The distance, rotation, and physical parameters of ζ Pup

Abstract

We scrutinize the Hipparcos parallax for the bright O supergiant ζ Pup and confirm that the implied distance of 332 ± 11 pc appears to be reliable. We then review the implications for the star's physical parameters and the consequences for the interpretation of P_phot, the 1.78-d photometric period. The inferred mass, radius, and luminosity are securely established to be less than canonical values for the spectral type and are not in agreement with single-star evolution models. The runaway status, rapid rotation, and anomalous physical properties are all indicative of an evolutionary history involving binary (or multiple-star) interaction. The equatorial rotation period is <3.7 d (with 95 per cent confidence), ruling out a proposed ∼5.1-d value. If the photometric period is the rotation period then i, the inclination of the rotation axis to the line of sight, is 33{^d_.}g2 ± 1{^d_.}g8. We perform simple star-spot modelling to show that the low axial inclination required if P_rot = 1.78 d has testable spectroscopic consequences, which have not been identified in existing time series. If P_phot is directly related to drivers of systematic, high-velocity stellar-wind variability (`discrete absorption components') in ζ Pup, antisolar differential rotation is required. Model line profiles calculated on that basis are at variance with observations.