  We present a GPU accelerated CUDA-C implementation of the Barnes Hut (BH)
tree code for calculating the gravitational potential on octree adaptive
meshes. The tree code algorithm is implemented within the FLASH4 adaptive mesh
refinement (AMR) code framework and therefore fully MPI parallel. We describe
the algorithm and present test results that demonstrate its accuracy and
performance in comparison to the algorithms available in the current FLASH4
version. We use a MacLaurin spheroid to test the accuracy of our new
implementation and use spherical, collapsing cloud cores with effective AMR to
carry out performance tests also in comparison with previous gravity solvers.
Depending on the setup and the GPU/CPU ratio, we find a speedup for the gravity
unit of at least a factor of 3 and up to 60 in comparison to the gravity
solvers implemented in the FLASH4 code. We find an overall speedup factor for
full simulations of at least factor 1.6 up to a factor of 10
