Astron. Astrophys. 335, L69-L72 (1998)

2. ISO-SWS observations of R Scl

Within our open time ISO observing program the spectral variations of a few representative AGB stars were monitored with the SWS. In this paper we concentrate on the semiregular variable R Scl which has the lowest amount of mass loss among our C-rich target stars. We have selected the two observations closest to visual maximum (JD 2450285.7, ) and minimum (JD 2450452.3, ) light. The phases were estimated from data provided by the AAVSO (visual, Mattei 1995-1997) and by P. Whitelock (JHKL-photometry, partly published in Whitelock et al. 1997 where the near IR light curve of this star is discussed). The spectra were obtained with SWS01, speed 2, so the typical resolution is about 600. The data were reduced with the ISO Spectral Analysis Package. The AOT-bands were combined with band 1A as a flux reference. The resulting SWS L-band flux agreed within with the above mentioned JHKL-photometry. The spectra were then rebinned to a resolution of 100 to make them comparable with our synthetic spectra.

In Fig. 1 we show two observations of R Scl together with the identification of the main features. The arrows indicate the extension of several molecular features beyond 4 µm as estimated from opacity sampling spectra (see below). The narrow features at 9.3 µm and 11.1µm are artefacts. It is interesting to note that in spite of a change in the bolometric magnitude by about 0:m 5, there is no large change in the overall flux level in the SWS range.

Fig. 1. ISO-SWS spectra of R Scl at visual phases 0.9 and 0.4 (thick and thin lines). The main molecular features are identified, the numbers in brackets are the quantum numbers of the upper vibrational levels for the polyatomic molecules.

An emission band around 8.6 µm has been reported in the IRAS-LRS spectra of carbon stars (Willems 1988), however its identification was not clear. A look at the ISO and the synthetic spectra shows that the emission is much more likely the red wing of an absorption due to strong combination bands of HCN and C₂H₂. In the ISO data only the 8.5-10 µm region shows a noticable change from maximum to minimum but in the opposite sense to the molecular features. One interpretation would be that of a constant molecular absorption feature between 6 µm and about 8.6 µm. At the same time there would be an enhanced emission in the blue wing of the SiC feature near minimum light but the 11 µm SiC peak stays constant. The reason for the enhanced emission could be a contribution from SiC which shows a broad shoulder in its absorption cross section (Borghesi et al. 1985). An alternative explanation would be an increasing 6-8.6 µm molecular absorption towards minimum light accompanied by a weaker SiC feature and a slightly higher continuous dust emission in the 8-10 µm region. Such a rather local dust emission makes the second interpretation less attractive. Since the 6-8.6 µm region is not well reproduced in our synthetic spectra (Sect. 5) a definite identification of the features cannot be given. We also note that PAH's show a broad and strong feature between 7 and 9 µm (e.g. Tielens 1997). The strong interaction between molecular and dust features in the 6-15 µm region of R Scl serves as a warning for the interpretation of ground based mid-IR data.

© European Southern Observatory (ESO) 1998

Online publication: June 26, 1998
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