Stellar and substellar companions of nearby stars from Gaia DR2 - Binarity from proper motion anomaly

Abstract: The census of stellar and substellar companions of nearby stars is largely incomplete. It is, however, fundamentally important in the understanding of the stellar and planetary formation and evolution mechanisms. We aim to characterize the presence of physical companions in orbit around nearby stars. Orbiting secondary bodies influence the proper motion (PM) of their parent star through their gravitational reflex motion. Using the Hipparcos catalog and Gaia's second data release (GDR2), we determined the long-term PM of the stars common to these two catalogs. We then searched for a PM anomaly (PMa) between the long-term PM vector and the GDR2 (or Hipparcos) measurements, indicative of the presence of a perturbing secondary object. We focussed our analysis on the 6741 nearby stars located within 50 pc, and we also present a catalog of the PMa for >99% of the Hipparcos catalog (>117000 stars). 30% of the stars studied present a PMa greater than 3 sigmas. The PMa allows us to detect orbiting companions, or set stringent limits on their presence. We present a few applications to interesting targets. We set upper limits of 0.1-0.3 M_Jup to potential planets orbiting Proxima between 1 and 10 au (P_orb=3 to 100 years). We confirm that Proxima is gravitationally bound to alpha Cen. We recover the masses of the known companions of epsilon Eri, epsilon Ind, Ross 614 and beta Pic. We also detect a possible planet of a few Jovian masses orbiting tau Ceti. The combination of the GDR2 with Hipparcos results in very high accuracy PMa vectors. The detection of tangential velocity anomalies at a median accuracy of sigma(Delta v_tan)=1.0 m/s per parsec of distance opens the possibility to identify long period orbital companions otherwise inaccessible. Gaia's complementarity to radial velocity and transit techniques already appears to be remarkably powerful.

Analyzing Stellar and Substellar Companions Using Gaia DR2 Data: Proper Motion Anomalies

This paper titled "Stellar and substellar companions of nearby stars from Gaia DR2" presents an astrometric analysis aimed at identifying potential companions to nearby stars through the detection of proper motion anomalies (PMa). Utilizing the Gaia Data Release 2 (GDR2) in conjunction with historical data from the Hipparcos mission, the researchers, led by Pierre Kervella and colleagues, investigate the gravitational impacts on stellar proper motions to infer the presence of orbiting objects.

Methodology and Data Analysis

The study focuses on stars located within 50 parsecs, leveraging the high-precision astrometry available from GDR2 and its combination with Hipparcos data to track long-term proper motion vectors. The central hypothesis is that deviations, or anomalies, in these vectors can suggest the gravitational influence of secondary bodies. The analysis includes 6,741 stars from the Hipparcos catalog and a broader statistical overview of approximately 117,000 stars.

The detection strategy involves comparing the long-term proper motion vectors to those observed at specific epochs (either Hipparcos or GDR2), identifying anomalies at a significance threshold of greater than 3 sigma. This approach allows the researchers to discern potential secondary companions, either confirming known bodies or proposing new candidates.

Key Results and Observations

1. PMa Frequency and Binarity: The paper finds that 30% of stars exhibit PMa significant at or above the 3 sigma level. This could suggest a broad presence of undetected companions, challenging estimates of binary frequencies.

2. Notable Stars: The study examines well-known stars like Proxima Centauri and Barnard's star. For Proxima Centauri, the analysis confirms no planetary companions above 0.1–0.3 Jupiter masses within 1–10 au, aligning with previous radial velocity observations. Interestingly, Proxima is confirmed to be gravitationally bound to the Alpha Centauri system.

3. Validation of Known Systems: Known systems such as ε Eridani and ε Indi show confirmations of mass estimates for companions via PMa, indicating the method’s utility in validating existing models.

4. Limits on Companion Detection: The PMa calculations are constrained by observational windows from the Hipparcos and GDR2 datasets, with biases in detecting long-period companions. However, the authors successfully recover known companionship dynamics, demonstrating the method's efficacy for periods longer than Gaia's operational time frame.

5. Potential New Discoveries: PMa analysis revealed potential companions in systems lacking prior detections, such as possible massive planets around stars like Ross 128 and AX Microscopii, thereby prompting further investigation and verification through complementary methods like radial velocity studies or direct imaging.

Implications and Future Work

This astrometric approach paves the way for enhancing our understanding of stellar systems by significantly refining companion mass-contraints and detecting wide-orbit substellar objects. The synergy between Gaia and Hipparcos data sets a precedent for future releases from Gaia, which promise even greater precision and potential for discovery.

Importantly, this research offers a novel tool in the detection of exoplanets and brown dwarfs by establishing constraints where traditional methods like transit or radial velocity detections are less effective for wide-orbit and low-mass companions.

In conclusion, the paper illustrates the power of high-precision astrometry in advancing our understanding of stellar domains, with promising avenues for future work as more precise data becomes available from ongoing and upcoming astronomical missions. The potential for it to fill gaps in the detection landscape, especially in locating elusive brown dwarfs, presents significant theoretical and practical implications for astrophysics.