Journal Article

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

S. A. Spreckley and I. R. Stevens

in Monthly Notices of the Royal Astronomical Society

Published on behalf of The Royal Astronomical Society

Volume 388, issue 3, pages 1239-1244
Published in print August 2008 | ISSN: 0035-8711
Published online July 2008 | e-ISSN: 1365-2966 | DOI:
The period and amplitude changes of Polaris (α UMi) from 2003 to 2007 measured with SMEI

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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 22 mmag 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.

Keywords: stars: evolution; stars: individual: α UMi (Polaris); Cepheids

Journal Article.  3891 words.  Illustrated.

Subjects: Astronomy and Astrophysics

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