Radiative ablation of disks around massive stars
Date
2015
Authors
Journal Title
Journal ISSN
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Publisher
University of Delaware
Abstract
Hot, massive stars (spectral types O and B) have extreme luminosities (104 −
106L ) 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. Extending the winddeveloped
Sobolev methods for line radiative transfer, 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 optically thin Be
disks can be explained by line-driven ablation without, as was done in previous work,
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
via “Pulsationally Driven Orbital Mass Ejection”. This “PDOME” model couples
observationally inferred non-radial pulsation modes and rapid stellar rotation to launch
material into orbiting Keplerian disks of Be-like densities.
In contrast to such Be decretion disks, star-forming accretion disks are much
denser and so are generally optically thick to continuum processes like electron scattering.
To circumvent the computational challenges associated with long-characteristic
radiation hydrodynamics through optically thick media, we develop an approximate
method for treating optically thick 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 is largely independent of disk mass, and depends
mainly on stellar properties like luminosity.
Finally, as a side problem, we discuss the role of “thin-shell mixing” in reducing
X-rays from colliding wind binaries. Laminar, adiabatic shocks produce well
understood X-ray emission, but the emission from radiatively cooled shocks is more
complex due to thin-shell instabilities. The parameter study conducted here systematically
varies colliding wind binary shock densities to determine scaling relations for
this emission. A key result is that, in the limit of strongly radiatively cooled shocks,
emission is reduced by a fixed factor ∼ 50 from analytic scalings that ignore thin-shell
structure.