Our Sun experiences an 11-year activity cycle. At the solar minimum, the Sun is nearly spot-free. At solar maximum, typically more than 100 sunspots cover less than one percent of the Sun’s surface on average.
A new study finds that Proxima Centauri undergoes a similar cycle lasting seven years from peak to peak. However, its cycle is much more dramatic. At least a full one-fifth of the star’s surface is covered in spots at once. Also, some of those spots are much bigger relative to the star’s size than the spots on our Sun.
“If intelligent aliens were living on Proxima b, they would have a very dramatic view,” says lead author Brad Wargelin of the Harvard-Smithsonian Center for Astrophysics (CfA).
Astronomers were surprised to detect a stellar activity cycle in Proxima Centauri because its interior is expected to be very different from the Sun’s. The outer third of the Sun experiences a roiling motion called convection, similar to water boiling in a pot, while the Sun’s interior remains relatively still. There is a difference in the speed of rotation between these two regions. Many astronomers think the shear arising from this difference is responsible for generating the Sun’s magnetic activity cycle.
In contrast, the interior of a small red dwarf like Proxima Centauri should be convective all the way into the star’s core. As a result, it shouldn’t experience a regular cycle of activity.
“The existence of a cycle in Proxima Centauri shows that we don’t understand how stars’ magnetic fields are generated as well as we thought we did,” says Smithsonian co-author Jeremy Drake.
Understanding stellar magnetic activity of stars is relevant to studies of exoplanets because starspots and flares can mimic or obscure the signatures of planets and may affect those planets’ habitability. This latter subject is especially interesting in light of the recent discovery of an exoplanet orbiting in the habitable zone of our Sun’s nearest neighbor, Proxima Centauri. Proxima b has a minimum mass of about 1.3 times that of the Earth and an orbital period of 11.2 days with a semi-major axis of only 0.049 AU, about one-eighth Mercury’s orbital radius. A key factor in planetary habitability is the effect on the atmosphere of X-ray/UV radiation and the stellar wind which are ultimately driven by the stellar magnetic field
Stars of stellar type later than about M3.5 are believed to be fully convective and therefore unable to support magnetic dynamos like the one that produces the 11-year solar cycle. Because of their intrinsic faintness, very few late M stars have undergone long-term monitoring to test this prediction, which is critical to our understanding of magnetic field generation in such stars. Magnetic activity is also of interest as the driver of UV and X-ray radiation, as well as energetic particles and stellar winds, that affect the atmospheres of close-in planets that lie within habitable zones, such as the recently discovered Proxima b. We report here on several years of optical, UV, and X-ray observations of Proxima Centauri (GJ 551; dM5.5e): 15 years of ASAS photometry in the V band (1085 nights) and 3 years in the I band (196 nights), 4 years of Swift XRT and UVOT observations (more than 120 exposures), and 9 sets of X-ray observations from other X-ray missions (ASCA, XMM-Newton, and three Chandra instruments) spanning 22 years. We confirm previous reports of an 83-day rotational period and find strong evidence for a 7-year stellar cycle, along with indications of differential rotation at about the solar level. X-ray/UV intensity is anti-correlated with optical V-band brightness for both rotational and cyclical variations. From comparison with other stars observed to have X-ray cycles we deduce a simple empirical relationship between X-ray cyclic modulation and Rossby number, and we also present Swift UV grism spectra covering 2300-6000
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