Rb-rich Asymptotic Giant Branch stars in the Magellanic Clouds

TL;DR

This study identifies Rb-rich massive AGB stars in the Magellanic Clouds, revealing extreme Rb enrichment and challenging existing s-process nucleosynthesis models, with implications for stellar evolution understanding.

Contribution

First high-resolution spectra of heavily obscured massive O-rich AGB stars in the Magellanic Clouds, showing extreme Rb enrichment and raising questions about s-process nucleosynthesis.

Findings

Discovery of strong Rb I lines in LMC AGB stars at high luminosities.

Extreme Rb enrichment up to 10^5 times solar in these stars.

High Rb/Zr ratios challenge current nucleosynthesis models.

Abstract

We present high-resolution (R~60,000) optical spectra of a carefully selected sample of heavily obscured and presumably massive O-rich Asymptotic Giant Branch (AGB) stars in the Magellanic Clouds (MCs). We report the discovery of strong Rb I lines at 7800 A in four Rb-rich LMC stars at luminosities equal to or greater than the standard adopted luminosity limit for AGB stars (Mbol~-7.1), confirming that "Hot Bottom Burning" (HBB) may produce a flux excess in the more massive AGB stars. In the SMC sample, just one of the five stars with Mbol<-7.1 was detected in Rb; the other stars may be massive red supergiants. The Rb-rich LMC AGB stars might have stellar masses of at least ~6-7 Msun. Our abundance analysis show that these Rb-rich stars are extremely enriched in Rb by up to 10^3-10^5 times solar but seem to have only mild Zr enhancements. The high Rb/Zr ratios, if real, represent a severe problem for the s-process, even if the 22Ne source is operational as expected for massive AGB stars; it is not possible to synthesize copious amounts of Rb without also overproducing Zr. The solution to the problem may lie with an incomplete present understanding of the atmospheres of luminous AGB stars.