2. Observations and reductions
The spectra of the seven M giants and the C star RV Aqr were observed with the Short-Wavelength Spectrometer (SWS, de Graauw, et al. 1996) on board ISO (Kessler et al., 1996). The spectrometer was used in the grating scan mode (SWS06), providing a resolution of , corresponding to or . Finally one observation of R Dor was performed using the high resolution SWS07 mode of the Fabry-Pérot interferometer with . The field of view of the SWS06 observation was times .
The data were partly reduced at the ISOSDC (ISO Spectroscopy Data Centre) at the Max-Planck Institut für Extraterrestrische Physik in Garching, using the 1997 version of the Interactive Analysis package (IA), and in Uppsala using ISAP 1.5 (ISO Spectral Analysis Package). The observed bands are not heavily crowded by cosmic ray hits, but are unfortunately severely affected by fringes due to regular interference patterns, which depend on the exact pointing of the satellite at the time of observation. Therefore, the fringes can not be reduced away directly. A possible way to artificially eliminate the fringes is, for example, to divide the signal by the simple harmonics that dominate the fringe pattern. This was done by using a defringing algorithm included in the IA package. Such a procedure is, however, somewhat questionable since real features, especially molecular lines occurring at frequencies close to those of the fringe pattern, are also taken away if not special care is taken. The flux calibration leads to a relative uncertainty of less than .
The M giants observed are given in Table 1 and the observations of these stars are shown in Fig. 1. The observations are all shown in the same figure and are therefore shifted and scaled arbitrarily in vertical direction to enable a comparison between the stars. Superimposed on the original, fringed data are the artificially defringed data. The carbon star RV Aqr, supposedly without any CO2 but with the same instrumental fringes, is plotted to give evidence for the hypothesis that the fringes are instrumental and not specific for M stars. The fringe pattern shows a clear resemblance among the M stars, and is convincingly similar between the M giant spectra and the carbon star spectrum. The spectra of R Cas and R Crt start from a little less than , which means that the band is included but is fairly close to the edge of the detected spectrum. This part of the spectrum is sampled only once, whereas the central regions are covered twelve times. Thus, the edges are more sensitive to cosmic hits and may therefore be noisier. The spectrum of RS CrA is of poor quality due to the low flux measured.
Table 1. Basic data for M giants with CO2 features, according to Loup et al. (1993)
The CO2 features are found around 13.9 and . Visible emission at is seen in the spectra of R Crt and R Dor, and possibly in the spectrum of R Cas. The feature at is seen as an emission in the same stars. In R Cas there is, in addition to the emission feature, an absorption part which is broader. In TX Cam, V1111 Oph, and V656 Cas and possibly RS CrA the feature is seen mainly as an absorption. In Fig. 2 the 13.9 and emission bands of R Dor show asymmetries, the former to the red and the latter to the blue. This is seen in all the stars that show CO2 emission.
© European Southern Observatory (ESO) 1999
Online publication: December 4, 1998