ACCRETION DISC BOUNDARY-LAYERS AROUND PRE-MAIN-SEQUENCE STARS

One-dimensional time-dependent calculations of geometrically thin accr etion disc boundary layers in pre-main-sequence stars are carried out for mass-accretion rates in the range M = 5 x 10(-7) to 10(-4) M circl e dot yr(-1), alpha = 0.005-0.3 and different inner boundary condition s on the temperature. Two kinds of solution are obtained: those with a distinct thermal boundary layer (BL) component and those without a th ermal boundary layer. For M up to approximate to 10(-5) M circle dot y r(-1), and for alpha > alpha(critic) approximate to 0.04, solutions wi th a thermal BL are obtained. For M approximate to 10(-4) M circle dot yr(-1) or for alpha < alpha(critic), solutions without a thermal BL a re obtained. The inner boundary condition F-T = sigma T-eff(4) leads t o hotter solutions and higher threshold values alpha(critic), while th e no-flux boundary condition dT/dr = 0 leads to cooler solutions. For a very low mass-accretion rate (M approximate to 5 x 10(-7) M circle d ot yr(-1)), the temperature in the disc drops below 10(4) K and the io nization front is adjacent to the outer edge of the hot thermal bounda ry layer. In the vicinity of the ionization front, the medium becomes slightly optically thin. For a very high mass-accretion rate, advectio n of energy (zeta = L(adv)/L(acc) approximate to 0.1-0.2) from the bou ndary layer into the inner boundary becomes important, and the boundar y layer luminosity is only a fraction of its expected value.