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Register a new userNuGrid: s process in massive stars
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Authors
Raphael Hirschi
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The s-process production in massive stars at very low metallicities is expected to be negligible due to the low abundance of the neutron source 22Ne, to primary neutron poisons and decreasing iron seed abundances. However, recent models of massive stars including the effects of rotation show that a strong production of 22Ne is possible in the helium core, as a consequence of the primary nitrogen production (observed in halo metal poor stars). Using the PPN post-processing code, we studied the impact of this primary 22Ne on the s process. We find a large production of s elements between strontium and barium, starting with the amount of primary 22Ne predicted by stellar models. There are several key reaction rate uncertainties influencing the s-process efficiency. Among them, 17O(alpha,gamma) may play a crucial role strongly influencing the s process efficiency, or it may play a negligible role, according to the rate used in the calculations. We also report on the development of a new parallel (MPI) post-processing code (MPPNP) designed to follow the complete nucleosynthesis in stars on highly resolved grids. We present here the first post-processing run from the ZAMS up to the end of helium burning for a 15 solar mass model.
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Context. Rotation is known to affect the nucleosynthesis of light elements in massive stars, mainly by rotation-induced mixing. In particular, rotation boosts the primary nitrogen production. Models of rotating stars are able to reproduce the nitrogen observed in low-Z halo stars. Aims. Here we present the first grid of stellar models for rotating massive stars at low Z, where a full s-process network is used to study the impact of rotation-induced mixing on the nucleosynthesis of heavy elements. Methods. We used the Geneva stellar evolution code that includes an enlarged reaction network with nuclear species up to bismuth to calculate 25 M$_odot$ models at three different Z and with different initial rotation rates. Results. First, we confirm that rotation-induced mixing leads to a production of primary $^{22}$Ne, which is the main neutron source for the s process in massive stars. Therefore rotation boosts the s process in massive stars at all Z. Second, the neutron-to-seed ratio increases with decreasing Z in models including rotation, which leads to the complete consumption of all iron seeds at Z < 1e-3 by the end of core He-burning. Thus at low Z, the iron seeds are the main limitation for this boosted s process. Third, as Z decreases, the production of elements up to the Ba peak increases at the expense of the elements of the Sr peak. We studied the impact of the initial rotation rate and of the uncertain $^{17}$O$(alpha,gamma)$ rate (which strongly affects the neutron poison strength of $^{16}$O) on our results. This study shows that rotating models can produce significant amounts of elements up to Ba over a wide range of Z. Fourth, compared to the He-core, the primary $^{22}$Ne production in the He-shell is even higher (> 1% in mass fraction at all Z), which could open the door for an explosive neutron capture nucleosynthesis in the He-shell, with a primary neutron source.
Current models of s-nucleosynthesis in massive stars ($Msim15 M_{odot}$ to $sim 30 M_{odot}$) are able to reproduce some main features of the abundance distributions of heavy isotopes in the solar system, at least in the $Asim 60-90$ mass range. The efficiency of the process and the above specified mass range for the s-nuclei are still heavily uncertain due to both nuclear reaction rates and stellar models uncertainties. A series of s-process simulations with stellar models in the $15-30 M_{odot}$ (mass at ZAMS) and metallicity $Z=0.02$ mass have been performed to analyse the impact of the overshooting model used on the s-process yields. As in a previous exploratory work performed with stellar models having $M_{ZAMS}=25 M_{odot}$ and $Z=0.02$, enhancements factors in the range 2-5 are found in the final s-process efficiency when overshooting is inserted in the models.
The goal of this paper is to analyze the impact of a primary neutron source on the s-process nucleosynthesis in massive stars at halo metallicity. Recent stellar models including rotation at very low metallicity predict a strong production of primary N14. Part of the nitrogen produced in the H-burning shell diffuses by rotational mixing into the He core where it is converted to Ne22 providing additional neutrons for the s process. We present nucleosynthesis calculations for a 25 Msun star at [Fe/H] = -3, -4, where in the convective core He-burning about 0.8 % in mass is made of primary Ne22. The usual weak s-process shape is changed by the additional neutron source with a peak between Sr and Ba, where the s-process yields increase by orders of magnitude with respect to the yields obtained without rotation. Iron seeds are fully consumed and the maximum production of Sr, Y and Zr is reached. On the other hand, the s-process efficiency beyond Sr and the ratio Sr/Ba are strongly affected by the amount of Ne22 and by nuclear uncertainties, first of all by the Ne22(alpha,n)Mg25 reaction. Finally, assuming that Ne22 is primary in the considered metallicity range, the s-process efficiency decreases with metallicity due to the effect of the major neutron poisons Mg25 and Ne22. This work represents a first step towards the study of primary neutron source effect in fast rotating massive stars, and its implications are discussed in the light of spectroscopic observations of heavy elements at halo metallicity.
A large sample of carbon enhanced metal-poor stars enriched in s-process elements (CEMP-s) have been observed in the Galactic halo. These stars of low mass (M ~ 0.9 Msun) are located on the main-sequence or the red giant phase, and do not undergo third dredge-up (TDU) episodes. The s-process enhancement is most plausibly due to accretion in a binary system from a more massive companion when on the asymptotic giant branch (AGB) phase (now a white dwarf). In order to interpret the spectroscopic observations, updated AGB models are needed to follow in detail the s-process nucleosynthesis. We present nucleosynthesis calculations based on AGB stellar models obtained with FRANEC (Frascati Raphson-Newton Evolutionary Code) for low initial stellar masses and low metallicities. For a given metallicity, a wide spread in the abundances of the s-process elements is obtained by varying the amount of 13C and its profile in the pocket, where the 13C(a, n)16O reaction is the major neutron source, releasing neutrons in radiative conditions during the interpulse phase. We account also for the second neutron source 22Ne(a, n)25Mg, partially activated during convective thermal pulses. We discuss the surface abundance of elements from carbon to bismuth, for AGB models of initial masses M = 1.3 -- 2 Msun, low metallicities ([Fe/H] from -1 down to -3.6) and for different 13C-pockets efficiencies. In particular we analyse the relative behaviour of the three s-process peaks: light-s (ls at magic neutron number N = 50), heavy-s (hs at N = 82) and lead (N = 126). Two s-process indicators, [hs/ls] and [Pb/hs], are needed in order to characterise the s-process distribution. In the online material, we provide a set of data tables with surface predictions. ...
In this paper we present a large-scale sensitivity study of reaction rates in the main component of the $s$ process. The aim of this study is to identify all rates, which have a global effect on the $s$ process abundance distribution and the three most important rates for the production of each isotope. We have performed a sensitivity study on the radiative $^{13}$C-pocket and on the convective thermal pulse, sites of the $s$ process in AGB stars. We identified 22 rates, which have the highest impact on the $s$-process abundances in AGB stars.
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