1995A&A...295L..43O

It has recently become possible to reduce the raw IRAS Low Resolution Spectrograph (LRS) data in an interactive way. In this paper several problems concerning the LRS data of extended objects are addressed. We investigate the presence of the 21 µm emission feature in HII regions as reported by Cox (1990A&A...236L..29C). These sources appear to be extended in the IRAS data. A careful re-analysis of the IRAS data shows that observing an extended source with a slit-less spectrograph like the LRS results in spurious features at the edges of the LRS band. The extent of an object results in a degradation of the wavelength resolution and a decrease of the LRS spectrum at the wavelength edges. Furthermore, the point source flux density listed in the IRAS Point Source Catalog (PSC) is under-estimated for extended objects. It is concluded that these effects can well explain the presence of the 21 µm emission feature in the HII regions discussed by Cox (1990A&A...236L..29C).