Astrophile: Runaway is star of cosmic whodunnit

Astrophile is our weekly column on curious cosmic objects, from the solar system to the far reaches of the multiverse

Object: Runaway star
Location: Orion Nebula, about 1300 light years from Earth

As whodunits go, this one's a smash hit. There's a m?nage ? trois that went horribly wrong, a near collision with an innocent bystander and a fugitive fleeing the scene of the crime. The cast doesn't include any famous human detectives or victims, though ? the stars are all of the astronomical variety. As a result, the detective work resolves a big question about where a superfast star got its speed.

The stellar tale begins in 1967, when astronomers Eric Becklin and Gerry Neugebauer stumbled upon an object in the Orion Nebula that was detectable only in the infrared. It was a star whose visible light was completely obscured by gas and dust. Over the last decade or so, it's become clear that the Becklin-Neugebauer (BN) object is a rare runaway star, moving away from the centre of the nebula at 30 kilometres per second.

One theory says that runaway stars start running away when one star of a pair of orbiting stars, known as a binary system, goes supernova and ejects its mate. But there's no supernova remnant in the Orion Nebula that could be the culprit.

So, what caused the BN object to flee? Fingers pointed to a massive object nearby called the Kleinmann-Low (KL) nebula protostar, which is about 15 solar masses in size, and is still growing by gulping gas from the disc of gas surrounding it. If one were to trace back BN's path, it passed perilously close to KL about 500 years ago.

Promiscuous proximity

There were other signs that KL spawned BN. The protostar's accretion disk is spewing out massive amounts of gas. This is normal, but in the case of this disk, the outflows are enormous, almost explosive. The cause could have been a violent collision with BN, which in turn could explain why the star ran away. It's what you would call circumstantial evidence.

Jonathan Tan of the University of Florida in Gainesville, Florida, though, has always eyed another suspect: Theta-1 Orionis C, the brightest star in the Trapezium, a cluster which also lies in the Orion Nebula. In 2009, astronomers got firm evidence that theta-1C is a binary made of a massive primary star of 38 solar masses tightly embracing a smaller secondary star of 9 solar masses.

The couple weren't always so cosy. At some point, they were joined in a messy threesome by BN (stars in the Trapezium are promiscuous in their proximities). "That's quite an unstable situation," says Tan. "Generally, the least massive of those three stars will undergo a gravitational slingshot, and will be ejected from the system."

Chaotic threesome

Could the least massive star be BN, at 8 solar masses? That would offer a new explanation for how it became a runaway.

Tan and colleague Sourav Chatterjee decided to settle the issue once and for all, using Chatterjee's forensically valuable ability to simulate the massively complex three-body interactions.

The three-star system is so chaotic that in order to explore all the possible outcomes the duo had to perform 10 million simulations. And voil? ? in some situations they found BN was ejected at 30 kilometres per second.

That still wasn't proof, however. Using the scenarios in which BN got ejected at the required speed and in the correct direction, the team calculated properties of the leftover binary system, theta-1C. They predicted seven properties, including how tight the orbit would be, the angle of the orbit relative to BN's direction and the recoil felt by theta-1C. Theta-1C's observed properties match these predictions.

Who framed KL?

The chances of this being just a coincidence are less than 1 in a 100,000, their calculations show. "The most likely explanation for the production of the runaway BN star is that it was ejected by a gravitational slingshot from theta-1C," says Chatterjee. The team presented the results at the American Astronomical Society meeting in Anchorage, Alaska.

Case closed? Not so fast. What about the KL protostar? How can we explain its massive, explosive outflows? It turns out that even though BN was ejected by theta-1C, the object still passed through the outskirts of KL's accretion disk, which happened to lie in its path. This created gravitational tidal waves in the disk, leading to faster accretion, which in turn led to the explosive outflows.

What's more, the runaway star's trajectory got bent by KL's gravity by about 10 degrees. Which is why if one merely traces BN's path backwards in time in a straight line, it comes close to the KL protostar, but is nowhere near theta-1C.

But though KL is still part of the story, the source of the mischief is clear: theta-1C. "All the evidence is pointing to theta-1C as the guilty party," says Tan. "KL is a victim. It was almost framed, if you like."

Reference: arxiv.org/abs/1203.0325v3

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