Calculations of Periodicity From H Alpha Profiles of Proxima Centauri

Abstract

In this report the stellar rotation signal for Proxima Centauri is investigated. High-resolution spectra taken with UVES and HARPS of Proxima Centauri over a 13-year period is used as well as photometric observations of Proxima Centauri from ASAS and HST. The Ha equivalent width and Ha index are measured (and found to be very similar), together with other measurements, skewness and kurtosis and a method that investigates the symmetry of the line, the Peak Ratio, is introduced which appears to return better results than the other measurements. The investigations return a most significant period of 82.6 0.1 days, confirming photometric results and ruling out a more recent result of 116.6 days which it is concluded is an artefact of the observation times. It is also concluded that whilst spectroscopic Ha measurements can be used for period recovery, in the case of Proxima Centauri the available photometric measurements are more reliable. By using 2D models of Proxima Centauri to generate simulated Ha, it is found that reasonable distributions of plage and chromospheric features are able to reproduce the equivalent width variations in observed data and recover the rotation period. The period recovery is still effective after simulated noise and the effects of flares similar to those in the observed data the are added to the modelling results. However the 2D models used fail to generate the observed variety of line shapes measured by the peak ratio. It is concluded that only 3D models which incorporate vertical motions in the chromosphere can achieve this.