Introduction

Athena is a grid-based code for astrophysical gas dynamics being developed with support of the NSF Information Technology Research (ITR) program. This User's Guide describes version 1.0 (hereafter referred to as athena1.0); the first publically released version of the code.

The primary goal of the Athena project is to develop a robust and flexible code with multiple-physics options in a variety of geometries, using flexible gridding methods, and optimized for modern shared or distributed memory parallel machines. This code is necessary to enable increasingly sophisticated investigations of a wide variety of problems in astrophysical gas dynamics. As part of the project, Athena will be made freely available to the astrophysics community, along with complete documentation and web-based training material. Although the capabilities of athena1.0 are quite restricted compared to the ultimate goals of the project, it is being freely distributed consistent with this open source philosophy.

The athena1.0 code contains algorithms for the following:

• compressible hydrodynamics and ideal MHD in one spatial dimension
• ideal gas equation of state with arbitrary $\gamma$ (including $\gamma = 1$, an isothermal EOS)
• second- or third-order characteristic interpolation in the primitive variables
• numerical fluxes computed using Roe's, or the HLLE, approximate Riemann solver (the HLLC solver is also available but for hydrodynamics only)

Since later versions of the code will always be backwards compatible (in particular, they will run one-dimensional problems simply and efficiently), it is strongly advised to use the latest release of the code for research problems (even those in one-dimension) to take advantage of improvements and potential bug fixes.

There are four basic sources of documentation for Athena

1. The Method Paper: (in preparation)
2. The User's Guide: (this document) gives an overview of how to install, configure, compile, and run athena1.0 and visualize the resulting output. Necessary reading for all users.
3. The Programmers's Guide: (in preparation)
4. Web-based Tutorials: (in preparation)

Since so much of this documentation is still in preparation, the equations of motion solved by athena1.0 are given below as a starting point.

Users of athena1.0 should have a basic, working knowledge of the Unix operating system, access to a C compiler, and a graphics package for plotting one-dimensional tabular data. Some familiarity with code management using Makefiles is helpful but not necessary.

The equations solved by athena1.0 can be written in conservative form as

$$\frac{\partial {\bf U}}{{\partial t}} + \frac{\partial {\bf F_{x}}{({\bf U})}}{{\partial x}} = 0,$$ (1)

where ${\bf U}$ is a vector of conserved quantities and ${\bf F_{x}}$ the vector of their fluxes. The number and form of the components of both ${\bf U}$ and ${\bf F_{x}}$ depend on the physics of the problem to be solved. For adiabatic MHD the vector of conserved quantities and their fluxes are

$${\bf U} = \left[ \begin{array}{c} \rho \\ \rho v_{x} \\ ... ...- B_{x}v_{y} \\ B_{z}v_{x} - B_{x}v_{z} \end{array} \right].$$ (2)

where

$$E = \frac{P}{\gamma -1} + \frac{1}{2}\rho v^{2} + \frac{B^{2}}{8\pi}$$ (3)

and $B^{2} = {\bf B} \cdot {\bf B}$.

For isothermal MHD, the fifth components of ${\bf U}$ and ${\bf F_{x}}$ are dropped, and equation (3) is replaced with $P=\rho C^{2}$, where $C$ is a constant (the isothermal speed of sound). For adiabatic hydrodynamics, the last two components of ${\bf U}$ and ${\bf F_{x}}$ are dropped, as well as all terms involving the magnetic field in equations (2) and (3). For isothermal hydrodynamics, the last three components of ${\bf U}$ and ${\bf F_{x}}$ are dropped, as well as all terms involving the magnetic field in equations (2) and (3).