Looking on the bright side: The story of AA Doradus as revealed by its cool companion

The effects of irradiation on the secondary stars of close binary systems are crucial for reliably determining the system parameters and for understanding the close binary evolution. They affect the stellar structure of the irradiated star and are reflected in the appearance of characteristic features in the spectroscopic and photometric data of these systems. We aim to study the light that originates from the irradiated side of the low-mass component of a close binary eclipsing system, which comprises a hot subdwarf primary and a low mass companion, to precisely interpret their high precision photometric and spectroscopic data, and accurately determine their system and surface parameters. We reanalyse the archival high-resolution time-resolved VLT/UVES spectra of AA Dor system, where irradiation features have already been detected. After removing the predominant contribution of the hot subdwarf primary, the residual spectra reveal more than 100 emission lines from the heated side of the secondary, which show maximum intensity close to the phases around the secondary eclipse. We analyse the residual spectrum to model the irradiation of the low-mass secondary. We perform a detailed analysis of 22 narrow emission lines of the irradiated secondary, mainly of O II, with a few significant C II lines. Their phase profiles constrain the emission region of the heated side to a radius >= 95% of the radius of the secondary, while the shape of their velocity profiles reveals two distinct asymmetry features, one at the quadrature and the other at the secondary eclipse. In addition, we identify weaker emission signatures originating from more than 70 lines, including lines from He I, N II, Si III, Ca II, and Mg II. From the emission lines of the heated side of the secondary star, we determine the radial velocity semi-amplitude of the centre-of-light and correct it to the centre-of-mass of the secondary which, in turn, gives accurate masses of both components of the AA Dor system. The resulting masses M-1 = 0.46 +/- 0.01 M-circle dot and M-2 = 0.079 +/- 0.002 M-circle dot are in perfect accordance with those of a canonical hot subdwarf primary and a low mass that is just at the substellar limit for the companion. We also compute a first generation atmosphere model of the low mass secondary, which includes irradiation effects and matches the observed spectrum well. We find an indication of an extended atmosphere of the irradiated secondary star.