Gaia's binary star renaissance (2403.12146v2)

Abstract: Stellar multiplicity is among the oldest and richest problems in astrophysics. Binary stars are a cornerstone of stellar mass and radius measurements that underpin modern stellar evolutionary models. Binaries are the progenitors of many of the most interesting and exotic astrophysical phenomena, ranging from type Ia supernovae to gamma ray bursts, hypervelocity stars, and most detectable stellar black holes. They are also ubiquitous, accounting for about half of all stars in the Universe. In the era of gravitational waves, wide-field surveys, and open-source stellar models, binaries are coming back stronger than a nineties trend. Much of the progress in the last decade has been enabled by the Gaia mission, which provides high-precision astrometry for more than a billion stars in the Milky Way. The Gaia data probe a wider range of binary separations and mass ratios than most previous surveys, enabling both an improved binary population census and discovery of rare objects. I summarize recent results in the study of binary stars brought about by Gaia, focusing in particular on developments related to wide ($a \gtrsim 100$ au) binaries, evidence of binarity from astrometric noise and proper motion anomaly, astrometric and radial velocity orbits from Gaia DR3, and binaries containing non-accreting compact objects. Limitations of the Gaia data, the importance of ground-based follow-up, and anticipated improvements with Gaia DR4 are also discussed.

• The paper shows that Gaia’s high-precision astrometry has revolutionized binary star detection, expanding discovery to wider separations and diverse mass ratios.
• The paper leverages astrometric noise metrics like RUWE to effectively identify binaries, including systems with non-accreting compact objects.
• The paper provides orbital solutions from Gaia’s DR3 that refine our understanding of stellar evolution and binary demographics.

Overview of "Gaia's Binary Star Renaissance"

This paper, authored by Kareem El-Badry, discusses significant advancements in our understanding of binary star systems, primarily fueled by the high-precision astrometric data from the Gaia mission. Binaries are fundamentally important in astrophysics as they provide critical data for measuring stellar masses and radii, which are essential for testing and refining stellar evolution models. Furthermore, binaries are the progenitors of various exotic astrophysical phenomena, including type Ia supernovae and gamma-ray bursts.

Gaia's Contributions to Binary Star Studies

The Gaia mission, with its unprecedented astrometric precision extending beyond a billion stars in the Milky Way, has significantly expanded the detection horizon for binary systems, probing wider separations and a broader range of mass ratios than previous surveys could achieve. Gaia's data enables better census of binary populations, contributing to the discovery of rare objects and a deeper understanding of stellar dynamics.

The paper highlights several specific areas where Gaia's data have been transformative:

• Wide Binaries: Gaia has expanded the ability to resolve binaries at unprecedented distances and separations, down to about 1000 AU for binaries at 1 kpc. It has provided insights into the formation environments of wide binaries by characterizing their separations and mass ratios, challenging previous notions about their formation and evolution.
• Astrometric Noise and Binarity: Binaries can be identified by deviations from expected single-star astrometric models, quantified through parameters like RUWE. Gaia enables the identification of binary systems where traditional imaging and spectroscopy might fail, especially highlighting binaries with non-accreting compact objects.
• Astrometric and Radial Velocity Orbits: Gaia's third data release included orbital solutions for astrometric and spectroscopic binaries, significantly increasing the sample of small-separation binaries with well-constrained orbits.

Key Findings and Implications

The research underscores that Gaia data has enabled more precise characterization of binary star systems across a range of separations. The implications of these results span both practical and theoretical realms. Practically, these insights enhance our understanding of the binary star demographics in the Milky Way, influence stellar population synthesis models, and improve predictions for merging compact objects that contribute to the gravitational wave signals.

Theoretically, the paper speculates that future Gaia data releases could further refine our understanding of binary interactions and evolution. The availability of epoch astrometry in future releases will likely enhance the discovery of systems with high dynamical complexity, such as hierarchical triples or compact object binaries that mimic binary signatures at certain periods.

In summary, the paper presents the Gaia mission as a pivotal step forward in binary star astrophysics, opening pathways for more exhaustive studies in binary formation, stellar dynamics, and the role of binaries in broader astronomical contexts. The anticipation for Gaia's continued contributions highlights the mission's potential to redefine the landscape of astrophysical research on binary systems.