Long-period variables in the Magellanic Clouds - Supergiants, AGB stars, supernova precursors, planetary nebula precursors, and enrichment of the interstellar medium

Abstract

IR JHK magnitudes and low dispersion red spectra for 90 long period variables (LPVs) in the Magellanic Clouds show the LPVs to fall into distinct groups of core He or C burning supergiants and asymptotic giant branch (AGB) stars. The existence of stars at the AGB limit provides direct evidence that the more massive AGB stars produce supernovae. Carbon star LPVs are confined to luminosities fainter than about -6, in agreement with other Magellanic cloud carbon star studies. The absence of carbon stars among the upper AGB stars is interpreted as due to CNO cycling of dredged-up C to N during quiescent AGB evolution. The AGB stars are therefore sources of primary N, as well as of C and s-process elements. It is deduced that stars with initial masses greater than about 5 solar masses produce supernovae, while less massive stars produce planetary nebulae with nebula masses from 0.1 to 2.1 solar masses.