Radial dependence of line profile variability in seven O9--B0.5 stars

TL;DR

This study investigates spectral variability in seven OB stars, revealing that wind lines exhibit significant variability on daily timescales, with variability amplitude correlating with the line formation region, but photospheric variability remains inconsistent.

Contribution

It provides a detailed analysis of the radial dependence of line profile variability in OB stars using high-resolution spectra and the TVS method, highlighting the stronger variability in wind-formed lines.

Findings

Wind lines show variability on daily timescales in giants and supergiants.

Photospheric lines sometimes vary, sometimes not, with no clear pattern.

Variability amplitude correlates with the fraction of the line formed in the wind.

Abstract

Massive stars show a variety of spectral variability: presence of discrete absorption components in UV P-Cygni profiles, optical line profile variability, X-ray variability, radial velocity modulations. Our goal is to study the spectral variability of single OB stars to better understand the relation between photospheric and wind variability. For that, we rely on high spectral resolution, high signal-to-noise ratio optical spectra collected with the spectrograph NARVAL on the Telescope Bernard Lyot at Pic du Midi. We investigate the variability of twelve spectral lines by means of the Temporal Variance Spectrum (TVS). The selected lines probe the radial structure of the atmosphere, from the photosphere to the outer wind. We also perform a spectroscopic analysis with atmosphere models to derive the stellar and wind properties, and to constrain the formation region of the selected lines. We show that variability is observed in the wind lines of all bright giants and supergiants, on a daily timescale. Lines formed in the photosphere are sometimes variable, sometimes not. The dwarf stars do not show any sign of variability. If variability is observed on a daily timescale, it can also (but not always) be observed on hourly timescales, albeit with lower amplitude. There is a very clear correlation between amplitude of the variability and fraction of the line formed in the wind. Strong anti-correlations between the different part of the temporal variance spectrum are observed. Our results indicate that variability is stronger in lines formed in the wind. A link between photospheric and wind variability is not obvious from our study, since wind variability is observed whatever the level of photospheric variability. Different photospheric lines also show different degrees of variability.