Cepheid Metallicity in the Leavitt Law (C- MetaLL) survey: IX: Spectroscopic detection of rare earth Dysprosium, Erbium, Lutetium and Thorium in Classical Cepheids

TL;DR

This study expands the C-MetaLL Cepheid survey by obtaining high-resolution spectroscopic metallicities for 60 Classical Cepheids (136 spectra) and introduces the first detections of Dysprosium, Erbium, Lutetium, and Thorium in Cepheids. Using updated line lists and LTE spectral synthesis across 32 elements, it derives consistent atmospheric parameters and abundances, revealing a Galactic $[Fe/H]$ gradient of $-0.064\pm0.003$ dex kpc$^{-1}$ and negative gradients for most elements, with heavier neutron-capture elements showing flattening. The outer Cepheids, analyzed under various spiral-arm models, tend to trace the Perseus and/or OSC arms, highlighting Cepheids as tracers of young disc chemistry and Galactic structure. These results refine the metallicity calibration of the Leavitt law and provide new constraints on the production of heavy elements in the Milky Way.

Abstract

Classical Cepheids are among the most important distance calibrators 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. We aim to increase the number of available DCEPS with high-resolution spectroscopic metallicities, to 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. 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 up to 33 different species. 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 survey and increase the number of available chemical species. For the first time (to our knowledge), we present the estimation of abundances for Dysprosium, as well as a systematic estimation of Erbium, Lutetium and Thorium abundances. We calculate a galactic radial gradient for [Fe/H] with a slope of $-0.064\pm0.002$, 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 (OSC) arms.

Figures (8)

• Figure 1: Distribution of the C-MetaLL sample along the Galactic disc. The points are colour-coded according to the pulsator's period. Targets presented in this work are highlighted with black contours. The position of the sun is shown with a yellow-black circle.
• Figure 2: Comparison of two neutral Zinc lines at $\lambda$ = 4810.532 (left panel) and $\lambda$ = 6362.338 (right panel)Å, for the same star (OGLE-GD-CEP-0228). The two spectral lines are highlighted with dashed black vertical lines, while the dashed black horizontal line defines the continuum (set at 1).
• Figure 3: Chemical elements in the form of [X/Fe] plotted against iron ([Fe/H]). Grey points represent stars already published in Ripepi2021aTrentin2023atrentin2024. Stars observed with UVES and published in Trentin2023atrentin2024 are coloured in yellow and red, respectively. New stars presented in this paper are coloured in blue, magenta and green for PEPSI, HARPS and UVES observations, respectively (see also the legend in \ref{['fig:iron_grad']}). Vertical and horizontal grey dashed lines highlight the solar abundance position in the plot.
• Figure 4: Observed spectrum of RY Cru (black solid line, left and centre panel) and Gaia DR2 4644478166748030976 (orange solid line, right panel) in the region of DyII (5090.386 Å, left panel), ErII (5028.905 Å, centre panel) and LuII (6221.89 Å, right panel) spectral lines. The position of these and other visible lines is reported and highlighted with grey dashed vertical lines. Synthetic spectra computed for the best estimated abundance are reported in blue, while the same spectra with [X/H]=$\pm$0.5 dex are plotted in red and green, respectively.
• Figure 5: Radial galactic gradients for each element in the form of [X/H]. Grey points represent stars already published in Ripepi2021aTrentin2023atrentin2024. Stars observed with UVES and published in Trentin2023atrentin2024 are coloured in yellow and red, respectively. New stars presented in this paper are coloured in blue, magenta and green for PEPSI, HARPS and UVES observations, respectively (see also the legend in \ref{['fig:iron_grad']}).