HST/COS Spectra of the Wind Lines of VFTS 102 and 285

TL;DR

This study analyzes ultraviolet spectra of two rapidly rotating O-type stars to understand how extreme rotation influences stellar wind structures and mass loss distribution, revealing a two-wind regime with implications for stellar angular momentum evolution.

Contribution

It provides the first detailed UV spectral analysis of the fastest rotating O-stars, demonstrating a two-wind regime with pole and equator mass loss differences linked to rapid rotation.

Findings

VFTS 285 shows a two-wind regime with pole and equator mass loss.

VFTS 102 exhibits equatorial mass loss into a circumstellar disk.

Most mass loss in these stars occurs as an equatorial outflow.

Abstract

Rapid rotation in massive stars imposes a latitudinal variation in the mass loss from radiatively driven winds that can lead to enhanced mass loss at the poles (with little angular momentum loss) and/or equator (with maximal angular momentum loss). Here we present an examination of the stellar wind lines of the two O-type stars with the fastest known equatorial velocities, VFTS 102 ($V\sin i = 610 \pm 30$ km/s; O9:Vnnne+) and VFTS 285 ($V\sin i = 609 \pm 29$ km/s; O7.5 Vnnn) in the Large Magellanic Cloud. Ultraviolet spectra of both stars were obtained with the Hubble Space Telescope Cosmic Origins Spectrograph. The spectrum of VFTS 285 displays a fast outflow in N V and a much slower wind in Si IV, and we argue that there is a two-wind regime in which mass loss is strong at the poles (fast and tenuous wind) but dominant at the equator (slow and dense winds). These ions and wind lines are not present in the spectrum of the cooler star VFTS 102, but the double-peaked H$\alpha$ emission in its spectrum implies equatorial mass loss into a circumstellar disk. The results suggest that in the fastest rotating O-stars, most mass is lost as an equatorial outflow, promoting angular momentum loss that contributes to a spin down over time.