Modeling UV Radiation Feedback from Massive Stars: I. Implementation of
  Adaptive Ray Tracing Method and Tests
  We present an implementation of an adaptive ray tracing (ART) module in the
Athena hydrodynamics code that accurately and efficiently handles the radiative
transfer involving multiple point sources on a three-dimensional Cartesian
grid. We adopt a recently proposed parallel algorithm that uses non-blocking,
asynchronous MPI communications to accelerate transport of rays across the
computational domain. We validate our implementation through several standard
test problems including the propagation of radiation in vacuum and the
expansions of various types of HII regions. Additionally, scaling tests show
that the cost of a full ray trace per source remains comparable to that of the
hydrodynamics update on up to $\sim 10^3$ processors. To demonstrate
application of our ART implementation, we perform a simulation of star cluster
formation in a marginally bound, turbulent cloud, finding that its star
formation efficiency is $12\%$ when both radiation pressure forces and
photoionization by UV radiation are treated. We directly compare the radiation
forces computed from the ART scheme with that from the M1 closure relation.
Although the ART and M1 schemes yield similar results on large scales, the
latter is unable to resolve the radiation field accurately near individual
point sources.
