  We present MAESTROeX, a massively parallel solver for low Mach number
astrophysical flows. The underlying low Mach number equation set allows for
efficient, long-time integration for highly subsonic flows compared to
compressible approaches. MAESTROeX is suitable for modeling full spherical
stars as well as well as planar simulations of dynamics within localized
regions of a star, and can robustly handle several orders of magnitude of
density and pressure stratification. Previously, we have described the
development of the predecessor of MAESTROeX, called MAESTRO, in a series of
papers. Here, we present a new, greatly simplified temporal integration scheme
that retains the same order of accuracy as our previous approaches. We also
explore the use of alternative spatial mapping of the one-dimensional base
state onto the full Cartesian grid. The code leverages the new AMReX software
framework for block-structured adaptive mesh refinement (AMR) applications,
allowing for scalability to large fractions of leadership-class machines. Using
our previous studies on the convective phase of single-degenerate progenitor
models of Type Ia supernovae as a guide, we characterize the performance of the
code and validate the new algorithmic features. Like MAESTRO, MAESTROeX is
fully open source.
