Speaker
Robert Izzard
(University of Utrecht)
Description
We study the effect of uncertainties in the proton-capture reaction rates
on nucleosynthesis due to the operation of the NeNa and MgAl chains during
hot bottom burning (HBB) in intermediate-mass asymptotic giant branch
(AGB) stars. This kind of nucleosynthesis is associated with the production
of sodium, of the radioactive nucleus Al26 and of the heavy magnesium isotopes, and
it is possibly responsible for the O, Na, Mg and Al abundance anomalies
observed in globular cluster stars. We model HBB with an analytic code
based on full stellar evolution models. In this way we can calculate a very large
number of stars in a relatively quick time (e.g. 10^6 stars in 12 hours).
We have computed stellar models at two different metallicities (0.02 and 0.004)
and two masses (5 and 6 solar masses). We vary in turn each of the p-capture rates
involved in the NeNa and MgAl chains of factors corresponding to their
current uncertainties in the range of temperatures relevant for HBB (T_9 ~
0.06 - 0.1 K).
We find large uncertainties, variations of up to one order of
magnitude, in the final yields of Na23 and Ne22, Al26, Al27, and Mg24,
because of uncertanties in the Ne22(p,g)Na23, Mg25(p,g)Al26,
Al26(p,g)Si27, Mg26(p,g)Al27 and Na23(p,g)Mg24 reaction rates.
The uncertainty ranges increase with decreasing metallicity and increasing
mass of the star, because the temperature at the base of the convective
envelope increases in these cases, making HBB more efficient.
We are in the process of modelling stars of lower metallicity (0.0001),
where the effect of HBB is even greater. Current work also involves the
computation of models in which we vary all the reaction rates in the same model.
We then calculate the variation of each isotopic yield due to the uncertainties
associated with all the reaction rates involved in the NeNa and MgAl chains.
Author
Robert Izzard
(University of Utrecht)
Co-authors
Dr
Amanda Karakas
(McMaster University)
Prof.
Christian Iliadis
(University of North Carolina)
Dr
Maria Lugaro
(University of Utrecht)
