Free Astronomy Magazine March-April 2022

14 MARCH-APRIL 2022 ASTRO PUBLISHING A LMA’s image of water- fountain star system W43A lies about 7000 light- years from Earth in the constel- lation Aquila, the Eagle. The double star at its center is much too small to be resolved in this image. However, ALMA’s measurements show the stars’ interaction has changed its immediate environ- ment. The two jets ejected from the central stars are seen in blue (approaching us) and red (receding). Dusty clouds en- trained by the jets are shown in pink. [ALMA (ESO/NAOJ/ NRAO), D. Tafoya et al.] terious phase” , he says. Understanding the typical envelope phase will also help scientists study what will happen in the very distant future, when the Sun too will be- come a more extensive, cooler star – a red giant – and engulf the inner- most planets. “Our research will help us under- stand how that might happen, but it gives me a more hopeful perspec- tive. When these stars embrace, they send dust and gas out into space that can become the ingredients for com- ing generations of stars and planets, and with them the potential for new life” , says Daniel Tafoya. Since the 15 stars seem to be evolv- ing on a human timescale, the team plans to keep monitoring them with ALMA and with other radio tele- scopes. With the future telescopes of the SKA Observatory, they hope to study how the stars form their jets and change their surroundings. They also hope to find more – if there are any. “Actually, we think the known ‘water fountains’ could be almost all the systems of their kind in the whole of our galaxy. If that’s true, then these stars are the key to un- derstanding the strangest, most wonderful, and most important process that two stars can experi- ence in their lives together” , con- cludes Theo Khouri. signals from different atoms (iso- topes) of carbon and oxygen. Unlike its sister molecule, carbon dioxide (CO 2 ), carbon monoxide is relatively easy to discover in space and is a fa- vorite tool for astronomers. “Thanks to ALMA’s exquisite sensitiv- ity, we were able to detect the very faint signals from several different molecules in the gas ejected by these stars. When we looked closely at the data, we saw details that we weren’t expecting to see” , says Theo Khouri. The observations confirmed that the stars were all blowing off their outer layers. But the proportions of the dif- ferent oxygen atoms in the mole- cules indicated that the stars were in another respect not as extreme as they had seemed, explains team member Wouter Vlemmings, an as- tronomer at Chalmers. “We realized that these stars started their lives with the same mass as the Sun or only a few times more. Now our measurements showed that they have ejected up to 50% of their total mass just in the last few hundred years. Something flamboyant must have happened to them” , he says. Why were such small stars come los- ing so much mass so quickly? The ev- idence all pointed to one explana- tion, the scientists concluded. These were all double stars, and they had all just been through a phase in which the two stars shared the same atmosphere, one star entirely em- braced by the other. “In this phase, the two stars orbit together in a sort of cocoon. This phase, we call it a ‘common enve- lope’ phase, is really brief and only lasts a few hundred years. In astro- nomical terms, it’s over in the blink of an eye” , says teammember Daniel Tafoya. Most stars in binary systems simply orbit around a common center of mass. These stars, however, share the same atmosphere. It can be a life- changing experience for a star and may even lead to the stars merging completely. Scientists believe that this sort of in- timate episode can lead to some of the sky’s most spectacular phenom- ena. Understanding how it happens could help answer some of the as- tronomers’ most important ques- tions about how stars live and die, Theo Khouri explains. “What happens to cause a super- nova explosion? How do black holes get close enough to collide? What’s makes the beautiful and symmetric objects we call planetary nebulae? Astronomers have suspected for many years that common envelopes are part of the answers to questions like these. Now we have a new way of studying this momentous but mys- !