PRECISE RADIAL VELOCITIES OF POLARIS: DETECTION OF AMPLITUDE GROWTH

Abstract

We present the first results of a long-term program of a radial velocity (RV) study of Cepheid Polaris (F7 Ib), with the aim of finding the amplitude and period of its pulsations and the nature of its secondary periodicities. A total of 264 new precise RV measurements were obtained during 2004-2007 with the fiber-fed echelle spectrograph Bohyunsan Observatory Echelle Spectrograph (BOES) of the 1.8 m telescope at Bohyunsan Optical Astronomy Observatory (BOAO) in Korea. We find a pulsational RV amplitude and period of Polaris for the three seasons 2005.183, 2006.360, and 2007.349 as 2K = 2.210 ± 0.048 km s–1, 2K = 2.080 ± 0.042 km s–1, and 2K = 2.406 ± 0.018 km s–1 respectively, indicating that the pulsational amplitudes of Polaris that had decayed during the last century are now increasing rapidly. The pulsational period was also found to be increasing. This is the first detection of a historical turnaround of a pulsational amplitude change in the Cepheids. We clearly find the presence of additional RV variations on a timescale of about 119 days and an amplitude of about ±138 m s–1, which is quasi-periodic rather than strictly periodic. From our data, we do not confirm the presence of the variation on a timescale of 34-45 days found in the RV data obtained in the 1980s and 1990s. We assume that both the 119 day quasi-periodic, noncoherent variations found in our data and the 34-45 day variations found previously can be caused by the 119 day rotation periods of Polaris and by surface inhomogeneities such as single- or multiple-spot configuration varying with time.