Astronomical Cardiology: Charting the Rhythms of Heartbeat Stars with Gaia and TESS
Heartbeat stars--named for their light curves resembling an electrocardiogram--are pairs of stars in eccentric orbits that briefly deform each other through tidal forces when they pass closely. In this study, Jowen Callahan and collaborators focus on finding new heartbeat star systems using data from two space missions: Gaia, which measures stellar motions, and TESS, which records changes in star brightness. These unusual stars offer a way to study how stars interact in binary systems and how such interactions change over time.
Data Sources and Initial Approach
To begin their search, the team focused on spectroscopic binaries--pairs of stars whose orbits can be inferred from their motion--already catalogued in Gaia's third data release. They separated these into two groups: double-line (SB2) binaries, where both stars’ motions can be tracked, and single-line (SB1) binaries, where only one star’s motion is visible. They then used brightness data from TESS to look for the telltale heartbeat signal: a dip and rise in brightness occurring during each orbit due to tidal distortion.
Manual and Automated Searches
For the smaller group of SB2 systems, the researchers manually inspected the TESS light curves and identified 10 candidates showing the heartbeat pattern. They modeled each system using a known mathematical formula from Kumar et al. (1995), which describes how a star’s brightness changes due to tidal forces during an orbit. To fit these models and extract information like the orbital period and eccentricity, they used a statistical method called Markov Chain Monte Carlo (MCMC).
Identifying New Heartbeat Stars
With over 180,000 SB1 systems, the team turned to automation. They first narrowed their search to systems with short periods (less than 13 days) to ensure multiple orbits would appear in each TESS observation. Then, they used algorithms to find likely heartbeat signals and narrowed down the list to 102 candidates through visual inspection. In total, they found 112 new heartbeat stars, with orbital periods ranging from 1.5 to 12.2 days and eccentricities between 0.10 and 0.57.
Comparison with Gaia Orbits
After identifying these new systems, the team compared their measurements to Gaia’s orbital solutions. They found strong agreement for 85% of the SB1 systems but only 2 out of 10 SB2 systems matched well. For these two successful cases, the team used a software tool called PHOEBE to calculate the stars’ masses and sizes. One system featured a pair of stars each about 2.6 times the mass of the Sun, while the other contained more massive stars--over 8 times the Sun’s mass.
Where Heartbeat Stars Live on the HR Diagram
The final part of the paper looks at trends among the discovered heartbeat stars. The researchers noticed that these stars tend to be brighter than average for their color, suggesting they are slightly evolved--having moved off the main sequence stage of their lives. They also found that heartbeat stars are far more common among hotter, more massive stars and nearly absent among cooler, lower-mass stars. This pattern likely results from how tidal forces and stellar evolution affect the visibility of heartbeat signals.
Conclusion and Future Prospects
In summary, this study significantly expands the known population of heartbeat stars by starting from known binaries and combining space-based measurements of motion and brightness. While some challenges remain--especially in confirming results for SB2 systems--this work shows the potential of combining Gaia and TESS data to study how stars influence one another in close, dynamic orbits.
Source: Callahan
