Rare find: Discovery and chemo-dynamical properties of two s-process-enhanced RR Lyrae stars

¹ Department of Physics, University of Rome Tor Vergata, Via della Ricerca Scientifica 1, 00133 Rome, Italy
² INAF Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
³ Department of Astronomy & McDonald Observatory, The University of Texas at Austin, 2515 Speedway, Austin, TX 78712, USA
⁴ Departament de Física Quàntica i Astrofísica, Universitat de Barcelona, c. Martí i Franquès, 1, 08028 Barcelona, Spain
⁵ Konkoly Observatory, HUN-REN Research Centre for Astronomy and Earth Sciences, MTA Centre for Excellence, Konkoly Thege Miklós út 15-17, Budapest 1121, Hungary
⁶ MTA-ELTE Lendület “Momentum” Milky Way Research Group, Szent Imre h. u. 112, 9700 Szombathely, Hungary
⁷ Eötvös Loránd University, Institute of Physics and Astronomy, H-1117 Pázmány Péter sétány 1/A, Budapest, Hungary
⁸ School of Physics and Astronomy, Monash University, Clayton, VIC 3800, Australia
⁹ OzGrav: Australian Research Council Centre of Excellence for Gravitational Wave Discovery, Clayton, VIC 3800, Australia
¹⁰ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, Monte Porzio Catone, Italy
¹¹ Space Science Data Center – ASI, Via del Politecnico SNC, 00133 Rome, Italy
¹² Department of Physics, North Carolina State University, Raleigh, NC 27695, USA
¹³ Max-Planck-Institut fur Astronomie, Konigstuhl 17, D-69117 Heidelberg, Germany

^⋆ Corresponding author: vdorazi@roma2.infn.it

Received: 10 September 2025
Accepted: 16 October 2025

Abstract

Aims. We report the serendipitous discovery of two RR Lyrae stars that exhibit significant s-process element enrichment, a rare class previously represented solely by TY Gruis. Our goal is to characterise these objects chemically and dynamically, and explore their origins and evolutionary histories.

Methods. Using high-resolution spectroscopy from HERMES@AAT and UVES@VLT, we derived detailed chemical abundances of key s-process elements (Y, Ba, La, Ce, Nd, and Eu) and carbon, along with α elements (Ca, Mg, and Ti). We also employed Gaia Data Release 3 astrometric data to analyse their kinematics, orbital properties, and classify their Galactic population membership. We compared observational results with theoretical asymptotic giant branch (AGB) nucleosynthesis models to interpret their enrichment patterns.

Results. Both stars exhibit clear signatures of s-process enrichment, with significant overabundances in second-peak elements such as Ba and La compared to first-peak Y and Zr. Comparison with AGB nucleosynthesis models suggests their progenitors experienced pollution of s-process-rich material, consistent with early binary interactions. However, notable discrepancies in dilution factors highlight the need for more refined low-metallicity AGB models. We also explore and discuss alternative scenarios, including sub-luminous post-AGB-like evolution or double episodes of mass transfer. In the latter case, the star initially undergoes a mass transfer when it is on the main sequence, accreting material from a former AGB companion. Subsequently, as the star evolves along the red giant branch, it may again transfer mass to its companion before becoming an RR Lyrae star.

Conclusions. Our findings confirm the existence of s-process-enhanced RR Lyrae stars and demonstrate the importance of combining chemical and dynamical diagnostics to unveil their complex evolutionary pathways. Future detailed binary evolution modelling and long-term orbital monitoring are essential to resolve their formation scenarios and assess the role of binarity in the evolution of pulsating variables.