RR Lyrae variables in the Small Magellanic Cloud - II. The extended area: chemical and structural analysis

TL;DR

This study analyzes RR Lyrae stars in the Small Magellanic Cloud to determine their metallicity, distance, and structural properties, revealing a metallicity gradient and complex three-dimensional structure including a thick disk, bulge, and halo.

Contribution

It provides the first detailed chemical and structural analysis of RR Lyrae variables across an extended area of the SMC using Fourier decomposition of light curves.

Findings

Average metallicity of RRab stars: [Fe/H] = -1.69 dex

Detected a metallicity gradient of -0.013 dex/kpc

SMC has a complex 3D structure with a thick disk, bulge, and halo

Abstract

We have performed the Fourier decomposition analysis of 8- and 13-year V-band light curves of a carefully selected sample of 454 fundamental-mode RR Lyrae variables (RRab type), detected in a 14 square degree area of the Small Magellanic Cloud (SMC) and listed in the Optical Gravitational Lensing Experiment, phase III, Catalogue of Variable Stars. The Fourier decomposition parameters were used to derive metal abundances and distance moduli, following the methodology described by Kapakos, Hatzidimitriou & Soszy\'nski. The average metal abundance of the RRab stars on the new scale of Carretta et al. was found to be <[Fe/H]C09> = -1.69pm0.41 dex (std, with a standard error of 0.02 dex). A tentative metallicity gradient of -0.013pm0.007 dex/kpc was detected, with increasing metal abundance towards the dynamical center of the SMC, but selection effects are also discussed. The distance modulus of the SMC was re-estimated and was found to be <\mu> = 19.13pm0.19 (std) in a distance scale where the distance modulus of the Large Magellanic Cloud (LMC) is \mu_LMC = 18.52pm0.06(std). The average 1-sigma line-of-sight depth was found to be sigma_int = 5.3pm0.4 kpc (std), while spatial variations of the depth were detected. The SMC was found to be deeper in the north-eastern region, while metal richer and metal poorer objects in the sample seem to belong to different dynamical structures. The former have smaller scale height and may constitute a thick disk, its width being 10.40pm0.02 kpc, and a bulge whose size (radius) is estimated to be 2.09pm0.81 kpc. The latter seem to belong to a halo structure with a maximum depth along the line of sight extending over 16 kpc in the SMC central region and falling to 12 kpc in the outer regions.