Academic literature on the topic 'Hydrogen nebulae'

Molecules and/or neutral hydrogen have been detected in a modest number of planetary nebulae. However, when detected, these indicators of a significant neutral component can provide fundamental information on the total mass of the envelope, its chemistry and kinematics, and on the morphology and evolutionary status of the planetary nebula. A review of recent results is presented, giving emphasis to CO (carbon monoxide), H2 (molecular hydrogen), OH (hydroxil), and HI (neutral hydrogen). A major development of the last few years has been the capability to map with considerable angular resolution these species in planetary nebulae. In the best studied cases, the neutral component appears to be located surrounding the “waist” of a bipolar planetary nebulae. These is evidence suggesting that molecules and neutral hydrogen are not as uncommon in planetary nebula as the present statistics suggest. Indeed, the outer parts of a considerable fraction of the known planetary nebulae could be neutral. It is possible that a combination of good selection criteria and long integrations with the best telescopes could increase largely the number of known cases.

AbstractNew spectra of A78 and A58 at different positions in the nebulae are presented. An abundance gradient is found in A78, extending quite close to the center. Similarly the nebulous knot near the center of A58 has considerably higher heavy element abundances than the outer regions of this nebula. The ionization state is considerably lower in A58 than A78. In A78 most of the neon is in the form of Ne+3 and Ne+4, indicating that the standard ionization correction factor as used by Jacoby and Ford, is substantially in error. Finally, the very high infrared excesses found in this nebulae are discussed.

The continuum radio spectra of planetary nebulae are discussed, and the structure of these objects is examined from the observed aperture synthesis brightness distributions determined with the Very Large Array. The use of radio observations in determining distances to planetary nebulae is examined. The detection of atomic neutral hydrogen at λ21 cm associated with planetary nebulae, as well as the associated CO and OH components are discussed. An upper limit, of the nebular magnetic field associated with the neutral material, of 1mG is reported for NGC 6302.

Studies of hot white dwarf samples constrain the properties and evolution of planetary nuclei and the nebulae. In particular, the white dwarf and planetary nebulae formation rates are compared. I discuss the overlap of the sequences of white dwarfs having hydrogen (DA) and helium-rich (DO) atmospheres with known central stars of high surface gravity. There is evidence that the hydrogen atmosphere nuclei have “thick” outer hydrogen layers (≳ 10−4 M⊙), but that DA white dwarfs may have surface hydrogen layers orders of magnitude thinner. Finally, a DA planetary nucleus is discussed (0950+139) which has undergone a late nebular ejection; this object may be demonstrating that a hydrogen layer can be lost even after the star has entered the white dwarf cooling sequence.

A non-negligible (∼ 15–20%) fraction of planetary nebulae is expected to be formed in close binaries in which one component fills its Roche lobe after the exhaustion of hydrogen or helium at its center. The nebula is ejected as a consequence of a frictional interaction between the stellar cores and a common envelope; the ionizing component of the central binary star may be a relatively high luminosity contracting star with a degenerate CO core, burning hydrogen or helium in a shell, or it may be a lower luminosity shell hydrogen-burning star with a degenerate helium core or a core helium-burning star. Even more exotic ionizing central stars are possible. Once the initial primary has become a white dwarf or neutron star, the secondary, after exhausting central hydrogen, will also fill its Roche lobe and eject a nebular shell in a common envelope event. The secondary becomes the ionizing star in a tight orbit with its compact companion. In all, there are roughly twenty different possibilities for the make-up of binary central stars, with the ionizing component being a post asymptotic giant branch star with a hydrogen- or helium-burning shell, a CO dwarf, a core helium-burning star, a shell helium-burning star with a degenerate CO core, a shell hydrogen-burning star with a degenerate helium core, or a helium degenerate dwarf, while its companion is a main sequence star, a CO degenerate dwarf, a helium star, a helium degenerate dwarf, or a neutron star. We estimate the occurrence frequency of several of these types and comment on the prior evolutionary history of 4 observed binary central stars.

Molecular hydrogen (H 2 ) emission is commonly detected in planetary nebulae (PNe), specially in objects with bipolar morphologies. New studies showed that H 2 gas is also packed in microstructures embedded in PNe of any morphological type. Despite the presence of H 2 in cometary knots being known for years, only in the last five years, much deeper imagery of PNe have revealed that H 2 also exists in other types of low-ionisation microstructures (LISs). Significant differences are found between the host PNe of cometary knots and other types of LISs, such as nebula age, central star temperature (evolutionary stage) and the absolute sizes of the microstructure itself.

We present the results of a spectroscopic study of planetary nebulae (PN) in the Magellanic Clouds. The optical survey of He, N, O, and Ne abundances by Monk et al. (1988) has been updated by higher S/N AAT optical data. In addition, carbon and other elemental abundances have been derived from the IUE spectra of 38 PN. Ionized nebular masses have been derived for 80 PN. The ionised mass versus nebular electron density plot shows that planetary nebulae become optically thin when their electron densities drop below 4500 cm--3. Below this density, the mean nebular hydrogen mass found for non-Type I PN is 0.22±0.08 M⊙. Using Zanstra and energy-balance methods, the mean central star mass found for 14 SMC and LMC PN is 0.59±0.02 M⊙.

Context. The first high-resolution X-ray spectroscopy of a planetary nebula, BD +30° 3639, opened the possibility to study plasma conditions and chemical compositions of X-ray emitting “hot” bubbles of planetary nebulae in much greater detail than before. Aims. We investigate (i) how diagnostic line ratios are influenced by the bubble’s thermal structure and chemical profile, (ii) whether the chemical composition inside the bubble of BD +30° 3639 is consistent with the hydrogen-poor composition of the stellar photosphere and wind, and (iii) whether hydrogen-rich nebular matter has already been added to the bubble of BD +30° 3639 by evaporation. Methods. We applied an analytical, one-dimensional (1D) model for wind-blown bubbles with temperature and density profiles based on self-similar solutions including thermal conduction. We also constructed heat-conduction bubbles with a chemical stratification. The X-ray emission was computed using the well-documented CHIANTI code. These bubble models are used to re-analyse the high-resolution X-ray spectrum from the hot bubble of BD +30° 3639. Results. We found that our 1D heat-conducting bubble models reproduce the observed line ratios much better than plasmas with single electron temperatures. In particular, all the temperature- and abundance-sensitive line ratios are consistent with BD +30° 3639 X-ray observations for (i) an intervening column density of neutral hydrogen, NH = 0.20-0.10+0.05 × 1022cm−2, (ii) a characteristic bubble X-ray temperature of TX = 1.8 ± 0.1 MK together with (iii) a very high neon mass fraction of about 0.05, virtually as high as that of oxygen. For lower values of NH, we cannot exclude the possibility that the hot bubble of BD +30° 3639 contains a small amount of “evaporated” (or mixed) hydrogen-rich nebular matter. Given the possible range of NH, the fraction of evaporated hydrogen-rich matter cannot exceed 3% of the bubble mass. Conclusions. The diffuse X-ray emission from BD +30° 3639 can be well explained by models of wind-blown bubbles with thermal conduction and a chemical composition equal to that of the hydrogen-poor and carbon-, oxygen-, and neon-rich stellar surface.

The Far Ultraviolet Spectroscopic Explorer (FUSE)> satellite provides a unique opportunity to obtain high-resolution far-UV spectra of a wide variety of astronomical objects, including planetary nebulae. Most FUSE observations of PNe to date have concentrated on the hot central star, providing a very effective way to study the atmosphere of the central star, the surrounding nebula through the absorption features from circumstellar gas. FUSE has found evidence of hot molecular hydrogen in several planetary nebulae, including M27 and BD+30° 3639. Central star spectra also reveal new information about stellar winds, mass loss, and photospheric abundances.

AbstractThe SMC harbours a class of hot nitrogen-sequence Wolf-Rayet stars (WNE) that display only relatively weak broad Heiiλ4686 emission indicative of their low mass-loss rates and which are therefore hard to detect. However, such stars are possible emitters of strong He+Lyman continua which in turn could ionize observable Heiiiregions, i.e. highly excited Hiiregions emitting nebular Heiiλ4686 emission. We here report the discovery of a second Heiiiregion in the SMC within OB association NGC 249 within which the weak-lined WN star SMC WR10 is embedded. SMC WR10 is of special importance since it is a single star showing the presence of atmospheric hydrogen. While analysing the spectrum in the framework of two popular, independent WR atmosphere models we found strongly discrepant predictions (by 1 dex) of the He+continuum for the same input parameters. A second interesting aspect of the work reported here concerns the beautiful MCELS images which clearly reveal a class of strongly Oiiiλ5007 emitting (blue-coded) nebulae. Not unexpectedly, most of the “blue” nebulae are known Wolf-Rayet bubbles, but new bubbles around a few WRs are also detected. Moreover, we report the existence of blue nebulae without associated known WRs and discuss the possibility that they reveal weak-wind WR stars with very faint stellar Heiiλ4686 emission. Alternatively, such nebulae might hint at the hitherto missing population of relatively low-mass, hot He stars predicted by massive binary evolution calculations. Such a binary system is probably responsible for the ionization of the unique Heiiλ4686-emitting nebula N 44C.

Central stars of planetary nebulae are low-mass stars on the brink of their final evolution towards white dwarfs. Because of their surface temperature of above 25,000 K their UV radiation ionizes the surrounding material, which was ejected in an earlier phase of their evolution. Such fluorescent circumstellar gas is called a "Planetary Nebula". About one-tenth of the Galactic central stars are hydrogen-deficient. Generally, the surface of these central stars is a mixture of helium, carbon, and oxygen resulting from partial helium burning. Moreover, most of them have a strong stellar wind, similar to massive Pop-I Wolf-Rayet stars, and are in analogy classified as [WC]. The brackets distinguish the special type from the massive WC stars. Qualitative spectral analyses of [WC] stars lead to the assumption of an evolutionary sequence from the cooler, so-called late-type [WCL] stars to the very hot, early-type [WCE] stars. Quantitative analyses of the winds of [WC] stars became possible by means of computer programs that solve the radiative transfer in the co-moving frame, together with the statistical equilibrium equations for the population numbers. First analyses employing models without iron-line blanketing resulted in systematically different abundances for [WCL] and [WCE] stars. While the mass ratio of He:C is roughly 40:50 for [WCL] stars, it is 60:30 in average for [WCE] stars. The postulated evolution from [WCL] to [WCE] however could only lead to an increase of carbon, since heavier elements are built up by nuclear fusion. In the present work, improved models are used to re-analyze the [WCE] stars and to confirm their He:C abundance ratio. Refined models, calculated with the Potsdam WR model atmosphere code (PoWR), account now for line-blanketing due to iron group elements, small scale wind inhomogeneities, and complex model atoms for He, C, O, H, P, N, and Ne. Referring to stellar evolutionary models for the hydrogen-deficient [WC] stars, Ne and N abundances are of particular interest. Only one out of three different evolutionary channels, the VLTP scenario, leads to a Ne and N overabundance of a few percent by mass. A VLTP, a very late thermal pulse, is a rapid increase of the energy production of the helium-burning shell, while hydrogen burning has already ceased. Subsequently, the hydrogen envelope is mixed with deeper layers and completely burnt in the presence of C, He, and O. This results in the formation of N and Ne. A sample of eleven [WCE] stars has been analyzed. For three of them, PB 6, NGC 5189, and [S71d]3, a N overabundance of 1.5% has been found, while for three other [WCE] stars such high abundances of N can be excluded. In the case of NGC 5189, strong spectral lines of Ne can be reproduced qualitatively by our models. At present, the Ne mass fraction can only be roughly estimated from the Ne emission lines and seems to be in the order of a few percent by mass. Furthermore, using a diagnostic He-C line pair, the He:C abundance ratio of 60:30 for [WCE] stars is confirmed. Within the framework of the analysis, a new class of hydrogen-deficient central stars has been discovered, with PB 8 as its first member. Its atmospheric mixture resembles rather that of the massive WNL stars than of the [WC] stars. The determined mass fractions H:He:C:N:O are 40:55:1.3:2:1.3. As the wind of PB 8 contains significant amounts of O and C, in contrast to WN stars, a classification as [WN/WC] is suggested.
Zentralsterne Planetarischer Nebel sind massearme Sterne kurz vor ihrer finalen Entwicklung zu Weißen Zwergen. Aufgrund ihrer Oberflächentemperatur von über 25 000 K sind sie in der Lage, durch Abstrahlung von UV-Licht das sie umgebende Material, welches in einer vorigen Phase ihrer Entwicklung abgestoßen wurde, zu ionisieren. Das solchermaßen zum Leuchten angeregte Gas bezeichnet man als Planetarischen Nebel. Etwa ein Zehntel der galaktischen Zentralsterne sind wasserstoffarm. Im Allgemeinen besteht die Oberfläche dieser Zentralsterne aus einer Mischung der Elemente Helium, Kohlenstoff und Sauerstoff, welche z.T. durch Heliumbrennen erzeugt wurden. Die meisten dieser Sterne haben darüberhinaus einen starken Sternwind, ähnlich den massereichen Pop-I-Wolf-Rayet-Sternen und werden in Analogie zu diesen als [WC] klassifiziert, wobei die eckigen Klammern der Unterscheidung von den massereichen WC-Sternen dienen. Qualitative Analysen der Spektren von [WC]-Sternen lassen eine Entwicklungssequenz dieser Sterne von kühleren sogenannten late-type [WC]-Sternen (kurz [WCL]) zu sehr heißen, early-type [WC]-Sternen (kurz [WCE]) vermuten. Mithilfe von Computerprogrammen, die den Strahlungstransport im mitbewegten Beobachtersystem zusammen mit den statistischen Gleichungen der Besetzungszahlen der Ionen im Sternwind rechnen können, wurden quantitative Untersuchungen der Winde von [WC]-Sternen möglich. Erste Analysen mit Modellen ohne Eisenlinien ergaben dabei systematisch unterschiedliche Häufigkeiten für [WCL]- und [WCE]-Sterne. Während sich für [WCL]-Sterne ein Verhältnis der Massenanteile von He:C von etwas 40:50 ergab, fand man für die [WCE]-Sterne ein mittleres Verhältnis von 60:30 für die He:C-Massenanteile. Dabei sollte die Entwicklung von [WCL] nach [WCE] innerhalb einer sehr kurzen Zeit durch Aufheizung infolge der Kontraktion der Hülle erfolgen und nicht mit einer wesentlichen Abnahme der Kohlenstoffhäufigkeit bei gleichzeitiger Zunahme der Heliumhäufigkeit an der Oberfläche einhergehen. Im Rahmen der vorgelegten Arbeit wird untersucht, ob sich mittels verbesserter Modelle für die Atmosphären von [WC]-Sternen das He:C-Häufigkeitsverhältnis der [WCE]-Sterne bestätigt. Elaboriertere Modelle, welche vom Potsdamer WR-Modelatmosphären-Code (PoWR) berechnet werden können, berücksichtigen Line-Blanketing aufgrund von Elementen der Eisengruppe, kleinskalige Windinhomogenitäten und die Elemente He, C, O, H, P, N und Ne. Unter Bezug auf Sternentwicklungsmodelle, die die Ursache der Wasserstoffunterhäufigkeit von [WC]-Sternen erklären, sind insbesondere die Neon- und Stickstoff-Häufigkeiten interessant. Von den drei möglichen Entwicklungskanälen für [WC]-Sterne führt lediglich das VLTP-Szenario zu einer Stickstoff-Überhäufigkeit von einigen Prozent bezogen auf die Masse. Bei einem VLTP, einem very late thermal pulse, handelt es sich um einen plötzlichen, starken Anstieg der Energieproduktion in der helium-brennenden Schale, während das Wasserstoffbrennen bereits zum Erliegen gekommen ist. Infolge eines VLTPs wird sämtlicher Wasserstoff kurz nach dem thermischen Puls in tiefere Schichten gemischt und in Anwesenheit von C, He und O verbrannt. Infolgedessen wird N und auch Ne erzeugt. Bei der Analyse von elf [WCE]-Sternen wurden für drei von ihnen, PB 6, NGC 5189 und [S71d]3, Stickststoffmassenanteile von 1,5 % bestimmt, während für drei andere Sterne solche hohen Stickstoffhäufigkeiten ausgeschlossen werden können. Für NGC 5189 gelang außerdem die qualitative Reproduktion der beobachteten, starken Ne-Spektrallinien mittels unserer Modelle. Zur Zeit lässt sich aus der Stärke der Ne-Emissionslinien der Ne-Massenanteil leider nur abschätzen, er scheint aber im Bereich einiger Prozent zu liegen. Mittels eines diagnostischen He-C-Linienpaares konnte das He:C-Massenverhältnis von 60:30 für [WCE]-Sterne bestätigt werden. Als Ergebnis der Analyse von PB 8 postulieren wir eine neue Klasse von wasserstoffarmen Zentralsternen, die in ihrer Elementzusammensetzung eher an massereiche WNL-Sterne als an [WC]-Sterne erinnern. Die ermittelten Massenanteile H:He:C:N:O betragen 40:55:1.3:2:1.3, der Wind von PB 8 enthält daher im Unterschied zu WN-Sternen signifikante Mengen von O und C. Es wird daher eine Klassifizierung als [WN/WC] vorgeschlagen.

This thesis is concerned with the structure of shocks occurring in dense regions of molec?ular clouds. These shocks are associated with the outflows from young stars, Herbig-Haro objects, expanding HII regions and the interaction of supernovae remnants with molecu?lar clouds. Momentum, mass and energy are imparted to the cloud. A full understanding of the shock process is thus needed if we are to understand the structure of molecular clouds and the impact on star formation. Emission from the near-infrared transitions of molecular hydrogen is commonly excited in these shocks. A major puzzle is that emis?sion is seen at velocities that would collisionally dissociate molecular hydrogen, and this is a central question that this thesis seeks to answer. This is approached observationally by trying to relate the observed emission to shock models. Fairly accurate semi-analytic derivations of the emission spectrum expected from hydrodynamic and magnetohydrodynamic molecular shocks are used to fully explore the parameter space of the initial conditions, without resort to expensive numerical calculations. The emission spectrum is then related to that observed. Most of this work is based on a spectroscopic multi-line study of the near-infrared H2 emission in two sources, the Orion outflow and the supernova remnant IC443. These observations are then compared with those expected from the models. In both sources it is found that planar hydrodynamic jump-type shocks (J) are consistent with the new observations. Whplanar magnetically moderated continuous shocks (C), which have been invoked to explain the emission from the shock in Orion, are not. Neither shock types can explain the intensities of CO rotational lines and the H2 line ratios simultaneously. The high velocities that are observed still present a problem. In IC443 the conclusion is the same but, in addition, the pressure needed to explain the observations is higher than that observed in the supernova remnant. It is suggested that this discrepancy may naturally occur when radiative shocks are driven through a clumpy medium. This approach of using line ratios as shock discriminators is extended by velocity resolved spectroscopy of three highly excited emission lines from Orion. These observa?tions demonstrate that there are no discernible differences in the line ratios with velocity despite the large change in the energies of the upper energy levels involved. It is discussed how this further constrains the shock type and limits the contribution from non-thermal excitation (such as fluorescence). The possible physical processes that could lead to high velocity, shocked molecular hydrogen are then discussed. Models proposed in the past are, it is argued, inadequate. It is then shown that the line ratios observed can be closely matched with non-planar continuous type shocks which occur in a bow shock. The densities and pressures needed are still high. The general conclusions are that previous plane parallel C-shock models invoked to explain the molecular shocks are inconsistent with the observations. The line ratios imply that either J-type shocks, in which the cooling takes a long time compared to the initial heating, or C-type bow shocks which produce a range of temperatures are responsible for the emission. It is finally suggested that C-shocks in gas with a very high magnetic field can produce the high velocity H2 emission observed without dissociating the molecules.

O objetivo deste trabalho é o estudo das condições de existência e a determinação da concentração da molécula H2 em diferentes condições típicas de nebulosas planetárias, dentro da região ionizada. Para este cálculo, desenvolvemos sub-rotinas computacionais que se acoplam ao código de fotoionização unidimensional Aangaba que, até agora, somente considerava espécies atômicas (H, He, C, N, O, Mg, Ne, Si, S, Ar, Cl e Fe) e seus íons. Inserimos nesse código os equilíbrios químico e de ionização envolvendo a molécula H2 e os demais compostos de hidrogênio, H-, H2+, H3+, além do H, H+ e dos elétrons que o código de fotoionização Aangaba já considerava em sua forma original. A molécula H3 não é considerada por ser instável. Levamos em conta 41 diferentes mecanismos de formação e destruição desses compostos do hidrogênio. Destacamos particularmente o efeito da reação de formação de H2 na superfície de grãos na produção global dessa molécula em nebulosas planetárias, considerada na literatura como a rota mais importante de formação dessa molécula no meio interestelar. Para isso, estudamos a possibilidade da sobrevivência de grãos dentro da região ionizada da nebulosa planetária. Analisamos também a influência das propriedades da estrela central e da densidade do gás, assim como das propriedades dos grãos astrofísicos, na concentração de H2. Demonstramos que quantidades significativas de H2 podem sobreviver dentro da região ionizada de nebulosas planetárias, principalmente na região de recombinação do hidrogênio. A concentração de H2 relativa à densidade total de H alcança valores de até 1E-4 e a razão entre a massa de H2 e a massa total de H da NP chega a valores de 4E-4. Verificamos que a razão entre a massa de H2 e a massa de H total da nebulosa aumenta significativamente com o aumento da temperatura de estrela central. Essa maior quantidade de H2 em nebulosas planetárias com estrela central mais quente pode explicar porque é mais comum encontrar emissão da molécula H2 em nebulosas planetárias com estrutura bipolar (regra de Gatley), já que nebulosas com esse tipo morfológico têm estrela central tipicamente mais quente. Na literatura, o valor 6,9E-5 é obtido para a razão entre a massa de H2 e a massa de H total da nebulosa planetária NGC 6720, a partir de dados observacionais. Usando os mesmos parâmetros deste artigo, calculamos com o código de fotoionização Aangaba o valor de 3,3E-5, que está razoavelmente próximo do valor da literatura.
The goal of this work is the study of the H2 molecule survival and the determination of its abundance in different typical planetary nebulae conditions inside the ionized region. In order to do these calculations, we developed Fortran subroutines for the Aangaba one-dimensional photoionization code that, until this work, only took into account the atomic species (H, He, C, N, O, Mg, Ne, Si, S, Ar, Cl, and Fe) and their ions. Ionization and chemical equilibria of H, H+, H-, H2, H2+, and H3+ are assumed. The H3 molecule is not included because it is unstable. Fortyone different reactions that could form and destroy these species are taken into account. Reaction on grain surfaces, the most important mechanism for the production of H2 molecules in the interstellar medium, is analyzed in detail in the conditions of planetary nebulae ionized regions. We make a careful analysis of the grain survival in these regions. We also study the influence of the central star properties and gas density, as well as the astrophysical grain properties in the obtained H2 concentration. It is shown that a significant concentration of H2 can exist inside the ionized region of planetary nebulae, mostly in the recombination zone. The H2 concentration relative to the total hydrogen concentration reaches values as high as 1E-4 and the H2 mass to total hydrogen mass ratio inside the ionized region reaches values as high as 4E-4. The ratio increases with increasing temperature. This fact can explain why the H2 emission is more often observed in bipolar planetary nebulae (Gatley?s rule), since this kind of object has typically hotter stars. In the literature a H2 mass to total hydrogen mass ratio equal to 6.9E-5 is estimated from observations for the planetary nebula NGC6720. With the same input parameters for the gas density and the stellar spectrum, we calculated a ratio equal to 3.3E-5, close to the observed value.

Na literatura, a análise e a interpretação das linhas de emissão de H2 em nebulosas planetárias são feitas, em geral, considerando que a molécula somente seja produzida em ambientes neutros, como as regiões de fotodissociação ou com choques. No entanto, existem fortes evidências observacionais de que ao menos parte da emissão seja proveniente da região ionizada desses objetos. Em trabalhos anteriores mostramos que quantidades significativas de H2 podem sobreviver dentro dessa região hostil. No presente trabalho nosso objetivo é calcular e estudar a emissão de H2 em linhas no infravermelho produzidas na região ionizada de nebulosas planetárias, utilizando o código de fotoionização unidimensional Aangaba. Para isso, desenvolvemos diversas sub-rotinas que determinam o povoamento em níveis de energia da molécula e calculam a intensidade das linhas de emissão de H2. Obtivemos a intensidade de diversas linhas produzidas pela molécula H2 em nebulosas planetárias cujos parâmetros característicos (temperatura e luminosidade da estrela central, densidade do gás, tipo e tamanho dos grãos, etc.) estão na faixa de valores conhecidos para esses objetos. Como resultado de nosso trabalho, mostramos que a contribuição da região ionizada para a emissão de H2 de nebulosas planetárias pode ser significativa em diversas situações, particularmente quando a temperatura da estrela central é alta. Esse resultado pode explicar porque a detecção de linhas de H2 é mais provável em nebulosas planetárias bipolares, que têm estrelas tipicamente mais quentes. Além disso, verificamos que na região ionizada a excitação e a desexcitação colisional são mecanismos importantes de povoamento de todos os níveis rovibracionais do estado fundamental eletrônico de H2. Os mecanismos radiativos são também importantes, particularmente para os níveis de energia excitados. Os mecanismos de formação em estados excitados podem ter alguma influência no espectro de linhas produzidas pela desexcitação de níveis rotacionais bastante elevados, principalmente em ambientes densos. Em nossos modelos incluímos o efeito da molécula H2 no equilíbrio térmico do gás, verificando que a molécula H2 somente tem influência significativa na temperatura do gás em casos de temperatura da estrela central muito alta ou grande quantidade de grãos, principalmente através da desexcitação colisional.
The analysis and the interpretation of the H2 line emission of planetary nebulae have been done in the literature assuming that the molecule survives only in neutral environments, as in photodissociation or shocked regions. However, there is strong evidence that at least part of the H2 emission is produced inside the ionized region of such objects. In previous work we showed that significant amounts of H2 can survive inside the ionized region of planetary nebulae. The aim of the present work is to calculate and study the infrared line emission of H2 produced inside the ionized region of planetary nebulae using the one-dimensional photoionization code Aangaba. For this, we developed several numerical subroutines in order to calculate the statistical population of the H2 energy levels, as well as the intensity of the H2 infrared emission lines in physical conditions typical of planetary nebulae. We show that the contribution of the ionized region for the H2 emission can be significant, particularly in the case of nebulae with high temperature central stars. This result explains why H2 emission is more frequently observed in bipolar planetary nebulae (Gatley\'s rule), since this kind of object typically has hotter stars. We show that collisional excitation plays an important role on the H2 population of the rovibrational levels of the electronic ground state. Radiative mechanisms are also important, particularly for upper levels. Formation pumping may have some effects on the line intensities produced by de-excitation from very high rotational levels, specially in dense environments. We include the effects of H2 in the thermal equilibrium of the gas, concluding that H2 only contributes to the thermal equilibrium in the case of very high temperature of the central star or high grain to gas ratio, mainly through collisional de-excitation.

Photodissociation regions (PDRs) are described in the context of the interstellar medium and star-forming regions. Observations of PDRs and molecular hydrogen are reviewed and the reflection nebula NGC 2023 is discussed in detail. NGC 2023 is a bright and well-studied reflection nebula at a distance of 450 parsecs in the Orion region. Illuminated primarily by a B-type star, it offers an ideal opportunity to study UV-excited molecular hydrogen. The theory of the hydrogen molecule is described: the energy states and their relationship with the quantum numbers which represent the vibratinal and rotational states of the molecule, the radiative processes which determine the optical and infrared emission spectrum of H₂, the effect collisions have on the excited states of the molecule and the processes which govern the formation and destruction of H₂. Particular attention is given to the process of formation on the surface of dust grains and the resulting energy states of the ejected H₂ molecule. Infrared and optical far-red observations of fluorescent H₂ line emission from NGC 2023 are presented. The resulting datasets contain flux measurements of over ninety lines. These are combined with published data to produce column densities for 81 energy states of the H₂ molecule, the most extensive dataset yet compiled for a PDR. The process of observing in the infrared red optical wavelength regimes are outlined. The emission lines of H₂ are intrinsically very faint and thus measurements require careful data reduction to minimise sources of noise wherever possible. The data reduction steps which were applied to observations are described in detail. An optical extinction of A_v = 5.7 ? 0.5 to the H₂ emission region and ortho/para abundance ratio of 2.0 ± 0.2 are derived from flux ratios of emission lines and by minimising the scatter on a diagram which plots the logarithm of the column density against the energy level of each state.

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.

Calculs scf et ci des courbes de potentiel du systeme s+h**(+) dans les symetriplets pi et delta internenant dans la reaction, avec des orbitales moleculaires occupees representees par des orbitales atomiques polarisees et des orbitales moleculaires virtuelles donnees par la projection d'orbitales atomiques orthogonalement aux orbitales occupees. Traitement de la dynamique de l'echange de charge dans une representation diabatique ou les couplages radiaux sont nuls et l'echange de charge est induit par des couplages de type electronique. Construction d'une representation effective de dimension reduite au nombre des voies ouvertes; calcul des couplages radiaux. Resolution des equations de collision par un traitement quantique dans chaque symetrie; deduction de la constante de vitesse

The motive for this work was to investigate whether small, isolated gas-rich galaxies show evidence of chemical evolution, by studying their nebular metallicities. I have identified a sample of 83 objects chosen for low luminosity and mass, the presence of active star formation, and isolation from other galaxies and galaxy clusters that might generate tidal effects or enrich the intergalactic medium. From these I have measured the spectra of 35 objects, using theWiFeS IFU spectrograph on the ANU 2.3m telescope at Siding Spring. In analysing spectra extracted from the WiFeS data cubes, I found that standard ‘strong line’ methods using emission line ratios to measure atomic abundances, gave either erratic or no results. I found that for those galaxies showing the [O iii] 4363Å auroral line, the metallicities determined using the standard ‘electron temperature’ methodwere inconsistent with previous published work. This led me to investigate the conventional assumption that electrons in Hii regions are in thermal equilibrium. I show that the non-equilibrium ‘ ’ electron energy distribution, found almost universally in solar system plasmas, can explain the long recognised ‘abundance discrepancy’ between recombination line and collisional line abundance calculations in nebular metallicity measurements. This has added an important new dimension to the analysis of nebular spectra. Using the extensively revised Mappings photoionisation modelling code and new atomic data to analyse the spectra of two exceptionally isolated dwarf galaxies, I find that they exhibit metallicities similar to galaxies in more crowded environments, and appear to have evolved quite normally, through periodic star formation and subsequent enrichment of their interstellar media. I present a new approach for calculating total oxygen abundance using electron temperatures that appears to give more consistent results than earlier methods. I apply this to my measured spectra, together with the revised Mappings photoionisation modelling code, to explore the physical parameters affecting the measurement of nebular metallicities. In particular, I find strong evidence for several of the observed nebulae being—in part—optically thin. I use the models to show that nebular optical depth affects measured abundances and temperatures, and that electron densities also have an important role. I develop models that give a very good match to the observations. I conclude that the measurement of abundances and temperatures in Hii regions is a more complex question than had generally been assumed, and important physical parameters affecting the measurement processes have in the past not been taken fully into account.