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

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 solar masses and metallicities between Z=0.0001 and 0.02. 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. 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. 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(p,g)23Na, 23Na(p,g)24Mg, 25Mg(p,g)26Al, 26Mg(p,g)27Al and 26Al(p,g)27Si. 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.Comment: accepted for publication on Astronomy & Astrophysic

Rb-rich Asymptotic Giant Branch stars in the Magellanic Clouds

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.Comment: accepted for publication in The Astrophysical Journal Letters (10 pages, 3 figures and 2 Tables

Reaction rate uncertainties and 26Al in AGB silicon carbide stardust

Stardust is a class of presolar grains each of which presents an ideally uncontaminated stellar sample. Mainstream silicon carbide (SiC) stardust formed in the extended envelopes of carbon-rich asymptotic giant branch (AGB) stars and incorporated the radioactive nucleus 26Al as a trace element. The aim of this paper is to analyse in detail the effect of nuclear uncertainties, in particular the large uncertainties of up to four orders of magnitude related to the 26Al_g+(p,gamma)27Si reaction rate, on the production of 26Al in AGB stars and compare model predictions to data obtained from laboratory analysis of SiC stardust grains. Stellar uncertainties are also briefly discussed. We use a detailed nucleosynthesis postprocessing code to calculate the 26Al/27Al ratios at the surface of AGB stars of different masses (M = 1.75, 3, and 5 M_sun) and metallicities (Z = 0.02, 0.012, and 0.008). For the lower limit and recommended value of the 26Al_g(p,gamma)27Si reaction rate, the predicted 26Al/27Al ratios replicate the upper values of the range of the 26Al/27Al ratios measured in SiC grains. For the upper limit of the 26Al_g(p,gamma)27Si reaction rate, instead, the predicted 26Al/27Al ratios are approximately 100 times lower and lie below the range observed in SiC grains. When considering models of different masses and metallicities, the spread of more than an order of magnitude in the 26Al/27Al ratios measured in stellar SiC grains is not reproduced. We propose two scenarios to explain the spread of the 26Al/27Al ratios observed in mainstream SiC, depending on the choice of the 26Al_g+p reaction rate. One involves different times of stardust formation, the other involves extra-mixing processes. Stronger conclusions will be possible after more information is available from future nuclear experiments on the 26Al_g+p reaction.Comment: 6 pages, 5 Postscript figures, accepted for publication in Astronomy and Astrophysic

Rubidium, zirconium, and lithium production in intermediate-mass asymptotic giant branch stars

A recent survey of a large sample of Galactic intermediate-mass (>3 Msun) asymptotic giant branch (AGB) stars shows that they exhibit large overabundances of rubidium (Rb) up to 100--1000 times solar. These observations set constraints on our theoretical notion of the slow neutron capture process (s process) that occurs inside intermediate-mass AGB stars. Lithium (Li) abundances are also reported for these stars. In intermediate-mass AGB stars, Li can be produced by proton captures occuring at the base of the convective envelope. For this reason the observations of Rb, Zr, and Li set complementary constraints on different processes occurring in the same stars. We present predictions for the abundances of Rb, Zr, and Li as computed for the first time simultaneously in intermediate-mass AGB star models and compare them to the current observational constraints. We find that the Rb abundance increases with increasing stellar mass, as is inferred from observations but we are unable to match the highest observed [Rb/Fe] abundances. Inclusion of a partial mixing zone (PMZ) to activate the 13C(a,n)16O reaction as an additional neutron source yields significant enhancements in the Rb abundance. However this leads to Zr abundances that exceed the upper limits of the current observational constraints. If the third dredge-up (TDU) efficiency remains as high during the final stages of AGB evolution as during the earlier stages, we can match the lowest values of the observed Rb abundance range. We predict large variations in the Li abundance, which are observed. Finally, the predicted Rb production increases with decreasing metallicity, in qualitative agreement with observations of Magellanic Cloud AGB stars. However stellar models of Z=0.008 and Z=0.004 intermediate-mass AGB stars do not produce enough Rb to match the observed abundances.Comment: 11 pages, 7 figures, accepted for publication on Astronomy & Astrophysic

Current hot questions on the s process in AGB stars

The version of record, M. Lugaro et al., 2016, 'Current Hot Questions on the s process in AGB stars', Journal of Physics: Conference Series, Vol. 665, 012021, published under licence by IOP Publishing Ltd, is available on line at doi: 10.1088/1742-6596/665/1/012021 Content from this work may be used under the terms of the Creative Commons Attribution 3.0 licence. Any further distribution of this work must maintain attribution to the author(s) and the title of the work, journal citation and DOI.Asymptotic giant branch (AGB) stars are a main site of production of nuclei heavier than iron via the s process. In massive (>4 M⊙) AGB stars the operation of the 22Ne neutron source appears to be confirmed by observations of high Rb enhancements, while the lack of Tc in these stars rules out 13C as a main source of neutrons. The problem is that the Rb enhancements are not accompanied by Zr enhancements, as expected by s-process models. This discrepancy may be solved via a better understanding of the complex atmospheres of AGB stars. Second- generation stars in globular clusters (GCs), on the other hand, do not show enhancements in any s-process elements, not even Rb. If massive AGB stars are responsible for the composition of these GC stars, they may have evolved differently in GCs than in the field. In AGB stars of lower masses, 13C is the main source of neutrons and we can potentially constrain the effects of rotation and proton-ingestion episodes using the observed composition of post-AGB stars and of stardust SiC grains. Furthermore, independent asteroseismology observations of the rotational velocities of the cores of red giants and of white dwarves will play a fundamental role in helping us to better constrain the effect of rotation. Observations of carbon-enhanced metal-poor stars enriched in both Ba and Eu may require a neutron flux in-between the s and the r process, while the puzzling increase of Ba as function of the age in open clusters, not accompanied by increase in any other element heavier than iron, require further observational efforts. Finally, stardust SiC provides us high-precision constraints to test nuclear inputs such as neutron-capture cross sections of stable and unstable isotopes and the impact of excited nuclear states in stellar environments.Peer reviewe

Nucleosynthesis Predictions for Intermediate-Mass Asymptotic Giant Branch Stars: Comparison to Observations of Type I Planetary Nebulae

Type I planetary nebulae (PNe) have high He/H and N/O ratios and are thought to be descendants of stars with initial masses of 3-8 M ⊙. These characteristics indicate that the progenitor stars experienced proton-capture nucleosynthesis at the base of t

Rubidium and zirconium production in massive AGB stars

A recent survey of a large sample of massive Galactic asymptotic giant branch (AGB) stars shows that significant overabundances of rubidium (up to 100 times solar), but merely solar zirconium, are present in these stars. These observations can set constraints on our theoretical notion of the slow neutron capture process (the s process) in AGB stars, as well as on the rates of the neutron capture reactions involved in the production of Rb and Zr. We use the Monash nucleosynthesis code with a recently extended network to try to reproduce these observations. We present results for AGB stars of masses 5, 6, and 6.5Mȯ and solar metallicity. We also show results for different available choices of the neutron capture rates, as well as for the possible inclusion of a partial mixing zone (PMZ), leading to the activation of the 13C neutron source. We find increasing Rb overabundances with increasing stellar mass, as observed, but we are far from matching the highest observed Rb enhancements. Inclusion of a PMZ increases the Rb abundance, but also produces an overabundance of Zr, contrary to what is observed. Only if the third dredge up efficiency remains as high as before the onset of the superwind phase during the final few pulses of a massive AGB star, can we match the highest [Rb/Fe] ratios observed by García-Hernández et al. [l]. A better understanding of the third dredge up efficiency with decreasing envelope mass for massive AGB stars is essential for further investigation of this issue

Rubidium, zirconium, and lithium production in massive AGB stars

A recent survey of a large sample of massive Galactic asymptotic giant branch (AGB) stars shows that significant overabundances of rubidium (up to 100 times solar) are present in these stars. Zirconium, on the other hand, is not enriched compared to the

A massive AGB star as source of short-lived nuclei in the early solar system

The origin of short-lived radioactive nuclei at the time of the formation of the Sun is a much debated mystery in modern astronomy. We show that abundance predictions for massive AGB stars (roughly 6Mȯ) of solar metallicity can self-consistently match t