Does the i-process operate at nearly solar metallicity?

¹ Department of Physics, University of Calicut, Thenhipalam, Malappuram 673635, India
e-mail: drdrisyak@uoc.ac.in
² Institut d’Astronomie et d’Astrophysique, Université Libre de Bruxelles (ULB) C.P. 226, 1050 Bruxelles, Belgium
e-mail: svaneck@astro.ulb.ac.be
³ European Southern Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago de Chile, Chile
⁴ Institute of Physics, Laboratory of Astrophysics, École polytechnique fédérale de Lausanne (EPFL), Observatoire de Sauverny, 1290 Versoix, Switzerland
⁵ Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium

Received: 25 January 2023
Accepted: 25 April 2023

Abstract

Context. A sample of 895 s-process-rich candidates has been found among the 454 180 giant stars surveyed by LAMOST at low spectral resolution (R ~ 1800). In a previous study, taking advantage of the higher resolution (R ~ 86 000) offered by the HERMES-Mercator spectrograph, we performed a reanalysis of 15 of the brightest stars of this sample. Among these 15 programme stars, which have close-to-solar metallicities, 11 showed mild to strong heavy-element overabundances. However, we did not investigate the nucleosynthetic process(es) at the origin of these overabundances in our former study.

Aims. We derive the abundances in s- and r-process elements of the 15 targets in order to investigate whether some stars also show an i-process signature, as sometimes found in their lower-metallicity counterparts (namely, the carbon-enhanced metal-poor (CEMP)-rs stars).

Methods. Abundances are derived from the high-resolution HERMES spectra for Pr, Nd, Sm, and Eu using the TURBOSPECTRUM radiative-transfer local thermodynamic equilibrium code with MARCS model atmospheres.

Results. Using the new classification scheme proposed in our recent study, we find that two stars show overabundances in both sand r-process elements that are well above the level expected from Galactic chemical evolution (GCE), an analogous situation to that of CEMP-rs stars at lower metallicities. We compare the abundances of the most enriched stars with the nucleosynthetic predictions from the STAREVOL stellar evolutionary code and find abundances compatible with an i-process occurring in asymptotic giant branch stars.

Conclusions. Despite there being a larger number of heavy elements with which to characterize the enrichment pattern, the limit between CEMP-s and CEMP-rs stars remains indistinct. Nevertheless, it is interesting to note that an increasing number of extrinsic stars are found to have abundances better reproduced by an i-process pattern even at close-to-solar metallicities.