Runaway Revelations: Tracking the Hidden Lives of Ejected Stars
Runaway stars are stellar speedsters—stars moving through space much faster than usual. These high-velocity stars are thought to have been ejected from their birthplaces by dramatic events like supernova explosions or violent encounters with other stars. In a study led by Guillermo Torres, astronomers conducted a long-term project to carefully measure the motion of 188 candidate runaway stars, hoping to learn more about where they came from and what kind of systems they belong to.
A Closer Look at the Sample
The stars in question were first identified in a 2011 catalog by Tetzlaff et al. as possible runaway stars, mostly because they had unusual tangential motions across the sky. However, the data at the time lacked reliable measurements of their movement along the line of sight (called radial velocities), which are crucial for fully understanding a star's 3D motion. Torres and his team began a 13-year spectroscopic monitoring campaign to fill in these missing pieces, obtaining thousands of high-resolution spectra using telescopes in Arizona and Massachusetts.
Focusing on Later-Type Stars
The team focused on stars of spectral type A and later (meaning cooler stars, like the Sun or cooler), a group that has been studied less often than the massive, hot O and B-type runaway stars. Each star’s velocity was measured multiple times to check for signs of binarity—that is, whether the star is part of a binary or multiple system. This is important because the presence of a companion can affect how a star moves, possibly mimicking or masking the signs of a true runaway.
Detecting Hidden Binary Systems
Of the 188 stars observed, more than 40 were found to be in binary systems, with over 30 of them having newly determined orbits. Some binaries were double-lined (both stars visible in the spectra), while others were single-lined (only one star detected). Additional signs of binarity came from space-based observations, such as Gaia's precise astrometry and measurements of subtle orbital motion. All this information allowed the authors to estimate each star’s velocity relative to the Local Standard of Rest (a reference frame that moves with the average motion of nearby stars in the galaxy).
How Fast Are These Stars Moving?
With this detailed velocity data, the authors determined that about 80% of the stars had velocities higher than 30 km/s—a typical cutoff for calling a star a "runaway." Two especially fast-moving stars stood out: HIP084038 and HIP004106, with velocities over 150 and 300 km/s, respectively. Interestingly, both are in long-period binary systems, suggesting that not all runaways are single stars, and that some can be ejected while still bound to a companion.
Looking Into the Past: A Traceback Study
In the final part of the paper, the authors carried out a pilot study to trace back the motion of these stars through the Milky Way. They looked for close past encounters with four well-known neutron stars—the remnants of supernova explosions—among the so-called “Magnificent Seven.” No convincing encounters were found, although one possible candidate, HIP076768, showed an intriguing (but unlikely) match with the neutron star RX J0720.4−3125.
Why This Work Matters
This work sets the stage for larger traceback studies, combining precise radial velocities with data from the Gaia mission. While this paper doesn’t find a smoking gun for the runaway origins of its targets, it adds vital data and confirms that high-speed stars can be found across a wide range of stellar types—many of them hiding in plain sight.
Source: Torres
