Infrared observations of 33 "classical" Be stars and 26 early-type comparison objects are presented. Excess infrared radiation in the 2.3-19.5 IL spectral region was observed for 28 of the 33 Be stars. Circumstellar-dust emission models are ruled out. Free-free emission from a hot (T > 10,000" K) circumstellar plasma shell is found to be the most likely source of the excess infrared radiation. A typical Be star shell, assuming a shell radius to thickness ratio of 5, is shown to be characterized by an electron density of 3.7 x 1011 , a radius of 2.5 x 1012 cm ( 4R*), and a Thomson optical depth of 0.16. The infrared excess in Be stars tends to decrease as spectral type becomes late. A number of the survey stars were tested for infrared variability with negative results. Subject headings: Be stars - circumstellar shells - infrared
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Gehrz et al. (1974) studied this question.