Astron. Astrophys. 333, 1025-1033 (1998)

4. Final remarks

The flux of low energy cosmic rays in the interstellar medium is not well known. A proton flux J(E = 1 MeV) 3 cm^-2 s^-1 has been estimated by Jenniskens et al. (1993). The specific energy loss of 1 MeV protons in a typical grain containing heavy atoms (C, N, O, Si) is S 5 10^-15 eV cm² /atom. Greenberg (1982) estimated lifetimes of dense clouds to be of years. Assuming a gas density n₀ 10⁴ cm^-3, it takes 10⁹ /n₀ 10⁵ years for icy mantles to condense on grains (Tielens & Allamandola 1987). These frozen mantles suffer ion irradiation for years ( sec). The former estimate refers to the case of mantles which evaporate immediately after formation while the latter refers to the limiting case of mantles which survive, at least partly, for all the cloud lifetime (Palumbo & Strazzulla 1993). Therefore, after a time t of sec the energy deposited on an icy grain mantle (dose) is given by and values 0.05-50 eV/atom(C,N,O,Si).

Laboratory experiments and theoretical models had predicted the presence of CH₄ in icy grain mantles. This had been confirmed by ground and airplane based observations first (Lacy et al., 1991; Boogert et al., 1997) and ISO observations recently (Boogert et al., 1996). They revealed the C-H deformation band of frozen CH₄, with a peak position of 7.67 µm and a FWHM of 0.062 µm, corresponding respectively to 1303.8 and 10.5 cm^-1. This fits nicely with the limits we found in our experiments, and especially with the position and FWHM we found for polar mixtures containing CH₃ OH where CH₄ is produced by ion irradiation. However, as was pointed out by Boogert et al. (1997), there are various ice mixtures containing CH₄ that are compatible with the observed spectra, so that no unique identification is possible at present. A more sensitive test would be to observe the interstellar C-H stretching band of CH₄, but this has not been achieved thus far. If this is finally done, the direct comparison of the two bands with our (and other) laboratory spectra will give precious information about the environment and irradiation history of the grain mantles containing CH₄. This will be possible when the data of the ISO observations will be publicly available.

© European Southern Observatory (ESO) 1998

Online publication: April 28, 1998

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