The moderate magnetic field on the flare star Proxima Centauri

We report moderate magnetic flux of 450G < Bf < 750G (3sigma) on the nearby M5.5 flare star Proxima Centauri. A high resolution UVES spectrum was used to measure magnetic flux from Zeeman broadening in absorption lines of molecular FeH around 1mu. The magnetic flux we find is relatively weak compared with classical strong flare stars, but so are Proxima's flaring rates and actual emission levels. We compare what is known about the rotation rate, Rossby number, and activity levels in this star to relations between these quantities that have been recently being developed more generally for M dwarfs. We conclude that the magnetic flux is higher than the best estimates of the Rossby number from period measurements. On the other hand, the activity levels on Proxima Centauri are at the high end of what could be expected based on the measured field, but not so high as to exceed the natural scatter in these relations (other stars lie along this high envelope as well).