hydrogen on a surface pre-dosed with molecules and quantified D₂formation as a non-thermal desorption mechanism. We have also measured the ortho-to-para ratio of newly formed molecular hydrogen on p-ASW, finding that it corresponds to the value expected at statistical equilibrium at high temperature. Silicate analog surfaces (forsterite and fayalite) have been fabricated to test the influence of their morphology and chemical composition on hydrogen formation. We have found that newly formed molecular hydrogen leaves rotationally cooler (with respect to the molecular beam rotational temperature) from crystalline surfaces, and that it is unaffected when it scatters from amorphous surfaces. We have also detected nuclear spin conversion of molecular hydrogen absorbed on bare silicates. Observational predictions from these experiments are tested using long slit near infrared spectroscopy available at the VLT and Keck telescopes. Planetary nebulae with H₂ and X-ray emission were chosen as ideal targets. H₂transitions have been detected throughout our targets. The intensity distribution of these transitions will be compared to models of formation pumping spectra. In addition, part of this thesis addresses the VUV high-resolution spectroscopy of CO and its isotopologues, using the Fourier Transform Spectrometer at the SOLEIL synchroton. This complements the work on hydrogen in the wider context of the astrochemistry of small molecules

Fundamental observations of molecular hydrogen (H₂) in dark clouds, star forming regions, and radiation-dominated environments are presented, modeled, and interpreted. Through a weak infrared absorption line spectrum, the abundance of cold H₂ in dark molecular clouds and star forming regions is measured directly and compared with the abundance of its most commonly cited surrogate, CO. The derived abundance of CO is between 1.5 and 2.5 x 10⁻⁴ for the sample. The CO molecule thus represents about ⅓ of the total carbon budget in dense clouds. Also detected via infrared line absorption is the pivotal molecular ion H⁺₃ , yielding a direct measure of the cosmic ray ionization rate of H₂ in dark molecular clouds (between 1 and 5 x 10⁻¹⁷ s⁻¹), a process that instigates the complex ion-neutral chemical pathways that form many of the 120+ known molecular species deep inside interstellar clouds. These timely tests of theory are applied to the detailed submillimeter-wave study of the ρ Ophiuchi star forming cloud and photodissociation front, allowing partial disentanglement of the complicated physical and chemical structure of a star forming cloud. Yet H₂ and H⁺₃ continue to surprise and delight us with more mysteries. The formation, excitation and survival of molecules in unusual & hostile environments is highlighted by the discoveries of H⁺₃ in circumstellar disks of early-type stars, and of fluorescing H₂ in two harshly-irradiated filaments of the Crab Nebula. The role of H⁺₃ as a possible tracer of planet formation, and the evolution of H₂ in the interstellar medium is discussed. The study of H₂ in hostile environments is extended to the ensemble properties of extragalactic star forming regions, and applied to the Arp 299 merger system as a unique probe of the feedback of newly-formed hot stars, their fossil remains, and the molecular material which formed them.

Parmi les différentes structures de l’univers existe ce qu’on appelle le milieu interstellaire (MIS). C’est un endroit où gaz et poussière co-existent et interagissent en parfaite harmonie. L’hydrogène moléculaire est l’espèce la plus abondante et de loin la plus importante du gaz interstellaire. Elle est à la base de trois sur quatre des molécules les plus essentielles à l’apparition de la vie : l’eau, le méthane, l’amine et le monoxyde de carbone. La physico-chimie du MIS qui mène à la formation de nouvelles molécules est divisée en deux : les réactions en phase gazeuse et les réactions sur les grains de poussière qui s’est révélée la voie de formation la plus efficace pour l’hydrogène moléculaire. Ce travail de thèse est une contribution expérimentale à l’étude de l’interaction et de la formation de l’hydrogène moléculaire sur les surface de glace d’eau amorphe qui couvrent les grains de poussière dans les nuages sombres du MIS. Dans ce but, en réunissant techniques ultravides, systèmes cryogéniques, jets atomiques et moléculaires, spectrométrie de masse et modélisation, plusieurs expériences ont été faites en utilisant le dispositif FORMOLISM (FORmation of MOLecules in the InterStellar Medium)
Among the different structures of the universe exists what we call the interstellar medium (ISM). It is a place where gas and dust coexist and interact in perfect harmony. In this medium, molecular hydrogen is the most abundant gaseous species and by far the most important one. It is the principal constituent of three of four molecules essential to the existence of life: water, methane, amine and carbon monoxide. The physico-chemistry of the ISM that leads to the formation of new molecules is divided in two: the gas-phase reactions and the gas-dust reactions. The second one being the most efficient route of molecular hydrogen in space. This thesis work is an experimental contribution to study the interaction and the formation of molecular hydrogen on the surface of amorphous water ice surfaces covering dust grains in dark clouds. For this, by uniting ultra-high vacuum techniques, cryogenic systems, atomic and molecular beams, mass spectroscopy and modelling, several experiments have been conducted by using the FOMOLISM experimental set-up (FORmation of MOLecules in the InterStellar Medium)

We present an analysis of the properties of HI holes detected in 20 galaxies that are part of “The HI Nearby Galaxy Survey” (THINGS). We detected more than 1000 holes in total in the sampled galaxies. Where they can be measured, their sizes range from about 100 pc (our resolution limit) to about 2 kpc, their expansion velocities range from 4 to 36 km/s, and their ages are estimated to range between 3 and 150 Myr. The holes are found throughout the discs of the galaxies, out to the edge of the HI disc; 23% of the holes fall outside R25. We find that shear limits the age of holes in spirals; shear is less important in dwarf galaxies which explains why HI holes in dwarfs are rounder, on average than in spirals. Shear, which is particularly strong in the inner part of spiral galaxies, also explains why we find that holes outside R25 are larger and older. We derive the scale height of the HI disc as a function of galactocentric radius and find that the disc flares at large radii in all galaxies. We proceed to derive the surface and volume porosity (Q2D and Q3D) and find that this correlates with the type of the host galaxy: later Hubble types tend to be more porous. The size distribution of the holes in our sample follows a power law with a slope of a=−2.9. Assuming that the holes are the result of massive star formation, we derive values for the supernova rate (SNR) and star formation rate (SFR) which scales with the SFR derived based on other tracers. If we extrapolate the observed number of holes to include those that fall below our resolution limit, down to holes created by a single supernova, we find that our results are compatible with the hypothesis that HI holes result from star formation. We use HI data from THINGS, 8μm, 24μm, 70μm and HI maps from SINGS, CO(2–1) data from HERACLES and FUV data from NGS to present a visual comparison of these maps with respect to the locations of HI holes. We find that the vast majority of HI holes are also prominent in the 8μm map and to some extent in the 24μm map. There is a lack of molecular gas from the interior of nearly all the holes, which is consistent with the idea that the latter are filled with hot gas. About 60% of young holes have FUV emission detected in their interiors highlighting the presence of the parent OB association. In addition, FUV is detected on the rims of some of the older HI holes, presumably due to the dispersion of the OB association with respect to the gas. We describe the development of a 2–D cross-correlation method to compare multi-wavelength maps in a quantitative way (quantified by Ccoef ) and give some first results from the application of this method to the nearby galaxy NGC2403. We find that the all the dust tracers are well correlated (Ccoef > 0.7) with the 8μm–24μm correlation being the highest (Ccoef > 0.88). Similarly all the star formation tracers are well linked as expected (Ccoef > 0.6). With respect to the relations between star formation and dust tracers we found that most are well matched (Ccoef > 0.7) as dust grains are heated by radiation in star forming regions. At smaller scales (15") FUV correlates poorly (Ccoef ~ 0.3) with the dust tracers, a direct consequence of the absorption of FUV photons by dust. We find that the HI is reasonably well correlated with the 8μm emission (Ccoef ~ 0.6) illustrating the fact that HI is mixed with PAH’s. Interestingly, the HI map shows some correlation with the SF map (Ccoef ~ 0.4) even though FUV and HI emissions were found to be completely uncorrelated (Ccoef ~ 0).

This thesis presents an observational study of molecular abundances in the dense cores of 27 prominent molecular clouds in the southern galactic plane.The molecular abundances and physical conditions in dense condensations have been derived from millimetre-wavelength observations of molecular rotational transitions.The study has produced a comprehensive data set of transition intensities and abundances for 10 different molecules in bright southern molecular clouds, and the general characteristics of emissions from these molecules such as optical depth, excitation and relative abundances are discussed. A comparison of different methods of calculating molecular hydrogen column density from observations of carbon monoxide emission is included.Both the analysis and the data collected provide an excellent starting point for further observational and theoretical studies of molecular clouds in the southern Milky Way utilising new instruments such as the millimeter-wave upgrade to the Australia Telescope Compact Array and the Attacama Large Millimetre Array (ALMA).
Doctor of Philosophy (PhD)

Entre les molècules de l'espai, l’H2 és una de les més rellevants de l'univers. És la més abundant en el medi interestel·lar i és un intermedi clau per a la formació de molècules més grans. La seva formació és complexa, però a causa de la seva inherent rellevància, la comprensió de la seva interacció i la seva formació pot ser considerat com un paradigma del procés astrofísic. La present tesi s'estructura en quatre capítols. El capítol 1 presenta el marc astroquímic en què es troba la tesi, mostrant la presència d'hidrogen interestel·lar en el medi interestel·lar i on es porta a terme, en els grans de pols interestel·lar circumdants. Després de presentar els objectius d'aquesta tesi, el capítol 2 repassa els aspectes teòrics generals que hi ha darrere, com per exemple l'estructura electrònica, els mètodes del funcional de la densitat, el modelització de sòlids i l’efecte túnel, proporcionant finalment els detalls computacionals implicats. El capítol 3 correspon als resultats obtinguts i a la seva discussió i es divideix en diferents seccions. La secció 3.1 presenta algunes de les propietats fisicoquímiques de l'estructura cristal·lina i les corresponents superfícies de Mg2SiO4 forsterita, així com dels sistemes que contenen ferro com la olivina Mg1.5Fe0.5SiO4. La secció 3.2 reporta l'adsorció d'hidrogen atòmic i la seva recombinació per formar una molècula de H2 sobre la superfície cristal·lina (010) de Mg2SiO4 forsterita i la secció 3.3 analitza la rellevància de la morfologia superficial considerant la formació de H2 sobre les superfícies cristal·lines (001) i (110) de Mg2SiO4 forsterita. Finalment, la secció 3.4 investiga la influència dels àtoms de Fe2+ tot modelant la fisisorció i quimisorció d’hidrogen atòmic sobre la superfície cristal·lina (010) de forsterita amb contingut de ferro, tenint lloc posteriorment la formació de H2. El capítol 4 aborda les conclusions generals de la present tesi i les possibles perspectives futures, mentre que el capítol 5 inclou les referències citades. Per últim, l'apèndix A i B aporta informació suplementària reportada en el capítol 3.
Among the molecules in space, H2 is one of the most relevant of the universe. It is the most abundant one in the interstellar medium and is a key intermediate for the formation of bigger molecules. Its formation is complex, but due to its inherent relevance understanding its interaction and its formation can be considered as a paradigm of the astrophysical process. The present thesis introduces in Chapter 1 the astrochemical framework in which the thesis is located, pointing out the presence of interstellar hydrogen in the interstellar medium and where it takes place, in the interstellar dust grains around. After presenting the goals this thesis aims, Chapter 2 overviews the general theoretical aspects behind it, such as electronic structure, density functional methods, solids modelling and tunnelling effects, providing finally the computational details entailed. Chapter 3 corresponds to results and discussion and is divided into different sections. Section 3.1 presents some physicochemical properties of the crystalline bulk structure and the corresponding surfaces of Mg2SiO4 forsterite and of the Fe-containing Mg1.5Fe0.5SiO4 olivine systems. Section 3.2 reports the adsorption of H atoms and their recombination to form a H2 molecule on the crystalline Mg2SiO4 forsterite (010) surface and Section 3.3 analyses the relevance of surface morphology by considering the H2 formation on the crystalline Mg2SiO4 forsterite (001) and (110) surfaces. Finally, Section 3.4 investigates the influence of Fe2+ atoms by modelling the physisorption/chemisorption of H atom on the Fe-containing (010) surface, subsequently taking place the formation of H2. Chapter 4 addresses the general conclusions of the present thesis and possible future perspectives, Chapter 5 includes the references cited and Appendix A and B supports the information given in Chapter 3.

We present a range of steady-state photoionization simulations, corresponding to different assumed shell geometries and compositions, of the unseen postulated rapidly expanding outer shell to the Crab Nebula. The properties of the shell are constrained by the mass that must lie within it, and by limits to the intensities of hydrogen recombination lines. In all cases the photoionization models predict very strong emission from high ionization lines that will not be emitted by the Crab’s filaments, alleviating problems with detecting these lines in the presence of light scattered from brighter parts of the Crab. The NIR [Ne VI] λ 7.652 mm line is a particularly good case; it should be dramatically brighter than the optical lines commonly used in searches. The C IV λ1549Å doublet is predicted to be the strongest absorption line from the shell, which is in agreement with HST observations. We show that the cooling timescale for the outer shell is much longer than the age of the Crab, due to the low density. This means that the temperature of the shell will actually “remember” its initial conditions. However, the recombination time is much shorter than the age of the Crab, so the predicted level of ionization should approximate the real ionization. In any case, it is clear that IR observations present the best opportunity to detect the outer shell and so guide future models that will constrain early events in the original explosion. Infrared observations have discovered a variety of objects, including filaments in the Crab Nebula and cool-core clusters of galaxies, where the H2 1-0 S(1) line is stronger than the infrared H I lines. A variety of processes could be responsible for this emission. Although many complete shock or PDR calculations of H2 emission have been published, we know of no previous simple calculation that shows the emission spectrum and level populations of thermally excited low-density H2. We present a range of purely thermal collisional simulations, corresponding to constant gas kinetic temperature at different densities. We consider the cases where the collisions affecting H2 are predominantly with atomic or molecular hydrogen. The resulting level population (often called “excitation”) diagrams show that excitation temperatures are sometimes lower than the gas kinetic temperature when the density is too low for the level populations to go to LTE. The atomic case goes to LTE at much lower densities than the molecular case due to larger collision rates. At low densities for the v=1 and 2 vibrational manifolds level populations are quasi-thermal, which could be misinterpreted as showing the gas is in LTE at high density. At low densities for the molecular case the level population diagrams are discontinuous between v=0 and 1 vibrational manifolds and between v=2, J=0, 1 and other higher J levels within the same vibrational manifold. These jumps could be used as density diagnostics. We show how much the H2 mass would be underestimated using the H2 1-0 S(1) line strength if the density is below that required for LTE. We give diagnostic diagrams showing level populations over a range of density and temperature. The density where the level populations are given by a Boltzmann distribution relative to the total molecular abundance (required to get the correct H2 mass), is shown for various cases. We discuss the implications of these results for the interpretation of H2 observations of the Crab Nebula and filaments in cool-core clusters of galaxies.

Le milieu interstellaire (MIS) est un système extrêmement complexe. Il correspond à une échelle intermédiaire entre les étoiles et les galaxies. Le gaz interstellaire est présent dans toute la galaxie, remplissant l’espace entre les étoiles. Une grande diversité de processus couplés, comme la gravité, le champs magnétiques, la turbulence et la chimie, participe à son évolution, faisant de la modélisation du MIS un problème ardu. Une description correcte du MIS nécessite un bon traitement des équations de la magnetohydrodynamique (MHD), de la gravité, du bilan thermique et de l’évolution chimique à l’intérieur du nuage moléculaire.L’objectif de ce travail de thèse est une meilleure compréhension de la formation et de l’évolution des nuages moléculaires, et plus particulièrement de la transition du gaz atomique en gaz moléculaire. Nous avons réalisé des simulations numériques de la formation des nuages moléculaires et de la formation de l’hydrogène moléculaire sous l’influence de la gravité et de la turbulence MHD, en utilisant des estimations précises de l’écrantage par les poussières et de l’auto-écrantage par la molécule H2. Ceci a été calculé grâce à une méthode en arbre, à même de fournir une rapide estimation des densités de colonne.Nous avons trouvé que l’hydrogène moléculaire se forme plus rapidement que prévu par les estimations classiques du fait de l’augmentation de densité locale provoquée par les fluctuations turbulentes du gaz. L’hydrogène moléculaire, formé à des densités plus élevées, peut alors migrer vers les régions plus chaudes et moins denses.Les densités de colonne totale d’hydrogène moléculaire montrent que la transition HI-H2 se produit à des densités de colonne de quelques 10^20 cm−2. Nous avons calculé les populations des niveaux rotationnels de H2 à l’équilibre thermique et intégré le long de plusieurs lignes de visée. Ces résultats reproduisent bien les valeurs observées par Copernicus et FUSE, suggérant que la transition observée et les populations excitées pourraient être une conséquence de la structure multi-phasique des nuages moléculaires. Comme la formation de H2 précède la formation des autres molécules, le H2 chaud pourrait permettre le développement d’espèces endothermiques et éventuellement expliquer certains aspects de la richesse moléculaire observée dans l’ISM
The interstellar medium (ISM) is a highly complex system. It corresponds to an intermediate scale between stars and galaxies. The interstellar gas is present throughout the galaxy, filling the volume between stars. A wide variety of coupled processes, such as gravity, magnetic fields, turbulence and chemistry, participate in its evolution, making the modeling of the ISM a challenging problem. A correct description of the ISM requires a good treatment of the magnetohydrodynamics (MHD) equations, gravity, thermal balance, and chemical evolution within the molecular clouds.This thesis work aims at a better understanding of the formation and evolution of molecular clouds, specially how they become "molecular", paying particular attention to the transition HI-to-H2. We have performed ideal MHD simulations of the formation of molecular clouds and the formation of molecular hydrogen under the influence of gravity and turbulence, using accurate estimates for the shielding effects from dust and the self-shielding for H2, calculated with a Tree-based method, able to provide fast estimates of column densities.We find that H2 is formed faster than predicted by the usual estimates due to local density enhancements created by the gas turbulent motions. Molecular hydrogen, formed at higher densities, could then migrate toward low density warmer regions.Total H2 column densities show that the HI-to-H2 transition occurs at total column densities of a few 10^20 cm−2. We have calculated the populations of rotational levels of H2 at thermal equilibrium, and integrated along several lines of sight. These two results reproduce quite well the values observed by Copernicus and FUSE, suggesting that the observed transition and the excited populations could arise as a consequence of the multi-phase structure of molecular clouds. As H2 formation is prior to further molecule formation, warm H2 could possibly allow the development of a warm chemistry, and eventually explain some aspects of the molecular richness observed in the ISM

Three separate topics are presented covering the temperature dependence of isotropic hyperfine coupling constants, an anharmonic analysis of molecular hydrogen in interstellar ice and the microscopic structure of doped crystalline silicon lattices. The temperature dependence and vibrational effects of isotropic hyperfine coupling constants are studied using the ab-initio density functional methods BLYP and B3LYP with the common double and triple zeta basis sets 6-31G(d,p) and 6-311(d,p). Harmonic oscillator wavefunction averages for all normal modes of a molecule are accounted for and the temperature dependence is developed from the Boltzmann distribution. An ab-initio study of the atomic displacements and microscopic structure of phosphorus and arsenic in doped silicon is carried out. The structures and relative displacements are estimated using SCF, DFT and MP2 methods with STO-3G, 3-21G, 3-21G(d,p) and 6-31G(d,p) basis sets with a 13 atom silicon molecular cluster. The calculations are carried out on clusters using empirical parameters for Si-Si and Si-H bond lengths in addition in fully optimised clusters. Following the discovery of a species thought to be molecular hydrogen in interstellar clouds by IR spectroscopy, ab-initio density functional B3LYP calculations are performed to back up experimental and semi-empirical studies of molecular hydrogen in amorphous ice. Anharmonic calculations are carried out on hydrogen molecules adsorbed onto ice and trapped in a cage. The issue of species other than molecular hydrogen and surface transport of hydrogen is addressed.

En Astrophysique, la compréhension du processus de formation d'étoiles reste l'une des principales questions. Elle est directement reliée à l'évolution du gaz interstellaire dans les galaxies, et en particulier aux processus de refroidissement et de condensation pour lesquels la turbulence et l'instabilité thermique jouent un rôle dominant. Ce travail se concentre sur l'évolution du gaz atomique et diffus qui fournit les conditions initiales à la formation des nuages moléculaires et se base sur une comparaison étroite entre observations à 21 cm et simulations numériques hydrodynamiques. Pour comprendre les rôles de l'instabilité thermique et de la turbulence dans la transition du gaz chaud (WNM, T ~ 8000 K, n = 0.5 cm-³) vers le gaz froid (CNM, T ~ 80 K, n = 50 cm-³), j'ai produit 90 simulations à basse résolution qui ont permis d'étudier l'influence de la densité initiale du WNM et de la compressibilité du forçage de la turbulence sur l'efficacité de la production de CNM. Un résultat important permet de conclure que le gaz chaud, dans les conditions de turbulence caractéristiques de ce qui est observé, ne transite pas vers le gaz froid quelque soit l'amplitude de la turbulence. Ces simulations à basse résolution ont aussi permis de déterminer quelles conditions initiales permettent de reproduire les propriétés déduites des observations telles que le nombre de Mach, la quantité de CNM en masse ou la dispersion de vitesse turbulente. Un processus de compression, que l'on peut reproduire soit en augmentant la densité initiale du WNM (n ≥ 1.5 cm-³) soit en appliquant un champ de forçage compressif, est nécessaire. Ces conditions initiales ont ensuite été utilisées pour produire deux simulations à haute résolution (1024³) pour lesquelles j'ai montré que les propriétés de la turbulence et de l'instabilité du milieu atomique neutre sont bien reproduites. Les histogrammes de température portent en effet la trace d'un milieu biphasique et les distributions de pression sont semblables aux observations. D'autre part, les spectres de puissance de la densité sont caractéristiques d'un milieu fortement contrasté alors que ceux de la vitesse restent caractéristiques d'une turbulence subsonique. Finalement, les structures froides de ces deux simulations reproduisent les relations masse-échelle et dispersion de vitesse-échelle observées dans les nuages moléculaires, suggérant que la structure des nuages moléculaires pourrait être héritée de celle des nuages de HI à partir desquels ils se sont formés. Le dernier aspect de mon travail est relié à la difficulté rencontrée lors de l'interprétation des données qui n'est possible qu'à partir de grandeurs projetées en deux dimensions. J'ai donc comparé en détails les deux simulations à haute résolution à des observations de cirrus en créant des observations artificielles à 21 cm. Les spectres d'émission et les cartes de densité de colonne ainsi produits sont semblables aux observations. De plus, les simulations donnant accès à l'information en trois dimensions, j'ai étudié les effets de l'auto-absorption dans la création de cartes de densité de colonne à partir de spectres de température de brillance. J'ai conclu de cette étude que l'auto-absorption ne peut être négligée mais qu'elle ne concerne que les lignes de visée les plus brillantes et les plus denses et que la correction habituellement appliquée sur les observations est efficace. Finalement, j'ai appliqué une méthode de décomposition en gaussiennes sur les spectres synthétiques. Cette méthode a pour objectif d'étudier les propriétés de chacune des deux phases thermiques du HI. Les résultats montrent qu'elle est prometteuse pour l'analyse des données de spectro-imagerie à 21 cm, bien que nécessitant des améliorations. Elle permet en effet de bien séparer les phases chaude et froide du milieu atomique et d'en déduire la distribution massique de chacune d'elles.

Der erste Teil der Aktivitäten dieser Arbeit bestand in der Entwicklung einer modernen Ionenspeicher Apparatur zur Untersuchung chemischer Prozesse mit atomarem Wasserstoff. Die Integration eines differentiell gepumpten Radikalenstrahls in eine vorhandene temperaturvariable 22-Pol Speicherapparatur erforderte größere Änderungen an dieser. Da astrophysikalische Fragestellungen im Vordergrund standen, führt die Einleitung zunächst in das Gebiet der Astrophysik und -chemie ein. Die Grundlagen der Ionenspeicherung in temperaturvariablen Hf-Speichern sind ausführlich in der Literatur dokumentiert. Daher ist die Beschreibung der Apparatur (Kapitel 2) relativ kurz gehalten. Viel Mühe wurde in die Entwicklung einer intensiven und stabilen Quelle für Wasserstoffatome aufgewandt, deren kinetische Energie variiert werden kann. Das Kapitel 3 beschreibt dieses Modul in vielen Details, wobei der Einsatz von magnetischen Hexapolen zum Führen der Atome und die chemische Behandlung der Oberflächen zur Reduzierung der H-H Rekombination einen wesentlichen Platz einnimmt. Durch die außergewöhnliche Empfindlichkeit der Speichertechnik kann das neue Instrument zur Untersuchung von vielen Reaktionen eingesetzt werden, die von astrochemischer und fundamentaler Bedeutung sind. Die Ergebnisse dieser Arbeit sind im Kapitel 4 zusammengestellt, einige Reprints und Entwürfe von Publikationen findet man im Anhang. Die Reaktionen von CO2+ mit Wasserstoffatomen und -molekülen erwiesen sich als sehr geeignet, um in situ H and H2 Dichten über den gesamten Temperaturbereich der Apparatur zu bestimmen (10 K - 300 K). Zum ersten mal wurden Reaktionen von H- and D-Atomen mit den Kohlenwasserstoffionen CH+, CH2+, and CH4+ bei Temperaturen des interstellaren Raums untersucht. Ein sehr interessantes, noch nicht ganz verstandenes Stoßsystem ist die Wechselwirkung von protoniertem Methan mit H-Atomen. Im Ausblick der Arbeit werden einige Ideen aufgezeigt, wie man das Instrument verbessern kann, und es werden einige Reaktionen erwähnt, die man als nächste untersuchen könnte. Diese Dissertation ist einen Beitrag zum Projekt 5 der Forschergruppe Laboratory Astrophysics: Structure, Dynamics and Properties of Molecules and Grains in Space, die von der DFG im Zeitraum von 2000 bis 2006 unterstützt wurde
The first part of the activities of this thesis was to develop a sophisticated ion storage apparatus dedicated to study chemical processes with atomic hydrogen. The integration of a differentially pumped radical beam source into an existing temperature variable 22-pole trapping machine has required major modifications. Since astrophysical questions have been in the center of our interest, the introduction first gives a short overview of astrophysics and -chemistry. The basics of ion trapping in temperature variable rf traps is well-documented in the literature; therefore, the description of the basic instrument (Chapter 2) is kept rather short. Much effort has been put into the development of an intense and stable source for hydrogen atoms the kinetic energy of which can be changed. Chapter 3 describes this module in detail with emphasis on the integration of magnetic hexapoles for guiding the atoms and special treatments of the surfaces for reducing H-H recombination. Due to the unique sensitivity of the rf ion trapping technique, this instrument allows one to study a variety of reactions of astrochemical and fundamental interest. The results of this work are summarized in Chapter 4, some reprints and drafts are reproduced in the appendix. Reactions of CO2+ with hydrogen atoms and molecules have been established as calibration standard for in situ determination of H and H2 densities over the full temperature range of the apparatus (10 K - 300 K). For the first time, reactions of H- and D-atoms with the ionic hydrocarbons CH+, CH2+, and CH4+ have been studied at temperatures of interstellar space. A very interesting, not yet fully understood collision system is the interaction of protonated methane with H. The outlook presents some ideas, how to improve the new instrument and a few reaction systems are mentioned which may be studied next. This thesis is a contribution to the project 5 of the research unit Laboratory Astrophysics: Structure, Dynamics and Properties of Molecules and Grains in Space which has been supported by the DFG from 2000 to 2006

Le milieu interstellaire (MIS) où se forment les étoiles est constitué de gaz très dilué dominé par l'hydrogène moléculaire, et de grains de poussière de taille submicrométrique. Ces poussières jouent un rôle crucial en atténuant la lumière des étoiles lointaines, protégeant ainsi les molécules du gaz des rayonnements ultra-violets, et en servant de catalyseurs à une chimie hétérogène à très basse température. Outre la synthèse de l'hydrogène moléculaire, la surface des grains permet de former des molécules organiques dites complexes comme le méthanol (CH3OH) à partir de l'hydrogénation (et la deutération) du monoxyde de carbone (CO). Les glaces ainsi formées participent à la complexification moléculaire du MIS et seront à terme intégrées au sein de disques de poussières, berceaux des astéroïdes, comètes et exo-planètes. L'objectif de cette thèse est l'étude des mécanismes d'échanges hydrogène-deuterium sur certains groupements fonctionnels de molécules organiques simples, méthanol par exemple, présentes à la surface ou dans les manteaux des grains interstellaires. La thèse est centrée sur une exploration expérimentale de ces processus en phase condensée, à l'aide d'une expérience de cryogénie synthétisant des glaces à très basse température (15K) couplée à un spectromètre infrarouge. Nous montrons que ces échanges se produisent avant la sublimation du manteau de glace sur des groupes fonctionnels capables d'établir des liaisons hydrogènes avec les molécules d'eau voisines. Le processus catalysant est vraisemblablement la cristallisation de la glace d'eau. Des études cinétiques nous permettent d'évaluer les énergies d'activation du transfert H/D (6745K) et de la transition amorphe-cristalline (8100K), et de déterminer la constante de vitesse d'échange dans le domaine de température 120-140~K. Cette constante de vitesse est, de plus, comparée à des calculs semi-classiques basés sur un traitement ab initio. En marge de ces expériences, des observations millimétriques de la molécule de méthanol en direction de proto-étoiles confirment une variabilité des abondances relatives des isotopologues simplement deutérés de cette molécule en fonction de la masse de la protoétoile.

Les barres jouent probablement un role important dans l'evolution des galaxies. Le potentiel gravitationnel de la barre est capable de concentrer de grandes quantites de gaz interstellaire dans le voisinage du noyau, fournissant ainsi du combustible aux activites nucleaires eventuelles (formation explosive d'etoiles ou disque d'accretion de trou noir). Le gaz tombe vers le centre de la galaxie le long de deux chocs presque paralleles a la barre. Ces chocs sont riches en poussieres et en gaz moleculaire. L'interferometre millimetrique et le telescope de 30m de l'iram ont permis une etude precise du gaz moleculaire dans la barre et le noyau d'une galaxie spirale barree typique, ngc 1530. Dans cette galaxie, nous avons detecte co(1-0) le long de deux bandes formees par les chocs dans le gaz moleculaire. Dans ces bandes, le gaz tombe vers le centre, avec une vitesse de chute typique de 100 kms#-#1. Nous avons montre l'anticorrelation entre le cisaillement du gaz dans ces chocs et l'efficacite de formation d'etoiles en comparant des images h et co. Puis nous avons etudie le centre de cette galaxie a plus haute resolution en #1#2co(1-0), #1#2co(2-1), #1#3co(1-0) et hcn(1-0). Le gaz a un mouvement centripete le long de deux arcs entourant un disque nucleaire gazeux, mais dans ce disque le mouvement est circulaire. Dans le disque, le gaz se distribue suivant un anneau dont les rayons externes et internes correspondent aux resonances interieures de lindblad ou suivant une spirale non resolue. Le disque nucleaire montre une grande quantite de gaz dense tracee par hcn et #1#3co, ainsi qu'une activite de formation d'etoiles intense detectee dans l'emission etendue du continuum centimetrique. Par contre les arcs sont pauvres en gaz dense et forment peu d'etoiles. La formation d'etoiles dans le disque nucleaire peut etre maintenue pendant une longue periode de temps, grace a la grande quantite de gaz moleculaire disponible.

L'observation radiomillimetrique a 110. 2 ghz du plan galactique a permis d'identifier 181 nuages interstellaires dont certains parametres sont listes. Deux bras spiraux sont mis en evidence: celui de persee d'inclinaison 12**(o) et un bras mineur lie au gaz local et incline de 22**(o). Les donnees moleculaires sont correlees aux donnees hi obtenues a arecibo et les resultats de cette comparaison sont presentes. D'autre part le nuage sombre l 134 est observe a 9 cm en ch et l'abondance relative de ce radical en fonction de la position est determinee

Les carbones amorphes hydrogénés (a-C:H ou HAC) constituent une composante importante de la poussière interstellaire. Ces grains hydrocarbonés sont observés au travers de bandes d'absorption IR à 3.4, 6.9 et 7.3 microns, caractéristiques des vibrations des liaisons C-H aliphatiques. Leurs signatures spectrales sont détectées dans le milieu interstellaire diffus de différentes lignes de visée de la Voie Lactée, mais aussi de nombreuses autres galaxies. Cette thèse porte sur l'étude de ces a-C:H interstellaires, à la fois au travers d'observations de ces poussières, et grâce à la synthèse et la caractérisation d'analogues de laboratoire.Une première partie de mon travail de thèse est consacrée à l'observation de la bande à 3.4 microns des a-C:H du milieu interstellaire diffus galactique en direction de la source IRAS 18511+0146. La bande d'absorption des modes d'élongation C-H détectée dans cette direction, vers différentes lignes de visée proches les unes des autres, présente des profondeurs optiques similaires et les plus fortes observées dans la Voie Lactée en dehors du centre galactique. Différentes interprétations de la profonde bande dans cette direction sont discutées.Des analogues de ces poussières carbonées aliphatiques ont été synthétisés en laboratoire, sous forme de films, grâce à un plasma, et reproduisent bien les bandes IR observées dans le milieu interstellaire diffus. Ces échantillons ont été caractérisés par spectroscopie d'absorption dans l'UV-visible et l'IR.Puisque les a-C:H émettent un rayonnement visible après absorption de photons UV ou visibles, une partie de la thèse est consacrée à une étude systématique de cette photoluminescence. Pour la première fois, les rendements absolus et intrinsèques de photoluminescence d'a-C:H sont déterminés pour une large gamme de longueurs d'onde d'excitation. Les propriétés de la photoluminescence des a-C:H sont confrontées aux observations de l'Emission Rouge Etendue, une large bande d'émission interstellaire dont les porteurs ne sont pas identifiés.Afin de déduire l'influence des rayons cosmiques sur ces poussières carbonées, les analogues produits ont été irradiés par différents ions énergétiques dont le dépôt d'énergie est similaire à celui du rayonnement cosmique interstellaire. Les effets induits ont été suivies par IR. L'analyse de la déshydrogénation des a-C:H observée au travers de la disparition progressive des bandes des C-H aliphatiques permet de déduire l'évolution de ces poussières interstellaires et de leurs signatures spectrales sous l'effet des rayons cosmiques. La destruction induite par les rayons cosmiques est comparée aux effets de l'exposition aux photons UV et aux atomes d'hydrogène afin d'interpréter l'évolution de la bande d'absorption à 3.4 microns observée dans le milieu interstellaire diffus, mais pas dans les nuages denses.

The interaction between the solar wind and the partially ionized gas of the local interstellar medium (ISM) creates a bubble known as the heliosphere. Classically, the shape of the heliosphere has been regarded as comet-like, with a long tail pointed in the direction opposite the Sun’s motion through the ISM. In this view, the solar magnetic field was assumed to have a negligible effect on the global structure of the heliosphere. Recent advances in numerical modeling have revealed the importance of the solar magnetic field in its ability to confine and collimate the solar wind plasma, and the shape of the heliosphere has been called into question. Energetic neutral atoms (ENAs) are created throughout the heliosphere via charge exchange. The separate contributions of the solar magnetic field topology and the solar wind structure to ENA observations is largely unexplored. The Interstellar Boundary Explorer (IBEX) has been providing a global perspective of the heliosphere through ENA maps with energies ranging from 0.2 to 6 keV. In this dissertation, three-dimensional magnetohydrodynamic simulations of the heliosphere are used as input to an ENA model designed to produce synthetic ENA maps. I compare modeled ENA maps with IBEX observations to investigate how different heliospheric conditions and properties affect ENAs created in the heliosphere, and therefore how ENA observations can be used to understand the heliosphere. First, I investigate the effect of the solar wind collimation by the solar magnetic field on ENA maps in the case of a solar wind without latitudinal variation. I find that even in the absence of variations of the solar wind, two lobes of strong ENA flux form at high latitudes, similar to what is observed by IBEX at high energies. Second, I test the effect of a latitudinally-varying solar wind on ENAs both with and without the inclusion of the solar magnetic field. I show that the latitudinal variations of the solar wind during solar minimum creates a structured ENA profile with latitude, corresponding to the profile observed at 1 AU, but that the solar magnetic field significantly enhances ENA flux in the region where the solar wind is confined. Lastly, I investigate the effect of the solar cycle on ENAs and how changing solar wind conditions (e.g. density, temperature, velocity) affect the heliosphere over time. I demonstrate that, given changes in the solar cycle, there is a significant evolution in the modeled ENA flux due to the changes in the solar wind profile and the solar magnetic field, which is also seen by ENA observations.

Initiated with the purpose of assigning the Fraunhofer lines in the solar spectrum to atomic transitions in the 18th century, the collaboration between spectroscopists and astrophysicists has remained fruitful, successful and ever fascinating. This collaboration has resulted in the unique detection of over 200 different molecular species in the interstellar medium (ISM). These interstellar molecular species play significant roles in diverse fields such as atmospheric chemistry, astrochemistry, prebiotic chemistry, astrophysics, astronomy, astrobiology, etc, and in our understanding of the solar system ''the world around us''. This Thesis work focuses on understanding of the different aspects of the chemistry of the various classes of these molecular species. Chapter one starts with an historical perspective of what is now regarded as Molecular Astrophysics or Astrochemistry and discusses the interstellar medium and its properties; interstellar molecular species and their importance; molecular spectroscopy as an indispensible tool in interstellar chemistry and the different formation routes of these molecular species. It also discusses hydrogen bonding which is one of the most important of all the intermolecular interactions. The chapter ends by setting the stage for the present investigations. The chapter two of the Thesis saddled with the task of describing the methodology employed in this Thesis begins by setting the stage on the importance of computational chemistry in interstellar chemistry. It discusses the Gaussian 09 suite of programs and the various theoretical methods used in all the quantum chemical calculations reported in this Thesis. The chapter ends with a brief summary on the homebuilt Pulsed Nozzle Fourier Transform Microwave (PN-FTMW) spectrometer used for the preliminary studies on Isoprene...Argon weakly bound complex reported in the appendix. After the introductory chapters, chapter three begins with what is unarguably one of the most important classes of interstellar molecular species - 'interstellar isomers'. In this chapter, the Energy, Stability and Abundance (ESA) relationship existing among interstellar molecular species has been firmly established using accurate thermochemical parameters obtained with the composite models and reported observational data. From the relationship, “Interstellar abundances of related species are directly proportional to their stabilities in the absence of the effect of interstellar hydrogen bonding”. The immediate consequences of the relationship in addressing some of the questions in interstellar chemistry such as: Where are Cyclic Interstellar Molecules? What are the possible candidates for astronomical observation? Why are more Interstellar Cyanides than isocyanides? among others are briefly discussed. Following the ESA relationship, other studies addressing some of the whys and wherefores in interstellar chemistry are discussed in details. From ESA relationship, though there has not been any successful astronomical observation of any heterocycle, the ones so far searched remain the best candidates for astronomical observation in their respective isomeric groups. The observation of the first branched chain molecule in ISM is in agreement with the ESA relationship and the C5H9N isomers have been shown to contain potential branched chain interstellar molecules. That molecules with the C-C-O backbone have less potential of formation in ISM as compared to their counterparts with the C-O-C backbone has been demonstrated not to be true following the ESA relationship. A detailed investigation on the relationship between molecular partition function and astronomical detection of isomeric species (or related molecules) shows that there is no direct correlation between the two rather there is a direct link between the thermodynamic stability of the isomeric species (or related molecules) and their interstellar abundances which influences the astronomical observation of some isomers at the expense of others. Chapter four presents an interesting and a fascinating phenomenon among the interstellar molecular species as it discusses for the first time, the existence and effects of Interstellar Hydrogen Bonding. This interstellar hydrogen bonding is shown to be responsible for the deviations from thermodynamically controlled processes, delayed observation of the most stable isomers, unsuccessful observations of amino acids among other happenings in interstellar chemistry and related areas. On the prediction that ketenes are the right candidates for astronomical searches among their respective isomers, a ketenyl radical; HCCO has recently been detected in line with this prediction. The deviation from the rule that the ratio of an interstellar sulphur molecule to its oxygen analogue is close to the cosmic S/O ratio is well accounted for on the basis of hydrogen bonding on the surface of the dust grains. Detecting weakly bound complexes in ISM has not been a major interest in the field so far but the detectability of weakly bound complexes in ISM is very possible as discussed in this chapter. Following the conditions in which these complexes are observed in the terrestrial laboratory as compared to the ISM conditions; it suffices to say that weakly bound complexes are present and are detectable in ISM. They could even account for some of the 'U' lines. Chapter five of the Thesis discusses the Linear Interstellar Carbon Chains which are the dominant theme in interstellar chemistry accounting for over 20% of all the known interstellar and circumstellar molecular species. Accurate spectroscopic parameters within experimental accuracy of few kHz which are the indispensable tools for the astronomical observation of these molecular species; are obtained for over 200 different species from the various chains using an inexpensive combined experimental and theoretical approach. With the availability of the spectroscopic parameters; thermodynamics is utilized in accounting for the known systems and in examining the right candidates for astronomical searches. These molecular species are shown to also obey the ESA relationship observed for the isomeric species discussed in chapter three of this work. The effect of kinetics on the formation processes of these molecular species is well controlled by thermodynamics as discussed in this chapter. Finally, the application of these studies in reducing the 'U' lines and probing new molecular species has been briefly summarized. Chapter six discusses Interstellar Ions and Isotopologues which are two unique classes of interstellar molecular species. Different studies on interstellar ions and isotopologues are presented. From the studies on interstellar protonated species with over 100 molecular species; protonated species resulting from a high proton affinity prefers to remain protonated rather than transferring a proton and returning to its neutral form as compared to its analogue that gives rise to a lower proton affinity from the same neutral species. The studies on detectable interstellar anions account for the known interstellar anions and predict members of the C2nO-, C2nS-, C2n-1Si-, HC2nN-, CnP-, and C2n chains as outstanding candidates for astronomical observation including the higher members of the C2nH- and C2n-1N- groups whose lower members have been observed. From high level ab initio quantum chemical calculations; ZPE and Boltzmann factor have been used to explain the observed deuterium enhancement and the possibility of detecting more deuterated species in ISM. Though all the heterocycles that have so far been searched for in ISM have been shown to be the right candidates for astronomical observation as discussed in the ESA relationship, they have also been shown to be strongly bonded to the surface of the interstellar dust grains thereby reducing their abundances, thus, contributing to their unsuccessful detection except for furan which is less affected by hydrogen bonding. The D-analogues of the heterocycles are shown from the computed Boltzmann factor to be formed under the dense molecular cloud conditions where major deuterium fractionation dominates implying very high D/H ratio above the cosmic D/H ratio which suggests the detectability of these deuterated species. Chapter seven examines the isomerization of the most stable isomer (which is probably the most abundant) to the less stable isomer(s) as one of the plausible formation routes for interstellar molecular species. An extensive investigation on the isomerization enthalpies of 243 molecular species from 64 isomeric groups is reported. From the results, the high abundances of the most stable isomers coupled with the energy sources in interstellar medium drive the isomerization process even for relative enthalpy difference as high as 67.4 kcal/mol. Specifically, the cyanides and their corresponding isocyanides pairs appear to be effectively synthesized via this process. The following potential interstellar molecules; CNC, NCCP, c-C5H, methylene ketene, methyl Ketene, CH3SCH3, C5O, 1,1-ethanediol, propanoic acid, propan-2-ol and propanol are identified and discussed. In all the isomeric groups, isomerization appears to be an effective route for the formation of the less stable isomers (which are probably less abundant) from the most stable ones. Chapter eight summarizes the conclusions drawn from the different studies presented in this Thesis and also highlights some of the future directions of these studies. The first appendix presents the preliminary study on Isoprene...Ar weakly bound complex while the second appendix contains a study on interstellar C3S describing the importance of accurate dipole moment in calculating interstellar abundances of molecular species and in astrophysical and astronomical models.

The heliosphere and solar magnetic field play an important role in protecting the solar system from harmful, high-energy Galactic radiation. Until recently, the magnetic field had been assumed to be passive, carried outwards by the solar wind. The influence of the solar magnetic field on the plasma has just begun to be understood. Among the consequences, the magnetic field could cause the heliotail to be short, collimating the flow into two lobes instead of the classical long, comet-like tail. In this dissertation, I investigate the role certain aspects of the magnetic field have on the heliosphere and detail how interstellar neutral particles alter its effect on the environment. From the observation by Voyager 1 (V1) and Voyager 2 (V2), it is clear that the plasma environment in the outer heliosphere is not fully understood. I present the first time-dependent model of the outer heliosphere that includes solar-cycle variations of the magnetic field strength. I find that the model can accurately predict the plasma environment at V2 but cannot describe all features observed at V1, suggesting additional processes are present. The effect of including the heliospheric current sheet (HCS) on large-scale modeling of the heliosphere is also studied. The inherent numerical dissipation in the HCS reduces the magnetic field strength in the heliosheath; however, the two-lobe structure of the heliotail remains. Neutral hydrogen has also been shown to greatly affect the location of the heliospheric boundaries. The large mean free path of these neutrals requires them to be described kinetically. To understand how the neutrals affect the influence of the solar magnetic field, I developed the Solar-wind with Hydrogen Ion Exchange and Large-scale Dynamics (SHIELD) model, a kinetic-magnetohydrodynamic model of the outer heliosphere. The model couples a 3D Monte-Carlo model to the magnetohydrodynamic solver. SHIELD reproduces the results of similar models, namely a higher filtration of neutrals into the heliosphere when compared to a fluid description of the atoms. When SHIELD is applied to the heliotail, the two-lobe structure persists even with kinetic neutrals. These results show that the solar magnetic field plays a crucial role in determining the heliospheric structure.

Der erste Teil der Aktivitäten dieser Arbeit bestand in der Entwicklung einer modernen Ionenspeicher Apparatur zur Untersuchung chemischer Prozesse mit atomarem Wasserstoff. Die Integration eines differentiell gepumpten Radikalenstrahls in eine vorhandene temperaturvariable 22-Pol Speicherapparatur erforderte größere Änderungen an dieser. Da astrophysikalische Fragestellungen im Vordergrund standen, führt die Einleitung zunächst in das Gebiet der Astrophysik und -chemie ein. Die Grundlagen der Ionenspeicherung in temperaturvariablen Hf-Speichern sind ausführlich in der Literatur dokumentiert. Daher ist die Beschreibung der Apparatur (Kapitel 2) relativ kurz gehalten. Viel Mühe wurde in die Entwicklung einer intensiven und stabilen Quelle für Wasserstoffatome aufgewandt, deren kinetische Energie variiert werden kann. Das Kapitel 3 beschreibt dieses Modul in vielen Details, wobei der Einsatz von magnetischen Hexapolen zum Führen der Atome und die chemische Behandlung der Oberflächen zur Reduzierung der H-H Rekombination einen wesentlichen Platz einnimmt. Durch die außergewöhnliche Empfindlichkeit der Speichertechnik kann das neue Instrument zur Untersuchung von vielen Reaktionen eingesetzt werden, die von astrochemischer und fundamentaler Bedeutung sind. Die Ergebnisse dieser Arbeit sind im Kapitel 4 zusammengestellt, einige Reprints und Entwürfe von Publikationen findet man im Anhang. Die Reaktionen von CO2+ mit Wasserstoffatomen und -molekülen erwiesen sich als sehr geeignet, um in situ H and H2 Dichten über den gesamten Temperaturbereich der Apparatur zu bestimmen (10 K - 300 K). Zum ersten mal wurden Reaktionen von H- and D-Atomen mit den Kohlenwasserstoffionen CH+, CH2+, and CH4+ bei Temperaturen des interstellaren Raums untersucht. Ein sehr interessantes, noch nicht ganz verstandenes Stoßsystem ist die Wechselwirkung von protoniertem Methan mit H-Atomen. Im Ausblick der Arbeit werden einige Ideen aufgezeigt, wie man das Instrument verbessern kann, und es werden einige Reaktionen erwähnt, die man als nächste untersuchen könnte. Diese Dissertation ist einen Beitrag zum Projekt 5 der Forschergruppe Laboratory Astrophysics: Structure, Dynamics and Properties of Molecules and Grains in Space, die von der DFG im Zeitraum von 2000 bis 2006 unterstützt wurde.
The first part of the activities of this thesis was to develop a sophisticated ion storage apparatus dedicated to study chemical processes with atomic hydrogen. The integration of a differentially pumped radical beam source into an existing temperature variable 22-pole trapping machine has required major modifications. Since astrophysical questions have been in the center of our interest, the introduction first gives a short overview of astrophysics and -chemistry. The basics of ion trapping in temperature variable rf traps is well-documented in the literature; therefore, the description of the basic instrument (Chapter 2) is kept rather short. Much effort has been put into the development of an intense and stable source for hydrogen atoms the kinetic energy of which can be changed. Chapter 3 describes this module in detail with emphasis on the integration of magnetic hexapoles for guiding the atoms and special treatments of the surfaces for reducing H-H recombination. Due to the unique sensitivity of the rf ion trapping technique, this instrument allows one to study a variety of reactions of astrochemical and fundamental interest. The results of this work are summarized in Chapter 4, some reprints and drafts are reproduced in the appendix. Reactions of CO2+ with hydrogen atoms and molecules have been established as calibration standard for in situ determination of H and H2 densities over the full temperature range of the apparatus (10 K - 300 K). For the first time, reactions of H- and D-atoms with the ionic hydrocarbons CH+, CH2+, and CH4+ have been studied at temperatures of interstellar space. A very interesting, not yet fully understood collision system is the interaction of protonated methane with H. The outlook presents some ideas, how to improve the new instrument and a few reaction systems are mentioned which may be studied next. This thesis is a contribution to the project 5 of the research unit Laboratory Astrophysics: Structure, Dynamics and Properties of Molecules and Grains in Space which has been supported by the DFG from 2000 to 2006.

Multiple coherent supernova explosions (SNe) in an OB association can produce a strong shock that moves through the interstellar medium (ISM). These shocks fronts carve out hot and tenuous regions in the ISM known as superbubbles. The density contour plot at three different times (0.5 Myr (left panel), 4 Myr (middle panel), 9.5 Myr (right panel)) showing different stages of superbubble evolution for n0 = 0.5 cm−3, z0 = 300 pc, and for NOB = 104. This density contour plot is produced using ZEUS-MP 2D hydrodynamic simulation with a resolution of 512 × 512 with a logarithmic grid extending from 2 pc to 2.5 kpc. For a detailed description of this figure, see Roy et. al., 2015. The evolution of a superbubble is marked by different phases, as it moves through the ISM. Consider an OB association at the center of a disk galaxy. Initially the distance of the shock front is much smaller than the disk scale height. The superbubble shell sweeps up the ISM material, and once the amount of swept up material becomes comparable to the ejected material during SNe, the superbubble enters a self-similar phase (analogous to the Sedov-Taylor phase of individual SNe). As the superbubble shell sweeps up material, its velocity decreases, and thus the corresponding post-shock temperature drops. At a temperature of ∼ 2 × 105 K (where the cooling function peaks), the superbubble shell becomes radiative and starts losing energy via radiative cooling. This radiative phase is shown in the left panel of Figure 1. The superbubble shell starts fragmenting into clumps and channels due to Rayleigh-Taylor instabilities (RTI) (which is seeded by the thermal instability; for details see Roy et. al., 2013) when the superbubble shell crosses a few times the scale height. This is represented in the middle panel of the same figure. At a much later epoch, RTI has a strong effect on the shell fragmentation and the top of the bubble is completely blown off (the right panel). In the first chapter of the thesis (reported in Sharma et. al., 2014), we show using ZEUS-MP hydrodynamic simulations that an isolated supernova loses almost all its mechanical energy within a Myr whereas superbubbles can retain up to ∼ 40% of the input energy over the lifetime of the starcluster (∼ few tens of Myr), consistent with the analytic estimate of the second chapter. We also compare different recipes (constant luminosity driven model (LD model), kinetic energy driven model (KE model) to implement SNe feedback in numerical simulations. We determine the constraints on the injection radius (within which the SNe input energy is injected) so that the supernova explosion energy realistically couples to the interstellar medium (ISM). We show that all models produce similar results if the SNe energy is injected within a very small volume ( typically 1–2 pc for typical disk parameters). The second chapter concentrates on the conditions for galactic disks to produce superbubbles which can give rise to galactic winds after breaking out of the disk. The Kompaneets formalism provides an analytic expression for the adiabatic evolution of a superbubble. In our calculation, we include radiative cooling, and implement the supernova explosion energy in terms of constant luminosity through out the life-time of the OB stars in an exponentially stratified medium (Roy et. al., 2013). We use hydrodynamic simulations (ZEUS-MP) to determine the evolution of the superbubble shell. The main result of our calculation is a clear demarcation between the energy scales of sources causing two different astrophysical phenomenon: (i) An energy injection rate of ∼ 10−4 erg cm−2 s−1 (corresponding Mach number ∼ 2–3, produced by large OB associations) is relevant for disk galaxies with synchrotron emitting gas in the extra-planar regions. (ii) A larger energy injection scale ∼ 10−3 erg cm−2 s−1, or equivalently a surface density of star formation rate ∼ 0.1 M⊙ yr−1 kpc−2 corresponding to superbubbles with high Mach number (∼ 5–10) produces galactic-scale superwinds (requires superstar clusters to evolve coherently in space and time). The stronger energy injection case also satisfies the requirements to create and maintain a multiphase halo (matches with observations). Roy et. al., 2013 also points out that Rayleigh-Taylor instability (RTI) plays an important role in the fragmentation of superbubble shell when the shell reaches a distance approximately 2–3 times the scale-height; and before the initiation of RTI, thermal instability helps to corrugate the shell and seed the RTI. Another important finding of this chapter is the analytic estimation of the energetics of superbubble shell. The shell retains almost ∼ 30% of the thermal energy after the radiative losses at the end of the lifetime of OB associations. The third chapter considers the escape of hydrogen ionizing (Lyc) photons arising from the central OB-association that depends on the superbubble shell dynamics. The escape fraction of Lyc photons is expected to decrease at an initial stage (when the superbubble is buried in the disk) as the dense shell absorbs most of the ionizing photons, whereas the subsequently formed channels (created by RTI and thermal instabilities) in the shell creates optically thin pathways at a later time (∼ 2–3 dynamical times) which help the ionizing photons to escape. We determine an escape fraction (fesc) of Lyc photons of ∼ 10 ± 5% from typical disk galaxies (within 0 ≤ z (redshift) ≤ 2) with a weak variation with disk masses (reported in Roy et. al., 2015). This is consistent with observations of local galaxies as well as constraints from the epoch of reionization. Our work connects the fesc with the fundamental disk parameters (mid-plane density (n0), scale-height (z0)) via a relation that fescαn20z03 (with a ≈ 2.2) is a constant. In the fourth chapter, we have considered a simple model of molecule formation in the superbubble shells produced in starburst nuclei. We determine the threshold conditions on the disk parameters (gas density and scale height) for the formation of molecules in superbubble shells breaking out of disk galaxies. This threshold condition implies a gas surface density of ≥ 2000 M⊙ pc−2, which translates to a SFR of ≥ 5 M⊙ yr−1 within the nuclear region of radius ∼ 100 pc, consistent with the observed SFR of galaxies hosting molecular outflows. Consideration of molecule formation in these expanding superbubble shells predicts molecular outflows with velocities ∼ 30–40 km s−1 at distances ∼ 100–200 pc with a molecular mass ∼ 106–107 M⊙, which tally with the recent ALMA observations of NGC 253. We also consider different combinations of disk parameters and predict velocities of molecule bearing shells in the range of ∼ 30–100 km s−1 with length scales of ≥ 100 pc, in rough agreement with the observations of molecules in NGC 3628 and M82 (Roy et. al., 2016, submitted to MNRAS).