Interstellar media

The parameters that characterize the surrounding medium (interstellar medium or matter from the binary companion) are also important. 

In the dense interstellar medium characteristic of sites of star fonuation, for example, scattering of visible/UV light by sub-micron-sized dust grains makes molecular clouds optically opaque and lowers their internal temperature to only a few tens of Kelvin. The thenual radiation from such objects therefore peaks in the FIR and only becomes optically thin at even longer wavelengths. Rotational motions of small molecules and rovibrational transitions of larger species and clusters thus provide, in many cases, the only or the most powerfiil probes of the dense, cold gas and dust of the interstellar medium. 

To date, researchers have identified more than 100 different molecules, composed of up to 13 atoms, in the interstellar medium [16]. Most were initially detected at microwave and (sub)millimetre frequencies, and the discoveries have reached far beyond the mere existence of molecules. Newly discovered entities such as difhise mterstellar clouds, dense (or dark) molecular clouds and giant molecular cloud complexes were characterized for the first time. Indeed, radioastronomy (which includes observations ranging from radio to submillunetre frequencies) has dramatically changed our perception of the composition of the universe. Radioastronomy has shown that most of the mass in the interstellar medium is contained in so-called dense... 

Herbst E 1995 Chemistry in the interstellar medium Ann. Rev. Rhys. Chem. 46 27-53... 

Comparison with the empirical Equation (1.4) shows that = /re /S/z eg and that n" = 2 for the Balmer series. Similarly n" = 1, 3, 4, and 5 for the Lyman, Paschen, Brackett and Pfimd series, although it is important to realize that there is an infinite number of series. Many series with high n" have been observed, by techniques of radioastronomy, in the interstellar medium, where there is a large amount of atomic hydrogen. For example, the (n = 167) — ( " = 166) transition has been observed with V = 1.425 GFIz (1 = 21.04 cm). 

Table 5.2 lists some of the molecules which have been detected. It is interesting to note that some of them, such as the linear triatomics C2H, HCO and N2H, were found in the interstellar medium before they were searched for and found in the laboratory. In all molecules, except OH and NH3, the transitions observed are rotational in nature. 

Identification of a molecule known in the laboratory is usually unambiguous because of the uniqueness of the highly precise transition frequencies. However, before frequencies detected in the interstellar medium can be compared with laboratory frequencies they must be corrected for the Doppler effect (see Section 2.3.2) due to the motion of the clouds. In Sagittarius B2 the molecules are found to be travelling fairly uniformly with a velocity of... 

Table 5.2 shows that quite large molecules, of which the cyanopolyacetylenes form a remarkable group, have been detected. The presence of such sizeable molecules in the interstellar medium came as a considerable surprise. Previously, it was supposed that the ultraviolet radiation present throughout all galaxies would photodecompose most of the molecules, and particularly the larger ones. It seems likely that the dust particles play an important part not only in the formation of the molecules but also in preventing their decomposition. 

The hydrogen atom and its spectrum are of enormous importance in astrophysics because of the large abundance of hydrogen atoms both in stars, including the sun, and in the interstellar medium. 

K. Narahad Rao and A. Weber, eds.. Spectroscopy of the Earth s Atmosphere and Interstellar Medium, Academic Press, Inc., Boston, Mass., 1992. 

There has been a resurgence of interest in atomic HF calculations because astrophysicists want to study highly ionized atomic species in the interstellar medium. They look to theory for their energy-level data rather than earth-bound experiments where the species are hard to prepare and study. 

And over all, as time and studies around this new interstellar component increase, it reveals to be possibly related to the polycyclic aromatic hydocarbon family (PAHs), those controversial molecules of prime interest which could be omnipresent in the interstellar medium and an essential link between simple molecules and grains. 

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]. 

The reactions of ground state atomic carbon, C(3Pj), with unsaturated hydrocarbons are another important class of reactions characterized by multiple pathways. These reactions, besides being of fundamental interest, are of great relevance in the chemistry of the interstellar medium and also in combustion.12,93-95... 

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. 

The Hj ion, recently detected in the interstellar medium via infrared transitions,25 can subsequently react with a variety of neutral atoms present in the gas. The reaction with oxygen leads to a chain of reactions that rapidly produce the hy-dronium ion H30+ via well-studied H atom-transfer reactions ... 

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. 

There are many situations in which scientists need to know how alike a number of samples are. A quality control technician working on the synthesis of a biochemical will want to ensure that each batch of product is of comparable purity. An astronomer with access to a large database of radiofrequency spectra, taken from observation of different parts of the interstellar medium, might need to arrange the spectra into groups to determine whether there is any correlation between the characteristics of the spectrum and the direction of observation. 

Ehrenfreund, P. and Charnley, S.B. (2000). Organic molecules in the interstellar medium, comets and meteorites. Annu. Rev. Astron. Astrophy., 38, 427-483... 

Linked to 1) is of course the enrichment of the interstellar medium, to which they are important contributors in nuclearly processed elements as He, C, N, s-elements (Ba etc). Goal 2) can be pursued with nuclearly unprocessed elements , the best accessible of them being O, Ne, Ar and S. 

If an external body is engulfed, it can enrich the star with the original interstellar medium abundances of 6Li, 7Li, 9Be and 10,11B (written here in increasing order of hardness to be destroyed by thermonuclear reactions). This mechanism is then supposed to produce stellar enrichment of these elements up to the maximum meteoritic value. Also, the engulfing star will suffer a rotational increase due to the gain of the planet momentum and a thermal expansion phenomenon due to the penetration of the body provoking mass loss phenomena (Siess Livio 1999). An extension to this scenario has been proposed by Denissenkov Weiss (2000) in order to explain supermeteoritic Li abundance values, via a combination of stellar rotation and activation of the 7Be mechanism at the base of the convective layer produced by the penetration of the external body. 

Dust is probably present in the DLA and significantly affects the observed abundances. I counted 55 systems for which both Fe and Zn are measured, which are plotted in Fig. 1. The abundance of Fe is always found below that of Zn. By analogy with the interstellar medium, this behaviour is interpreted as the effect of some Fe being locked into dust grains. Other indicators for the presence of dust... 

The idea behind this hypothesis is that an early generation of AGBs might have evolved and polluted the interstellar medium with nuclearly processed material, so that the new generations of stars were born from an already contaminated gas (D Antona et al. 1983 Cottrell Da Costa 1981). 

Investigate the creation of stars and their interaction with the interstellar medium. 

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