Abstract
Due to the low excitation requirements and easy observation from the ground, molecular line observations probably constitute our main source of empirical knowledge on circumstellar envelopes (CSEs) around AGB stars. The CO rotational transitions, the most intense ‘thermal’ lines, are efficiently used to determine the total gas content and its spatial distribution in wide samples of objects. Thermal emission from other molecules is mainly useful in order to study their abundances and the chemical reactions taking place in CSEs. Maser lines are easily observed due to their high intensity and the flux distribution in very compact spatial spots and narrow profile spikes, characteristic of the exponential amplification; however the data interpretation is difficult due to the intricate pumping processes. The most important maser lines (of SiO, H2O and OH) arise from very different regions, which allows the study of various components of the CSEs. We will focus here on SiO masers.
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