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${}_{\ast }$ 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.

Accepted in ApJ

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• The 1600 Angstrom Emission Bump in Protoplanetary Disks: A Spectral Signature of H${}_2$O Dissociation
• Structure of a protobinary system: an asymmetric circumbinary disk and spiral arms
• Recurring Occultations of RW Aurigae by Coagulated Dust in the Tidally Disrupted Circumstellar Disk
• A Search For X-ray Emission From Colliding Magnetospheres In Young Eccentric Stellar Binaries
• Constraints to the magnetospheric properties of T Tauri stars - II. The Mg II ultraviolet feature
• Protoplanetary disk shadowing by gas infalling onto the young star AK Sco
• Weak Magnetic Fields in Two Herbig Ae Systems: The SB2 AK Sco and the Presumed Binary HD 95881
• Orbital motions and light curves of young binaries XZ Tau and VY Tau
• Evidence of hot high velocity photoionized plasma falling on actively accreting T-Tauri Stars
• Accretion and inter-cycle variations in the PMS interacting binary AK Sco
• Excitation of turbulence in accretion disks of binary stars by non-linear perturbations