Transitions among the ground state fine structure levels of abundant atoms and ions are bright and important in astrophysical environments. However, their wavelengths are difficult to determine independently since they usually are not directly observable under laboratory conditions. Historically, it was necessary to derive the energies of these levels from the difference of transitions connecting the fine structure levels to a common level at much higher energies. These differences of two similar numbers induced considerable uncertainties in the derived fine structure wavelengths, hampering the interpretation or even observation of fine structure spectra. In particular, accurate pre-ISO wavelengths were lacking for high ionization species prominent in the spectra of AGNs.
Since a good in-orbit wavelength calibration has been obtained rapidly for the ISO Short Wavelength Spectrometer (Valentijn et al. 1996), a program has been initiated to quickly provide improved wavelengths for a number of bright lines that may be of relevance in a variety of sources being observed with ISO-SWS. Both the SWS grating spectrometer (2.4-45 µm) and the high resolution SWS Fabry-Perot spectrometer (12-45 µm) have been used for dedicated observations of suitable sources, taking special care to minimize errors due to pointing problems or small calibration inaccuracies. The figure shows an example of Fabry-Perot observations of a planetary nebula. The Fabry-Perot observations resulted in about 10 new wavelength determinations typically accurate to 1 part in 50000, while grating observations provided about 20 new wavelength typically accurate to 1 part in 10000. Good wavelengths are now available for all bright fine structure lines observable with ISO-SWS.
Feuchtgruber,H., Lutz,D., Beintema,D., et al., accepted for publication
in ApJ (1997)
Valentijn,E.A., Feuchtgruber,H., Kester,D.J., et al.: Astron. Astrophys. 315, L60-L63 (1996)