The period and amplitude changes of Polaris (alpha UMi) from 2003 to 2007 measured with SMEI

Abstract

We present an analysis of 4.5 years of high precision (0.1%) space-based photometric measurements of the Cepheid variable Polaris, obtained by the broad band Solar Mass Ejection Imager (SMEI) instrument on board the Coriolis satellite. The data span from April 2003 to October 2007, with a cadence of 101 minutes and a fill factor of 70%. We have measured the mean peak to peak amplitude across the whole set of observations to be 25 mmag. There is, however, a clear trend that the size of the oscillations has been increasing during the observations, with peak to peak variations less than 22 mmag in early 2003, increasing to around 28 mmag by October 2007, suggesting that the peak to peak amplitude is increasing at a rate of 1.39 \pm 0.12 mmag yr^{-1}. Additionally, we have combined our new measurements with archival measurements to measure a rate of period change of 4.90 \pm 0.26 s yr^{-1} over the last 50 years. However, there is some suggestion that the period of Polaris has undergone a recent decline, and combined with the increased amplitude, this could imply evolution away from an overtone pulsation mode into the fundamental or a double pulsation mode depending on the precise mass of Polaris.