Astron. Astrophys. 328, 107-120 (1997)

9. Summary

We have calculated the gamma-ray line production induced by energetic particles (EPs) in active molecular clouds, assuming that the composition of the EPs reflects that of the combined wind material of Wolf-Rayet stars (W-R). The resulting composition is found to be enriched in C and O, but not completely devoid of protons and particles, which ensures a non negligible contribution of the narrow line component to the induced nuclear de-excitation lines.

As far as Orion is concerned, we have shown that any of our mean-wind or mean-OB compositions satisfies the observational constraint provided by the COMPTEL measurement of the (1-3 MeV)/(3-7 MeV) band ratio, R. Our calculations also revealed the importance of the unresolved gamma-ray lines from nuclei heavier than ²⁰ Ne, since the exclusion of specific EP compositions (such as SS or CRS) on the basis of the R band ratio actually depends on the distribution of these unresolved lines in the energy range (0.7-7 MeV). Accurate measurements of the corresponding cross sections would therefore be of astrophysical interest. We found that the ¹² C /¹⁶ O line ratio obtained with SS, GR, SN35 and late-WC compositions is hardly compatible with estimates from COMPTEL's data. By contrast, our mean-wind and mean-OB compositions seem to satisfy all the available constraints. Similarly, we found that the inverse-to-direct component ratio is very sensitive to the assumed EP composition, and therefore represents a powerful observational constraint.

We have shown that the ¹⁶ O line should be significantly narrower than the ¹² C line, because of the larger contribution of the inverse reactions. The ¹⁶ O narrow line component should even dominate at twice solar metallicity, which is relevant to the gamma-ray emission from the inner Galaxy. Moreover, both (0.2-1 MeV)/(3-7 MeV) and (1-3 MeV)/(3-7 MeV) band ratios are larger by a factor of 1.5-3 at twice solar metallicity, so that we predict a variation with Galactic longitude of the gamma-ray line spectrum, of interest for the INTEGRAL mission.

We also found that the gamma-ray emission in the ⁷ Li-⁷ Be feature at MeV should be measurable by INTEGRAL in Orion, with a flux of . This flux, as compared to that in the (3-7 MeV) band, should intensify toward the inner regions of the Galaxy by a factor of about 2. Other spallation lines should provide a significant contribution to the gamma-ray emission at MeV energies, like the ¹⁰ B line at 0.717 MeV which even exceeds the ¹⁶ O line for high values of the injection break energy . In this respect, measurements of the production cross sections of excited fragments would be of great astrophysical interest. Indeed, in light of our calculations, the future detection of a ¹¹ B line of spallative origin is not unrealistic. This would provide a `live record' of ongoing spallative nucleosynthesis of light elements in the ISM.

© European Southern Observatory (ESO) 1997

Online publication: March 24, 1998

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