Multi-dimensional radiative transfer in the expanding atmospheres of massive stars
Final Report Abstract
This project has accomplished its main objective, namely to develop a 3D radiative transfer code that is capable of calculating continuum and line scattering problems in asymmetric winds of OB stars with nonmonotonic velocity fields. The code can be used with two different methods, the finite-volume method (FVM) and the short-characteristics (SC) method, with faster turn-around times of the FVM when compared to the SC methods (however at the cost of lower accuracy). Both methods have been augmented by an accelerated Λ-iteration scheme using newly developed non-local operators. Since the basic finitevolume method has been already applied (though in a different implementation) to non-spherical winds by Lobel & Blomme (2008), the low accuracy of this method was certainly surprising. Indeed, the predicted synthetic spectra can only be used for qualitative interpretations (unless considering very weak line transitions). Moreover, the FVM breaks down completely for optically thick continua. For such problems, and when aiming at a quantitative analysis of line profiles, the SC methods should be used. All methods have been extensively tested, such that the 3D code can be readily applied to calculate resonance-line transitions in arbitrary wind structures with non-monotonic velocity fields. An extension of the code to treat multi-level atoms (e.g., required for calculating Hα ) is planned for future projects. As first applications to non-spherical winds, we considered the resonance-line formation in (i) rapidly rotating and (ii) (slowly rotating) magnetically confined winds. For the latter, we were able to show that a commonly used description of the wind structure (the so-called ‘analytic dynamical magnetosphere’) shows severe deviations when compared with line profiles obtained from numerical magneto-hydrodynamic models. Thus, the ADM model needs to be adjusted in order to be used for calculating wind lines. To analyze a large sample of magnetic stars, an analytic description is required due to the computationally very expensive MHD simulations. For rapidly rotating stars, we only considered a prolate wind model (as expected in fast rotating O stars), and predict that the resonance-line profiles depend in specific ways on both the stellar angular velocity and the inclination towards the observer. Thus, future investigations of such objects may help to derive the actual rotational velocities (instead of the projected ones). Moreover, the developed code will certainly help to answer important questions related to theory: (i) What is the actual behaviour of the gravity darkening law? (ii) What is the actual wind structure (prolate vs. oblate wind) of rapidly rotating stars, and at which temperatures would we observe a (potential) transition?
Publications
- (2019) The Low Molecular Mass Photosystem II Protein PsbTn Is Important for Light Acclimation. Plant physiology 179 (4) 1739–1753
Hennicker, L.; Puls, J.; Kee, N. D.; Sundqvist, J. O.
- 2018: ”3D radiative transfer: Continuum and line scattering in non-spherical winds from OB stars”, A&A, 616, A140
Hennicker L., Puls J., Kee N. D., Sundqvist J. O.
(See online at https://doi.org/10.1051/0004-6361/201731858)
