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2016 ; 458
(3
): 2323-2335
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Line-driven ablation of circumstellar discs - I Optically thin decretion discs
of classical Oe/Be stars
#MMPMID27346978
Kee ND
; Owocki S
; Sundqvist JO
Mon Not R Astron Soc
2016[May]; 458
(3
): 2323-2335
PMID27346978
show ga
The extreme luminosities of massive, hot OB stars drive strong stellar winds
through line-scattering of the star's UV continuum radiation. For OB stars with
an orbiting circumstellar disc, we explore here the effect of such
line-scattering in driving an ablation of material from the disc's surface
layers, with initial focus on the marginally optically thin decretion discs of
classical Oe and Be stars. For this we apply a multidimensional
radiation-hydrodynamics code that assumes simple optically thin ray tracing for
the stellar continuum, but uses a multiray Sobolev treatment of the line
transfer; this fully accounts for the efficient driving by non-radial rays, due
to desaturation of line-absorption by velocity gradients associated with the
Keplerian shear in the disc. Results show a dense, intermediate-speed surface
ablation, consistent with the strong, blueshifted absorption of UV wind lines
seen in Be shell stars that are observed from near the disc plane. A key overall
result is that, after an initial adjustment to the introduction of the disc, the
asymptotic disc destruction rate is typically just an order-unity factor times
the stellar wind mass-loss rate. For optically thin Be discs, this leads to a
disc destruction time of order months to years, consistent with observationally
inferred disc decay times. The much stronger radiative forces of O stars reduce
this time to order days, making it more difficult for decretion processes to
sustain a disc in earlier spectral types, and so providing a natural explanation
for the relative rarity of Oe stars in the Galaxy. Moreover, the decrease in
line-driving at lower metallicity implies both a reduction in the winds that help
spin-down stars from near-critical rotation, and a reduction in the ablation of
any decretion disc; together these provide a natural explanation for the higher
fraction of classical Be stars, as well as the presence of Oe stars, in the lower
metallicity Magellanic Clouds. We conclude with a discussion of future extensions
to study line-driven ablation of denser, optically thick, accretion discs of
pre-main-sequence massive stars.
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