Accretion onto a binary from a polar circumbinary disc

TL;DR

This study uses hydrodynamical simulations to explore how a polar circumbinary disc feeds a highly eccentric binary, revealing that the disc remains low eccentricity and accretion rates are balanced, contrasting with coplanar cases.

Contribution

It provides the first detailed hydrodynamical modeling of accretion onto a binary from a polar circumbinary disc, highlighting differences from coplanar configurations.

Findings

Disc remains low eccentricity in polar configuration.

Accretion rates onto primary and secondary are nearly equal.

Eccentric orbit of outer companion increases accretion by ~20% after periastron.

Abstract

We present hydrodynamical simulations to model the accretion flow from a polar circumbinary disc onto a high eccentricity ($e=0.78$) binary star system with near unity mass ratio ($q=0.83$), as a model for binary HD 98800 BaBb. We compare the polar circumbinary disc accretion flow with the previously studied coplanar case. In the coplanar case, the circumbinary disc becomes eccentric and the accretion alternates from being dominant onto one binary member to the other. For the polar disc case involving a highly eccentric binary, we find that the circumbinary disc retains its initially low eccentricity and that the primary star accretion rate is always about the same as the secondary star accretion rate. Recent observations of the binary HD 98800 BaBb, which has a polar circumbinary disc, have been used to determine the value of the $\rm H\alpha$ flux from the brighter component. From this value, we infer that the accretion rate is much lower than for typical T Tauri stars. The eccentric orbit of the outer companion HD 98800 A increases the accretion rate onto HD 98800 B by $\sim 20$ per cent after each periastron passage. Our hydrodynamical simulations are unable to explain such a low accretion rate unless the disc viscosity parameter is very small, $\alpha < 10^{-5}$. Additional observations of this system would be useful to check on this low accretion rate.