Something is threatening red dwarfs. Human eyes are accustomed to our lovely yellow sun and the warm light that shines on our glorious life-covered planet. But red dwarfs can seem moody, ill-tempered, and even alarming.
For long periods of time, they can be quiet, but then they can flare up violently, sounding a warning to any life that might be gaining a foothold on a nearby planet.
Red dwarfs (M dwarfs) are the most common type of star in the Milky Way. This means that most of the outer planets rotate red dwarfs, not cute, G-type stars like our Sun. As astronomers study red dwarfs in more detail, they are finding that red dwarfs may not be the best star hosts when it comes to exoplanet habitability. Multiple studies have shown that red dwarfs can erupt violently, emitting enough powerful radiation to render nearby planets uninhabitable, even when they are stationary in the potentially habitable zone.
But there is still a lot that astronomers don’t know about red dwarfs and their wild nature. A new study examined 177 M-dwarfs to better understand their long-term variability. Researchers have found that red dwarf behavior is more complex than previously thought, and even the quieter red dwarfs are wilder than the sun.
The study is titled “Characterization of Stellar Activity for MI Dwarfs by Long Time Scale Variability in a Large Sample and Detection of New Cycles.” The paper will be published in the journal Astronomy and astrophysics It is available on a prepress server arXiv. The lead author is Lucile Mignon, a postdoctoral researcher from the University of Grenoble Alpes and the French National Center for Scientific Research (CNRS).
All stars are variable to one degree or another. The Sun follows an 11-year cycle during which the number of sunspots on our star’s surface diminishes. It’s all related to magnetic activity. But habitability hinges on longer-term cycles. Life progresses in time frames much longer than a few years. It took billions of years for life on Earth to really get started.
This is one reason for astrophysicists’ long-term interest in red dwarfs and their diversity. Life appeared on Earth about 3.5 billion years ago, but complex life actually appeared about 540 million years ago during the Cambrian Explosion. If life generally follows a similar time frame, could red dwarf variability prevent life from surviving?
Observing red dwarfs and drawing any conclusions is a challenge. We can watch our sun in great detail, especially in recent years. A fleet of spacecraft—including the Parker Solar Probe, Solar Orbiter, Heliospheric Orbiter, and others—is dedicated to observing it in detail. We have also observed the sun and its activity over a long period of time.
Unfortunately, we haven’t been able to observe individual red dwarfs for very long periods of time. Instead, researchers have to make do with it datasets over two decades or so. In this new paper, Menon and her co-authors examined 177 million dwarfs observed by HARPS (High-Resolution Radial Velocity Finder) from 2003 to 2020. Activity on this timescale contains clues to how these stars behave over longer periods.
HARPS is essentially a spectrometer, and from it, the authors of this study obtained the chromospheric emissions of red dwarfs. The chromospheric emissions stem from the activity of the star’s magnetic field rather than their own fusion. Burning is caused by magnetic activity, so studying ignition means studying a star’s chromosphere. The team also analyzed the optical properties of red dwarfs along with their chromospheric emissions.
The difficulty in studying red dwarf variance stems from our limited long-term data. “A clear definition of a cycle requires measurements that show its recurrence over several periods. This requires data taken over a long period of time,” they explained.
Absent that, the researchers worked with the idea of what they call seasons. “By identifying the seasons of individual stars, they can better analyze the data.” We defined these seasons as bins of 150 days (for the average rotation adjustment as best as possible.) with at least five observations (150 days is the typical maximum period for M dwarfs rotation), and the gaps between observations are shorter than 40 days within a 150 bin. day,” they explain.
This determined a subsample of 57 stars.
The results show that variability is a defining feature among M dwarfs. “We found that most stars are highly variable, even the quietest ones,” the researchers wrote. “Most stars in our sample (75%) show long-term variability, which is mostly manifested by linear or quadratic variation, although the real behavior may be more complex.” (Linear variance is more simple, while quadratic variance indicates a cycle.)
The researchers found cycles in their sample ranging from several years to more than 20 years. But they are quick to point out that their findings have limitations and that their study is just an initial step toward a better understanding of red dwarfs. For many stars, this is a strong indication of long-term volatility. “…better sampled stars might show more complex behavior if they were sampled better,” they wrote. Nevertheless, their results “…indicate the strong presence of long-term variability, however, and suggest that these stars have strong long-term variability, which is important when searching for exoplanets.”
There can be multiple layers of cycles and variability that influence each other, making it very difficult to decipher the behavior of stars. Their puzzling behavior “…may be due to complex fundamental volatility in different time scales simultaneously,” the authors write.
Even with their limited data, the researchers say, they’ve made progress. “Even if the temporal coverage is not sufficient for some stars, however, our data can be used to estimate the lowest period of the cycle if any.” But some conclusions are elusive at the moment. Their analysis “…is not sufficient to guarantee that the signal is periodic or even quasi-periodic.”
The “slam dunk” answer to a red dwarf’s habitability is currently elusive. It may also be the case, as this study suggests, that there are many more variability Among the eternally unpredictable red dwarfs. But don’t bet on science revealing more details.
The burning of a red dwarf is well documented. The most powerful stellar glow ever detected came from a red dwarf. In 2019, Proxima Centauri, a red dwarf and our closest stellar neighbor, emitted a flare 14,000 times brighter than its pre-flare, and it only took a few seconds for that bubblegum to light up. The exoplanet Proxima Centauri b is located in the star’s potentially habitable zone, and a bright flare could eliminate the possibility of life or even liquid water on the planet. Even if Proxima Centauri were to burn brightly once every million years, or even longer, it could wipe out the possibility of life.
The search for life or habitability on other worlds inevitably includes a focus on red dwarfs. Its abundance means that it must be studied in greater depth. It may end up that many planets that we think could be habitable, such as the well-known TRAPPIST-1 planets, are simply exposed to intense radiation from their red dwarf hosts. The more variable it is, the less likely it is that life will survive and even thrive on the exoplanets around red dwarfs.
L.Mignon et al, Characterization of the stellar activity of M. 1 dwarfs. Long time-scale variability in a large sample and discovery of new cycles, arXiv (2023). doi: 10.48550/arxiv.2303.03998
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