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The elemental abundances between strontium and silver (Z= 38–47) observed in the atmospheres of very metal-poor stars in the Galaxy may contain the fingerprint of the weakr-process andνp-process occurring in early core-collapse supernovae explosions. In this work, we combine various astrophysical conditions based on a steady-state model to cover the richness of the supernova ejecta in terms of entropy, expansion timescale, and electron fraction. The calculated abundances based on different combinations of conditions are compared with stellar observations, with the aim of constraining supernova ejecta conditions. We find that some conditions of the neutrino-driven outflows consistently reproduce the observed abundances of our sample. In addition, from the successful combinations, the neutron-rich trajectories better reproduce the observed abundances of Sr–Zr (Z= 38–40), while the proton-rich ones, Mo–Pd (Z= 42–47).

 

Aims. An analysis of the methylidyne (CH) radical in non-local thermodynamic equilibrium (NLTE) is performed for the physical conditions of cool stellar atmospheres typical of red giants (log ɡ = 2.0, T eff = 4500 K) and the Sun. The aim of the present work is to explore whether the G band of the CH molecule, which is commonly used in abundance diagnostics of carbon-enhanced metal-poor stars, is sensitive to NLTE effects. Methods. LTE and NLTE theoretical spectra were computed with the MULTI code. We used one-dimensional (1D) LTE hydrostatic MARCS model atmospheres with parameters representing eleven red giant stars with metallicities ranging from [Fe/H] = −4.0 to [Fe/H] = 0.0 and carbon-to-iron ratios of [C/Fe] = 0.0, +0.7, +1.5, and +3.0. The CH molecule model was represented by 1981 energy levels, 18 377 radiative bound-bound transitions, and 932 photo-dissociation reactions. The rates due to transitions caused by collisions with free electrons and hydrogen atoms were computed using classical recipes. Results. Our calculations suggest that NLTE effects in the statistical equilibrium of the CH molecule are significant and cannot be neglected for precision spectroscopic analysis of C abundances. The NLTE effects are mostly driven by radiative over-dissociation, owing to the very low dissociation threshold of the molecule and significant resonances in the photo-dissociation cross-sections. The NLTE effects in the G band increase with decreasing metallicity. When comparing the C abundances determined from the CH G band in LTE and in NLTE, we show that the C abundances are always under-estimated if LTE is assumed. The NLTE corrections to C abundance inferred from the CH feature range from +0.04 dex for the Sun to +0.21 dex for a red giant with metallicity [Fe/H] = −4.0. Conclusions. Departures from the LTE assumption in the CH molecule are non-negligible, and NLTE effects have to be taken into account in the diagnostic spectroscopy based on the CH lines. We show here that the NLTE effects in the optical CH lines are non-negligible for the Sun and red giant stars, but further calculations are warranted to investigate the effects in other regimes of stellar parameters. 

Abstract A promising astrophysical site to produce the lighter heavy elements of the first r -process peak ( Z = 38 − 47) is the moderately neutron-rich (0.4 < Y e < 0.5) neutrino-driven ejecta of explosive environments, such as core-collapse supernovae and neutron star mergers, where the weak r -process operates. This nucleosynthesis exhibits uncertainties from the absence of experimental data from ( α , xn ) reactions on neutron-rich nuclei, which are currently based on statistical model estimates. In this work, we report on a new study of the nuclear reaction impact using a Monte Carlo approach and improved ( α , xn ) rates based on the Atomki-V2 α optical model potential. We compare our results with observations from an up-to-date list of metal-poor stars with [Fe/H] < −1.5 to find conditions of the neutrino-driven wind where the lighter heavy elements can be synthesized. We identified a list of ( α , xn ) reaction rates that affect key elemental ratios in different astrophysical conditions. Our study aims to motivate more nuclear physics experiments on ( α , xn ) reactions using the current and new generation of radioactive beam facilities and also more observational studies of metal-poor stars. 

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