Optical, UV, and X-ray evidence for a 7-yr stellar cycle in Proxima Centauri

Abstract

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 2300-6000Å.

THE SPACE WEATHER OF PROXIMA CENTAURI b

“…The true Alfvén surface will be fairly dynamic and depend on the exact wind conditions at any given time. The strong dependence of the Alfvén surface size on the magnetic field strength and the likely variation of this, together with the recently detected magnetic cycles (Wargelin et al 2016), suggest Proxima b is likely to encounter transitions between subsonic and supersonic wind conditions quite frequently. Cohen et al (2014) found that the lack of a bow shock in subsonic conditions leads to much deeper penetration of the wind into the magnetosphere than in supersonic conditions.…”

“…Magnetic field strength scales with rotation period, and to account for this we have scaled the magnetic map to match the field strength measured for Proxima of ∼600 G (Reiners & Basri 2008). Since this is only a single measurement of the average magnetic field strength and magnetic cycles have been detected on Proxima (Wargelin et al 2016), we probe two different scalings. In one, the amplitude (or maximum strength) of the magnetic field is 600 G (see Figure 1) and in the other the mean magnetic field is 600 G and the maximum value is 1200 G (and looks exactly like the magnetogram in Figure 1 but with a color bar going up to 1200 G).…”

Magnetic Cycles in a Dynamo Simulation of Fully Convective M-Star Proxima Centauri

“…An earlier reporting (Cincunegui et al 2007) suggested an activity cycle length of about 1.2 years. Recent work based on a longer data set could not confirm the 1.2 year period but found strong evidence for cycles with a length of approximately 7 years (Mascareño et al 2016;Wargelin et al 2017).…”

Strong dipole magnetic fields in fast rotating fully convective stars

“…This would explain the difference in magnetic fields between individual objects. Note that the signs of activity cycles were found in some M dwarfs stars based on photometric/spectroscopic observations 27 , including a recent detection of a clear 7 year photometric cycle in fully convective and slowly rotating (P ≈ 83d) M dwarf Proxima Centauri 28 . By computing a model for this star it was shown that dynamo in fully convective M dwarfs can indeed generate magnetic cycles with repeated reversals of the magnetic polarities at different latitudes with time, provided that these stars rotate slow enough to develop differential rotation in their convection zones 29 .…”