We have investigated the nature of IRAS 03201+5459 by analysis of its near- and mid-IR spectrum covering 2.3 to 45 µm using a full grating scan taken with the SWS spectrometer on board of ISO with the second fastest speed. The obtained spectrum shows the features of both C-based molecules (at 3.02 and 13.07 µm attributed to the transitions in HCN and/or C2H2) and O-based ones (at 9.7 µm related to amorphous silicates and at 4.27 µm caused by solid CO2). The broad feature at 3.02 µm can be explained only if HCN and/or C2H2 molecules are formed near the stellar surface, making the star genuinely carbon-rich. However, just from the SWS 01 spectrum we cannot solve the puzzle of the location of O-rich material: whether it is only of interstellar origin, or freely flowing out of the star or more confined to some disk-like configuration. Without further observations we can not completely exclude a possibility that to this object is underestimated and/or the interstellar extinction law in the direction of our source is unusual and, in consequence, that 9.7 µm band is solely of interstellar origin. However, at the moment, we are not able to fully explain the 9.7 µm absorption by interstellar matter and we have considered the hypothesis that IRAS 03201+5459 is presently a carbon-rich AGB star which is experiencing or has recently experienced the fatal thermal pulse that changed the star chemistry. In principle, our numerical radiative transfer modelling shows that it is possible to explain the complex nature of IRAS 03201 on the basis of an outflow model in which a still optically thick O-rich envelope is followed by the newly created C-rich one. Nevertheless this model has difficulty explaining the similarity between the IRAS and ISO spectra, since it implies a very recent transition of the star into a carbon star. It is possible that the O-rich material is located in some disk-like configuration around IRAS 03201 so that it can be stable for long periods of time, but only careful investigation of the molecular cloud in the line-of-sight of IRAS 03201+5459 could solve this puzzle.
© European Southern Observatory (ESO) 2000
Online publication: October 30, 19100