Before the recent American Astronomical Society meeting in Seattle gets too far behind us, I want to be sure to include an interesting story on red dwarfs in the coverage here. The story involves an extrasolar planet survey called SWEEPS — Sagittarius Window Eclipsing Extrasolar Planet Search, which used the Hubble Space Telescope to monitor 215,000 stars in the so-called Sagittarius Window (also called Baade’s Window, after Walter Baade, who discovered it with the 18” Schmidt camera on Mt. Palomar). The ‘window’ offers a view of the Milky Way’s central bulge stars, which are otherwise blocked by dark clouds of galactic dust.
M-dwarfs are by far the most common type of star in the Milky Way, and therefore have major implications for the search for extraterrestrial life. We now know from SWEEPS data that these small stars are given to stellar flares that can have major effects on a planetary atmosphere. Flares have often been mentioned as a serious problem for the development of life on M-dwarf planets, but the new data tell us they may be more dangerous than we had thought, occurring on a regular and frequent basis. This BBC story quotes planet hunter Geoff Marcy:
> “Such powerful flares bode ill for any possible biology, life, on any planet that happens to be close to that flaring star. It’s extraordinary to think that the most numerous stars, the smallest ones in our galaxy, pose this threat to life.”
How Red Dwarf Flares Happen
The threat, vividly portrayed in the results presented by Adam Kowalski (University of Washington) at the conference, involves an eruption of hot plasma that happens when magnetic field lines in a stellar atmosphere reconnect and release an amount of energy that can surpass that of 100 million atomic bombs. From the perspective of life on a planet orbiting an M-dwarf, the planetary surface is blasted with ultraviolet light and a bath of X-rays, along with the charged particles of the stellar wind. The SWEEPS study, with observations over a seven-day period, found 100 stellar flares in this largest continuous monitoring of red dwarfs ever undertaken.
You wouldn’t think small M-dwarfs would pack an impressive punch, but it turns out they have a deep convection zone where cells of hot gas can bubble to the surface in a process Rachel Osten (Space Telescope Science Institute) likens to ‘boiling oatmeal.’ It’s within this zone that the magnetic field is generated that produces the flare, a magnetic field stronger than our Sun’s. I learned from reading papers related to this topic that while sunspots cover less than one percent of the Sun’s surface, the star spots that cover a red dwarf can occupy fully half their surface. And it’s not just young, active stars that pose the threat, according to Osten:
> “We know that hyperactive young stars produce flares, but this study shows that even in fairly old stars that are several billion years old, flares are a fact of life. Life could be rough for any planets orbiting close enough to these flaring stars. Their heated atmospheres could puff up and might get stripped away.”
Most flares last for only a few minutes, but some have been observed to persist for up to eight hours. Older stars do seem to flare less frequently than younger ones, but this survey, taken from data originally compiled in 2006 as part of an exoplanet hunt, tells us that flares continue to be an issue for M-dwarfs that have moved past their youth. Some of the surveyed stars grew as much as 10 percent brighter in a short period of time, making their flares much brighter than those from our Sun, and a few of the stars surveyed produced more than one flare.
Waiting for Stellar Maturity
I haven’t found the paper on this work, but related papers using other surveys include Hilton et al., “The Galactic M Dwarf Flare Rate,” from the Proceedings of the Cool Stars 16 Workshop (preprint) and Hilton et al., “M Dwarf Flares from Time-Resolved SDSS Spectra,” accepted for publication in The Astrophysical Journal (preprint). The latter gets into the age issue re flares. From the abstract: “We find that the flare duty cycle is larger in the population near the Galactic plane and that the flare stars are more spatially restricted than the magnetically active but non-flaring stars. This suggests that flare frequency may be related to stellar age (younger stars are more likely to flare) and that the flare stars are younger than the mean active population.”
With red dwarfs comprising 75 percent and perhaps more of the stars in our galaxy, the question of life around them may come down to how long it takes a flare star to attain a more sedate existence, with flare activity slowing to less threatening levels. On that score, we have much to learn. Because their lifetimes are far longer than the current age of the universe, we have no senescent red dwarfs to study.