Free Astronomy Magazine May-June 2018

37 MAY-JUNE 2018 SPACE CHRONICLES are the most common in our galaxy. “March 24, 2017, was no ordinary day for Proxima Cen,” said Meredith MacGregor, an astronomer at the Carnegie Institution for Science, De- partment of Terrestrial Magnetism in Washington, D.C., who led the re- search with fellow Carnegie as- tronomer Alycia Weinberger. Along with colleagues from the Harvard- Smithsonian Center for Astrophysics’ David Wilner and Adam Kowalski and Steven Cranmer of the Univer- sity of Colorado Boulder—they dis- covered the enormous flare when they reanalyzed ALMA observations taken last year. The flare increased Proxima Cen- tauri’s brightness by 1,000 times over 10 seconds. This was preceded by a smaller flare; taken together, the whole event lasted fewer than two minutes of the 10 hours that ALMA observed the star between January and March of last year. Stellar flares happen when a shift in the star’s magnetic field accelerates electrons to speeds approaching that of light. The accelerated electrons interact with the highly charged plasma that makes up most of the star, causing an eruption that produces emission across the entire electromagnetic spectrum. “It’s likely that Proxima b was blasted by high energy radia- tion during this flare,” MacGregor explained, adding that it was al- ready known that Proxima Centauri experienced regular, although smaller, X-ray flares. “Over the bil- lions of years since Proxima b formed, flares like this one could have evaporated any atmosphere or ocean and sterilized the surface, suggesting that habitability may in- volve more than just being the right distance from the host star to have liquid water.” An earlier paper that also used the same ALMA data interpreted its av- erage brightness, which included the light output of both the star and the flare together, as being caused by multiple disks of dust encircling Proxima Centauri, not unlike our own solar system’s asteroid and Kuiper belts. But when MacGregor, Weinberger, and their team looked at the ALMA data as a func- tion of observing time, in- stead of averaging it all together, they were able to see the transient explosion of radiation emitted from Proxima Centauri for what it truly was. “There is now no reason to think that there is a substantial amount of dust around Proxima Cen,” Weinberger said. “Nor is there any informa- tion yet that indicates the star has a rich planetary sys- tem like ours.” A rtist impression of a red dwarf star like Proxima Centauri, the nearest star to our sun. New analysis of ALMA observations reveal that Proxima Centauri emitted a powerful flare that would have created inhospitable conditions for plan- ets in that system. [NRAO/AUI/NSF; D. Berry] T he brightness of Proxima Centauri as observed by ALMA over the two minutes of the event on March 24, 2017. The massive stellar flare is shown in red, with the smaller earlier flare in or- ange, and the enhanced emission surrounding the flare that could mimic a disk in blue. At its peak, the flare increased Proxima Centauri’s brightness by 1,000 times. The shaded area represents uncertainty. [Meredith Mac- Gregor, Carnegie] !