Circumstellar medium

Circumstellar medium The environment around a star that has chemistry driven by the photon flux or radiation field. 

The study of the effects induced by ion irradiation of solid materials, in particular solid carbons, is relevant in many fields of science and technology. Here we focus on its relevance in astrophysics. Solid carbon-bearing species are extremely abundant in space both in the gas and in the solid phases. A wide variety of solid carbons are observed in the interstellar and circumstellar medium as well as in many objects of the Solar System including those collected at or nearby Earth (interplanetary dust particles and meteorites). Observed and/or predicted carbon-bearing solids (or large molecules) include species with different hybridizations (sp, sp, sp" ) such as amorphous carbons, polycyclic aromatic hydrocarbons, fullerenes, nanodiamonds, graphite, and carbon chain molecules. The literature in the field is enormous interesting reviews can be found in a recent special volume of Speetroehimica Acta [1]. 

How can a scientist remain indifferent in light of the fact that carbon chains have been detected by radio astronomers in the molecular clouds present in the interstellar medium or in the circumstellar medium of carbon-rich stars or in the atmosphere of certain bodies of the Solar System such as Titan, Saturn s giant moon The wonder increases further when it is realized that about one thousand organic molecules classified as polyynes are produced by plants, fungi, and microorganisms and play a biological role in the biosphere and may be used in the treatment of diseases as antibiotics, anticancer or, more simply, as anti-infective agents. 

The problem of evolution of stars to their explosion and subsequent interaction with the circumstellar medium has many time scales (ranging from tens of milliseconds to tens of thousands of years) and macroscopic length scales (from dimensions effectively that of a white dwarf to that of a supernova remnant, i.e from few thousand kilometers to many tens of light years). The physics of supernova explosions is complex and diverse and in many parts, the explosion mechanism is still an unsolved problem. Even the constraining parameters and ingredients which makes the SN explode are still controversial (see e.g. the discussion in [96]). It is possible that the identification of the key... 

X-rays from a supernova explosion arise from the interaction of the supersonic ejecta with the circumstellar medium (CSM). The CSM typically consists of a slow-moving wind. When the ejecta collides with the CSM, it creates two shocks a high-temperature, low-density, forward-shock ploughing through... 

Pre-solar processes occurring either in the interstellar or circumstellar medium. 

The first question to ask about the formation of interstellar molecules is where the formation occurs. There are two possibilities the molecules are formed within the clouds themselves or they are formed elsewhere. As an alternative to local formation, one possibility is that the molecules are synthesized in the expanding envelopes of old stars, previously referred to as circumstellar clouds. Both molecules and dust particles are known to form in such objects, and molecular development is especially efficient in those objects that are carbon-rich (elemental C > elemental O) such as the well-studied source IRC+10216.12 Chemical models of carbon-rich envelopes show that acetylene is produced under high-temperature thermodynamic equilibrium conditions and that as the material cools and flows out of the star, a chemistry somewhat akin to an acetylene discharge takes place, perhaps even forming molecules as complex as PAHs.13,14 As to the contribution of such chemistry to the interstellar medium, however, all but the very large species will be photodissociated rapidly by the radiation field present in interstellar space once the molecules are blown out of the protective cocoon of the stellar envelope in which they are formed. Consequently, the material flowing out into space will consist mainly of atoms, dust particles, and possibly PAHs that are relatively immune to radiation because of their size and stability. It is therefore necessary for the observed interstellar molecules to be produced locally. 

A significant fraction of the fullerenes and buckyonions in the interstellar medium could be hydrogenated as discussed by Webster in 1992 (see Fig. 1.4). These molecules, named genetically as fulleranes have deserved attention as potential carriers of diffuse intestellar bands and other interstellar and circumstellar features (Webster 1991, 1992, 1993a). Both, fullerenes and fulleranes have been detected in samples of the Allende meteorite (Becker et al. 1994), see Fig. 1.3b. 

Petrie and Bohme (2000) and Millar (1992) have studied the interstellar fullerene chemistry focusing on ion/molecule chemistry in various astrophysical environments. Fullerene based molecules could play a relevant role in the chemistry and physics of the interstellar medium and circumstellar environments (see also Watson et al. 2005 Webster 1991 Cataldo 2003). 

Mixtures of fulleranes produced by hydrogenation of solid C60 films under atomic H flux have revealed spectral features that bear striking similarity to those observed in the diffuse interstellar medium, both in the far IR and in the UV spectral windows. Of course, one must be cautious not to overextend the interpretation of laboratory data, for a number of reasons firstly, because electron spectroscopy, the experimental technique used in these studies, differs in several important aspects from the spectroscopic methods employed in observational astronomy, and secondly, because of the specifics of specimen preparation and environmental conditions. In this regard, there is a need to explore the stability of fulleranes to energetic and corpuscular radiation (Cataldo et al. 2009). Nonetheless, our findings lend support to the suggestion of fulleranes as candidates for unidentified emission and absorption features of interstellar and circumstellar media. Whether or not they exist in sufficient abundance is still unclear however, their spectral features make them undoubtedly an ideal model system for laboratory studies of these fascinating astrophysical phenomena. 

Linear hydrocarbon radicals have been the subject of intensive laboratory spectroscopic and radio-astronomical research since the early 1980s. In recent years, a considerable number of rotational spectroscopic studies of medium to longer hydrocarbon chains such as C5H, CeH, CgH, and ChH have been carried out using a pulsed molecular beam FTMW spectrometer. The high resolution offered by such a spectrometer allowed the detection of the hyperfine sphtting of rotational transitions. These measurements improved fine and hyperfine coupling constants and provided rest frequencies with accuracies better than 0.30 km s in equivalent radial velocity up to 50 GHz. Indeed, some of the small C H radicals with n < 9 have subsequently been detected in space, in molecular cloud cores, and in certain circumstellar shells. These hydrocarbon chains are among the most abundant reactive space molecules known. 

The ISM is the medium between the stars. For present purposes, we will also consider the media immediately surrounding stars (generally considered as circumstellar media) as part of the ISM. Most of the matter in the ISM is in the form of a very tenuous gas with densities of less than one hydrogen atom per cm to perhaps a million hydrogen atoms per cm. For comparison the terrestrial atmosphere contains about 10 hydrogen atoms per cm. ... 

These stars have been of central concern in a myriad of observational and theoretical works. No wonder They indeed play a key role in many chapters of astrophysics. In particular, they influence the physical and chemical states of their circumstellar environments or of the interstellar medium through their intense radiation and mass losses during their non-explosive phases of evolution, and even more so, as a result of their final supernova explosions. They may act as triggers of star formation, are essential agents of the evolution of the nuclidic content of the galaxies, accelerate particles to cosmic ray energies, and leave neutron stars or black holes at the end of their evolution. They are also the progenitors of certain 7-ray bursts. 

AGB stars return -0.3 solar masses per year to the interstellar medium (ISM) in our galaxy (65), of which C stars contribute about 10-50% (14, 30). Thus, dust condensation in the circumstellar envelopes of C stars is a plausible source of the presolar C-bearing grains found in meteorites. Although... 

The interstellar medium (ISM) contains different environments showing large ranges in temperature between 10 K in dense clouds and up to 200 K in warm circumstellar envelopes and densities between 100 to 10 hydrogen atoms cm. Dense clouds are characterized by very low temperatures (10-30 K) and... 

A number of molecules have been detected in the interstellar medium, in circumstellar envelopes around evolved stars, and comae and tails of comets through observation of their microwave, infrared, or optical spectra. The following list gives the molecules and the particular isotopic species that have been reported so far. Molecules are listed by molecular formula in the Hill order. All species not footnoted otherwise are observed in interstellar clouds, while some are also found in comets and circumstellar clouds. The list was last updated in October 2008 and lists 162 molecules (298 isotopic forms). 

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