Proxima Centauri’s flaring activity has been well known to astronomers using visible wavelengths, but new observations with the Atacama Large Millimeter/submillimeter Array (ALMA) highlight the star’s extreme activity at radio and millimeter wavelengths.
An artist’s conception of a violent stellar flare from Proxima Centauri. Image credit: S. Dagnello, NRAO / AUI / NSF.
Proxima Centauri is a red dwarf star located about 4.24 light-years away in the constellation of Centaurus.
Discovered in 1915 by Scottish astronomer Robert Innes, the star is not visible to the naked eye.
Its average luminosity is very low, and it is quite small compared to other stars, at only about an eighth of the mass of the Sun.
Proxima Centauri is also known as Alpha Centauri C, because it is actually part of a triple star system.
The star’s separation from its larger companions, Alpha Centauri A and B, is about 0.2 light years — equivalent to 400 times the size of Neptune’s orbit.
Proxima Centauri hosts a terrestrial exoplanet, Proxima b, in its habitable zone at 0.0485 AU.
The star has been well established as a highly active star, making it a prime target to investigate the effects of stellar activity on the habitability of planets orbiting red dwarfs.
In a new study, University of Colorado astronomer Kiana Burton, Johns Hopkins University astronomer Meredith MacGregor and their colleagues used archival data and new ALMA observations to study the millimeter-wavelength flare activity of Proxima Centauri.
Proxima Centauri’s small size and strong magnetic field indicate that its entire internal structure is convective (unlike the Sun, which has both convective and non-convective layers), making the star much more active.
Its magnetic fields become twisted, develop tension, and eventually snap, sending streams of energy and particles outward in what is observed as flares.
“Our Sun’s activity doesn’t remove Earth’s atmosphere and instead causes beautiful auroras because we have a thick atmosphere and a strong magnetic field to protect our planet,” Dr. MacGregor said.
“But Proxima Centauri’s flares are much more powerful, and we know it has rocky planets in the habitable zone.”
“What are these flares doing to their atmospheres? Is there such a large flux of radiation and particles that the atmosphere is getting chemically modified, or perhaps completely eroded?”
This 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-m array as well as the 7-m Atacama Compact Array (ACA), a total of 463 flare events were reported at energies ranging from 1024 to 1027 erg, and with a brief duration ranging from 3 to 16 seconds.
“When we see the flares with ALMA, we see the electromagnetic radiation — the light in various wavelengths,” Dr. MacGregor said.
“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.”
To do so, the astronomers characterized the star’s (so-called flare frequency distribution) to map out the number of flares as a function of their energy.
Typically, the slope of this distribution tends to follow a power law function: smaller (less energetic) flares occur more frequently, while larger, more energetic flares occur less regularly.
Proxima Centauri experiences so many flares that the researchers detected many flares within each energy range.
Furthermore, they were able to quantify the asymmetry of the star’s highest energy flares, describing how the flares’ decay phase was much longer than the initial burst phase.
Radio and millimeter-wavelength observations help constrain the energies associated with these flares and their associated particles.
“The millimeter flaring seems much more frequent,” Dr. MacGregor said.
“It’s a different power law than we see at the optical wavelengths.”
“If we only look at optical wavelengths, we’re missing critical information.”
“ALMA is the only millimeter interferometer sensitive enough for these measurements.”
The team’s findings were published in the Astrophysical Journal.
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Kiana Burton et al. 2025. The Proxima Centauri Campaign – First Constraints on Millimeter Flare Rates from ALMA. ApJ 982, 43; doi: 10.3847/1538-4357/ada5f2
