AN INFRARED STUDY OF THE REMARKABLE DUSTY M-STAR HR-3126

HR 3126 is a unique M giant star embedded in the bipolar reflection ne bula IC 2220. Its evolutionary status is uncertain, and both the compo sition of the dust shell and the mechanism responsible for its ejectio n have proved controversial. In this paper we present new photometric and spectroscopic observations of HR 3126 and combine them with existi ng data from the literature in an extensive reappraisal of the star's properties at infrared wavelengths. The spectral energy distribution i s consistent with an optical classification of MO-3 II with infrared e xcess. The dust shell cannot be fitted by a single-temperature blackbo dy: at least three components are required, with temperatures in the r ange 35-1300 K. On the basis of spectroscopy at 1-4 and 7-25 mum, comb ined with an assessment of various color-color diagrams, we are able t o reject the hypothesis that HR 3126 is carbon-rich. Weak silicate emi ssion features are detected at 10 and 19 mum, and a previous report of silicon carbide emission at 11.2 mum is not substantiated. Our result s are discussed with a view to discrimination between proposed scenari os for the evolutionary status of the star. Although it is not yet pos sible to draw definitive conclusions, it seems likely that HR 2136 is in a phase of advanced and rapid post-main-sequence evolution, possibl y beginning its ascent of the asymptotic giant branch.