Oxygen, sulfur, and iron radial abundance gradients of classical Cepheids across the Galactic thin disk^{★,★★,★★★}

¹ INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00078 Monte Porzio Catone, Italy
e-mail: ronaldo.oliveira@inaf.it
² Agenzia Spaziale Italiana, Space Science Data Center, via del Politecnico snc, 00133 Rome, Italy
³ Department of Physics, University of Rome Tor Vergata, via della Ricerca Scientifica 1, 00133 Rome, Italy
⁴ INAF – Osservatorio Astronomico di Padova, vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁵ Sterrenkundig Observatorium, Ghent University, Krijgslaan 281 - S9, 9000 Gent, Belgium
⁶ INAF – Osservatorio Astronomico di Bologna, via Gobetti 93/3, 40129 Bologna, Italy
⁷ Astronomisches Rechen-Institut, ZAH, Universität Heidelberg, Mönchhofstr. 12–14, 69120 Heidelberg, Germany
⁸ INAF, Osservatorio Astronomico di Trieste, via G. B. Tiepolo 11, 34131, Trieste, Italy
⁹ Dipartimento di Fisica, Sezione di Astronomia, Università degli Studi di Trieste, via G. B. Tiepolo 11, 34131 Trieste, Italy
¹⁰ INFN, Sezione di Trieste, via A. Valerio 2, 34100 Trieste, Italy
¹¹ Materials Science and Applied Mathematics, Malmö University, 205 06 Malmö, Sweden
¹² Astronomical Observatory, Odessa National University, Shevchenko Park, 65014 Odessa, Ukraine
¹³ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, Firenze, Italy
¹⁴ Max Planck Institute for Astronomy, 69117 Heidelberg, Germany
¹⁵ Niels Bohr International Academy, Niels Bohr Institute, Blegdamsvej 17, 2100 Copenhagen Ø, Denmark
¹⁶ INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
¹⁷ GEPI, Observatoire de Paris, PSL Research University, CNRS, 61 avenue de l’Observatoire, 75014 Paris, France
¹⁸ UPJV, Université de Picardie Jules Verne, 33 rue St Leu, 80080 Amiens, France
¹⁹ Science and Technology Department, Parthenope University of Naples, Naples, Italy
²⁰ LMU München, Universitätssternwarte, Scheinerstr. 1, 81679 München, Germany
²¹ Institute for Astronomy, University of Hawaii at Manoa, 2680 Woodlawn Drive, Honolulu, HI 96822, USA
²² Department of Astronomy, School of Science, The University of Tokyo, 7-3-1, Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
²³ Laboratory of Infrared High-Resolution spectroscopy (LiH), Koyama Astronomical Observatory, Kyoto Sangyo University, Motoyama, Kamigamo, Kita-ku, Kyoto 603-8555, Japan
²⁴ Instituto de Astrofísica de Canarias (IAC), C. Vía Láctea s/n, 38205 La Laguna, Spain
²⁵ Departamento de Astrofísica, Universidad de La Laguna (ULL), 38200 La Laguna, Spain
²⁶ Department of Astronomy and McDonald Observatory, The University of Texas, Austin, TX 78712, USA
²⁷ Leibniz-Institut für Astrophysik Potsdam (AIP), An der Sternwarte 16, 14482, Potsdam, Germany
²⁸ Université Côte d’Azur, Observatoire de la Côte d’Azur, CNRS, Lagrange UMR 7293, CS 34229, 06304 Nice Cedex 4, France
²⁹ National Astronomical Observatory of Japan, Mitaka, Tokyo 1818588, Japan
³⁰ Department of Astrophysics, University of Vienna, Türkenschanzstraße 17, 1180 Vienna, Austria

Received: 23 May 2023
Accepted: 25 July 2023

Abstract

Context. Classical Cepheids (CCs) are solid distance indicators and tracers of young stellar populations. Dating back to the beginning of the 20th century, they have been safely adopted to trace the rotation, kinematics, and chemical enrichment history of the Galactic thin disk.

Aims. The main aim of this investigation is to provide iron, oxygen, and sulfur abundances for the largest and most homogeneous sample of Galactic CCs analyzed so far (1118 spectra of 356 objects). The current sample, containing 70 CCs for which spectroscopic metal abundances are provided for the first time, covers a wide range in galactocentric distances, pulsation modes, and pulsation periods.

Methods. Optical high-resolution spectra with a high signal-to-noise ratio that were collected with different spectrographs were adopted to provide homogeneous estimates of the atmospheric parameters (effective temperature, surface gravity, and microturbulent velocity) that are required to determine the abundance. Individual distances were based either on trigonometric parallaxes by the Gaia Data Release 3 (Gaia DR3) or on distances based on near-infrared period-luminosity relations.

Results. We found that iron and α-element radial gradients based on CCs display a well-defined change in the slope for galactocentric distances larger than ~12 kpc. We also found that logarithmic regressions account for the variation in [X/H] abundances from the inner to the outer disk. Radial gradients for the same elements, but based on open clusters covering a wide range in cluster ages, display similar trends. This means that the flattening in the outer disk is an intrinsic feature of the radial gradients because it is independent of age. Empirical evidence indicates that the S radial gradient is steeper than the Fe radial gradient. The difference in the slope is a factor of two in the linear fit (−0.081 vs. −0.041 dex kpc⁻¹) and changes from −1.62 to −0.91 in the logarithmic distance. Moreover, we found that S (explosive nucleosynthesis) is underabundant on average when compared with O (hydrostatic nucleosynthesis). The difference becomes clearer in the metal-poor regime and for the [O/Fe] and [S/Fe] abundance ratios. We performed a detailed comparison with Galactic chemical evolution models and found that a constant star formation efficiency for galactocentric distances larger than 12 kpc accounts for the flattening observed in both iron and α-elements. To further constrain the impact of the predicted S yields for massive stars on radial gradients, we adopted a toy model and found that the flattening in the outermost regions requires a decrease of a factor of four in the current S predictions.

Conclusions. CCs are solid beacons for tracing the recent chemical enrichment of young stellar populations. Sulfur photospheric abundances, when compared with other α-elements, have the key advantage of being a volatile element. Therefore, stellar S abundances can be directly compared with nebular sulfur abundances in external galaxies.