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

We present an analysis of 4.5 yr of high precision (0.1 per cent) 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 2003 April to 2007 October, with a cadence of 101 min and a fill factor of 70 per cent. 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 22mmag in early 2003, increasing to around 28 mmag by 2007 October, suggesting that the peak-to-peak amplitude is increasing at a rate of 1.39 +/- 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 +/- 0.26 s yr(-1) over the last 50 yr. 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.