Radiative Ablation of Disks Around Massive Stars

TL;DR

This paper investigates how stellar radiation causes ablation of disks around massive stars, explaining observed disk decay and the absence of Oe stars, and introduces models for disk formation and an approximation method for optically thick disks.

Contribution

It provides a radiation-driven ablation model explaining disk decay and star observations, and develops an approximate method for continuum absorption in optically thick disks.

Findings

Classical Be disk decay explained by line-driven ablation

O star luminosity causes rapid ablation of thin disks

Optical thickness reduces ablation rate by less than 50%

Abstract

Hot, massive stars (spectral types O and B) have extreme luminosities ($10^4 -10^6 L_\odot$) that drive strong stellar winds through UV line-scattering. Some massive stars also have disks, formed by either decretion from the star (as in the rapidly rotating "Classical Be stars"), or accretion during the star's formation. This dissertation examines the role of stellar radiation in driving (ablating) material away from these circumstellar disks. A key result is that the observed month to year decay of Classical Be disks can be explained by line-driven ablation without, as previously done, appealing to anomalously strong viscous diffusion. Moreover, the higher luminosity of O stars leads to ablation of optically thin disks on dynamical timescales of order a day, providing a natural explanation for the lack of observed Oe stars. In addition to the destruction of Be disks, this dissertation also introduces a model for their formation by coupling observationally inferred non-radial pulsation modes and rapid stellar rotation to launch material into orbiting Keplerian disks. In contrast to such Be decretion disks, star-forming accretion disks are much denser and so are generally optically thick to continuum processes. To circumvent the computational challenges associated with radiation hydrodynamics through optically thick media, we develop an approximate method for treating continuum absorption in the limit of geometrically thin disks. The comparison of ablation with and without continuum absorption shows that accounting for disk optical thickness leads to less than a 50$\%$ reduction in ablation rate, implying that ablation rate depends mainly on stellar properties like luminosity.