Phase-resolved ultraviolet spectroscopy of the magnetic Of?p star HD 191612

TL;DR

This study presents the first phase-resolved UV spectroscopic analysis of the magnetic Of?p star HD 191612, revealing coherent variability in spectral features linked to its magnetic and rotational properties.

Contribution

It provides new phase-resolved UV data for HD 191612 and models the star's magnetosphere using non-spherical MHD simulations to explain observed wind variability.

Findings

UV line strengths vary with the star's rotation period

Magnetic and wind features show coherent variability

Non-spherical MHD models explain wind profile variations

Abstract

We present for the first time phase-resolved UV spectroscopy of an Of?p star, namely, HD 191612. The observations were acquired with the Space Telescope Imaging Spectrograph (STIS) on-board the Hubble Space Telescope (HST). We report the variability observed in the main photospheric and wind features and compare the results with previous findings for the Of?p star HD 108. We show that UV line strengths, H(alpha), and longitudinal magnetic field, vary coherently according to the rotational period (P = 537.6d), providing additional support for the magnetic oblique rotator scenario. The stellar and wind parameters of HD 191612 are obtained based on NLTE expanding atmosphere models. The peculiar wind line profile variations revealed by the new STIS data - not reproduced by 1D atmosphere models - are addressed through non-spherical MHD simulations coupled with radiative transfer. The basic aspects of the UV variability observed are explained and the structure of the dynamical magnetosphere of HD 191612 is discussed.