Distribution of iron, oxygen, and sulfur in the direction of the outer arms of the Galaxy^★

¹ Astronomical Observatory, Odessa National University, Shevchenko Park, 65014 Odessa, Ukraine
² Department of Physics and Astronomy, University of Hawai’i at Hilo, Hilo, HI 96720, USA
³ Crimean Astrophysical Observatory, Nauchny 298409, Republic of Crimea

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

Received: 3 October 2025
Accepted: 5 December 2025

Abstract

Aims. We conducted a high-resolution spectroscopic study of 51 distant Cepheids located in the outer part of the Galactic disk in the direction of the Perseus Arm. The aim of this investigation is to search for a possible observational manifestation of the influence of spiral arms or other dynamical processes on the chemical properties in this region of low-density gas.

Methods. The effective temperature for each Cepheid was obtained from the line-depth-ratio dependencies. Abundances of three chemical elements – oxygen, sulfur, and iron – were obtained. We used the local-thermodynamic-equilibrium (LTE) approximation for iron, and the nonlocal thermodynamic equilibrium (non-LTE) approximation for oxygen and sulfur.

Results. The abundances of iron, oxygen, and sulfur in our program stars were plotted as a function of galactocentric distances determined using heliocentric distances based on parallaxes from Gaia DR3, corrected for parallax bias. The Locally Weighted Scatterplot-Smoothing (LOWESS) method was used for statistical analysis of the plotted dependencies. We found a clear sign of flattening of the radial abundance distribution (plateau-like structure) for the studied elements, starting at a galactocentric distance of approximately 14 kpc. Formally dividing the overall iron abundance distribution into two parts and applying a linear regression for each section yields the following results on the gradient: –0.072 dex kpc⁻¹ for the 8–14 kpc range and –0.006 dex kpc⁻¹ for the 14–22 kpc range, with a formal intercept at 14 kpc. Similar gradients were also found for sulfur.

Conclusions. We believe this flattening is the result of the dynamical influence of the Perseus and other outer spiral arms on the global star formation processes in the gas disk at the outskirts of our Galaxy.