A&A 620, A127 (2018) Astronomy https://doi.org/10.1051/0004-6361/201833514 & © ESO 2018 Astrophysics Gaia Data Release 2 Special issue Gaia Data Release 2 Validating the classification of RR Lyrae and Cepheid variables with the Kepler and K2 missions? László Molnár1,2, Emese Plachy1,2, Áron L. Juhász1,3, and Lorenzo Rimoldini4 1 Konkoly Observatory, MTA CSFK, 1121, Budapest, Konkoly Thege Miklós út 15-17, Hungary 2 MTA CSFK Lendület Near-Field Cosmology Research Group, 1121, Budapest, Konkoly Thege Miklós út 15-17, Hungary e-mail: [email protected] 3 Department of Astronomy, Eötvös Loránd University, 1117, Budapest, Pázmány Péter sétány 1/a, Hungary 4 Department of Astronomy, University of Geneva, Chemin d’Ecogia 16, 1290, Versoix, Switzerland Received 28 May 2018 / Accepted 22 October 2018 ABSTRACT Context. The second data release of the Gaia mission (DR2) includes an advance catalogue of variable stars. The classifications of these stars are based on sparse photometry from the first 22 months of the mission. Aims. We set out to investigate the purity and completeness of the all-sky Gaia classification results with the help of the continuous light curves of the observed targets from the Kepler and K2 missions, focusing specifically on RR Lyrae and Cepheid pulsators, outside the Galactic bulge region. Methods. We cross-matched the Gaia identifications with the observations collected by the Kepler space telescope. We inspected the light curves visually, then calculated the relative Fourier coefficients and period ratios for the single- and double-mode K2 RR Lyrae stars to further classify them. Results. We identified 1443 and 41 stars classified as RR Lyrae or Cepheid variables in Gaia DR2 in the targeted observations of the two missions and 263 more RR Lyre targets in the full-frame images (FFI) of the original mission. We provide the cross-match of these sources. We conclude that the RR Lyrae catalogue has a completeness between 70–78%, and provide a purity estimate of between 92 and 98% (targeted observations) with lower limits of 75% (FFI stars) and 51% (K2 worst-case scenario). The low number of Cepheids prevents us from drawing detailed conclusions, but the purity of the DR2 sample is estimated to be about 66%. Key words. stars: variables: general – stars: variables: RR Lyrae – stars: variables: Cepheids 1. Introduction is now nearly complete, concluding the work started a century ago by Leavitt(1908). RR Lyrae stars are large-amplitude pulsating stars with easily The Gaia Data Release 2 (DR2; Gaia Collaboration 2018) recognisable light-curve shapes. They can be used to measure represents a new step forward in mapping the distribution of distances, to trace structures in the Milky Way and other galax- RR Lyrae and Cepheid stars, as well as other variable stars in ies, and to investigate stellar pulsation and evolution. Large sky the Milky Way and its vicinity. However, these classifications surveys have mapped the distribution of RR Lyrae stars in par- are based on sparse photometry, and those classifications can ticular to large numbers and depths (see e.g. Drake et al. 2013; be affected by poor phase coverage and/or confusion between Sesar et al. 2013; Beaton et al. 2016; Hernitschek et al. 2016; variable types. Minniti et al. 2017, for some examples). Space photometric missions, such as MOST, CoRoT, Kepler, Cepheids, an umbrella term used to describe the classical δ or BRITE, provide dense, continuous photometry of variable Cephei stars, the Type II Cepheids, and the anomalous Cepheids, stars that might in principle be used to validate the results from are even more important tools for mapping the structure of other surveys. However, most missions have either small aper- the cosmos and individual galaxies: although they are less fre- tures or very limited sample sizes or both, therefore their use quent than RR Lyrae stars, they are more luminous, and hence for validation purposes is very limited. The only exception is the more easily detectable. The OGLE survey has very thoroughly Kepler space telescope, which observed hundreds of thousands searched both the Galactic bulge and the Magellanic Clouds for of targets so far, including thousands of RR Lyrae and hundreds all members of the Cepheid and RR Lyrae families. According of Cepheid stars (Szabó et al. 2017; Molnár 2018). Our prelim- to Soszynski´ et al.(2017), their collection of classical Cepheids inary studies with the PanSTARRS PS1 survey by Hernitschek et al.(2016) indicate that a comparison with data from Kepler is ? Full Tables A1, A4, and A5 are only available at the CDS via feasible, but the first results indicate that eclipsing binary stars anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via may contaminate the RR Lyrae sample in the Galactic disc in http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/620/A127 that survey (Juhász & Molnár 2018). Article published by EDP Sciences A127, page 1 of 16 A&A 620, A127 (2018) These findings led us to use the observations of Kepler to In this study, we used the following information available provide an independent validation of the classifications of the in the Gaia DR2 archive (fields and table names specified in various RR Lyrae and Cepheid-type variables in Gaia DR2. footnotes): celestial coordinates1, median G brightnesses2, the Conversely, input from Gaia DR2 will allow us in the future to number of FoV transits in the G band3, classification classes4 correctly identify as RR Lyrae or Cepheid stars targets that hide and scores5 ranging between 0 and 1, provided by the all-sky in the Kepler and K2 databases. classification pipeline (Rimoldini et al. 2018). The paper is structured as follows: in Sect.2 we intro- duce the variable star observations made by Gaia and Kepler, 2.2. Kepler and K2 missions respectively; in Sect.3 we describe the classification of the Kepler light curves into the various subclasses; in Sect.4 we The Kepler space telescope was designed to detect transiting present the results of the validation, and in Sect.5 we provide exoplanets and determine the occurrence of Earth-like, temper- our conclusions. ate rocky planets around Sun-like stars (Borucki et al. 2010; Borucki 2016). To achieve this, it collected quasi-continuous photometry of approximately 170 000 stars in a field between 2. Observations Lyra and Cygnus, spanning about 115 deg2. The majority of stars 2.1. Variable stars in Gaia DR2 were observed with a 29.4 min sampling, called long cadence (LC), and the telescope was rolled by 90 deg every quarter Holl et al.(2018) summarized the results for the more than half year. Not all stars were continuously observed: the target list a million variable stars published in Gaia DR2, which include was updated for every quarter. The prime mission lasted for candidates of RR Lyrae stars, Cepheids, long-period variables, four years, until the breakdown of two reaction wheels on board. rotation modulation (BY Draconis) stars, δ Scuti & SX Phoenicis Afterwards, Kepler was reoriented to observe in shorter, 60- stars, and short-timescale variables. to 80-day-long campaigns along the ecliptic, using only the two Different techniques were employed in the variability remaining reaction wheels. The new mission, named K2, had pipeline depending on the variability type. Here, we focus no core science program, and the space telescope has been uti- on the Cepheids and RR Lyrae stars identified by the lized as a space photometric observatory instead, carrying out all-sky classification (Rimoldini et al. 2018), available in a very broad science program that extends beyond exoplanets the gaiadr2.vari_classifier_result table of the Gaia and stellar physics (Howell et al. 2014). Coordination and target archive. Such classifications are normally verified by a subse- selection is managed by the Kepler Guest Observer Office6. Stel- quent pipeline module dedicated to the RR Lyrae and Cepheid lar physics studies are coordinated by the Kepler Asteroseismic variables (Clementini et al. 2018), but only for sources with Science Consortium (KASC), which has a dedicated RR Lyrae at least 12 field-of-view (FoV) transits in the G band. The and Cepheids Working Group. variable classes that we use are the four RR Lyrae subtypes, During the original mission, Kepler observed about 50 RRAB, RRC, and RRD/ARRD (corresponding to fundamental- RR Lyrae stars, one classical Cepheid, V1154 Cyg, and a mode, first-overtone, and double-mode stars with either normal Type-II Cepheid of the RVb subtype, DF Cyg (see e.g. Benko˝ or anomalous period ratios), and various Cepheid types, CEP, et al. 2010; Nemec et al. 2013; Bódi et al. 2016; Derekas et al. T2CEP, and ACEP (corresponding to classical or δ Cephei stars, 2017; Vega et al. 2017, and references therein). The K2 mission Type II Cepheids, and anomalous Cepheids). Throughout the greatly expanded the spatial coverage of the telescope, and pro- paper, we refer to the variable classifications in Gaia DR2 in all pelled the numbers of observed RR Lyrae and Cepheid stars into capitals (RRAB, RRC, RRD), and use the regular RRab, RRc, several thousands and hundreds, respectively, greatly exceeding RRd notations for our classifications based on the Kepler and the observations of previous space photometric missions (Szabó K2 light curves. et al. 2017). Our goal is to provide an independent assessment of the com- An important aspect of the Kepler space telescope that sets pleteness and purity rates of the all-sky classification results of it apart from other space photometric missions is the large opti- RR Lyrae and Cepheid candidates based on the fine light-curve cal aperture (95 cm), allowing us to reach targets fainter than Kp sampling of Kepler and K2 targets, regardless of the number of 20 mag (where Kp refers to brightness in the wide passband of observations and other features or limitations related to the Gaia Kepler∼ ).