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 \text { M } _ { \odot } 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 ^ { 14 } N and ^ { 22 } Ne 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 ^ { 22 } Ne .
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 to reproduce the observed large range of the [ Sr/Ba ] ratios in ( carbon-enhanced and normal ) EMP stars .