6. C and O lines and line ratio
Since 12 C and 16 O are the most abundant nuclei in the winds of massive stars after protons and particles, the two most intense gamma-ray lines predicted are the 12 C line at 4.438 MeV and the 16 O line at 6.129 MeV, in agreement with the COMPTEL observations in Orion. These two lines also constitute the main contribution to the total gamma-ray flux emitted in the range 3-7 MeV, to which we have normalised all our calculated fluxes. We have used the updated cross sections of Tatischeff et al. (1996).
6.1. The 12 C line at 4.438 MeV
The first excited state of 12 C can be populated by direct excitation from EP-ISM interactions, or by the breaking of heavier nuclei in spallation reactions, mainly of 16 O and 14 N. For all the compositions considered here, however, the main contribution to the 4.438 MeV line is the in-flight excitation of energetic 12 C nuclei on ISM protons, leading to essentially broad line emission. Figs. 6 and 7 show the detailed contribution of all the production mechanisms representing more than of the total gamma-ray line flux, in two extreme cases. The first corresponds to the late-WC composition, which is so poor in protons and alpha particles that more than of the 4.438 MeV line flux is due to inverse reactions involving carbon and oxygen; and the second corresponds to the mean-wind composition of an OB association with twice solar initial metallicity and enhanced mass loss rates (Geneva group's model E). In this case direct reactions contribute to about of the total 12 C production, and EP's 14 N nuclei excitation provides the second most important contribution to the gamma-ray line flux, for any break energy MeV/n.
In order to distinguish the broad and narrow components of the 4.438 MeV line (leaving aside the possible line splitting; Bykov et al. 1996, Ramaty et al. 1997), we show in Fig. 8 the ratio of inverse to direct contributions for various EP compositions, excluding GR and late-WC compositions for which this ratio is greater than 100, as will as solar composition for which it is of order 0.3. The wide range of variation of this ratio demonstrates that any instrument capable of measuring the 12 C line profile, such as INTEGRAL's SPI, would be able to set strong constraints on the actual EP composition.
6.2. The 16 O line at 6.129 MeV
Fig. 9 and 10 show the detailed production of the 6.129 MeV line resulting from the de-excitation of 16 O nuclei produced in their first excited level by direct excitation and 20 Ne spallation for two EP compositions corresponding to the same model (mean-OB composition with models C and IMF index ), but with either solar or twice solar initial metallicity. The differences are quite striking.
In the case of solar initial metallicity, the EPs are much richer in 16 O than the ambient ISM (O/H versus ). As a result, the encounters of energetic 16 O nuclei with ISM protons are much more frequent than that of energetic protons with ISM 16 O nuclei. This is not anymore the case at twice solar metallicity, because the EPs are richer in protons (O/H versus ). As a consequence, direct and inverse excitations involving protons contribute to the gamma-ray line flux at about the same level (Fig. 10). This effect is even more pronounced for the -16 O reactions, because of the strong enhancement of the mean-wind helium abundance when passing from solar to twice solar metallicity. The direct-to-inverse ratio then reverses, resulting in a dominating narrow line emission. This prediction could be tested by the variation of the 16 O line profile as a function of the Galactic longitude, since most of the metal-rich W-R stars should be concentrated in the 4 kpc ring.
Fig. 11 shows the global inverse-to-direct ratio, or equivalently the broad-to-narrow line component ratio for various EP compositions. As in the case of the 12 C line at 4.438 MeV, it can be seen that this ratio is very sensitive to the source composition.
At any rate, we predict a 6.129 MeV line significantly narrower than the 4.438 MeV line for any of our mean wind compositions (from either individual stars or OB associations). In particular, the direct reactions are always found to contribute to at least of the total line emission, and actually dominate in the case of a twice solar metallicity (inner Galaxy) or for models with enhanced mass loss rates during the main-sequence phase. This represents a distinctive feature of our models with respect to, e.g., the late-WC model which presents a very high 16 O abundance.
6.3. The 12 C /16 O line ratio
Apart from the line profiles discussed above, the 12 C /16 O gamma-ray line ratio is one of the most relevant observable for gamma-ray spectroscopy. It should indeed be accessible quite easily to the INTEGRAL's spectrometer SPI in Orion, and also presumably in the diffuse Galactic emission. The data collected by COMPTEL provide a first constraint on this line ratio in Orion (Bloemen et al. 1997). Our rough guess estimate is C /16 O .
As can be seen in Fig. 12 and Fig. 13, the 12 C /16 O ratio is quite sensitive to the chemical composition of the EPs. If our rough estimate is correct, one can already exclude many compositions, among them the solar system (SS), grain (GR), supernova ejecta (SN35), as well as late-WC compositions, whatever the value of the break energy may be. On the contrary, our mean-OB compositions seem to provide a more adequate line ratio. In the case of a star with enhanced mass loss rate (model E), a break energy MeV/n is required.
Although our estimate is admitedly uncertain, we point out that whatever the value of the 12 C /16 O line ratio will prove to be, its measurement will allow us to distinguish between mean-WC and late-WC compositions, and will provide a strong argument to exclude (or favour) compositions such as SS, GR or SN35.
Finally, we note that in a general way, the 12 C /16 O line ratio decreases for increasing values of the injection break energy . This is due to the high excitation threshold of 16 O. However, this does not apply to the SN35 composition. In this case, indeed, the 16 O abundance is so high with respect to 12 C ( times greater) that the main contribution to the 4.438 MeV line is of spallative origin (except for the lowest values of ). Both 12 C and 16 O lines are thus due to the same collisions, namely 16 O p, so that the previous argument doesn't hold.
© European Southern Observatory (ESO) 1997
Online publication: March 24, 1998