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

Novae were thought to be promising sources to produce the 2-3 solar ma sses (M.) interstellar Al-26 needed to account for the 1.8 MeV gamma r ay line observed to come from the central region of our Galaxy. Explos ive hydrogen burning during novae bursts should produce not only Al-26 but also N-15. We used simple models for the chemical evolution of th e Galaxy to study whether the novae hypothesis satisfies the interstel lar N-15/N-14 observations. We found that, on a per N-15 basis, conven tional C-O novae were not efficient producers of Al-26. It would requi re thousands of novae bursts per year to produce the required amount o f Al-26. Moreover, they would have also over-produced N-15 by two orde rs of magnitude. Ne-Mg novae have high Al-26/N-15 production ratios be cause they started with more Mg seeds but fewer C and N seeds. They wo uld 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.