Astronomers have had a huge surprise with the discovery that a star known as WL 20S, which has been studied extensively since the 1970s, is actually not one but a pair of stars.
The discovery of the surprise twins was made with the James Webb Space Telescope during a study of a group of young stars called WL 20.
What's more, observations with the Atacama Large Millimeter/submillimeter Array (ALMA) show disks of dust and gas around both stars and, given the stars are relatively young at 2 - 4 million years old, it's possible planets could be forming within the disks.
The discovery that WL 20S is a pair of stars was made using the Webb Telescope's Mid-Infrared Instrument (MIRI) and presented at the 244th meeting of the American Astronomical Society on 12 June 2024.
MIRI data also shows the twin stars have matching jets of gas ejecting into space from their north and south poles.
"Our jaws dropped," says astronomer Mary Barsony, lead author of a paper describing the results.
"After studying this source for decades, we thought we knew it pretty well. But without MIRI we would not have known this was two stars or that these jets existed. That’s really astonishing. It’s like having brand new eyes."
Discovering the twins and jets
WL 20 is located in a star-forming region of our Galaxy known as Rho Ophiuchi, just 400 lighyears from Earth.
Like a typical star-forming region, it's a huge cloud of gas and dust, and therefore difficult to examine internally in visible light.
Because the James Webb Space Telescope observes in infrared, it's able to peer through the dust and see what's lurking inside star-forming regions like WL 20.
The same is true with ALMA, which observes at a wavelength called 'submillimeter', and can also penetrate the gas clouds of a star-forming region.
ALMA was able to to observe the two disks of gas and dust surrounding the WL 20S stars but, say the astronomers behind the study, this could have been interpreted as a single disk with a gap in it.
It's the discovery of the two stellar jets by MIRI that shows WL 20S is a pair of twin stars.
The jets of gas are composed of ions, or individual atoms with some electrons stripped away, and they glow at mid-infrared, but not a submillimeter, meaning Webb's MIRI could see them, but ALMA couldn't.
ALMA, however, is able to observe the leftover materials that surround very young stars following their formation.
The absence of such clouds in ALMA observations shows the stars are beyond their initial formation phase.
All of which means the twin stars are transitioning from youth into adulthood, giving astronomers the chance to observe this key period in the lifecycle of a star.
"The power of these two telescopes together is really incredible," says Mike Ressler, project scientist for MIRI at NASA’s Jet Propulsion Laboratory and co-author of the new study.
"If we hadn’t seen that these were two stars, the ALMA results might have just looked like a single disk with a gap in the middle. Instead, we have new data about two stars that are clearly at a critical point in their lives, when the processes that formed them are petering out.
"It’s amazing that this region still has so much to teach us about the life cycle of stars. I’m thrilled to see what else Webb will reveal."
