  Radiation pressure on dust is thought to play a crucial role in the formation
process of massive stars by acting against gravitational collapse onto the
central protostar. However, dust properties in dense regions irradiated by the
intense radiation of massive protostars are poorly constrained. Previous
studies usually assume the standard interstellar dust model to constrain the
maximum mass of massive stars formed by accretion, which appears to contradict
with dust evolution theory. In this paper, using the fact that stellar
radiation exerts on dust simultaneous radiation pressure and radiative torques,
we study the effects of grain rotational disruption by radiative torques (RATs)
on radiation pressure and explore its implications for massive star formation.
For this paper, we focus on the protostellar envelope and adopt a spherical
geometry. We find that original large grains of micron-sizes presumably formed
in very dense regions can be rapidly disrupted into small grains by RATs due to
infrared radiation from the hot dust shell near the sublimation front induced
by direct stellar radiation. Owing to the modification in the size distribution
by rotational disruption, the radiation pressure opacity can be decreased by a
factor of $\sim 3$ from the value expected from the original dust model.
However, to form massive stars via spherical accretion, the dust-to-gas mass
ratio needs to be reduced by a factor of $\sim 5$ as previously found.
