Orbits and outflows of emission-line stars

Thesis

The optical spectra of some stars exhibit spectral lines in emission, which is characteristic of them having powerful radiatively driven winds. These stars are known as emission-line stars. In this thesis, I study the orbits and outflows of the most luminous and massive emission-line stars: Luminous Blue Variables and Wolf-Rayet stars.

Orbital parameters are typically determined by fitting Keplerian models to radial velocity measurements. For stars with powerful winds, the broad emission lines are formed far out in the stellar wind, encoding dynamical information from an extensive volume of outflowing gas, in a complex and stratified circumstellar environment, which may lead to systematics in the observables. I tackle these problems twofold. (1) I formulate a novel semi-analytical model which encapsulates both the star's orbital motion and the propagation of the wind. (2) I devise a statistical framework for validating the performance of algorithms, and utilise a marginalised Gaussian process to account for residual systematics. I benchmark our models with synthetic data and apply them to several prototypical emission-line binaries, namely Eta Carinae, GG Carinae, WR 140, and WR 133. I find our models identify a consistent set of orbital parameters, independent of the emission line used, and given validation on benchmark datasets, the estimated parameter distributions and dynamical masses are more accurate than those previously calculated.

The Global Jet Watch affords unprecedented coverage of Eta Carinae throughout apastron, revealing the distinct dynamics of the emitting versus the absorbing components of the He I profiles. Cognisant of the known Helium ionisation maps, I model these absorption velocities as tracers of the geometry of the post-shock gas in the colliding winds. I find the modelling is concordant with the panchromatic observations and simulations of Eta Carinae and demonstrate how the model can be used to probe the latitudinal dependence of stellar mass loss. Finally, I describe the Global Jet Watch's photometric datastream, and detail the innovation of new algorithms and the resulting light curves.

Authors: Grant, D

• Type of award: DPhil
• Level of award: Doctoral
• Awarding institution: University of Oxford
• Language: English
• Subjects: Astrophysics