Towards an understanding of the Of?p star HD 191612: optical spectroscopy

TL;DR

This study provides detailed optical spectroscopy of the magnetic star HD 191612, revealing periodic emission variability likely caused by rotational modulation of magnetically confined plasma, and identifies the star as an O8 giant with a B1 main-sequence companion.

Contribution

It offers the first extensive optical spectroscopic analysis of HD 191612, linking emission variability to magnetic and rotational phenomena rather than orbital motion.

Findings

Reproducible 538-day emission variability unrelated to orbital motion

Identification of the star as an O8 giant with a B1 main-sequence companion

Detection of a double-lined binary orbit with a 1542-day period

Abstract

We present extensive optical spectroscopy of the early-type magnetic star HD 191612 (O6.5f?pe-O8fp). The Balmer and HeI lines show strongly variable emission which is highly reproducible on a well-determined 538-d period. Metal lines and HeII absorptions (including many selective emission lines but excluding He II 4686A emission) are essentially constant in line strength, but are variable in velocity, establishing a double-lined binary orbit with P(orb) = 1542d, e=0.45. We conduct a model-atmosphere analysis of the primary, and find that the system is consistent with a O8: giant with a B1: main-sequence secondary. Since the periodic 538-d changes are unrelated to orbital motion, rotational modulation of a magnetically constrained plasma is strongly favoured as the most likely underlying `clock'. An upper limit on the equatorial rotation is consistent with this hypothesis, but is too weak to provide a strong constraint.