Hubble Space Telescope Trigonometric Parallax of Polaris B, Companion of the Nearest Cepheid

TL;DR

This paper presents a new, independent measurement of Polaris B's parallax using Hubble's FGS, suggesting Polaris is farther and less luminous than previously thought, which impacts its pulsation mode and age estimates.

Contribution

The study provides a novel parallax measurement of Polaris B using HST FGS, challenging prior measurements and implications for Polaris's distance, luminosity, and pulsation mode.

Findings

Polaris B's parallax is 6.26+/-0.24 mas, smaller than Hipparcos.

Polaris may pulsate in the second overtone, implying higher luminosity.

Discrepancies exist between the ages of Polaris A and B.

Abstract

Polaris, the nearest and brightest Cepheid, is a potential anchor point for the Leavitt period-luminosity relation. However, its distance is a matter of contention, with recent advocacy for a parallax of ~10 mas, in contrast with the Hipparcos measurement of 7.54+/-0.11 mas. We report an independent trigonometric parallax determination, using the Fine Guidance Sensors (FGS) on the Hubble Space Telescope. Polaris itself is too bright for FGS, so we measured its 8th-magnitude companion Polaris B, relative to a network of background reference stars. We converted the FGS relative parallax to absolute, using estimated distances to the reference stars from ground-based photometry and spectral classification. Our result, 6.26+/-0.24 mas, is even smaller than found by Hipparcos. We note other objects for which Hipparcos appears to have overestimated parallaxes, including the well-established case of the Pleiades. We consider possible sources of systematic error in the FGS parallax, but find no evidence they are significant. If our "long" distance is correct, the high luminosity of Polaris indicates that it is pulsating in the second overtone of its fundamental mode. Our results raise several puzzles, including a long pulsation period for Polaris compared to second-overtone pulsators in the Magellanic Clouds, and a conflict between the isochrone age of Polaris B (~2.1 Gyr) and the much younger age of Polaris A. We discuss possibilities that B is not a physical companion of A, in spite of the strong evidence that it is, or that one of the stars is a merger remnant. These issues may be resolved when Gaia provides parallaxes for both stars.