Polaris

Villanova astronomy students, Scott Engle (Villanova), James Davis (now SDSU) and Jeffrey Tracey (now Catholic Univ.) with Guinan continue to carried out photoelectric photometry of Polaris during 2002 and 2003. These observations were used to study light variations and period changes of this nearby, low-amplitude, pulsating classical Cepheid. Previous studies have found a steady decline of Polaris light and radial velocity amplitudes since the early twentieth century. An analysis of the times of maximum light (or corresponding times derived from radial velocity data) show that Polaris is undergoing an increase in pulsation period of dP/dt = +3.2 sec/yr. Our photometry, when combined with previous results, shows that the light amplitude change of Polaris appears to be holding steady from 1994 until early 2002 with a light amplitude of Amp(V) $= 0.028\pm 0.002$ mag. But photometry secured recently during Fall 2003 indicates a possible increase in the light amplitude to Amp(V) $= 0.035\pm 0.00$ mag. In addition an analysis of the all of photometric and spectroscopic timings shows that the period continues to increase at the rate of +3.51 sec/ yr (using a cubic least squares fit to the data). These observations (together with interferometric diameter measures and Hipparcos parallaxes) support the overtone nature of Polaris pulsations. The transition from a moderate to a low amplitude pulsator is delineated in the literature (e.g., see Evans et al.). This work is a continuation of the previous work done by K.W. Kamper and J.D. Fernie, whose invaluable study of the radial velocity/light amplitude relationship of Polaris serves as the basis for this continuing monitoring of Polaris after the minimum in the light amplitude occurred during the mid-1990s.

The increase in Polariss pulsation period, however, continues unabated. In addition to the long term secular period increase, a detailed analysis of the times of light maximum possibly shows a small cyclic (sinusoidal) oscillation of the apparent period on time scales of several decades. A formal least squares analysis of the residuals yields a period of $P' \sim 53.6\pm 1.4$ years. The semi-amplitude of this variation is $\delta (O-C) = 0.314\pm0.033$ days. This amplitude is too large to be produced by the light travel time effect from a massive companion. Possible explanations for this are small cyclic variations in the star's pulsation that could be produced by small cyclic changes in its average radius. These radius changes could arise from cycles of magnetic activity of the order of 54 yrs. Also these oscillations could arise from interactions of the fundamental and overtone+ pulsation modes. The preliminary results of this study were presented at the January 2003 Meeting of the AAS (Seattle, WA.) and a detailed paper is being prepared for publication.