Metallicities of Classical Cepheids in the Inner Galactic Disk

Abstract

Metallicity gradients refer to the sloped radial profiles of the 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 the 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 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 > 6.5 kpc. The gradient in the inner disk may be shallower or even flat, but the small sample does not allow the determination of 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, the 3D velocities of our inner-disk Cepheids show a kinematic pattern that indicates noncircular 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.

WARP: The Data Reduction Pipeline for the WINERED Spectrograph

“…The high throughput and resolution of WINERED have advanced the observational studies of various astronomical objects: metallicities and chemical abundances of various types of stars (D'Orazi et al 2018;Kondo et al 2019;Jian et al 2020;Fukue et al 2021;Taniguchi et al 2021;Matsunaga et al 2022Matsunaga et al , 2023, the diffuse interstellar bands and interstellar molecules (Hamano et al 2019(Hamano et al , 2022, young stellar objects (Yasui et al 2019), AGN (Mizumoto et al 2024), luminous blue variable stars (Mizumoto et al 2018), quasars (Sameshima et al 2020) and comets (Shinnaka et al 2017).…”

The globular cluster VVV CL002 falling down to the hazardous Galactic centre

“…On June 10, 2023, we used the WINERED spectrograph (Ikeda et al 2022;Matsunaga et al 2023) attached to the Magellan Clay telescope in Chile to obtain high- resolution spectra of VIRAC2 ID=9327562027332 and VIRAC2 ID=9327562015390. WINERED is a NIR high-resolution spectrograph covering 0.90-1.35 µm (z , Y, and J bands), with a resolution of R = λ/∆λ = 28 000 with the WIDE mode (Ikeda et al 2022).…”