Interstellar gas

The gas component, which is the most important form in which matter is found in outer space (up to 98-99%), is dominated by the element hydrogen, which makes up 70% of the mass and 90% of the particles. In ionized form (the H-II regions) the gas can be recognized by its recombination and fluorescence light emission. The hydrogen is mainly present in neutral form (H-I regions), at a mean density of 2x 107 particles per cubic metre and a mean temperature of about 80 K. 

It was not possible to detect neutral hydrogen until the 21-cm spectral line (in the radio region), predicted by van de Hulst, was discovered in 1951. Since this radiation is unaffected by dust, regions become available which cannot be studied 

The chemical reactions which take place under the extreme conditions of outer space are complex and not always comparable to those which can be simulated under laboratory conditions Eric Herbst (1990) provides a survey. 

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There is a very low cosmic abundance of boron, but its occurrence at all is surprising for two reasons. First, boron s isotopes are not involved in a star s normal chain of thermonuclear reactions, and second, boron should not survive a star s extreme thermal condition. The formation of boron has been proposed to arise predominantly from cosmic ray bombardment of interstellar gas in a process called spallation (1). 

The darkness associated with dense interstellar clouds is caused by dust particles of size =0.1 microns, which are a common ingredient in interstellar and circum-stellar space, taking up perhaps 1% of the mass of interstellar clouds with a fractional number density of 10-12. These particles both scatter and absorb external visible and ultraviolet radiation from stars, protecting molecules in dense clouds from direct photodissociation via external starlight. They are rather less protective in the infrared, and are quite transparent in the microwave.6 The chemical nature of the dust particles is not easy to ascertain compared with the chemical nature of the interstellar gas broad spectral features in the infrared have been interpreted in terms of core-mantle particles, with the cores consisting of two populations, one of silicates and one of carbonaceous, possibly graphitic material. The mantles, which appear to be restricted to dense clouds, are probably a mixture of ices such as water, carbon monoxide, and methanol.7... 

Figure 2. The new species added to our chemical models of interstellar clouds. The species range in complexity from 10-64 carbon atoms and comprise the following groups of molecules linear carbon chains, monocyclic rings, tricyclic rings, and fullerenes. The synthetic pathways are also indicated. See ref. 83. Reproduced from the International Journal of Mass Spectrometry and Ion Processes, vol. 149/150, R.P.A. Bettens, Eric Herbst "The interstellar gas phase production of highly complex hydrocarbons construction of a model", pp 321-343 (1995) with kind permission from Elsevier Science-NL, Sara Burgerhartstraat 25,1055 KV, Amsterdam, The Netherlands.

In specific applications, it is critically important to know which isomer is produced in a particular situation in order to ascertain its further reactivity. Indeed, further reactivity, in the form of rate coefficients and product ion distributions, both identifies which reactions generate the same isomeric forms and gives information to enable the isomeric forms to be identified (often by determining the energetics and comparing them with theoretical calculations). One such application is to molecular synthesis in interstellar gas clouds. In the synthesis of the >115 molecules (mainly neutral -85%) detected in these clouds,14 a major production route is via the radiatively stabilized analog of the collisional association discussed above,15 viz. ... 

This would reveal possible routes to the C2H5CNH+ that is believed to be produced in the analogous radiative association reactions that occur in interstellar gas clouds. 

There has been a consistent motivation for the work presented in this chapter the application to molecular synthesis in interstellar gas clouds (see, for example, Herbst,22 this volume). The species in these regions are detected spectroscopically and are thus automatically isomerically identified. The routes to the observed neutral species consistently involve ion-molecule reactions followed by dissociative electron-ion recombination.18 The first step in this process is to determine whether an isomeric ion can be formed which is likely to recombine to an observed neutral species. The foregoing discussion has shown that whether this occurs depends on the detailed nature of the potential surface. Certainly, this only occurs in some of the cases studied. Much more understanding will be required before the needs of this application are fulfilled. 

The abundance patterns of individual stars of different ages and environments enable us to unlock the evolutionary history of galaxies. Many physical characteristics of a galaxy may change over time, such as shape and colour, however the metal content and abundance ratios of stellar atmospheres are not so easy to tamper with. Stars retain the chemical imprint of the interstellar gas out of which they formed, and metals can only increase with time. This method to study galaxy evolution has been elegantly named Chemical Tagging [2],... 

Radioastronomers first learned of 3He in 1955 at the fourth I.A.U. Symposium in Jodrell Bank, when the frequency of the hyperfine 3He+ line at 8.666 GHz (3.46 cm) was included by Charles Townes in a list of radio-frequency lines of interest to astronomy (Townes 1957). The line was (probably) detected for the first time only twenty years later, by Rood, Wilson Steigman (1979) in W51, opening the way to the determination of the 3He abundance in the interstellar gas of our Galaxy via direct (although technically challenging) radioastronomical observations. In the last two decades, a considerable collection of 3He+ abundance determinations has been assembled in Hi I regions and planetary nebulae. The relevance of these results will be discussed in Sect. 4 and 5 respectively. 

Rotational spectra provide measurement of the moments of inertia of a chemical species. Bond angles and bond lengths can be derived by making isotopic substitutions and measuring the resulting changes in the moments of inertia. A major drawback of rotational spectroscopies is the limited information contained in a measurement of the moment of inertia. Consequently, while quite precise, it is generally limited to smaller molecules. It is the chief technique used to identify molecules in outer space, such as the components of interstellar gas clouds. 

Because of the destruction of D when interstellar gas is recycled through stars, its present-day abundance is a firm lower limit to the primordial one. Adopting the local bubble value as representative of the Galaxy places an upper limit on the baryonic density parameter,... 

Line absorption in the intervening interstellar gas, or Earth s atmosphere. 

Fig. 12.14. Metallicity evolution in DLAs. Curves show predicted mean metallic-ity in the interstellar gas relative to solar predicted by chemical evolution models of Pei, Fall and Hauser (1999), Pei and Fall (1995), Malaney and Chaboyer (1996) and Somerville, Primack and Faber (2001) respectively. Data points giving column-density weighted metallicities based on zinc only (filled circles) or other elements (open circles) are plotted in the upper panel taking upper limits as detections and in the lower panel taking upper limits as zeros. Horizontal error bars show the redshift bins adopted. After Kulkarni et al. (2005).

A. S. Eddington develops theory of radiative equilibrium (building on earlier work by A. Schuster and K. Schwarzschild) and applies it to internal constitution of stars. He also pioneers physics of interstellar gas. 

Deuterium discovered in interstellar gas (Copernicus satellite) and quantitatively estimated in early Solar System, restricting baryonic density in Big Bang nucleosynthesis (BBNS) theory. 

Advances in Gas Phase Ion Chemistry is different from other ion chemistry series in that it focuses on reviews of the author s own work rather than give a generai review of the research area. This allows for presentation of some current work in a timely fashion which marks the unique nature of this series. Emphasis is placed on gas phase ion chemistry in its broadest sense to include ion neutral, ion electron, and ion-ion reactions. These reaction processes span the various disciplines of chemistry and include some of those in physics. Within this scope, both experimental and theoretical contributions are included which deal with a wide variety of areas ranging from fundamental interactions to applications in real media such as Interstellar gas clouds and pleismas used in the etching of semiconductors. The authors are scientists who are leaders in their fields and the series will therefore provide an up-to-date analysis of topics of current importance. This series is suitable for researchers and graduate students working in ion chemistry and related fields and will be an invaluable reference for years to come. The contributions to the series embody the wealth of molecular information that can be obtained by studying chemical reactions between ions, electrons and neutrals in the gas phase. 

Dust grains act like stones in the desert. They accumulate heat and restore it to the medium in the form of infrared radiation. They are intermediaries between light from stars and interstellar gas, for they absorb stellar photons in a most efficient manner. This is why these clouds appear so dark in photographs. In fact, they shine in the infrared. The dust strewn across the Galaxy trades the big money of stellar light for the small change of the infrared. 

Neon was discovered by Ramsay and Travers in 1898.1ts name comes from the Greek word neos, which means new. It is present in the atmosphere at a concentration of 0.00182% by volume (dry atmosphere). This element also is found in stars and interstellar gas clouds. Earth s earliest crust probably contained neon occluded in minerals. The gas later escaped into the atmosphere. 

The shock wave of fresh debris from a supernova explosion travels a great distance. For example, it has been calculated that the Crab Nebula will attain a diameter of 70 light years 23,000 years from now 140 light years in 260,000 years 210 light years in 1.3 million years and 280 light years in 4 million years. After that the expanding shell will begin to dissipate because the velocity of expansion drops to about 1-2 miles/sec. (3,600-7,200 miles/hr.), which is the velocity of the molecules of interstellar gas. 

According to Opik (13), the supernova explosion sweeps up and compresses into an expanding shell a mass of interstellar gas of the order of 15,000 solar masses, 1000 times greater than the original mass of the supernova. This means that the fresh debris from a supernova is diluted 1000 times by old debris. 

This image of the Crab Nebula taken by the Hubble Space Telescope shows enormous interstellar gas clouds, in which spallation reactions may be taking place. 

The Lagoon Nebula in the constellation Sagittarius. These interstellar gas clouds consist largely of atomic hydrogen, the most abundant element in the universe. The gas is heated by radiation from nearby stars. Can you explain its characteristic red glow (Recall Section 5.3.)... 

Among optical DIBs, the 4,430 A band is the strongest. This band is remarkably broad with a width (FWHM) of order a few tens of A. Krelowski and Walker (1987) assign the two broad DIBs 4,430 and 6,177 A to the same family and Krelowski et al. (1989) and McIntosh and Webster (1993) note that the carrier of this family appears to prefer denser interstellar gas than other carriers. There is also evidence of a positive correlation between the 4,430 A band and the strongest feature in the interstellar extinction curve, the UV bump at 2,175 A (Webster 1992 Nandy and Thompson 1975). It is therefore plausible that these two bands are produced by the same type of molecule. 

You ve got it Another name for heliosphere is magnetosphere. Again, the heliopause is the boundary between the Sun s magnetosphere (i.e., the solar wind) and interstellar gas. ... 

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