The influence of rotation on optical emission profiles of O stars

TL;DR

This paper investigates how stellar rotation influences the formation and appearance of emission lines in O stars, explaining observed profile shapes without needing additional density structures.

Contribution

It demonstrates that rotation alone can produce rectangular and double-peaked emission profiles, challenging previous assumptions about stellar atmospheres.

Findings

Rotation causes rectangular and double-peaked emission profiles.

Limb darkening laws are non-standard for certain emission lines.

Rotation affects wind diagnostic line profiles, impacting wind structure inferences.

Abstract

We study the formation of photospheric emission lines in O stars and show that the rectangular profiles, sometimes double peaked, that are observed for some stars are a direct consequence of rotation, and it is unnecessary to invoke an enhanced density structure in the equatorial regions. Emission lines, such as N IV 4058 and the N III 4634-4640-4642 multiplet, exhibit non-standard "limb darkening" laws. The lines can be in absorption for rays striking the center of the star and in emission for rays near the limb. Weak features in the flux spectrum do not necessarily indicate an intrinsically weak feature -- instead the feature can be weak because of cancellation between absorption in "core" rays and emission from rays near the limb. Rotation also modifies line profiles of wind diagnostics such as He II 4686 and Halpha and should not be neglected when inferring the actual stratification, level and nature of wind structures.