Stokes imaging of AM Her systems using 3D inhomogeneous models – I. Description of the code and an application to V834 Cen

• Published in: Monthly notices of the Royal Astronomical Society Vol. 398; no. 1; pp. 240 - 248
• Main Authors: Costa, J. E. R., Rodrigues, C. V.
• Format: Journal Article
• Language: English
• Published: Oxford, UK Blackwell Publishing Ltd 01.09.2009; Wiley-Blackwell; Oxford University Press
• Subjects: Astronomy; cataclysmic variables; Earth, ocean, space; Exact sciences and technology; Magnetic fields; methods: numerical; novae; novae, cataclysmic variables; polarization; radiative transfer; Stars & galaxies; Statistical analysis; polarization; magnetic fields; methods: numerical; radiative transfer; novae, cataclysmic variables; Polarization; Accretion; White dwarf stars; Optical models; Bremsstrahlung; Novae; Three-dimensional systems; Numerical method; Genetic algorithms; cataclysmic variables; Stokes parameters; Electron temperature; Magnetic line; High field; Cataclysmic variable stars; Radiative transfer; Magnetic fields; Dipole field
• ISSN: 0035-8711; 1365-2966
• DOI: 10.1111/j.1365-2966.2009.15130.x

Polars (or AM Her systems) are cataclysmic variables without a disc due to the strong magnetic field of the white dwarf. Most of their emission comes from the region where the accretion column impacts the white dwarf and cools through cyclotron and bremsstrahlung processes. We present a new code, cyclops, to model the optical emission from these systems including the four Stokes parameters. It considers a three-dimensional region with the electronic density and temperature varying following a shock-like profile and a dipolar magnetic field. The radiative transfer is solved in steps considering the solution with non-null input radiation. The footprint of the column in the white dwarf surface is determined by the threading region in the equatorial plane, i.e. the region from where the flow follows the magnetic lines. The extinction caused by the Thomson scattering above the emitting region is optionally included. The search for the model parameters that best fit an observational data set is carried out using a hybrid approach: a genetic algorithm is used to seek for the regions of the parameter space having the best models and then an amoeba code refines the search. An example of the application to multi-wavelength data of V834 Cen is presented. The fit found is consistent with previous parameter estimates and is able to reproduce the features of V834 data in three wavebands.