Accretion disc boundary layers around pre-main-sequence stars

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Abstract
One-dimensional time-dependent calculations of geometrically thin accretion disc boundary layers in pre-main-sequence stars are carried out for mass-accretion rates in the range M=5 × 10−7 to 10− M⊙ yr−1, α = 0.005−0.3 and different inner boundary conditions on the temperature. Two kinds of solution are obtained: those with a distinct thermal boundary layer (BL) component and those without a thermal boundary layer. For Ṁ up to ≈ 10−5 M⊙ yr− 1, and for α > αcritic ≈ 0.04, solutions with a thermal BL are obtained. For Ṁ ≈ 10−4 M⊙ yr−1 or for α < αcritic, solutions without a thermal BL are obtained. The inner boundary condition F = σTeff4 leads to hotter solutions and higher threshold values αcritic, while the no-flux boundary condition dT/dr = 0 leads to cooler solutions. For a very low mass-accretion rate (Ṁ≈5× 10−7 M⊙ yr−), the temperature in the disc drops below 104 K and the ionization front is adjacent to the outer edge of the hot thermal boundary layer. In the vicinity of the ionization front, the medium becomes slightly optically thin. For a very high massaccretion rate, advection of energy (ζ=Ladv/Lacc ≈ 0.1−0.2) from the boundary layer into the inner boundary becomes important, and the boundary layer luminosity is only a fraction of its expected value.

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