Weeds is an extension of the CLASS radio astronomy software which is intended to ease the analysis of spectral surveys or spectral lines observations with large bandwidths. Weeds provides several commands to identify lines on a spectrum using spectral catalogs, which are accessed on-line using the VO-compliant SLAP protocol. In addition, Weeds can perform a simple modeling of the observed spectra, under the assumption of local thermodynamic equilibrium. Together with the catalog query commands, the modeling tool allows to efficiently identify lines in crowded spectra, where the line overlap can be important.
Weeds development was inspired by other packages, such as XCLASS and CASSIS. Both Weeds and XCLASS are extensions of the widely used CLASS spectra reduction software, which is part of GILDAS; this allows to reduce and analyze spectral surveys within the same environment. However, unlike XCLASS, Weeds is distributed with GILDAS as a contributed extension, and it uses the most recent CLASS version (CLASS90). Another difference between Weeds and both XCLASS and CASSIS is that Weeds accesses to spectral catalogs on-line. Consequently any changes in these catalogs (e.g. line additions) are readily available in Weeds. Nevertheless, Weeds can make a local copy a spectral line catalog, for offline searches. This is useful when you want to use Weeds with no (or a slow) internet connection. All this makes Weeds a lightweight, easy-to-install and efficient alternative to other spectral line analysis packages.
Weeds is named after some species that are detected in the interstellar medium, such as methanol, methyl formate or dimethyl ether. Because of internal motions, these species have an extremely rich line spectrum at millimeter and sub-millimeter wavelengths. Often, in order to analyze a spectral survey, one needs to identify the weeds first before picking-up the flowers (that is the lines of other species of interest). This is precisely the purpose of this extension.
This document describes how to use Weeds to analyze a spectral survey. It is written as a tutorial; each of step of the data analysis is explained through examples.