Gaia Data Release 1. Testing parallaxes with local Cepheids and RR Lyrae stars

Context. Parallaxes for 331 classical Cepheids, 31 Type II Cepheids, and364 RR Lyrae stars in common between Gaia and the Hipparcos and Tycho-2catalogues are published in Gaia Data Release 1 (DR1) as part of theTycho-Gaia Astrometric Solution (TGAS). Aims: In order to testthese first parallax measurements of the primary standard candles of thecosmological distance ladder, which involve astrometry collected by Gaiaduring the initial 14 months of science operation, we compared them withliterature estimates and derived new period-luminosity (PL),period-Wesenheit (PW) relations for classical and Type II Cepheids andinfrared PL, PL-metallicity (PLZ), and optical luminosity-metallicity(MV-[Fe/H]) relations for the RR Lyrae stars, with zeropoints based on TGAS. Methods: Classical Cepheids were carefullyselected in order to discard known or suspected binary systems. Thefinal sample comprises 102 fundamental mode pulsators with periodsranging from 1.68 to 51.66 days (of which 33 withσϖ/ϖ<0.5). The Type II Cepheids include atotal of 26 W Virginis and BL Herculis stars spanning the period rangefrom 1.16 to 30.00 days (of which only 7 withσϖ/ϖ<0.5). The RR Lyrae stars include 200sources with pulsation period ranging from 0.27 to 0.80 days (of which112 with σϖ/ϖ<0.5). The new relations werecomputed using multi-band (V,I,J,Ks) photometry andspectroscopic metal abundances available in the literature, and byapplying three alternative approaches: (I) linear least-squares fittingof the absolute magnitudes inferred from direct transformation of theTGAS parallaxes; (II) adopting astrometry-based luminosities; and (III)using a Bayesian fitting approach. The last two methods work in parallaxspace where parallaxes are used directly, thus maintaining symmetricalerrors and allowing negative parallaxes to be used. The TGAS-basedPL,PW,PLZ, and MV- [Fe/H] relations are discussed bycomparing the distance to the Large Magellanic Cloud provided bydifferent types of pulsating stars and alternative fitting methods. Results: Good agreement is found from direct comparison of theparallaxes of RR Lyrae stars for which both TGAS and HST measurementsare available. Similarly, very good agreement is found between the TGASvalues and the parallaxes inferred from the absolute magnitudes ofCepheids and RR Lyrae stars analysed with the Baade-Wesselink method.TGAS values also compare favourably with the parallaxes inferred bytheoretical model fitting of the multi-band light curves for two of thethree classical Cepheids and one RR Lyrae star, which were analysed withthis technique in our samples. The K-band PL relations show thesignificant improvement of the TGAS parallaxes for Cepheids and RR Lyraestars with respect to the Hipparcos measurements. This is particularlytrue for the RR Lyrae stars for which improvement in quality andstatistics is impressive. Conclusions: TGAS parallaxes bring asignificant added value to the previous Hipparcos estimates. Therelations presented in this paper represent the first Gaia-calibratedrelations and form a work-in-progress milestone report in the wait forGaia-only parallaxes of which a first solution will become availablewith Gaia Data Release 2 (DR2) in 2018.Full Tables A.1-A.3 are only available at the CDS via anonymous ftp tohttp://cdsarc.u-strasbg.fr (http://130.79.128.5) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/605/A79