Long- and short-term variability in O-star winds II. Quantitative analysis of DAC behaviour

TL;DR

This study quantitatively analyzes UV spectral variability in 10 O-type stars, focusing on discrete absorption components (DACs), their recurrence, and wind structure implications, revealing links to stellar rotation and wind geometry.

Contribution

It provides a detailed modeling of DAC behavior, recurrence timescales, and evidence for curved wind structures like corotating interaction regions in O-star winds.

Findings

DAC column density varies with time, increasing then decreasing.

DAC recurrence times are linked to stellar rotation periods.

Evidence of curved wind structures in at least one star.

Abstract

A quantitative analysis of time series of ultraviolet spectra from a sample of 10 bright O-type stars (cf. Kaper et al. 1996, Paper I) is presented. The migrating discrete absorption components (DACs), responsible for the observed variability in the UV resonance doublets, are modeled. It turns out that the column density of a DAC first increases and subsequently decreases with time when the component is approaching its asymptotic velocity. In some cases this velocity systematically differs from event to event. The recurrence timescale of DACs is derived for most targets, and consistent results are obtained for different spectral lines. The DAC recurrence timescale is interpreted as an integer fraction of the stellar rotation period. In some datasets the variability in the blue edge of the P Cygni lines exhibits a longer period than the DAC variability. This might be related to the systematic difference in asymptotic velocity of successive DACs. The phase diagram for the O giant xi Persei shows clear evidence for so-called ``phase bowing'', which is an observational indication for the presence of curved wind structures like corotating interaction regions. No other O stars in our sample convincingly show phase bowing, but this could be simply due to the absence of periodic signal and hence coherent phase behaviour at low wind velocities.