title: Line-driven ablation of circumstellar discs - I. Optically thin decretion discs of classical Oe/Be stars.
abstract: 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 anablationof material from the disc's surface layers, with initial focus on the marginallyoptically thin decretiondiscs 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.
authors.affiliation: Department of Physics and Astronomy, Bartol Research Institute, University of Delaware, Newark, DE 19716, USA.
authors.forename: Nathaniel Dylan
authors.initials: ND
authors.lastname: Kee
authors.affiliation: Department of Physics and Astronomy, Bartol Research Institute, University of Delaware, Newark, DE 19716, USA.
authors.forename: Stanley
authors.initials: S
authors.lastname: Owocki
authors.affiliation: Centro de Astrobiologia, Instituto Nacional de Tecnica Aerospacial, E-28850 Torrejon de Ardoz, Madrid, Spain; Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D, B-3001 Leuven, Belgium.
authors.forename: J O
authors.initials: JO
authors.lastname: Sundqvist
keywords: circumstellar matter; hydrodynamics; stars: early-type; stars: emission-line, Be; stars: mass-loss; stars: winds, outflows