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

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1. Introduction

The gamma-ray emission in the 3-7 MeV range observed by COMPTEL from the Orion complex (Bloemen et al. 1994) and recently confirmed at a level of [FORMULA] (Bloemen et al. 1997) has given rise to many analyses and discussions (e.g. Bykov & Bloemen 1994, Cassé et al. 1995, Ramaty et al. 1995). It is generally believed that this gamma-ray emission is due to in-flight de-excitation of fast 12 C and 16 O nuclei following their excitation to the 4.4 MeV and 6.1 MeV levels, respectively, by collision with the cloud material. The energetic particles involved seem to constitute a component on its own, distinct from the usual Galactic cosmic rays (GCRs), having notably a lower caracteristic energy ([FORMULA] -30 MeV/n), a composition richer in carbon and oxygen, and an energy density higher by at least a factor of 10 (Ramaty 1996). We shall denote by EPs these energetic particles presumably associated with the intense mechanical activity (SNe and stellar winds, essentially) in regions of massive star formation, of which the Orion complex is the most vivid example.

Various energetic particle (EP) compositions have been studied in the context of the Orion gamma-ray emission (Cassé et al. 1995; Ramaty et al. 1995, 1996). Energetic considerations as well as the absence of any detected line between 1 and 3 MeV suggest that the EPs are helium poor and almost devoid of protons (Ramaty et al. 1995ab). On this basis, three compositions have been elected by Ramaty and co-workers, corresponding to three possible origins: carbon rich Wolf-Rayet (W-R) stars, supernovae with massive progenitors ([FORMULA]) and interstellar grains (GR). SN and W-R compositions have also been used by Cassé et al. (1995) and Vangioni-Flam et al. (1996, 1997) to evaluate the Li, Be and B (LiBeB) production in the interstellar medium (ISM) as a result of spallation reactions induced by EPs. The most attractive feature of such compositions enriched in freshly synthesized C and O is that they provide a natural understanding of the observed linear growth of the Be and B abundances in the early Galaxy, with respect to Fe, whereas GCR-produced Be and B grow quadratically with metallicity (Cassé et al. 1995).

In this article, we analyse in detail the expected composition of the EPs, considering that they result from the acceleration of the cumulated wind material blown by massive stars, and investigate the induced gamma-ray line emission. Although we have developed elsewhere time-dependent models adapted to the study of individual W-R stars and SNe (Parizot et al. 1997 a,b,c), we assume here that nuclei injected by mass losing stars are continuously accelerated by large scale motions in regions experiencing a large energy release in the form of strong stellar winds and multiple supernova explosions (Bykov 1995, Bykov & Fleishman 1992). Therefore we use a steady-state, thick target model, and study systematically the influence of both EP composition and target metallicity.

The main features of the present work are that: 1) we use integrated wind compositions instead of the surface composition of a WC star at the very end of its life (as done by Ramaty et al.), which represents only a negligible mass fraction of the wind, 2) we consider the mean composition of the combined ejecta of a whole OB association, 3) we extend our calculations to metallicities higher than solar in a consistent way, changing both the target composition (ambient material) and the EP composition. Indeed, the composition of a stellar wind depends on the initial metallicity of the star considered, while the target composition is basically the same as that of the material from which the star was formed, since the ambient chemical composition is nearly constant during the lifetime of the most massive stars. Accordingly, we use EP compositions deduced from massive star evolutionary models calculated at the same initial metallicity as that assumed for the target. In particular, the calculations at solar metallicity apply to Orion, while those at supersolar metallicity are assumed to describe the inner Galaxy.

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© European Southern Observatory (ESO) 1997

Online publication: March 24, 1998