ISO-LWS two colours diagram of young stellar objects

We present a [60-100] versus [100-170]ìm two-colour diagram for a sample of 61 young stellar objects (YSOs) observed with the Long Wavelength Spectrometer (LWS) on-board the Infrared Space Observatory (ISO ). The sample consists of 17 Class 0 sources, 15 Class I, nine Bright Class I (L bol >104 Lsolar ) and 20 Class II (14 Herbig Ae/Be stars and six T Tauri stars). We find that each class occupies a well-defined region in our diagram with colour temperatures increasing from Class 0 to Class II. Therefore the [60-100] versus [100-170] two-colour diagram is a powerful and simple tool to derive from future (e.g. with the Herschel Space Observatory ) photometric surveys the evolutionary status of YSOs. The advantage over other tools already developed is that photometry at other wavelengths is not required: three flux measurements are enough to derive the evolutionary status of a source. As an example we use the colours of the YSO IRAS 18148-0440 to classify it as Class I. The main limitation of this work is the low spatial resolution of the LWS which, for some objects, causes a high uncertainty in the measured fluxes due to background emission or to source confusion inside the LWS beam.