The stars are plowing through regions of dense interstellar gas, creating brilliant arrowhead structures and trailing tails of glowing gas. These arrowheads, or bow shocks, form when the stars' powerful stellar winds, streams of matter flowing from the stars, slam into surrounding dense gas. The phenomenon is similar to that seen when a speeding boat pushes through water on a lake.
The astronomers can only estimate the ages, masses and velocities of these renegade stars. The stars appear to be young -- just millions of years old. Their ages are based partly on their strong stellar winds.
Most stars produce powerful winds either when they are very young or very old. Only very massive stars greater than 10 times the sun's mass have stellar winds throughout their lifetimes.
The stars in these Hubble images are among 14 young runaway stars spotted by the Advanced Camera for Surveys between October 2005 and July 2006. Image credit: NASA/ESA/JPL
But the objects observed by Hubble are not very massive because they do not have glowing clouds of ionized gas around them. They are medium-sized stars that are a few to eight times more massive than the sun. The stars are not old because the shapes of the nebulae around aging, dying stars are very different, and old stars are almost never found near dense interstellar clouds.
Depending on their distance from Earth, the bullet-nosed bow shocks could be 100 billion to a trillion miles wide (the equivalent of 17 to 170 solar system diameters, measured out to Neptune's orbit). The bow shocks indicate that the stars are traveling fast, more than 180,000 kilometers an hour (more than 112,000 miles an hour) with respect to the dense gas they are plowing through, which is roughly five times faster than typical young stars.
"We think we have found a new class of bright, high-velocity stellar interlopers," said Sahai. "Finding these stars is a complete surprise because we were not looking for them. When I first saw the images, I said, 'Wow. This is like a bullet speeding through the interstellar medium.' Hubble's sharp 'eye' reveals the structure and shape of these bow shocks."
"The high-speed stars were likely kicked out of their homes, which were probably massive star clusters."
There are two possible ways this stellar expulsion could have happened. One way is if one star in a binary system exploded as a supernova and the partner got kicked out. Another scenario is a collision between two binary-star systems or a binary system and a third star. One or more of these stars could have picked up energy from the interaction and escaped the cluster.
Assuming their youthful phase lasts only a million years and they are moving at roughly 180,000 kilometers an hour (about 112,000 mph), the stars have traveled about 160 light-years.
Runaway stars have been seen before. The joint European-NASA Infrared Astronomical Satellite, which performed an all-sky infrared survey in 1983, spied a few similar-looking objects. The first observation of these objects was in the late 1980s. But those stars produced much larger bow shocks than the stars in the Hubble study, suggesting that they are more massive stars with more powerful stellar winds.
"The stars in our study are likely the lower-mass and/or lower-speed counterparts to the massive stars with bow shocks detected by the Infrared Astronomical Satellite," Sahai explained. "We think the massive runaway stars observed before were just the tip of the iceberg. The stars seen with Hubble may represent the bulk of the population, both because many more lower-mass stars inhabit the universe than higher-mass stars, and because a much larger number are subject to modest speed kicks."
Sahai presented his results at the American Astronomical Society meeting in Long Beach, Calif. The science team also includes M. Morris of the University of California, Los Angeles; M. Claussen of the National Radio Astronomy Observatory in Socorro, N.M.; and R. Ainsworth of the University of Tennessee in Knoxville.