Diffuse interstellar medium

The rotational spectrum has been calculated accuratly by ab-initio methods [2], and has been measured in the laboratory with high precision [3,4], so that the radio detection of C3H2can be done without ambiguity, encouraging its search in different environments as dense dark clouds [5], diffuse interstellar medium [6] or Hll regions [7]. 

What is the ultimate fate of the molecular material formed in the envelopes of carbon-rich stars as it heads out into space The dust grains will be processed only slowly by the interstellar radiation held and survive almost intact until they become part of an interstellar cloud. The survival of individual PAHs depends on their size the larger ones withstand radiation much better than do the smaller ones.115 By survival we are referring to the aromatic skeleton the interstellar radiation field will efficiently break H bonds and cause ionization so that unsaturated, ionized PAHs are likely to dominate those found in the diffuse interstellar medium. Such species have been suggested as a source of the DIBs.118,123 Small molecules photodissociate in the interstellar radiation field before the material becomes part of an interstellar cloud. 

Diffuse interstellar medium the atomic density can be of order 1-102 atoms cm-3 and temperature is hard to define. The translational temperatures of... 

Diffuse interstellar medium The most tenuous regions of space with perhaps one molecule per cm3. 

In recent decades, spectroscopy has revealed that the elemental and isotopic abundances in the galaxy vary with radial position and that the Sun has a somewhat different composition than the molecular clouds and diffuse interstellar medium in the solar neighborhood. For this reason, we can no longer think of the solar system abundances as truly cosmic abundances. 

Abstract We review the potential contribution of single fullerenes and buckyonions to interstellar extinction. Photoabsorption spectra of these molecules are compared with some of the most relevant features of interstellar extinction, the UV bump, the far UV rise and the diffuse interstellar bands. According to semiempirical models, photoabsorption by fullerenes (single and multishell) could explain the shape, width and peak energy of the most prominent feature of the interstellar absorption, the UV bump at 2,175 A. Other weaker transitions are predicted in the optical and near-infrared providing a potential explanation for diffuse interstellar bands. In particular, several fullerenes could contribute to the well known strong DIB at 4,430 A comparing cross sections and available data for this DIB and the UV bump we estimate a density of fullerenes in the diffuse interstellar medium of 0.1-0.2 ppm. These molecules could then be a major reservoir for interstellar carbon. 

The comparison of the computed cross sections of fullerenes and buckyonions with observations of the UV bump for Ry = 3.1 allow an estimate of the number of these molecules in the diffuse interstellar medium. Let us describe the extinction curve as a + a2x + a37Tx) where 7Tx) is the theoretical cross section computed for each fullerene or buckyonion. Here we assume that indeed the extinction at the energy of the bump is the result of the fullerene plus silicate contributions. We obtain via a least squared fit the relative contribution of the two components (see Fig. 1.6b). The coefficients of this lineal component do not depend significantly on the particular fullerene under consideration taking typical values of a, 1.6 and a2 = 0.07 with a relative error of 20%. 

Hydrogenated fullerenes and buckyonions may produce rotationally based electric dipole microwave radiation under the conditions of the diffuse interstellar medium. These molecules are potential carriers for the anomalous Galactic micro-wave emission recently detected by several cosmic microwave experiments. Their precise contribution to this emission should be fully investigated. 

Various forms of molecular carbon, from ions to radicals, have been detected in the diffuse interstellar medium (ISM) using electronic, rotational, and vibrational spectroscopies (Henning and Salama 1998 Snow and Witt 1995). Discrete absorption and emission bands seen toward diffuse interstellar clouds indicate the presence of numerous two-atom molecules such as CO, CN and C2. In addition to these interstellar features, a large family of spectral bands observed from the far-UV to the far-IR still defies explanation. Currently, it is the general consensus that many of the unidentified spectral features are formed by a complex, carbonaceous species that show rich chemistry in interstellar dust clouds (Ehrenfreund... 

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. 

While dense IS regions are generally well-shielded against high-energy photons, the interaction between molecular ions and photons is relevant to the question of the survival of such ions in the diffuse interstellar medium, where UV irradiation might be expected to be a powerful destructive force. Such effects are, of course, important also for the fate of neutral molecules in the diffuse interstellar radiation field, but UV photo absorption by molecules of moderate size is often more likely to lead to photoionization (itself an important topic, but not covered herein) than to photodissociation. 

The 11.2 pm fine structure on the Si-O silicate feature has provided interesting insight into the relationship between cometary and interstellar materials, because IR observations of silicates in the diffuse interstellar medium and molecular clouds do not show the feature (Molster et al., 2002a,b). Searches for the 11.2 pm fine structure towards the Galactic center indicates that less than 0.5% of interstellar silicates are crystalline (Kemper and Tielens, 2003). The crystalline olivine feature is, however, seen in certain astronomical objects, stars surrounded with disks. It has been seen in Beta Pictoris (Knacke et al., 1993) and Herbig Ae/Be stars... 

It is particularly intriguing that Fo and En are also seen in the IR spectra of young stars with disks and also in dust formed from gas outflows from evolved stars (Molster et al., 2002a,b Waters and Molster, 2002 Jaeger et al., 1998). Forsterite and enstatite are common minerals around stars, in LP comets and in the most primitive meteorites even though they are not seen in the diffuse interstellar medium or in SP Kuiper Belt comets. 

However, the existence of interstellar matter is not limited to these tenuous, transparent regions. Much material is concentrated in large, relatively opaque, interstellar molecular clouds (Plate 1) (5-7). In contrast with the diffuse interstellar medium which is characterized by very low hydrogen number densities (-1-10 H atoms cm ), the number densities in molecular clouds are much higher (>10" H atoms cm ). Astronomers often express measurements in terms of hydrogen because, as illustrated in Figure 1, it is the most abundant of the cosmic elements. 

Cosmic abundances in the interstellar medium are derived by measuring elemental abundances in stellar photospheres, the atmospheric layer just above the stellar surface. Such measurements indicate the amount of elements available for the formation of molecules and particles. Cosmic dust models indicate that up to 80% of the carbon in the photon-dominated diffuse interstellar medium is incorporated into solid aromatic macromolecules and gaseous polycylic aromatic hydrocarbons (41,30). CO gas and C-based ice species (such as CO, CO2, CH3OH and others) may be responsible for up to -25 % of the carbon in cold dense interstellar regions. 

We may, therefore, envisage a cycle of events in the diffuse interstellar medium (i) typical diffuse material is shocked (ii) the post shock cooling induces a substantial compression and densities 10 - 100 times the pre-shock values may occur (iii) a rarefaction wave penetrates the dense shell and tends to restore density and temperature to pre-shock values (iv) the material maintains those values until a new shock arrives. It is assumed that the shock, (i), releases substantial amounts of carbon to the gas phase. 

The presence of reactive ions like CH in the diffuse interstellar medium has been a challenge for interstellar chemistry since CH is easily destroyed by reactions with H2 but slow to form under the known physical conditions of the diffuse interstellar medium. It now appears that a warm chemistry can develop in the tiny dissipative structures of the interstellar turbulence, enabling the formation of transient species like CH and SH [43], The opening up of the sub-millimetre sky by the Herschel telescope has led to the discovery of several new reactive ions, enabling a better characterization of their chemistry. In the future, these tracers should bring interesting constraints on the properties of the interstellar turbulence. 

MivUle-Deschenes M-A, Martin PG, Abergel A et al (2010) HCTschel-SPIRE observations of the Polaris flare structure of the diffuse interstellar medium at the sub-parsec scale. Astron Astrophys 518 L10... 

Indriolo N, McCall BJ (2012) Investigating the cosmic-ray ionization rate in the galactic diffuse interstellar medium through observations of Hs. Astrophys J 745 91... 

Godard B, Falgarone E, Pineau Des Forets G (2009) Models of turbulent dissipation regions in the diffuse interstellar medium. Astron Astrophys 495 847... 

Planck Collaboration, Abergel A, Ade PAR, Aghanim N et al (2011) Planck early results. XXTV. Dust in the diffuse interstellar medium and the galactic halo. Astron Astrophys 536 24... 

Leach S (2012) Why COBE and CN spectroscopy cosmic background radiation temperature measurements differ, and a remedy. Mon Not R Astron Soc 421 1325-1330 Indriolo N, McCall BJ (2012) Investigating the cosmic-ray ionization rate in the galactic diffuse interstellar medium through observation of Ha" ". Astrophys J 745 91-1-17... 

Grains provide a solid surface on which chemical reactions take place. In fact, they are the primary site for H2 synthesis. In addition, metallic ions deplete onto the grains. This is evident from the lower-than-stellar abundances observed for the heavy metals, such as iron and calcium, in the diffuse interstellar medium. It appears that most of the heavy metals in the diffuse and molecular cloud phases of the ISM may be tied up in the grains, which nonetheless constitute only about 10 , by number, of the ISM. CO and H2O may also stick to the grain surfaces, and models and laboratory simulations show a host of complex organic molecules can be synthesized in the resultant mantle. It remains to be determined whether these simulations are relevant for interstellar conditions they do seem to mimic many of the reaction products observed in situ in comet Halley. 

The first observations of diatomic species in the diffuse interstellar medium several decades ago posed serious challenges for theorists because of the extremely low densities which are found there. Radiative association seemed unable to produce any of the observed species, most importantly CH, and this meant that exotic mechanisms were initially held responsible for the presence of such molecules. Work on the abundance of H2, following the observation of the molecule in the diffuse medium in the ultraviolet by the Copernicus satellite in the mid-1970s, and the discovery of... 

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