Photometry of Proxima Centauri and Barnard's Star Using [ITAL]Hubble[/ITAL] [ITAL]Space[/ITAL] [ITAL]T[/ITAL][ITAL]elescope[/ITAL] Fine Guidance Sensor 3: A Search for Periodic Variations

Abstract

We have observed Proxima Centauri and Barnards star with the Hubble Space T elescope Fine Guidance Sensor 3. Proxima Cen exhibits small-amplitude, periodic photometric variations. Once several sources of systematic photometric error are corrected, we obtain 2 mmag internal photometric precision. We identify two distinct behavior modes over the past 4 years : higher amplitude, longer period and smaller amplitude, shorter period. Within the errors, one period (P D 83 days) is twice the other. Barnards star shows very weak evidence for periodicity on a timescale of approximately 130 days. If we interpret these periodic phenomena as rotational modulation of starspots, we identify three discrete spots on Proxima Cen and possibly one spot on Barnards star. We nd that the disturbances change signicantly on timescales as short as one rotation period.