Gaia Data Release 2. Specific characterisation and validation of all-sky Cepheids and RR Lyrae stars

Abstract

Gaia second Data Release (DR2) presents a first mapping of full-sky RR Lyrae stars and Cepheids observed by the spacecraft during the initial 22 months of observations and publishes characteristic parameters derived for these sources by the Specific Objects Study (SOS) pipeline developed to validate and fully characterise Cepheids and RR Lyrae stars (SOS Cep&RRL) observed by Gaia. The SOS Cep&RRL processing uses tools such as the period-amplitude (PA) and the period-luminosity (PL) relations in the G-band. For the Gaia DR2 data processing we also used tools based on the G_BP and G_RP photometry, such as the period-Wesenheit (PW) relation in G,G_RP. Furthermore, we implemented the use of parallaxes working directly in parallax space and applied different PL, PW relations depending on the source position on sky, whether in the LMC, in the SMC or outside them. G, G_BP and G_RP time series photometry and characterisation by the SOS Cep&RRL pipeline (mean magnitudes and pulsation characteristics) are published in Gaia DR2 for a total of 150,359 sources distributed all over the sky: 9,575 are classified as Cepheids and 140,784 as RR Lyrae stars. These samples include also variables in 87 globular clusters and 12 dwarf galaxies. To the best of our knowledge, as of 25 April 2018, 50,570 of these variables (about 350 Cepheids and 50,220 RR Lyrae stars) do not have a known counterpart in the literature, hence they are likely new discoveries by Gaia. Furthermore, an estimate of the interstellar absorption is published for 54,272 fundamental mode RR Lyrae stars from a relation based on the amplitude of the light variation in the G-band and the star period. Photometric metal abundances ([Fe/H]) derived from the Fourier parameters of the light curves are also released for 64,932 RR Lyrae stars and for 3,738 fundamental-mode classical Cepheids with period shorter than 6.3 days.