Multiplicity of Galactic Cepheids and RR Lyrae stars from Gaia DR2

I. Binarity from proper motion anomaly^⋆

¹ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Univ. Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
e-mail: Pierre.Kervella@obspm.fr
² European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Santiago, Chile
³ Smithsonian Astrophysical Observatory, MS 4, 60 Garden Street, Cambridge, MA 02138, USA
⁴ Konkoly Observatory, MTA CSFK, Konkoly Thege M. út 15-17, 1121, Hungary
⁵ GEPI, Observatoire de Paris, Université PSL, CNRS, 5 Place Jules Janssen, 92190, Meudon, France
⁶ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
⁷ Ecole Normale Supérieure de Lyon, 15 parvis René Descartes, 69342 Lyon, France
⁸ Warsaw University Observatory, Al. Ujazdowskie 4, 00-478 Warsaw, Poland
⁹ Université Côte d’Azur, OCA, CNRS, Lagrange, France
¹⁰ Universidad de Concepción, Departamento de Astronomía, Casilla 160-C, Concepción, Chile
¹¹ Nicolaus Copernicus Astronomical Centre, Polish Academy of Sciences, Bartycka 18, 00-716 Warszawa, Poland

Received: 8 September 2018
Accepted: 23 January 2019

Abstract

Context. Classical Cepheids (CCs) and RR Lyrae stars (RRLs) are important classes of variable stars used as standard candles to estimate galactic and extragalactic distances. Their multiplicity is imperfectly known, particularly for RRLs. Astoundingly, to date only one RRL has convincingly been demonstrated to be a binary, TU UMa, out of tens of thousands of known RRLs.

Aims. Our aim is to detect the binary and multiple stars present in a sample of Milky Way CCs and RRLs.

Methods. In the present article, we combine the HIPPARCOS and Gaia DR2 positions to determine the mean proper motion of the targets, and we search for proper motion anomalies (PMa) caused by close-in orbiting companions.

Results. We identify 57 CC binaries from PMa out of 254 tested stars and 75 additional candidates, confirming the high binary fraction of these massive stars. For 28 binary CCs, we determine the companion mass by combining their spectroscopic orbital parameters and astrometric PMa. We detect 13 RRLs showing a significant PMa out of 198 tested stars, and 61 additional candidates.

Conclusions. We determine that the binary fraction of CCs is likely above 80%, while that of RRLs is at least 7%. The newly detected systems will be useful to improve our understanding of their evolutionary states. The discovery of a significant number of RRLs in binary systems also resolves the long-standing mystery of their extremely low apparent binary fraction.