Small star, mighty flares: A new view of Proxima Centauri

Washington, DC—Talk about a noisy neighbor! Recent research about Proxima Centauri, our Sun’s star next door offered new details about the twisting tension in its magnetic fields that results in daily flare outbursts, as well as what these streams of energy and particles mean for the habitability of its planets.

Similar to flares on our own Sun, Proxima Centauri’s outbursts release light energy across wavelengths from the X-ray to radio. A small planet orbits the star, and is blasted by stellar radiation from these flares, which may strip its atmosphere of necessary ingredients for habitability such as ozone and water.

A team of astronomers including Carnegie Science’s Alycia Weinberger and former Carnegie Postdoctoral Fellows Meredith MacGregor, now a professor at Johns Hopkins University, and Evgenya Schkolnik, now a professor at Arizona State University, mounted a campaign to study these bursts of radiation from Proxima Centauri. Their findings, published last week in a paper lead authored Kiana Burton of the University of Colorado, used both archival data and new observations from the Atacama Large Millimeter/submillimeter Array (ALMA) in Chile to study the millimeter-wavelength flare activity of Proxima Centauri.

The star’s small size and strong magnetic field indicate that its entire internal structure is likely convective—like a pot of boiling water. In this way it differs from the Sun, which has both convective and non-convective layers. As a result, Proxima Centauri is much more active than our Solar System’s central star. Its magnetic fields become twisted, develop tension, and eventually snap, sending streams of energy and particles outward in what astronomers observe as flares.

Weinberger summarized the study’s key question: “How often do flares that can erode an exoplanet atmosphere occur on Proxima Centauri?”

Added MacGregor: “Our Sun’s activity doesn't remove Earth’s atmosphere and instead cause beautiful auroras, because we have a thick atmosphere and a strong magnetic field to protect our planet. But Proxima Centauri’s flares are much more powerful.”

The team’s research represents the first multi-wavelength study using millimeter observations to uncover a new look at the physics of flares. Combining 50 hours of ALMA observations using both the full 12-meter array as well as the 7-meter Atacama Compact Array, a total of 463 flare events were reported at energies ranging from 1024 to 1027 erg. The flares themselves were short events, ranging from 3 to 16 seconds.

“When we see the flares with ALMA, what we’re seeing is the electromagnetic radiation–the light in various wavelengths. But looking deeper, this radio wavelength flaring is also giving us a way to trace the properties of those particles and get a handle on what is being released from the star,” MacGregor explained.

To do so, the astronomers characterized the star’s so-called flare frequency distribution in order to map out the number of flares as a function of their energy. As is typical, they showed that smaller, less energetic, flares occurred more frequently while larger, more energetic flares occurred less frequently. Proxima Centauri experiences so many flares that the team detected many flares within each energy range.

Radio- and millimeter-wavelength observations help to put constraints on the energies associated with these flares and their associated particles. The team had previously shown that these long wavelengths are emitted together with atmosphere-damaging UV light.

Weinberger highlighted ALMA’s key role: “The millimeter flares show a different rate than flares seen at visible wavelengths. The small millimeter flares are incredibly common. Previously, we were missing the full picture of how energy is released in Proxima Centauri’s flares.”

MacGregor noted: “ALMA is the only millimeter interferometer sensitive enough for these measurements.”