1. Introduction


NEMO is a toolbox with many programs to perform various stellar dynamics functions.

For the user NEMO is a collection of programs, running under a standard UNIX shell, capable of running various stellar dynamical codes and related utilities (initialization, analysis, gridding, orbits). It can be thought of as a collection of various groups (packages) of programs, a group being defined by their common file structure, described below.

A common command line User Interface is defined with which the user communicates with a program. In order to run NEMO programs, your shell environment has to be modified. See Using NEMO on how to setup NEMO, and of course Installation if that had not been done yet. There is also a section with many Examples. NEMO stores its information in binary files, obeying a Filestructure.

Here are the main groups of programs, clearly showing the structure of NEMO:

  • The N-body group is defined by a common file structure of snapshots. In this group we find various programs to create N-body systems (spherical, elliptical, disk), methods to compute the gravitational field (softened Newtonian, hierarchical, Fourier expansion), and time-integrators (leapfrog, Runge-Kutta). Many utilities exist to manipulate, analyze and display these data.

  • The Orbit group is defined by a common file structure of orbits. It is mainly intended to calculate the path of an individual orbit in a static potential and then analyze it. This group is closely related to the before mentioned N-body group, and utilities in both groups can interact with each other. For example, it is possible to freeze the potential of an N-body snapshot, and calculate the path of a specific star in it, now conserving energy exactly. Or to extract the path of a selected star in a simulation, and extract an orbit from it.

  • The Image group is defined by a common file structure of images, i.e. two dimensional rectangular pixel arrays with a ‘value’ defined for every pixel. Actually an image may also have a third axis, although this axis often has a slightly different meaning e.g. Doppler velocity. It is possible to generate arbitrary two-(and three-) dimensional images from snapshots, FITS files of such images can be created, which can then be exported to other familiar astronomical data reduction packages. There exists a variety of programs in the astronomical community to manipulate data through the FITS format.

  • The Table group appears quite commonly among application programs in all of the above mentioned groups. Most of the time it is a simple ASCII file in the form of a matrix of numbers (like a spreadsheet). A few programs in NEMO can manipulate, display and plot such table files, although there are many superior programs and packages outside of NEMO available with similar functionality. It is mostly through these table files that we leave the NEMO environment, and persue analysis in a different environment. The obvious advantage of storing tables in binary form is the self-documenting nature of NEMO’s binary files. For historical reasons, most tables are displayed and created in ASCII, though you will find a few binary tables as well.

More groups and file structures are readily defined, as NEMO is also an excellent development system. We encourage users to define their own (or extend existing) data structures as the need arises. In Programmers Guide we will detail some ‘rules’ how to incorporate/add new software into the package, and extend your own NEMO environment.

The remaining chapters of this manual outline various concepts that you will find necessary to work with NEMO. The User Interface outlines the user interface (commandline, shells etc.). NEMO stores most data in files, and Filestructure explains how data is stored on disk and can be manipulated, including the concept of function descriptors in NEMO. In Graphics and Image Display we details how data can be graphically displayed, either using NEMO itself or programs outside of NEMO.