Runaway Star Points to Stellar Nursery Free-for-all

The discovery of a runaway star in Hubble's image of the Orion Nebula suggests a stellar tussle ejected three stars 540 years ago.

Within the bedlam that is the Orion Nebula, newborn stars fling about heated and ionized gas, creating an appearance of turbulent chaos. Yet the stars themselves generally move with slow dignity across the field. The Hubble Space Telescope revealed these stars' steady proper motions across the sky in two images, one taken in 1998 using the NICMOS camera and the other in 2015 using the Wide Field Camera 3 (pictured here).

But there are exceptions. Two stars discovered decades ago, the infrared-radiating Becklin-Neugebauer (BN) object and a radio emitter dubbed Source I, are outwinging their protostellar neighbors in Orion at 60,000 mph and 22,000 mph, respectively. They're zooming in opposite directions from the centrally located Kleinmann-Low Nebula.

Now, Kevin Luhman (Penn State University) and colleagues report in the March 20th Astrophysical Journal Letters (full text here), new examinations of the Hubble images have revealed a third star, "Source x," moving at more than 120,000 mph across the sky, apparently from the same origin point. (Luhman was originally looking for rogue planets, but the discovery of a runaway star didn't disappoint!)

The astronomers collected an infrared spectrum of the object, which shows that the protostar weighs in at 2 or 3 solar masses, lower than its runaway companions. (BN is probably 20 solar masses and Source I is 7 solar masses). The three may have been part of a single multi-star system — but they would have parted ways 540 years ago.

Source x is the missing link to this equation. While astronomers had already suspected that the Becklin-Neugebauer object and Source I had been involved in some kind of stellar tussle centuries ago, the system's energy didn't add up. The new measurement of Source x's proper motion accounts for the missing energy and seals the deal: all three stars were originally part of the same system. (In this scenario, the radio emission from Source I may be coming from two sources in a single tight binary or from a stellar merger.)

Source x may clear up the runaway stars' origin, but it raises another question of its own. Along with the other stars in this stellar mashup, it is young enough to still be enshrouded in a dusty disk. But how did the stars retain (or reform) their disks as they were ejected? The answer may need to wait for Hubble's succcessor, the James Webb Space Telescope, scheduled to launch in October 2018.

To read more about this system and find high-resolution images, fly over to the Hubble team's press release.