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.
Highlights
We present the sample of Classical Cepheids (CCs) and RR Lyrae stars (RRLs) selected for our present PM analysis (Paper I) and for the search for resolved common proper motion companions presented in Paper II
The smoothly decreasing shape of the binary fraction curve shown in Fig. 4 is due to the the sensitivity of the proper motion anomalies (PMa) technique in terms of companion mass being a linear function of the parallax
We detected a significant number of new candidate companions of CCs and RRLs from the signature of their orbital motion on the proper motion vector of the targets
Summary
The remarkable correlation of the intrinsic luminosity of classical Cepheids (CCs; Leavitt 1908; Leavitt & Pickering 1912; Fouqué et al 2007) and RR Lyrae stars (RRLs; Catelan et al 2004; Neeley et al 2017) respectively with their pulsation period and metallicity makes these two classes of variable stars essential standard candles for Galactic (Drake et al 2013), globular cluster (Carney et al 1992), and extragalactic distance measurements (Clementini et al 2003; Riess et al 2011, 2016). From the analysis of the light curves of a large sample of nearly 2000 RR Lyrae stars, Hajdu et al (2015) identified 12 binary candidates that display phase shifts of their light curves that can be attributed to lighttime effect (LiTE) that point to the presence of an orbiting companion. We postpone the discussion of individual stars to Paper II
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