Huge Flares from a Tiny Star

Just a few days ago we looked at evidence that Kepler-438b, thought in some circles to be a possibly habitable world, is likely kept out of that category by flare activity and coronal mass ejections from the parent star. These may well have stripped the planet’s atmosphere entirely (see A Kepler-438b Caveat – and a Digression). Now we have another important study, this one out of the Harvard-Smithsonian Center for Astrophysics, taking a deep look at the red dwarf TVLM 513–46546 and finding flare activity far stronger than anything our Sun produces.

Led by the CfA’s Peter Williams, the team behind this work used data from the Atacama Large Millimeter/submillimeter Array (ALMA), examining the star at a frequency of 95 GHz. Flares have never before been detected from a red dwarf at frequencies as high as this. Moreover, although TVLM 513 is just one-tenth as massive as Sol, the detected emissions are fully 10,000 times brighter than what our star produces. The four-hour observation window was short, which may be an indication that we’re looking at a star that is frequently active.

Now considered an M9 dwarf, TVLM 513 is about 35 light years away in the constellation Boötes. It is believed to be on the borderline between red and brown dwarfs, with a radius 0.11 that of the Sun, a temperature of 2500 K, and a rotation rate of a scant two hours (the Sun takes almost a month for a complete rotation). For a habitable planet to exist here — one with temperatures allowing liquid water on the surface — it would need to orbit at about 0.02 AU. That’s obviously a problem, as Williams explains in this CfA news release:

“It’s like living in Tornado Alley in the U.S. Your location puts you at greater risk of severe storms. A planet in the habitable zone of a star like this would be buffeted by storms much stronger than those generated by the Sun.”

Image: Artist’s impression of red dwarf star TVLM 513-46546. ALMA observations suggest that it has an amazingly powerful magnetic field (shown by the blue lines), potentially associated with a flurry of solar-flare-like eruptions. Credit: NRAO/AUI/NSF; Dana Berry / SkyWorks.

Another unusual aspect of TVLM 513 is its magnetic field. Data from the Very Large Array in New Mexico had previously shown a magnetic field several hundred times stronger than the Sun’s. The paper argues that the emissions observed in the ALMA data are the result of synchrotron emission — radiation generated by the acceleration of high-velocity charged particles through magnetic fields — associated with the small star’s magnetic activity.

We have a lot to learn about small stars, their magnetic fields and their flare processes, and even in this study, the paper offers a caveat:

… confident inferences based on the broadband radio spectrum of TVLM 513 are precluded because the ALMA observations were not obtained contemporaneously with observations at longer wavelengths, and TVLM 513’s radio luminosity, and possibly its radio spectral shape, are variable. Additional support from the Joint ALMA Observatory to allow simultaneous observations with other observatories would be highly valuable.

The authors add that while it has long been known that both stars and gas giant planets have magnetic fields, the mechanisms at work are different and it is unclear what kind of magnetic activity we should expect from objects of intermediate size. Learning more about magnetic processes in small stars should help us understand more about exoplanets and their magnetic activity. This first result at millimeter wavelengths thus points to the work ahead:

Modern radio telescopes are capable of achieving ∼µJy sensitivities at high frequencies (≿20 GHz), raising the possibility of probing the means by which particles are accelerated to MeV energies by objects with effective temperatures of ≾2500 K.

So we’re going to learn a lot more about small red dwarfs as we study whether or not such stars can host habitable planets. The argument against red dwarfs and astrobiology used to focus on tidal lock and the problems of atmospheric circulation, but we’re now wondering whether, particularly in young red dwarfs, flare activity may not be the key factor. If TVLM 513 is representative of a category of flare-spitting stars, the smallest red dwarfs may be hostile to life.

The paper is Williams et al., “The First Millimeter Detection of a Non-Accreting Ultracool Dwarf,” in press at The Astrophysical Journal (preprint).