A fresh look at the RR Lyrae population in the Draco dwarf spheroidal galaxy with Gaia

TL;DR

This study catalogs 285 RR Lyrae stars in the Draco galaxy using Gaia data, introduces a new Gaia-based distance measurement method, and explores the galaxy's structure and interaction clues.

Contribution

It provides the first Gaia G-band luminosity-metallicity relation for RR Lyrae stars and applies it to determine Draco's structure and distance.

Findings

Average distance modulus of 19.53 mag for Draco

Detection of spatial variation suggesting interaction with the Milky Way

Confirmation of Draco's Oosterhoff-intermediate classification

Abstract

We present a catalogue of 285 RR Lyrae stars (RRLs) in the Draco dwarf spheroidal galaxy (dSph), obtained by combining data from a number of different surveys including the second data release (DR2) of the European Space Agency (ESA) cornerstone mission Gaia. We have determined individual distances to the RRLs in our sample using for the first time a Gaia G-band luminosity-metallicity relation (MG - [Fe/H]) and study the structure of the Draco dSph as traced by its RRL population. We find that the RRLs located in the western/south-western region of Draco appear to be closer to us, which may be a clue of interaction between Draco and the Milky Way (MW). The average distance modulus of Draco measured with the RRLs is 19.53 +/- 0.07 mag, corresponding to a distance of 80.5 +/- 2.6 kpc, in good agreement with previous determinations in the literature. Based on the pulsation properties of the RRLs we confirm the Oosterhoff-intermediate nature of Draco. We present an additional sample of 41 candidate RRLs in Draco, which we selected from the Gaia DR2 catalogue based on the uncertainty of their G-band magnitudes. Additional epoch data that will become available in the Gaia third data release (DR3) will help to confirm whether these candidates are bona-fide Draco RRLs.