Presentasjon av masteroppgave: Marius Berge Eide
Towards a More Realistic Treatment of Polarization in Lyman Alpha Radiative Transfer
Lyman α radiation can be used as an astronomical and cosmological observable, being the strongest line among the hydrogen transitions. Using modern radiative transfer routines, it is possible to create simulations of accurate physical conditions and create predictions on observable properties of Lyman α radiation. Until recently has most information been obtained from the intensity and the spectrum, but a few observations (Hayes et al. 2011; Oesch et al. 2015) have been made which indicate that the polarisation also is an observable property. These observations yielded results that were in accordance with the simulated predictions by Dijkstra & Loeb (2008). However, the simulations by Dijkstra & Loeb (2008) are unique among the increasingly physically complex radiative transfer routines of recent times, as these largely ignore polarisation. Despite being rather unique, the treatment of Dijkstra & Loeb (2008) was not accurate. An approach that is accurate quantum mechanically, based on the density matrix formalism as a description of quantum systems, was devised and applied by Lee & Ahn (2002).In this thesis is this formalism incorporated into an existing radiative transfer routine developed by Gronke & Dijkstra (2014), called tlac. The method is applied to analytical scattering scenarios, single scattering events of polarised and unpolarised radiation and multiple scatterings in a plane-parallel, semi-infinite slab of line centre optical depths Ƭ0 = 2 ×10²,2 ×10³,2×10⁴,2×10⁶. We show that the density matrix formalism is coordinate dependent, and produce a polarisation signal specific to the scattering medium described by Lee & Ahn (2002). We reproduce some key results from Lee & Ahn (2002); Chandrasekhar (1960). We show that the intrinsic degree of polarisation associated with each photon increases as a function of number of wing scatterings, which again depends on the optical thickness of the scattering medium. This photon-intrinsic increase in polarisation does not necessarily correspond to a detectable polarization signal, as the latter depends on (not exclusively) the scattering geometry, the viewing angle and the measurement method. We find that scattering of polarised light follows the same angular distribution as scattering of unpolarised light, as predicted by Dijkstra & Loeb (2008). However, the degree of polarisation obtained from scattering of polarised light differs from the degree obtained when unpolarised light scatters. Future applications of the modified tlac on physically realistic scattering media may provide observational predictions on the polarisation signal that future telescopes could detect.
Veileder: Førsteamanuensis Mark Dijkstra,Institutt for teoretisk astrofysikk , UiO
Intern sensor: Førsteamanuensis Boris Vilhelm Gudiksen, Institutt for teoretisk astrofysikk, UiO
Ekstern sensor: Universitetslektor Matthew Hayes, Institutionen för astronomi, Stockholm University