Hot bottom burning and s-process nucleosynthesis in massive AGB stars at the beginning of the thermally-pulsing phase^⋆

¹ Instituto de Astrofísica de Canarias, C/ Via Láctea s/n, 38200 La Laguna, Spain
e-mail: agarcia@iac.es
² Departamento de Astrofísica, Universidad de La Laguna (ULL), 38206 La Laguna, Spain
³ INAF-Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio, Italy
⁴ University of Vienna, Department of Astrophysics, Türkenschanzstraße 17, 1180 Vienna, Austria
⁵ Research School of Astronomy & Astrophysics, Australian National University, ACT, 2611 Canberra, Australia
⁶ Monash Centre for Astrophysics (MoCA), Monash University, 3800 Victoria, Australia
⁷ W. J. McDonald Observatory. The University of Texas at Austin. 1 University Station, C1400. Austin, TX 78712−0259, USA

Received: 2 May 2013
Accepted: 9 June 2013

Abstract

We report the first spectroscopic identification of massive Galactic asymptotic giant branch (AGB) stars at the beginning of the thermal pulse (TP) phase. These stars are the most Li-rich massive AGBs found to date, super Li-rich AGBs with log ε (Li) ~ 3−4. The high Li overabundances are accompanied by weak or no s-process element (i.e. Rb and Zr) enhancements. A comparison of our observations with the most recent hot bottom burning (HBB) and s-process nucleosynthesis models confirms that HBB is strongly activated during the first TPs but the ²²Ne neutron source needs many more TP and third dredge-up episodes to produce enough Rb at the stellar surface. We also show that the short-lived element Tc, usually used as an indicator of AGB genuineness, is not detected in massive AGBs, which is in agreement with the theoretical predictions when the ²²Ne neutron source dominates the s-process nucleosynthesis.