What an 'oddball' star in the Cygnus cluster can teach us about how masers are made

MWC 349A is 30 times bigger than our own sun and one of the brightest radio sources in the sky.


Like going to the store to buy dog food and coming back with a duck, researchers with the National Radio Astronomy Observatory may have uncovered a significant insight into how masers (nature's lasers) are formed while conducting a routine study of the "oddball" star MWC 349A using the Atacama Large Millimeter/submillimeter Array (ALMA). It came in the form of a previously unseen jet of ejected material being launched away from the star at "impossibly high speeds," according to the NRAO.

MWC 349A, which resides 3,900 light years away from us in the Cygnus constellation, earned its oddball moniker by being 30 times larger than our own star as well as one of the brightest radio sources in the sky. It's also one of the only observed celestial objects that's known to have a hydrogen maser. Those are as cool as they sound, being radio wavelength analogs to lasers that emit powerful, narrow beams of radiation instead of coherent light. Naturally occurring masers are valuable research tools as they amplify radio wave emissions which enables researchers to study processes that are too far or obscured to observe visually — think star-sized bullhorns in space.

“A maser is like a naturally occurring laser,” Sirina Prasad, primary author of the study and an undergraduate research assistant at the Center for Astrophysics, said in a release Monday. “It’s an area in outer space that emits a really bright kind of light. We can see this light and trace it back to where it came from, bringing us one step closer to figuring out what’s really going on.”

The scientific community has been aware of MWC 349's existence since 1989 when they observed that it had, "some of the characteristics of a molecular maser source: It was extremely bright, and it varied in time, the result of sensitivity to changes in the detailed excitation processes," Ignacio Diaz Bobillo at the Center for Astrophysics wrote in 2013.

He notes that the maser source offered three valuable features:

The first is that the excited atoms produced a series of masers at a series of wavelengths from the corresponding set of hydrogen lines – some even at wavelengths short enough to be trumpeted as being natural lasers. The second is that the numerous lines allowed scientists to model the emitting region in detail. It is an edge-on disk rotating in so-called Keplerian fashion, that is, like the planets orbit in the solar system with those near the Sun orbiting faster than those far from the Sun (very different from the rotation of a solid disk). The final, mysterious point was that this first hydrogen maser source seemed to be unique.

No one understands why, but despite decades of searching for other hydrogen maser sources, only two other possible examples have been proposed, though they remain uncertain at best.

“Our previous understanding of MWC 349A was that the star was surrounded by a rotating disk and photo-evaporating wind," Prasad continued. "Strong evidence for an additional collimated jet had not yet been seen in this system." But that is what they stumbled upon this time around.

The collimated jet is streaking away from the star and its gas disk at a blistering 500km/s — at those speeds you can get from San Diego, California to Phoenix, Arizona faster than you can say "please, no, anywhere but Phoenix." Literally. Prasad's team believes that the material is accelerating to such high speeds with the help of the star's immensely powerful magnetic field which is generating powerful magnetohydrodynamic winds.

"Although we don’t yet know for certain where it comes from or how it is made, it could be that a magnetohydrodynamic wind is producing the jet, in which case the magnetic field is responsible for launching rotating material from the system," Prasad noted. "This could help us to better understand the disk-wind dynamics of MWC 349A, and the interplay between circumstellar disks, winds, and jets in other star systems."

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