Cepheid Metallicity in the Leavitt Law (C- MetaLL) survey

VIII. Spectroscopic detection of rare earth dysprosium, erbium, lutetium, and thorium in classical Cepheids

¹ INAF-Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Naples, Italy
² INAF-Osservatorio Astrofisico di Catania, Via S.Sofia 78, 95123 Catania, Italy
³ Università di Salerno, Dipartimento di Fisica “E.R. Caianiello”, Via Giovanni Paolo II 132, 84084 Fisciano (SA), Italy
⁴ INAF-Osservatorio di Astrofisica e Scienza dello Spazio, Via Gobetti 93/3, 40129 Bologna, Italy
⁵ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482 Potsdam, Germany
⁶ Inter-University Center for Astronomy and Astrophysics (IUCAA), Post Bag 4, Ganeshkhind, Pune 411 007, India
⁷ Istituto Nazionale di Fisica Nucleare (INFN)-Sez. di Napoli, Via Cinthia, 80126 Napoli, Italy
⁸ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching bei München, Germany
⁹ Scuola Superiore Meridionale, Largo San Marcellino10 80138 Napoli, Italy
¹⁰ INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00078 Monte Porzio Catone, Italy

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 6 November 2025
Accepted: 9 January 2026

Abstract

Context. Classical Cepheids (DCEPs) are among the most important distance calibrators thanks to the correlation between their period and luminosity (PL relation), and play a crucial role in the calibration as the first rung of the extragalactic distance ladder. Given their typical age, they also constitute an optimal tracer of the young population in the Galactic disc.

Aims. We aim to increase the number of available DCEPs with high-resolution spectroscopic metallicities, study the galactocentric radial gradients of several chemical elements, and analyse the spatial distribution of the Galactic young population of stars in the Milky Way disc.

Methods. We performed a complete spectroscopical analysis of 136 spectra obtained from three different high-resolution spectrographs, for a total of 60 DCEPs. More than half have pulsational periods longer than 15 days, up to 70 days, doubling the number of stars in our sample with P > 15d. We derived radial velocities, atmospheric parameters, and chemical abundances for up to 33 different species.

Results. We present an updated list of trusted spectroscopic lines for the detection and estimation of chemical abundances. We used this new set to revisit the abundances already published in the context of the C-MetaLL (Cepheids-Metallicity in the Leavitt Law) survey and increase the number of available chemical species. For the first time (to our knowledge), we present the estimation of abundances for Cepheids for dysprosium (Dy, Z = 66), as well as a systematic estimation of erbium (Er, ZZ = 68), lutetium (Lu, Z = 71), and thorium (Th, Z = 90) abundances.

Conclusions. We calculated a galactic radial gradient for [Fe/H] with a slope of −0.064 ± 0.002 dex kpc⁻¹, in good agreement with recent literature estimation. The other elements also exhibit a clear negative radial trend, with this effect diminishing and eventually disappearing for heavier neutron-capture elements. Depending on the proposed spiral arms model present in several literature sources, our most external stars agree on tracing either the Perseus, the Norma-Outer, or both the Outer and the association Outer-Scutum-Centaurus arms.