Proxima Centauri's no good, very bad day

A team of astronomers led by Carnegie's Meredith MacGregor and Alycia Weinberger detected a massive stellar flare -- an energetic explosion of radiation -- from the closest star to our own Sun, Proxima Centauri, which occurred last March. This finding, published in Astrophysical Journal Letters, raises questions about the habitability of our Solar System's nearest exoplanetary neighbor, Proxima b, which orbits Proxima Centauri.

An artist's impression of a flare from Proxima Centauri, modeled after the loops of glowing hot gas seen in the largest solar
 flares. An artist's impression of the exoplanet Proxima b is shown in the foreground.Proxima b orbits its star 20 times closer
 than the Earth orbits the Sun. A flare 10 times larger than a major solar flare would blast Proxima b with 4,000 times more
 radiation than the Earth gets from our Sun's flares [Credit: Roberto Molar Candanosa/Carnegie Institution for Science,
NASA/SDO, NASA/JPL]

MacGregor, Weinberger and their colleagues -- the Harvard-Smithsonian Center for Astrophysics' David Wilner and Adam Kowalski and Steven Cranmer of the University of Colorado Boulder -- discovered the enormous flare when they reanalyzed observations taken last year by Atacama Large Millimeter/submillimeter Array, or ALMA, a radio telescope made up of 66 antennae.

At peak luminosity it was 10 times brighter than our Sun's largest flares when observed at similar wavelengths. Stellar flares have not been well studied at the wavelengths detected by ALMA, especially around stars of Proxima Centauri's type, called M dwarfs, which are the most common in our galaxy.

"March 24, 2017 was no ordinary day for Proxima Cen," said lead author MacGregor.

The 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 orange, 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 [Credit: Meredith MacGregor]

The flare increased Proxima Centauri'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 radiation during this flare," MacGregor explained, adding that it was already known that Proxima Centauri experienced regular, although smaller, x-ray flares. "Over the billions 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 involve more than just being the right distance from the host star to have liquid water."

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