The ACS LCID Project: RR Lyrae stars as tracers of old population gradients in the isolated dwarf spheroidal galaxy Tucana

TL;DR

This study uses RR Lyrae stars to trace the early metallicity and age gradients in the isolated dwarf galaxy Tucana, revealing a central concentration of older, metal-rich stars formed over 10 billion years ago.

Contribution

It provides the first detailed analysis of RR Lyrae spatial distribution in Tucana, linking pulsational properties to metallicity and age gradients in an isolated dwarf galaxy.

Findings

Fainter RR Lyrae stars are more centrally concentrated.

Fainter RR Lyrae stars are more metal-rich.

The metallicity gradient formed over 10 Gyr ago.

Abstract

We present a study of the radial distribution of RR Lyrae variables, which present a range of photometric and pulsational properties, in the dwarf spheroidal galaxy Tucana. We find that the fainter RR Lyrae stars, having a shorter period, are more centrally concentrated than the more luminous, longer period RR Lyrae variables. Through comparison with the predictions of theoretical models of stellar evolution and stellar pulsation, we interpret the fainter RR Lyrae stars as a more metal-rich subsample. In addition, we show that they must be older than about 10 Gyr. Therefore, the metallicity gradient must have appeared very early on in the history of this galaxy.