Capturing a Flare of Proxima Centauri
Low mass M-type stars are the most common stars in the Galaxy and have a high frequency of Earth-sized planets. There is debate, however, as to what extent planets around M dwarfs would be amenable to life. Late M-type stars have long pre-main sequence phases during which the stellar luminosity can change significantly, as well as high stellar activity throughout their lifetimes. Proxima Centauri (spectral type M5.5V) has garnered tremendous recent interest due to the radial velocity detection of a potentially Earth-mass planet within the habitable zone and a new candidate transit event. At a distance of just 1.3 parsecs, Proxima Centauri b (mp sin I = 1.3 M⊕, a = 0.05 AU) is the closest extrasolar planet to the Solar System. Proxima Centauri has long been known as a flare star.
MacGregor et al. present new analyses of ALMA 12m and Atacama Compact Array (ACA) observations at 233 GHz (1.3 mm) of the Proxima Centauri system acquired 21 January – 25 April 2017. These analyses reveal that the star underwent a significant flaring event during one of the ACA observations on 24 March 2017. The event lasted ~1 minute and reached a peak flux density nearly 1000X times brighter than the star’s quiescent emission. At the flare peak, the continuum emission is characterized by a steeply falling spectral index with frequency Fν ∝ να with α = −1.77 ± 0.45, and a lower limit on the fractional linear polarization of ∣Q / I∣ = 0.19 ± 0.02. Because the ACA observations do not show any quiescent excess emission, the authors conclude that there is no need to invoke the presence of a dust belt at 1–4 AU. They also posit that the slight excess flux density of 101 ± 9 μJy observed in the 12m observations, compared to the photospheric flux density of 74 ± 4 μJy extrapolated from infrared wavelengths, may be due to coronal heating from continual smaller flares, as is seen for AU Mic, another nearby well-studied M dwarf flare star.
Image: Millimeter emission from Proxima Centauri is detected only in a small subset of the ACA data. The left image combines all 13 ACA observations. The middle image shows the first 12 observations combined; the right image shows only the final observation. A point source is clearly detected at the stellar location only in the final observation. The blue star indicates the stellar position; the 7.27 x 5.51 arcsecond synthesized beam is indicated by the ellipse in the lower-left corner.
Publication: Meredith MacGregor (Carnegie) et al., Detection of a Millimeter Flare from Proxima Centauri, Astrophysical Journal Letters, 855, L2 (1 March 2018).
