Using Shell models to investigate clumping in the wind of the O7Iaf + supergiant AzV83

Abstract

ABSTRACT Hot massive stars exhibit strong stellar winds that enrich the surrounding interstellar medium and affect the stars’ evolution. However, the winds are inhomogeneous (clumped) and are difficult to model in radiative transfer codes. To produce more realistic spectra, many codes use a volume-filling factor (VFF) approach to incorporate the effects of clumping. While this approach is convenient, it is simplistic. We introduce an alternative approach to incorporate clumping by assuming the wind is composed of dense spherical shells. Using this approach in the radiative transfer code cmfgen, we produce synthetic spectra for AzV83, an O7Iaf + supergiant located in the Small Magellanic Cloud. The spectrum of AzV83 is rich in both photospheric and wind features, making it an ideal candidate with which to investigate the physical characteristics of stellar winds. Synthetic spectra are compared to the star’s observed spectrum to better characterize the influence of clumped winds on spectral features, and to better understand the limitations of the VFF approach. The approach using spherical shells yields similar wind parameters to those obtained using the VFF approach although a slightly higher mass-loss rate is required to fit Hα. As expected, the interclump medium in the model with shells allows the high ionization resonance transitions of N v and O vi to be fitted using LX-ray/LBol ≈ 10−7, which is typically observed for O stars, and which is a factor of 10 lower than needed with the VFF approach.