2002A&A...387.1022M

We analyze ISO/SWS spectra of the RV Tau star R Scuti. The infrared spectra are dominated by H₂O emission bands. The near- and mid-infrared excess is attributed to H₂O; the dust contribution is less important. We also identify CO, SiO and CO₂ bands. The various molecular emission bands originate from an extended atmosphere, an atmosphere above the photosphere. The extended atmosphere of R Sct is formed from matter which gradually have lifted up from the photosphere through the pulsations of the star. In contrast to the abundant molecules around the star, the silicate dust feature is weak and the dust mass-loss rate is only {dot}(M)_d=10^–11M_☉yr^–1. This implies that there might be a process to inhibit dust formation from molecules. RV Tau stars are commonly considered as post-AGB stars. While a detached dust envelope around R Sct is consistent with such an interpretation, we show that its period evolution is slower than expected. We argue that R Sct may be a thermal-pulsing AGB star, observed in a helium-burning phase.