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

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7. The main other gamma-ray lines

Other gamma-ray lines produced by EP interactions in the ISM may be observed in the future thanks to more sensitive instruments such as INTEGRAL, or with longer COMPTEL exposures. In Fig. 14, we show the gamma-ray fluxes calculated for the most intense lines as a function of the break energy [FORMULA], for different EP compositions relevant to the Orion complex as well as to the inner Galaxy. The fluxes are normalised to the Orion flux in the band 3-7 MeV. Only the lines with fluxes greater than [FORMULA] are shown, recalling that INTEGRAL's sensitivity is expected around [FORMULA] for narrow lines and [FORMULA] for broad lines (Mandrou et al. 1997). Note for example that the lines from 24 Mg (1.369 MeV) and 28 Si (1.779 MeV) which are observed in solar flares (e.g. Murphy et al. 1991) are always weaker than [FORMULA] in the considered scenarii.

[FIGURE] Fig. 14. Gamma-ray fluxes of the most intense lines arising from EPs-ISM interactions as a function of the injection energy break [FORMULA], for different compositions and metallicities. The numbers labeling the curves indicate the energy (in MeV) of the gamma-ray line in the rest frame of the emitting nuclei. Solid lines correspond to the cases when the inverse process (broad line component) dominates, and dashed lines when the direct process does (narrow line component). In the case when both components are approximately equal, we use alternate dashed lines.

7.1. The 7 Li-7 Be feature at [FORMULA] 0.450 MeV

The [FORMULA] fusion reactions lead to significant production of 7 Li, either directly or through the decay of the unstable mirror nuclei 7 Be. In any case, these reactions are accompanied by gamma-ray emission. Some of the 7 Li nuclei are indeed produced in an excited state at 0.478 MeV, while [FORMULA] of the 7 Be nuclei decay toward this excited state too. 7 Be nuclei produced in their first excited state also give rise to a line at 0.429 MeV. Because of their Doppler broadening, these two lines melt together in a broad feature around 0.450 MeV (e.g. Murphy et al. 1990) which we refer to as the 7 Li-7 Be feature. The total emission rate is [FORMULA] of the total 7 Li production rate (e.g. RKL79).

As can be seen in Fig. 14, the 7 Li-7 Be feature (labeled as 0.450) is quite intense and probably observable for some of our EP compositions. This constitutes a distinctive prediction of our mean-wind models as compared with late-WC EP composition, for which the gamma-ray flux in the 7 Li-7 Be feature is very low, notably below INTEGRAL's thresholds. This is because the surface composition of completely evolved WC stars is much poorer in helium than the WC wind itself. In the case of models E, i.e. with enhanced mass loss rates during the MS phase, the flux in the 7 Li-7 Be feature is even higher than the flux in the 6.129 MeV line of 16 O. This is also true for models C at twice solar metallicity and with an EPs' injection break energy between 8 and 20 MeV/n.

In any case, we predict a flux of 1- [FORMULA] at [FORMULA] MeV from the Orion complex, which is below the current OSSE upper limit, but above INTEGRAL's expected sensitivity.

7.2. The 10 B spallation lines at 0.717 MeV and 1.023 MeV

The spallation product 10 B can also be generated, either directly or from the decay of 10 C, through several excited levels, the most probable being at 0.717 MeV (RKL79). As we show in Fig. 14, the gamma-ray flux in the resulting de-excitation line should be rather intense ([FORMULA]), especially for high values of [FORMULA]. However, the main contributions to this line are the inverse reactions [FORMULA] and, to a lower extent, [FORMULA], so that the line should be broad, lowering the INTEGRAL SPI sensitivity by about a factor of 10.

Comparing the 0.717 MeV line fluxes for solar and twice solar metallicities, one can see that the (0.717 MeV)/(4.438 MeV) line ratio is unchanged. This is because both of these lines are produced by the same collisions, namely [FORMULA] X. However, this line ratio depends on the EP spectrum, because the threshold energy is higher for the 0.717 MeV line production than for 12 C excitation.

The same behavior is observed at a slightly lower level for the 1.023 MeV line, resulting from the de-excitation of another level of 10 B (see Fig. 14).

7.3. What about the 11 B spallation lines?

In the light of the two preceding sections, we point out that the spallation reactions induced by the EPs should also lead to rather intense de-excitation lines from 11 B, since the production rate of this isotope is always greater than that of 10 B by a factor of 2.5 or more. It is even greater than the 7 Li production rate for EP spectra with [FORMULA] MeV/n.

In the case of 11 B, measurements have been made for the main reaction, [FORMULA], at a bombarding energy of 50 MeV, showing evidence for gamma-ray emission from four excited levels at 2.12, 4.45, 5.01 and 6.79 MeV, with approximately equal fluxes (Pugh et al. 1967). RKL79 also include lines from 11 B spallation, using some cross sections of Zobel et al. (1968), but with large uncertainties, up to a factor of 10 near the resonance peak at [FORMULA] -40 MeV/n. We used their estimates to calculate the gamma-ray flux emitted near 2 MeV as a result of 12 C spallation. One component comes from 11 B de-excitation (2.124 MeV), and a second one comes from 11 C de-excitation (1.995 MeV) preceding the decay toward 11 B. The sum of these two components is shown in Fig. 14 with the label 2.000. The fluxes are quite low ([FORMULA]). However, these estimates are very uncertain and new measurements of these cross sections, especially near the reaction thresholds, would be of great astrophysical interest. In Orion, if one assumes that a significant fraction of 11 B is actually produced in an excited state (as 7 Li), then the de-excitation fluxes should be of order a few [FORMULA] and could even provide a non negligible contribution to the detected flux in the 3-7 MeV band.

7.4. Other 16 O lines

Fig. 14 also shows the contributions of two additional lines from the excited states at 6.917 and 7.117 MeV of 16 O. The emitted fluxes in these lines are of course lower than that arising from the first excited state, but would be among the most intense lines in the case when the 12 C [FORMULA] /16 O [FORMULA] line ratio is close to unity, i.e. for compositions with 12 C [FORMULA] O, like solar (SS), dust grain (GR), SN35 or late-WC compositions, but unlike our mean wind compositions. For the sake of completeness, we also mention the 2.741 MeV line arising from the partial de-excitation of the 8.872 MeV level of 16 O toward the 6.129 level, with a branching ratio of [FORMULA] (Lederer et al. 1978). The gamma-ray flux in this line reaches [FORMULA] (with the `Orion normalisation') for the late-WC composition, and [FORMULA] for the GR composition (not shown here).

Concerning the line width, all that we said for the 6.129 MeV line (see Sect.  6.2) holds also for the other 16 O lines.

7.5. The 14 N lines at 2.313 and 5.105 MeV

The first excited level of 14 N is at 2.313 MeV. It can be reached either by direct excitation or by spallation reactions involving 16 O nuclei. As can be seen in Fig. 14, the gamma-ray flux in the corresponding de-excitation line is always low: [FORMULA] with the Orion normalisation. This is due to the low 14 N abundance in the EPs, a consequence of the smaller contribution of the WN phase than the WC phase to the total stellar wind. One exception should however be noted, for the model with twice solar metallicity and an enhanced mass loss rate during the MS phase and, precisely, the WNL phase. In this case, the gamma-ray line flux reaches [FORMULA] for small values of the break energy [FORMULA].

An additional line at 5.105 MeV has also been considered by RKL79, and we therefore included it. It corresponds to the fourth excited level of 14 N (Lederer et al 1978). The corresponding flux is therefore always smaller than that of the 2.313 MeV line, and should not be detectable, expect for exceptionaly 14 N rich compositions. Note however that 14 N has two other levels at 3.948 MeV and 4.915 MeV, which should provide gamma-ray fluxes larger than that in the line at 5.105 MeV, and contribute to the total emission between 3 and 7 MeV. Unfortunately, the corresponding excitation cross sections are lacking.

7.6. 20 Ne and 22 Ne lines

Neon is known to have two isotopes (20 Ne and 22 Ne) characterised by distinct production mechanisms. This makes this element very interesting in the context of nucleosynthesis. Indeed, 22 Ne is produced during the helium burning phase from the 14 N nuclei (synthesized in the CNO cycle during the H burning phase), while 20 Ne is produced from 16 O during the carbon and the oxygen burning phases. As a consequence, since the winds of W-R stars only eject elements resulting from the helium burning, no freshly synthesized 20 Ne nuclei are present in the mean-wind (or mean-OB) compositions.

Indeed, as can be checked from Table 1, the EP compositions that we propose are richer in 22 Ne than in 20 Ne, whereas the usual ISM composition, i.e. solar, is made of about 10 times more 20 Ne than 22 Ne. The detection of the 22 Ne line at 1.275 MeV at the same flux as the 20 Ne line at 1.634 MeV would therefore constitute a clear signature of the link between the winds of Wolf-Rayet stars and the EPs.

Unfortunately, both lines have rather low fluxes, and should not be detected as individual lines. We further note that even in the hypothetical case of a 20 Ne line detection, the absence of the 22 Ne line would not be conclusive. Indeed, while both lines are expected to have approximately equal fluxes for all our mean wind compositions, the 20 Ne line is always narrow (due to the excitation of 20 Ne nuclei in the ISM) contrary to the 22 Ne line, which is broad (due to the 22 Ne nuclei in the EPs) and most certainly swamped in the rest of the gamma-ray emission.

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© European Southern Observatory (ESO) 1997

Online publication: March 24, 1998

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