Goodbye to Polaris the Cepheid

Abstract

New radial velocities and uvby photometry for Polaris are presented. These reveal that the pulsational amplitude of Polaris had declined to 0.59±0.19 km s<SUP>-1</SUP> and 0.010±0.002 mag in V by 1992. Comparison with earlier determinations reveals an exponential change with time rather than any sudden onset of the decline, and suggests that Polaris had an amplitude near 0.12 mag (6.0 km s 1) prior to 1900. There is no sign of the amplitude changing in a manner consistent with a beat phenomenon; instead, the star seems destined to stop pulsating in 1994. <P />Our data add new points to the O-C diagram of Polaris and show a continuation of the period increase known previously. We point out that departures of this diagram from a true parabola do not necessarily imply that factors other than stellar evolution are at work. <P />Finally, we lay to rest the myth that the amplitude of Polaris has declined because it is evolving across the red edge of the Cepheid instability strip. We show Polaris to be well within the strip and surrounded there by Cepheids of substantial amplitude. In particular, Polaris is virtually identical to RT Aur in M<SUB>V</SUB> and color, yet RT Aur has an amplitude of 0.8 mag. We conclude that there is as yet no adequate understanding of what sets the amplitude of a Cepheid