A Search for the Binary Companion of Polaris

Abstract

Polaris has a spectroscopic orbit determined from an extensive series of observations as well as a more uncertain astrometric orbit. The determination of its mass and evolutionary state is of considerable interest because it is a low-amplitude classical Cepheid with unusual period and amplitude variations. In this study, IUE spectra are investigated to search for light from the companion. The spectra of Polaris from 1600 A to 3200 A are a good match for nonvariable supergiants of similar spectral type. The lack of any excess flux at the shortest wavelengths implies that a main-sequence companion must be later than A8 V. Although this is the most likely companion, the ultraviolet observations cannot rule out a white dwarf 15,000 K or cooler. Both these companions are consistent with either an evolutionary mass or a smaller pulsation mass for the Cepheid.