NOVAE, INTERSTELLAR AL-26, AND GALACTIC NITROGEN ISOTOPIC RATIO

Novae were thought to be promising sources to produce the 2-3 solar masses (M.) interstellar Al-26 needed to account for the 1.8 MeV gamma ray line observed to come from the central region of our Galaxy. Explosive hydrogen burning during novae bursts should produce not only Al-26 but also N-15. We used simple models for the chemical evolution of the Galaxy to study whether the novae hypothesis satisfies the interstellar N-15/N-14 observations. We found that, on a per N-15 basis, conventional C-O novae were not efficient producers of Al-26. It would require thousands of novae bursts per year to produce the required amount of Al-26. Moreover, they would have also over-produced N-15 by two orders of magnitude. Ne-Mg novae have high Al-26/N-15 production ratios because they started with more Mg seeds but fewer C and N seeds. They would thus satisfy the nitrogen constraint and produce enough Al-26. The inferred Ne-Mg novae rate would be about 100 per year for the Galaxy, a value still perhaps ten times too high.