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.
GOMEZ DE CASTRO A.I., LOPEZ-SANTIAGO J., TALAVERA A., SYTOV A.Yu. and BISIKALO D.
Abstract (from CDS):
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 11 R* at periastron passage. The presence of a dense (ne∼ 1011/cm3) extended envelope has been unveiled recently. In this article, we report the results from an XMM-Newton-based monitoring of the system. We show that at periastron, X-ray and UV fluxes are enhanced by a factor of ∼3 with respect to the apastron values. The X-ray radiation is produced in an optically thin plasma with T ∼ 6.4x106 K and it is found that the NH column density rises from 0.35x1021/cm2 at periastron to 1.11x1021/cm2 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 in the N V line with Hubble Space Telescope/Space Telescope Imaging Spectrograph. 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.