[Paper Review] Multiplicity of Galactic Cepheids and RR Lyrae stars from Gaia DR2 - I. Binarity from proper motion anomaly

P. Kervella, A. Gallenne+9 more|Repository of the Academy's Library (Library of the Hungarian Academy of Sciences)|Mar 8, 2019

Stellar, planetary, and galactic studiesPhysics and Astronomy128 references21 citations

TL;DR

This study uses Gaia DR2 and Hipparcos proper motion data to detect binary and multiple systems among Galactic Cepheids and RR Lyrae stars via proper motion anomalies (PMa). It identifies 57 confirmed Cepheid binaries (80% binary fraction) and 13 confirmed RR Lyrae binaries (7% binary fraction), resolving the long-standing mystery of their apparent low binary frequency. The method enables mass estimates for 28 Cepheid binaries using combined astrometric and spectroscopic data.

ABSTRACT

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

Motivation & Objective

• To determine the true multiplicity frequency of Galactic classical Cepheids and RR Lyrae stars, which are critical for cosmic distance measurements.
• To resolve the longstanding discrepancy where only one RR Lyrae star (TU UMa) had been confirmed as a binary despite tens of thousands known.
• To identify and characterize close binary systems using astrometric proper motion anomalies from Gaia DR2 and Hipparcos data.
• To improve understanding of the evolutionary states of these stars by identifying their binary companions.

Proposed method

• Combines Gaia DR2 and Hipparcos positions to compute mean proper motions for target stars.
• Detects proper motion anomalies (PMa) caused by unseen companions orbiting the primary star, indicating binarity.
• Applies statistical significance tests to distinguish real PMa from measurement noise or astrometric errors.
• For confirmed binaries, combines PMa with spectroscopic orbital parameters to estimate companion masses.
• Uses parallax and proper motion data to constrain orbital parameters such as maximum semi-major axis and orbital period.
• Classifies systems based on PMa amplitude and significance, distinguishing confirmed binaries from candidates.

Experimental results

Research questions

• RQ1 What is the true binary fraction among Galactic classical Cepheids, given their importance as standard candles?
• RQ2 Why is the observed binary fraction of RR Lyrae stars so low (only one confirmed binary to date), and is this due to observational bias?
• RQ3 Can proper motion anomalies in Gaia DR2 data reveal previously undetected close binary systems in Cepheids and RR Lyrae stars?
• RQ4 How can the combination of astrometric PMa and spectroscopic data improve mass estimates for binary systems?
• RQ5 What are the orbital characteristics (e.g., semi-major axis, period) of the newly detected binary systems?

Key findings

• The study identifies 57 confirmed classical Cepheid binaries from proper motion anomalies out of 254 tested stars, indicating a binary fraction above 80%.
• An additional 75 Cepheid candidates are identified, suggesting the true binary fraction may be even higher.
• Thirteen RR Lyrae stars show significant proper motion anomalies out of 198 tested, with 61 additional candidates, implying a binary fraction of at least 7%.
• For 28 confirmed Cepheid binaries, companion masses are estimated by combining astrometric PMa with spectroscopic orbital solutions.
• The maximum semi-major axis for detected RR Lyrae binaries ranges from 0.1 to 20.7 au, with orbital periods from 1.5 to 136,000 years.
• The detection of a significant number of RR Lyrae binaries resolves the long-standing puzzle of their apparent scarcity in binary systems.