The Influence of Stellar Radiation on the Rate of Accretion

Bondi's discussion of spherically symmetric accretion ( 1 ) is extended to take account of the heating of the cloud by ultra-violet radiation from the star. It is shown that if the accretion rate is to be high, the radius of the H II zone must not be more than about one thousand stellar radii. The absorbing power of a cloud is enormously increased by the high densities near the star, so that a cloud of moderately high density at infinity is able to keep the H II zone small enough for rapid accretion to take place. For a star of given type, there is a lower limit to the cloud density that will keep the H II zone small; when a star, accreting from a cloud of given density, reaches a critical mass, the H II zone increases rapidly to Strömgren's value, and accretion effectively ceases. The conditions under which the state of rapid accretion can be set up are investigated : it is shown, for example, that this state can be reached with the cloud of density 10 3 particles/cm 3 and temperature 100 deg. K, provided the star entering the cloud is of mass less than 1·75 ⊙ .