Hydrogen in the interstellar medium

Surfaces Involved in the Synthesis of Molecular Hydrogen in the Interstellar Medium... 

Here Ni E) is the spatial density of cosmic-ray nuclei of mass i, and rip is the number density of target nuclei (mostly hydrogen) in the interstellar medium, Qi(E) is the number of primary nuclei of type i accelerated per cm3 per second, and Oi and are respectively the total and partial cross sections for interactions of cosmic-ray nuclei with the gas in the interstellar medium. The second term on the r.h.s. of Eq. 1 represents losses due to interactions with cross section a, and decay for unstable nuclei with lifetime t,. The energy per nucleon, E, remains constant to a good approximation in the transition from parent nuclei to nuclear spallation products, which move with velocity (5c and Lorentz factor 7 = E/mp. 

The Copernicus satellite also made the first direct observations of deuterium ( heavy hydrogen ) in the interstellar medium, both in atomic form and in the molecule HD (Fig. 19). Accurate knowledge of the ratio of D to H in the interstellar gas is of relevance to theories of the origin and evolution of the universe. 

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). 

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. 

Clouds of gas in the interstellar medium are called gaseous nebulas. These nebulas are regions of the interstellar medium with above-average density. The proportions of elements in the interstellar medium conform to the abundances in the table, that is, 90% hydrogen atoms, 9% helium atoms and less than 1% heavier atoms, where these percentages now refer to relative numbers of atoms rather than relative mass. 

Finally, hydrogenated fullerenes have been proposed as carriers of the anomalous microwave emission recently detected by several experiments on the Cosmic Microwave Background (Iglesias-Groth 2005, 2006). In the interstellar medium these molecules should spin with rates of several to tens of gigaHertz, if as expected they have a small dipole moment, then they would emit electric dipole radiation in a frequency range very similar to that observed for the anomalous microwave emission. 

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. 

Schematically, during main sequence evolution, the fast wind creates a cavity in the interstellar medium and sweeps out a shell of compressed gas. After departure from the main sequence, the nature of the mass loss changes and the star loses chemically enriched material. When the star reaches the Wolf-Rayet phase, its outer layers are almost hydrogen free. This material is lost at high velocity and catches up with material lost in previous stages (see Chu 1991 or Marston 1999 for a review).

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