tokens: grains (Fujiya et al. 2013 ; Jadhav et al. 2013 ; Liu et al. 2014 ). Stellar evolution theory predicts that, besides the H-ingesting He-shell flash convection on white dwarfs, the i process may also occur in metal-poor low-mass thermally pulsing AGB stars (Lugaro et al. 2012 ), during a He-core flash in metal-poor RGB stars (Campbell, Lugaro Karakas 2010 ), in super-AGB stars (Jones et al. 2016 ), and in Population-III massive stars when their H- and He-burning shells merge (Banerjee Qian Heger 2018 ; Clarkson, Herwig Pignatari 2019 ). In this work, we investigate the robustness of the weak i-process elemental abundances and their ratios that are proposed to contribute to the anomalous chemical composition of the star HD94028. The predicted abundances are affected by uncertainties of the (n,γ) reaction rates for the unstable isotopes involved in this nucleosynthesis. By ‘weak’ with typical neutron exposure τ ∼ 1 we mean an i process that, in spite of having a high neutron density, does not reach a large neutron exposure (time-integrated neutron flux), e.g. because it is terminated by the violent, global, non-radial instability that can be induced by H ingestion into the He-shell convection (Herwig et al. 2014b ), as probably happened in Sakurai’s object Herwig et al. ( 2011 ). As a result of this ‘weakness’ (a low neutron exposure), only the first n-capture peak elements (those with the neutron number close to the magic one N = 50) are produced in a significant amount by the weak i process. We employ the same computational methods and analysis tools that Denissenkov et al. ( 2018 ; hereafter, Paper I ) used in a similar study of i process producing the first peak elements observed in Sakurai’s object. Unlike in Paper I , we do not know the i-process site that contributed to the composition of HD94028. Therefore, we have to use a site-independent one-zone model with the temperature T , density ρ, and initial chemical composition adjusted to mimic the i-process physical conditions appropriate for this case, like it was done by Roederer et al. ( 2016 ). We also deploy constant neutron–density equilibrium models that are independent of the neutron-source reactions. 2 METHODS 2.1 The benchmark simulation For the one-zone simulations of the i-process nucleosynthesis, we use the NuGrid code ppn (Pignatari et al. 2016 ) with the same fixed temperature T = 2 × 10 8 K and density as in Paper I . The initial chemical composition is prepared using the Solar system abundances of Asplund et al. ( 2009 ) scaled to the metallicity of the star HD94028 ([Fe/H] =−1.6), assuming that the α-element abundances are enhanced with the mean value of [α/Fe] equal to +0.4. The initial mass fraction of 12 C is increased to X ( 12 C) = 0.5 to be close to the values found in He-flash convective zones. We also reduce the initial hydrogen abundance to the value of X ( 1 H) = 0.2 adjusted to guarantee that the neutron density will reach the values of N n = 10 13 –10 16 cm −3
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