Hubble catches ballistic stars

BY DR EMILY BALDWIN

ASTRONOMY NOW

Posted: 08 January, 2009

NASA's Hubble Space Telescope has captured 14 young, runaway stars slicing through regions of dense interstellar gas, creating brilliant bow shocks and leaving glowing tendrils in their wake.

"We think we have found a new class of bright, high-velocity stellar interlopers," says astronomer Raghvendra Sahai of NASA's Jet Propulsion Laboratory, and leader of the study. The team were using Hubble to search for long-lived pre-planetary nebula, swelling aging stars on the cusps of expelling their outer layers to become glowing nebulae. But instead they stumbled upon these youthful stellar runaways. "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."

Stellar interlopers caught speeding through space. Bright shocks can be seen as the stars' stellar winds collide with the dense surrounding gas. Image: NASA, ESA, and R. Sahai (NASA's Jet Propulsion Laboratory).

The bow shocks result from the stars' powerful stellar winds –streams of matter flowing from the stars – slamming into the surrounding dense gas. Depending on their distance from Earth, these bow shocks could be the equivalent of 17 to 170 Solar System diameters wide, as measured out to Neptune's orbit. The bow shocks can be used as a speed gauge, and indicate that the stars are whipping through space at more than 180,000 kilometers an hour with respect to the gas they are ploughing through, which is roughly five times faster than typical young stars. Their strong stellar winds combined with observations of the stars’ masses – they are medium sized stars up to eight times the Sun’s mass – indicates they are only a few million years old. Putting all this data together suggests that the stars could have covered a distance of about 160 light years (1.51 x 10^15 kilometres).

"The high-speed stars were likely kicked out of their homes, which were probably massive star clusters," says Sahai. This could have happened if one star in a binary system exploded as a supernova, kicking the companion star out of its gravitational grasp. Another scenario could have seen a collision between two binary star systems or a binary system and a third star. One or more of these stars could have gained enough energy from the interaction to escape the cluster.

This isn’t the first time that runaway stars have been seen running amok through the Universe. The Infrared Astronomical Satellite (IRAS), which performed an all-sky infrared survey in 1983, spied similar-looking objects but with 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 IRAS," Sahai explains. "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."

The team is planning to probe selected objects from this survey in greater detail to understand their effects on their environment. "One of the questions that these very showy encounters raise is what effect they have on the clouds," says team member Mark Morris of the University of California, Los Angeles. "Is it an insignificant flash in the pan, or do the strong winds from these stars stir up the clouds and thereby slow down their evolution toward forming another generation of stars?"

The team also plans to build up the database with more interstellar interlopers, but the first challenge will be to track these runaways down; so far all of the known examples have been identified serendipitously.