We investigate temporally changing variability amplitudes and the multi-periodicity of the type-I Cepheid Polaris using 161 high-precision radial velocity ( RV ) and bisector inverse span ( BIS ) measurements based on optical spectra recorded using Hermes at the 1.2 m Flemish Mercator telescope on La Palma , Canary Islands , Spain .
Using an empirical template fitting method , we show that Polaris ’ RV amplitude has been stable to within \sim 30 m s ^ { -1 } between September 2011 and November 2018 .
We apply the template fitting method to publicly accessible , homogeneous RV data sets from the literature and provide an updated solution of Polaris ’ eccentric 29.3 yr orbit .
While the inferred pulsation-induced RV amplitudes differ among individual data sets , we find no evidence for time-variable RV amplitudes in any of the separately considered , homogeneous data sets .
Additionally , we find that increasing photometric amplitudes determined using SMEI photometry are likely spurious detections due to as yet ill-understood systematic effects of instrumental origin .
Given this confusing situation , further analysis of high-quality homogeneous data sets with well-understood systematics is required to confidently establish whether Polaris ’ variability amplitude is subject to change over time .
We confirm periodic bisector variability periods of 3.97 d and 40.22 d using Hermes BIS measurements and identify a third signal at a period of 60.17 d. Although the 60.17 d signal dominates the BIS periodogram , we caution that this signal may not be independent of the 40.22 d signal .
Finally , we show that the 40.22 d signal can not be explained by stellar rotation .
Further long-term , high-quality spectroscopic monitoring is required to unravel the complete set of Polaris ’ periodic signals , which has the potential to provide unprecedented insights into the evolution of Cepheid variables .