Cosmic ray ionisation

The choice of chemical networks is complicated and even for simple clouds such as TMC the species list is 218 species, with 2747 chemical reactions linking them. Network reduction mechanisms have been employed to reduce the number of reactions but preserve the chemical composition of at least the major species. All models must include simple ion-molecule chemistry with UV and cosmic ray ionisation initiation reactions, as shown in Figure 5.20. 

The ion HCO is formed and destroyed by proton transfer reactions in the inner region of the envelope and formed by other ion-molecule reactions at r 10 cm and has a column density of 3x10 cm 2. Glassgold et al. (1987) have presented an extensive discussion of the HCO abundance and its sensitivity to parameters such as mass-loss rate and the cosmic-ray ionisation rate. In particular, their calculated antenna temperature for the J = 1 - 0 line is consistent with the upper limit obtained by Lucas et al. 

As is the case for carbon-rich CSEs, cosmic-ray ionisation produces H3+ and, via proton transfer reactions, HCO+ and, most importantly, HsO- -, in the inner envelope. Figure 2, taken from Mamon et al. (1987), shows the fractional abundance of H30 for a number of mass-loss rates. Deep in the envelope the almost linear dependence of x(H30 ) with radial distance can be understood in terms of the formation and destruction reactions H3+ + H2O —> H3O+ + H and H30 + e —> products. 

As the radial distance Increases, the external radiation field dominates cosmic-ray ionisation and the radical OH becomes the dominant reactive species. In addition to loss through photoprocesses, OH can react with a large number of species and, in particular, with 0 and N atoms, the latter formed in the photodissociation of N2, to produce O2 and NO. The reaction 0 + OH —> O2 + H... 

Since this ion does not react with ubiquitous H2, it has a higher abundance than and has been detected via vibrational transitions in both dense and diffuse clouds. Cosmic ray ionisation of helium is also a key process the resulting He+ ion does not react rapidly with H2. 

Cosmic ray-induced heating refers to the passage of a high energy cosmic ray through a grain that heats material along its path of interaction and allows evaporation to occur. This process has a slow rate as the cosmic-ray ionisation rate is small and such events relatively rare. ... 

As discussed in Sec. 1.1, cosmic-ray ionisation of H2 produces secondary electrons which excite H2 and cause it to fluoresce in the Lyman and Werner bands. Since cosmic rays penetrate cold cores easily, these UV photons are generated internally throughout the cloud and are not subject to the levels of extinction suffered by externally-generated photons. In principle these photons can both dissociate gas-phase molecules and photodesorb mantle material the latter process is known as cosmic ray-induced photodesorption. 

The state of matter within these regions needs to be determined before the balance of energy and chemistry can be understood. Extreme photon fluxes break all chemical bonds, prevent molecule formation and ionise atoms but as the density of species increases the UV and far-UV photons are absorbed and molecules begin to form. Chemical reactions are, however, slow in the gas phase due to the low temperature, and molecules condense out on the surface of dust particles, perhaps forming ice grains. Once on the surface, molecules continue to be photoprocessed by the starlight as well as by the continual bombardment of cosmic rays. 

It is then possible to construct reaction schemes to build all of the hydrocarbon molecules observed to date. The reactions with N2 require much shorter wavelength photons to break the N=N triple bond and the chemistry is initiated by cosmic ray (cr) ionisation, with the reactions leading to HCN ... 

Cosmic rays like nothing better than to share their fabulous energies. They dole it out in various ways, ionising and heating matter encountered along their path. 

Air is ionised by radiation from natural activity in the air and on the ground, and by cosmic rays. Production of one ion pair requires 32.5 eV if ionisation is by fast electrons, 35.6 eV if by alpha rays. The total energy dissipated in air per decay of 222Rn depends on the equilibrium ratio of the decay products. Taking the mean of the results in Table 1.7, it is 10.3 MeV, and the rate of production of ion pairs is approximately... 

The description is based on reports from the Swedish National Institute of Radiation Protection (SSI, 1986a and 1986b). Since the end of the 1950s, 25 stations equipped with ionisation chambers 2.5 m above ground have been in operation by the Swedish National Institute of Radiation Protection (SSI). They register continuously the gamma radiation from both ground and cosmic rays. Only three stations transmit data automatically via telephone to a computer at SSI. 

The first source is installed in the secondary beam lines (H6) from the Super Proton Synchrotron (SPS). A proton beam is stopped in a copper target, 7 cm in diameter and 50 cm in lei jh. These roof-shields produce almost uniform radiation fields over two areas of 2 x 2 m, each divided into 16 squares of 50x50 cm. Each element of these grids represents a reference exposure location. The intensity of the primary beam is monitored by an air-filled, precision ionisation chamber (PIC) at atmospheric pressure. One PIC-count corresponds to 2.2 x 10 particles (error 10%) impii ng on the target. Typical values of dose equivalent rates are 1-2 nSv per PIC-count on top of die 40 cm iron roof-shield and 0.3 nSv per PIC-count outside the 80 cm concrete shields (roof and side). Behind the 80 cm concrete shield, the neutron spectrum has a second pronounced maximum at about 70 MeV and resembles the high-energy component of the radiation field created by cosmic rays at commercial flight altitude. ... 

The density is sufficiently high that ionisation by cosmic rays is unimportant (Millar and Nejad, 1985) while X-rays and UV radiation can be neglected because of the very high extinction. Hence the abundance of ions in the hot core phase remains low. 

ABSTRACT. This review is concerned with the chemistry occurring in the expanding envelopes around cool, late-type stars. We show that, although many different chemical processes help determine the overedl chemical composition of such envelopes, the detailed composition, as evidenced by the many molecules detected at radio wavelengths, is best explained by a complex ion-molecule chemistry driven by ionisation caused by the ambient external ultraviolet radiation field and by cosmic-rays. 

The ionisation provided by cosmic-rays and UV photons drives an ion-molecule chemistry in CSEs which leads to a rich variety of molecular species. In order to be important, chemical processes must occur on a time-scale faster than that of the expansion. Figure 1 from Nejad (1986) shows a number of such time-scales for the case of lRC+10216. One sees that a number of fast processes such as reaction with Hj and dissociative recombination can occur faster than the expansion time but that grain surface processes will be unimportant in the outer envelope as discussed in 2.2. An important point to note in this figure is that fast reaction of a molecular ion with H2 can dominate even dissociative recombination out to a radial distance of - 10 cm. 

The most important ionisation caused by cosmic ray protons (CRP) in dense clouds is that of H2, the dominant species ... 

Cosmic-ray particles (with energies in the MeV to GeV range) produce the ionisation of molecular and atomic hydrogen and helium. The high-energy electrons from this process can in turn excite H2, which then emits UV photons. This process, first proposed by Prasad and Tarafdar [27], is an efficient source for the... 

Another datum that has proven elusive to pin down accurately, by theoretical calculations or by observations, is the ionisation rate due to cosmic rays impinging... 

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