H$_{\alpha}$ line as an indicator of envelope presence around the Cepheid Polaris Aa ($\alpha~ UMi$)

TL;DR

This study analyzes radial velocity measurements of Polaris Aa over 16 years, revealing that changes in the H$_{eta}$ line core asymmetries indicate dynamical envelope activity around the Cepheid star.

Contribution

It provides evidence linking H$_{eta}$ line core asymmetries to envelope presence and activity around Polaris Aa, a novel observational insight.

Findings

H$_{eta}$ line core asymmetries correlate with envelope activity

H$_{eta}$ RV differences increase with pulsational activity

Envelope dynamics influence spectral line profiles

Abstract

We present the results of the radial velocity ($RV$) measurements of metallic lines as well as H$_{\alpha}$ (H$_{\beta}$) obtained in 55 high-resolution spectra of the Cepheid $\alpha$ 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$_{\alpha}$ 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$_{\alpha}$ line core as dynamical changes in the envelope around Polaris Aa.