XMM-Newton monitoring of the close pre-main-sequence binary AK Sco. Evidence of tide driven filling of the inner gap in the circumbinary disk

Abstract

AK~Sco stands out among pre-main sequence binaries because of its prominent ultraviolet excess, the high eccentricity of its orbit and the strong tides driven by it. AK Sco consists of two F5 type stars that get as close as 11R$_*$ at periastron passage. The presence of a dense ($n_e \sim 10^{11}$~cm$^{-3}$) extended envelope has been unveiled recently. In this article, we report the results from a XMM-Newton based monitoring of the system. We show that at periastron, X-ray and UV fluxes are enhanced by a factor of $\sim 3$ with respect to the apastron values. The X-ray radiation is produced in an optically thin plasma with T$\sim 6.4\times 10^{6}$ K and it is found that the N$_H$ column density rises from 0.35$\times 10^{21}$~cm$^{-2}$ at periastron to 1.11$\times 10^{21}$~cm$^{-2}$ at apastron, in good agreement with previous polarimetric observations. The UV emission detected in the Optical Monitor band seems to be caused by the reprocessing of the high energy magnetospheric radiation on the circumstellar material. Further evidence of the strong magnetospheric disturbances is provided by the detection of line broadening of 278.7~km~s$^{-1}$ in the N~V line with HST/STIS. Numerical simulations of the mass flow from the circumbinary disk to the components have been carried out. They provide a consistent scenario with which to interpret AK~Sco observations. We show that the eccentric orbit acts like a gravitational piston. At apastron, matter is dragged efficiently from the inner disk border, filling the inner gap and producing accretion streams that end as ring-like structures around each component of the system. At periastron, the ring-like structures come into contact, leading to angular momentum loss, and thus producing an accretion outburst.