  The mass of a protostar is calculated from the infall and dispersal of an
isothermal sphere in a uniform background. For high contrast between peak and
background densities and for short dispersal time t_d, the accretion is
"self-limiting": gas beyond the core is dispersed before it accretes, and the
protostar mass approaches a time-independent value of low mass. For lower
density contrast and longer dispersal time, the accretion "runs away": gas
accretes from beyond the core, and the protostar mass approaches massive star
values. The final protostar mass is approximately the initial gas mass whose
free-fall time equals t_d. This mass matches the peak of the IMF for gas
temperature 10 K, peak and background densities 10^6 and 10^3 cm^-3, and for
t_d comparable to the core free-fall time t_core. The accretion luminosity
exceeds 1 L-Sun for 0.1 Myr, as in the "Class 0" phase. For t_d/t_core=0.4-0.8
and temperature 7-50 K, self-limiting protostar masses are 0.08-5 M-Sun. These
protostar and core masses have ratio 0.4 +- 0.2, as expected if the core mass
distribution and the IMF have the same shape.
