Looking at the bright side – The story of AA Dor ⋆ as revealed by its cool companion

Irradiation effects in secondary stars of close binary systems are crucial f or reliable determination of system parameters and understanding the close binary evolution. They a ffect the stellar structure of the irradiated star and are refle ct d in the appearance of characteristic features in the spectroscopic and photomet ric data of these systems. We aim to study the light originati ng from the irradiated side of the low mass component of close binary ecl ipsing system comprising a hot subdwarf primary and a low mas s companion, in order to precisely interpret their high precisio n photometric and spectroscopic data, and accurately deter min their system and surface parameters. We re-analyse the archival high-re solution time-resolved VLT /UVES spectra of AA Dor system where irradiation features have already been detected. After removin g the predominant contribution of the hot subdwarf primary, the residual spectra reveal more than 100 emission lines from the heated s id of the secondary that show maximum intensity close to the phases around secondary eclipse. We analyse the residual spectrum in order to model the irradiation of the low mass secondary. W e perform a detailed analysis of 22 narrow emission lines of the irradi ated secondary, mainly of O ii, with a few significant Cii 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 o ne at the quadrature and the other at the secondary eclipse. I n addition, we identify weaker emission signatures originating from more than 70 lines including lines from He i, N ii, Siiii, Caii and Mgii. From the emission lines of the heated side of the secondary star we determine the radial velocity semi-amplitude of the center -of-light and correct it to the centre-of-mass of the secondary which in tu r gives accurate masses of both components of the AA Dor syst em. The resulting masses M1 = 0.46± 0.01M⊙ and M2 = 0.079± 0.002M⊙ are in perfect accordance with those of a canonical hot subdw arf primary and a low mass just at the substellar limit for the com panion. We also compute a first generation atmosphere model o f the low mass secondary, which includes irradiation e ffects and matches the observed spectrum well. We find an indica tion of an extended atmosphere of the irradiated secondary star.