Metallicities of Classical Cepheids in the Inner Galactic Disk

TL;DR

This study measures the metallicities of 16 classical Cepheids in the inner Galactic disk using high-resolution near-infrared spectroscopy, revealing the metallicity gradient in this region and insights into stellar kinematics influenced by the Galactic bar.

Contribution

First measurement of the metallicity gradient in the inner Galactic disk using Cepheids with high-resolution near-infrared spectra, expanding understanding of Galactic chemical distribution.

Findings

Inner disk Cepheids have metallicities mostly between 0.1 and 0.3 dex in [Fe/H].

The metallicity gradient in the inner disk may be shallower or flat.

Inner-disk Cepheids show non-circular velocities influenced by the Galactic bar.

Abstract

Metallicity gradients refer to the sloped radial profile of metallicities of gas and stars and are commonly seen in disk galaxies. A well-defined metallicity gradient of the Galactic disk is observed particularly well with classical Cepheids, which are good stellar tracers thanks to their period-luminosity relation allowing precise distance estimation and other advantages. However, the measurement of the inner-disk gradient has been impeded by the incompleteness of previous samples of Cepheids and limitations of optical spectroscopy in observing highly reddened objects. Here we report the metallicities of 16 Cepheids measured with high-resolution spectra in the near-infrared YJ bands. These Cepheids are located at 3-5.6 kpc in the Galactocentric distance, R(GC), and reveal the metallicity gradient in this range for the first time. Their metallicities are mostly between 0.1 and 0.3 dex in [Fe/H] and more or less follow the extrapolation of the metallicity gradient found in the outer part, R(GC) larger than 6.5 kpc. The gradient in the inner disk may be shallower or even flat, but the small sample does not allow to determine the slope precisely. More extensive spectroscopic observations would also be necessary for studying minor populations, if any, with higher or lower metallicities that were reported in previous literature. In addition, three-dimensional velocities of our inner-disk Cepheids show the kinematic pattern that indicates non-circular orbits caused by the Galactic bar, which is consistent with the patterns reported in recent studies on high-mass star-forming regions and red giant branch stars.