Astronomers are now finding dozens of fast-moving runaway stars by searching for the bow shocks they create in interstellar gas and dust.
The apparent stillness of the night sky is deceiving. In reality, some stars zip through the cosmos quiet fast — at least 30 kilometers per second relative to their surroundings. These speedsters tend to be moving in isolated regions of the galaxy, though their paths often lead directly away from stellar clusters or even the supermassive black hole at the center of the galaxy.
Through pure luck, astronomers have discovered more than 20 so-called runaway stars. But there is strength in numbers. If astronomers could systematically search for — and therefore discover more — runaway stars, they might be able to better understand what boosted those stars to such high velocities in the first place.
Enter Zeta Ophiuchi, a hot, massive O star traveling around the galaxy at a distance from us of about 366 light-years. Its relative motion is fast enough, roughly 24 km/s (54,000 miles per hour), to be "supersonic." As this massive star plows through space, its strong outflowing wind causes interstellar gas and dust to stack up in front of it, like water that piles ahead of a ship or air that piles ahead of a supersonic jet. This arc-shaped gas then compresses, heats up, and shines with infrared light — a direct signature of the mass and speed of the star coursing through it.
With this example in hand, a team of astronomers decided to probe bow shocks in search for runaway stars. They turned to archival infrared data from the Spitzer and WISE space observatories and found more than 200 images of fuzzy red arcs such as the examples below. They then used the Wyoming Infrared Observatory — a 2.3-meter telescope perched atop Mount Jelm — to search for the culprits behind the shocks. The team was surprised to find that “more than 95% of these bow shocks have a hot, massive, runaway candidate at their center,” said William Chick, a graduate student at the University of Wyoming, when he presented the results at the American Astronomical Society meeting in Kissimmee, Florida. “It may be that our Milky Way is swarming with these hot runaway stars.”
Moreover, he added, “It appears that bow-shock nebulae are an extremely efficient method at locating these hot runaway stars.” His team plans to extend its search to include the entire galactic plane. With more runaways spotted, the researchers will be able to trace the stars’ motions backward in order to find the source of their accelerating kicks.
Two mechanisms have been offered to fling a star into the void with that much speed. If a binary star falls close to the supermassive black hole at the Milky Way’s center, then it can be disrupted such that one star remains trapped around the black hole and the other gets slung away rapidly. Alternatively, a supernova in a binary system might free the companion, flinging it outward into the galaxy. Chick suspects that both mechanisms likely play a role.