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How binary stars change their stellar dance with age

Figure 1. Orbital periods and eccentricities of binary systems. The colour code indicates the stage of evolution (age) of the bright component of the system, as inferred by using “star-quake” analysis, or asteroseismology. Main sequence and subgiants (MS+SG), red giant branch (RGB), and secondary red clump stars (RC+2RC) indicate respectively the “young”, “old” and “very old” stars. The background color map represents the distribution of the ~4000 well-studied binary stars from ground-based spectroscopy (SB9 catalogue), whereby white regions mark well-populated regions of the parameter space. The white vertical dashed line represents the 1034 day timebase of Gaia DR3. Credits: Figure adapted from the paper “Constraining stellar and orbital co-evolution through ensemble seismology of solar-like oscillators in binary systems - A census of oscillating red-giants and main-sequence stars in Gaia DR3 binaries” by Beck et al. 2024, A&A 682, A7.

The third Gaia Data Release (Gaia DR3) included, for the first time, a new and vast catalogue of about 440,000 orbital solutions for binary star candidates based on Gaia’s spectroscopy, astrometry, and photometry (Gaia Collaboration, Arenou, et al. 2023). Binary stars are systems comprising two stars that are gravitationally bound. This dataset includes systems with components of all possible stellar masses and stages of stellar evolution.

Binary stars are treasure troves for stellar astrophysics. The two stars in the system were born together from the same interstellar cloud and, therefore, have the same age, chemical composition, and distance. This means that we can derive their fundamental properties, such as mass and age, with high certainty and test our knowledge of stellar physics. Studying the star-star interactions in such systems is another aspect of astrophysics that helps us better understand the interaction between planets and their host star. If one or both system components show signs of stellar vibrations or oscillations, then a comprehensive picture of the stellar structure and evolution can be drawn from the inference of these oscillations using what is known as asteroseismology. To date, finding oscillating stars like our Sun in binary systems is a very rare find. The NASA Kepler mission provided so far the greatest yield of about 100 such systems.

A study led by Beck et al. (2024), recently published in A&A, exploited the Gaia DR3 binary catalogue (Gaia Collaboration, Arenou et al. 2023) and the NASA Kepler and TESS catalogues of 35,000 solar-like oscillators (Yu et al. 2018, Mackereth et al. 2021, Mathur et al. 2022) to identify stars in these systems that represent the present and the future of the Sun. Just as we use seismology of the Earth to better understand the inner structure of our own planet, asteroseismology allows us to infer the internal structure and dynamics of stars by studying the periodic variations in brightness caused by stellar oscillations. By searching the binary catalogue of Gaia, Beck and collaborators found nearly 1000 additional solar-like oscillators in binary systems, thereby increasing the size of the known sample by an order of magnitude.

Animation 1. The life of a solar-like star, a star similar to our Sun. This is an extract of a video published along with Gaia Data Release 3, with the extract also published here. Credits: ESA/Gaia/DPAC - CC BY-SA 3.0 IGO. Acknowledgements: Animation produced by Laurent Rohrbasser, Krzysztof Nienartowicz, in collaboration with Orlagh Creevey, Laurent Eyer, Celine Reylé.

When stars like the Sun age, they are subject to dramatic changes. Once the Sun has exhausted its core hydrogen, it will turn into a red-giant star. While the central regions contract, the outer regions of the star will expand up to about 200 times the current solar radius until the star's core is hot enough to ignite helium. When entering the He-core burning phase, the star will recede to a typical radius of about 10 times the current solar value.

The substantial changes in the size of a red-giant star will affect the dance of the two gravitationally bound stellar companions around their common center of mass. If the stars in a system are close enough, they will start interacting through tides. On Earth, this effect is best seen from the ebb and flow at the ocean shore due to the interplay between the Moon and our planet. Over time, tides reduce the eccentricity of the orbits of a system, making them more and more circular. For giant stars, the main factor that governs the strength of the interaction is the size of the star, in relation to the orbital separation of the two components. Consequently, from theory, we expect that evolved stars, which have gone through that significant expansion and now are in their helium-core burning phase show evidence of tidal interactions and therefore a trend to lower orbital eccentricities. This evolutionary effect has not yet been detected observationally.

While the star undergoes tremendous changes in size, the individual phases of the red-giant evolution are impossible to identify from the radius change alone. Using asteroseismic techniques to distinguish between less evolved red giants that have not reached their maximum radius and the more evolved giants that have already ignited their helium core, Beck and collaborators demonstrate, as depicted in the animated artist's impression below (Animation 2), that, indeed, the more evolved giants are found on orbits with lower eccentricities and longer orbits. These are the accumulated effects of tidal interaction and stellar evolution. Combining data from ESA Gaia with data from NASA Kepler and TESS has provided a unique dataset to study this unsolved mystery in binary star evolution.

Animation 2. The artist’s impression illustrates the evolution of a binary system from the early phases of the red giant phase, when the red-giant star just starts to expand (left panel, red giant phase), to the more advanced phases, where the red giant already has ignited its core helium (right panel, red clump phase). Each panel shows on the left the orbits of the stellar components around the common centre of gravity of the system. The right diagram shows the evolution of the radius for two stars of approximately the mass of the Sun. Credits: Animation created by Lukas Steinwender based on P. Beck et al. 2024, A&A 682, A7 - CC BY-SA 3.0 IGO.

Thanks to the fundamental work and the data products provided by Gaia's Data Processing and Analysis Consortium (Gaia DPAC), the ESA Gaia mission is an essential tool for state-of-the-art stellar astrophysics. In addition to ongoing research, Data Release 3 is vital for preparing the ESA’s asteroseismology and exoplanet mission PLATO (M3). It allows the targeted inclusion of binary systems of interest in the space telescope observing program. With Gaia Data Release 4, the number of binary star orbits provided will be much larger and extend the data set to even longer periods. The extended timebase will further increase the accuracy of the orbital eccentricities, periods, and inclinations. With Gaia DR4 and PLATO, even larger datasets will be established, which are essential ingredients for further studies of the co-evolution of stars and their hosting binary systems, which contribute to a better understanding of the tidal star-planet interaction in planetary systems.

Further reading:

• “Constraining stellar and orbital co-evolution through ensemble seismology of solar-like oscillators in binary systems -- A census of oscillating red-giants and main-sequence stars in Gaia DR3 binaries” by Beck et al. 2024, A&A 682, A7
• Gaia Data Release 3: Stellar multiplicity, a teaser for the hidden treasure by Gaia Collaboration, F. Arenou, et al.
• Is it a double star - discover Gaia's non-single star catalogue
• Gaia sees strange stars in most detailed Milky Way survey to date
• Gaia sees starquakes

Story written by Paul Beck, Tineke Roegiers, Orlagh Creevey

Credits: ESA/Gaia/DPAC, Paul Beck, Tineke Roegiers, Orlagh Creevey, Lukas Steinwender

[Published: 23/07/2024]