ON THE 21-MU-M FEATURE IN HII-REGIONS

It has recently become possible to reduce the raw IRAS Low Resolution Spectrograph (LRS) data in an interactive way. In this paper several p roblems concerning the LRS data of extended objects are addressed. We investigate the presence of the 21 mum emission feature in HII regions as reported by Cox (1990). These sources appear to be extended in the IRAS data. A careful re-analysis of the IRAS data shows that observin g an extended source with a slit-less spectrograph like the LRS result s 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 dec rese of the LRS spectrum at the wavelength edges. Furthermore, the poi nt source flux density listed in the IRAS Point Source Catalog (PSC) i s under-estimated for extended objects. It is concluded that these eff ects can well explain the presence of the 21 mum emission feature in t he HII regions discussed by Cox (1990).