Reaction rate uncertainties and the operation of the NeNa and MgAl chains during HBB in intermediate-mass AGB stars ^*

¹ Sterrekundig Instituut, University of Utrecht, Postbus 80000 3508 TA Utrecht, The Netherlands e-mail: [R.G.Izzard;M.Lugaro;raai]@phys.uu.nl
² Origins Institute, Department of Physics & Astronomy, McMaster University, Hamilton ON, Canada e-mail: karakas@physics.mcmaster.ca
³ Department of Physics and Astronomy, University of North Carolina, Chapel Hill, NC 27599-3255, USA e-mail: iliadis@unc.edu
⁴ Triangle Universities Nuclear Laboratory, PO Box 90308, Durham, NC 27708-0308, USA

Corresponding author: akarakas@mso.anu.edu.au

Received: 8 December 2006
Accepted: 7 February 2007

Abstract

Context.We test the effect of proton-capture reaction rate uncertainties on the abundances of the Ne, Na, Mg and Al isotopes processed by the NeNa and MgAl chains during hot bottom burning (HBB) in asymptotic giant branch (AGB) stars of intermediate mass between 4 and 6  and metallicities between and 0.02.

Aims.We provide uncertainty ranges for the AGB stellar yields, for inclusion in galactic chemical evolution models, and indicate which reaction rates are most important and should be better determined.

Methods.We use a fast synthetic algorithm based on detailed AGB models. We run a large number of stellar models, varying one reaction per time for a very fine grid of values, as well as all reactions simultaneously.

Results.We show that there are uncertainties in the yields of all the Ne, Na, Mg and Al isotopes due to uncertain proton-capture reaction rates. The most uncertain yields are those of ²⁶Al and ²³Na (variations of two orders of magnitude), ²⁴Mg and ²⁷Al (variations of more than one order of magnitude), ²⁰Ne and ²²Ne (variations between factors 2 and 7). In order to obtain more reliable Ne, Na, Mg and Al yields from IM-AGB stars the rates that require more accurate determination are: ²²Ne()²³Na, ²³Na()²⁴Mg, ²⁵Mg()²⁶Al, ²⁶Mg()²⁷Al and ²⁶Al()²⁷Si.

Conclusions.Detailed galactic chemical evolution models should be constructed to address the impact of our uncertainty ranges on the observational constraints related to HBB nucleosynthesis, such as globular cluster chemical anomalies.