Mon. Not. R. Astron. Soc. 388 (3), 1239−1344, 2008
© Royal Astronomical Society
The period and amplitude changes of Polaris (α UMi) from
2003 to 2007 measured with SMEI
S.A. Spreckley and I.R. Stevens
School of Physics and Astronomy, Univ. of Birmingham, Birmingham, UK
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 ± 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 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.