Homing in on Polaris: A 7 M$_\odot$ first-overtone Cepheid entering the instability strip for the first time

TL;DR

This paper reinterprets Polaris as a 7 solar mass, first-overtone classical Cepheid near the first instability strip boundary, aligning multiple observational data with stellar models and confirming the physical association with its companion.

Contribution

It provides a simplified interpretation of Polaris's pulsation mode and evolutionary status, resolving previous discrepancies and supporting the accuracy of recent parallax measurements.

Findings

Polaris is consistent with a 7 M_sun, first-overtone Cepheid.

The new parallax aligns with models and observational data.

An age discrepancy suggests a possible stellar merger history.

Abstract

A recently presented HST/FGS parallax measurement of the Polaris system has been interpreted as evidence for the Cepheid Polaris Aa to be pulsating in the second overtone. An age discrepancy between components A and B has been noted and discussed in terms of a stellar merger. Here I show that the new parallax of Polaris is consistent with a simpler interpretation of Polaris as a $7 \rm{M}_\odot,$ first-overtone, classical Cepheid near the hot boundary of the first instability strip crossing. This picture is anchored to rates of period change, the period-luminosity relation, the location in color-magnitude space, the interferometrically determined radius, spectroscopic N/C and N/O enhancements, and a dynamical mass measurement. The detailed agreement between models and data corroborates the physical association between the Cepheid and its visual companion as well as the accuracy of the HST parallax. The age discrepancy between components A and B is confirmed and requires further analysis, for example to investigate the possibility of stellar mergers in an evaporating birth cluster of which the Polaris triple system would be the remaining core.