H$_α$ line as an indicator of envelope presence around the Cepheid Polaris Aa ($α~ UMi$)

We present the results of the radial velocity ($RV$) measurements of metallic lines as well as H$_α$ (H$_β$) obtained in 55 high-resolution spectra of the Cepheid $α$ UMi (Polaris Aa) in 1994-2010. While the $RV$ amplitudes of these lines are roughly equal, their mean $RV$ begin to differ essentially with growth of the Polaris Aa pulsational activity. This difference is accompanied by the H$_α$ line core asymmetries on the red side mainly (so-called knife-like profiles) and reaches 8-12 km/s in 2003 with a subsequent decrease to 1.5-2 km/s. We interpret a so unusual behaviour of the H$_α$ line core as dynamical changes in the envelope around Polaris Aa.