s-process production in rotating massive stars at solar and low metallicities

Frischknecht, Urs; Hirschi, Raphael; Pignatari, Marco; Maeder, André; Meynet, George; Chiappini, Cristina; Thielemann, Friedrich-Karl; Rauscher, Thomas; Georgy, Cyril; Ekström, Sylvia

Abstract

Rotation was shown to have a strong impact on the structure and light element nucleosynthesis in massive stars. In particular, models including rotation can reproduce the primary nitrogen observed in halo extremely metal poor (EMP) stars. Additional exploratory models showed that rotation may enhance s-process production at low metallicity. Here we present a large grid of massive star models including rotation and a full s-process network to study the impact of rotation on the weak s-process. We explore the possibility of producing significant amounts of elements beyond the strontium peak, which is where the weak s-process usually stops. We used the Geneva stellar evolution code coupled to an enlarged reaction network with 737 nuclear species up to bismuth to calculate 15–40 M⊙ models at four metallicities (Z = 0.014, 10⁻³, 10⁻⁵ and 10⁻⁷) from the main sequence up to the end of oxygen burning. We confirm that rotation-induced mixing between the convective H-shell and He-core enables an important production of primary ¹⁴N and ²²Ne and s-process at low metallicity. At low metallicity, even though the production is still limited by the initial number of iron seeds, rotation enhances the s-process production, even for isotopes heavier than strontium, by increasing the neutron-to-seed ratio. The increase in this ratio is a direct consequence of the primary production of ²²Ne. Despite nuclear uncertainties affecting the s-process production and stellar uncertainties affecting the rotation-induced mixing, our results show a robust production of s-process at low metallicity when rotation is taken into account. Considering models with a distribution of initial rotation rates enables us to reproduce the observed large range of the [Sr/Ba] ratios in (carbon-enhanced and normal) EMP stars.

Citation

Frischknecht, U., Hirschi, R., Pignatari, M., Maeder, A., Meynet, G., Chiappini, C., Thielemann, F.-K., Rauscher, T., Georgy, C., & Ekström, S. (2016). s-process production in rotating massive stars at solar and low metallicities. Monthly notices of the Royal Astronomical Society, 456(2), 1803-1825. https://doi.org/10.1093/mnras/stv2723

• Journal Article Type: Article
• Acceptance Date: Nov 18, 2015
• Online Publication Date: Dec 28, 2015
• Publication Date: Feb 21, 2016
• Deposit Date: Mar 31, 2016
• Publicly Available Date: Nov 23, 2017
• Journal: Monthly notices of the Royal Astronomical Society
• Print ISSN: 0035-8711
• Electronic ISSN: 1365-2966
• Publisher: Oxford University Press
• Peer Reviewed: Peer Reviewed
• Volume: 456
• Issue: 2
• Pages: 1803-1825
• DOI: https://doi.org/10.1093/mnras/stv2723
• Keywords: Stars: abundances, Stars: chemically peculiar, Stars: massive, Stars: population II , Stars: rotation, Galaxy: abundances
• Public URL: https://hull-repository.worktribe.com/output/434534
• Publisher URL: http://mnras.oxfordjournals.org/content/456/2/1803
• Additional Information: This article has been accepted for publication in Monthly notices of the Royal Astronomical Society. ©2015 The Authors. Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.
• Contract Date: Nov 23, 2017