The Radial Velocity and Spectral Line Bisector Variability of Polaris

Abstract

We present precise stellar radial velocity measurements for the Cepheid-type star Polaris taken in 1992–1993. The peak-to-peak amplitude of the pulsations was 1.555 ± 0.056 km s-1 in epoch 1992.4 and 1.517 ± 0.047 km s-1 in epoch 1993.2. These amplitudes are comparable to the one measured by Dinshaw et al. in 1987.75 and recent measurements by Kamper &Fernie for 1995–1997. Consequently, these do not support the continued drop in pulsational amplitude first extrapolated by Dinshaw et al. A periodogram analysis for our data yields a pulsational period of 39726769 ± 000011, slightly higher than the value found by Dinshaw et al. and consistent with the value derived from the Kamper &Fernie data. The pulsational period of Polaris may thus be increasing. Residual radial velocity measurements after removal of the component due to radial pulsations show the presence of a 40 day period with a 2K amplitude of about 400 m s-1, first reported by Dinshaw et al. Spectral line bisectors of the Sc II λ5526 and Mg I λ5528 lines also show variations with the 40 day period, strongly indicating that this period is indeed real. The presence of line bisector variations excludes a low-mass companion object as a cause of the residual radial velocity variations. Two models are considered for the residual radial velocity and bisector span variations: starspots and nonradial pulsations. Although both can reproduce the amplitude and shape of the residual radial velocity variations, the spot models considered were unable to reproduce the overall shape and amplitude of the bisector span variations. The best-fit spot model was provided by a single spot with a temperature 500 K cooler than the photosphere with a macroturbulent velocity of zero in the spot. If surface features do exist on Polaris and are responsible for the residual radial variations for this star, then possibly they have a temperature structure and distribution more complicated than the simple models considered here. The best fit to all the observed variations was provided by a nonradial m = 4 mode, although these were not entirely satisfactory having a phase shift of about 0.25 between the observed and predicted bisector variations. Possible explanations for this shift include temperature effects, a source function originating in a dynamic stellar atmosphere, or a different pulsation mode than the one that was considered. Although the exact pulsational mode is yet to be identified, we believe that the residual radial velocity variations are due to a long-period pulsation mode. Supporting evidence comes from the fact that three different residual variations with periods near 40 days have now been seen in Polaris and it is unlikely that these variations are due to rotational modulation by surface features on a star that is differentially rotating.