Radiation-Driven Warping of Circumbinary Disks Around Eccentric Young Star Binaries

TL;DR

This paper investigates the conditions under which circumbinary disks around young eccentric binary stars become unstable to radiation-driven warping, highlighting the influence of disk properties and stellar luminosity.

Contribution

It introduces a model for radiation-driven warping instability in circumbinary disks, analyzing the dependence on various physical parameters and the effects of tidal torques.

Findings

Warping occurs beyond a certain radius depending on disk properties.

High stellar luminosity (>10^4 Lsun) favors warping on AU to kilo-AU scales.

Warping is suppressed in disks around less luminous stars.

Abstract

We study a warping instability of a geometrically thin, non-self-gravitating, circumbinary disk around young binary stars on an eccentric orbit. Such a disk is subject to both the tidal torques due to a time-dependent binary potential and the radiative torques due to radiation emitted from each star. The tilt angle between the circumbinary disk plane and the binary orbital plane is assumed to be very small. We find that there is a radius within/beyond which the circumbinary disk is unstable to radiation-driven warping, depending on the disk density and temperature gradient indices. This marginally stable warping radius is very sensitive to viscosity parameters, a fiducial disk radius and the temperature measured there, the stellar luminosity, and the disk surface density at a radius where the disk changes from the optically thick to thin for the irradiation from the central stars. On the other hand, it is insensitive to the orbital eccentricity and binary irradiation parameter, which is a function of the binary mass ratio and luminosity of each star. Since the tidal torques can suppress the warping in the inner part of the circumbinary disk, the disk starts to be warped in the outer part. While the circumbinary disks are most likely to be subject to the radiation-driven warping on a AU to kilo-AU scale for binaries with young massive stars more luminous than 10^4Lsun, the radiation driven warping does not work for those around young binaries with the luminosity comparable to the solar luminosity.