ISO-LWS two colour diagram of young stellar objects

We present a [60--100] vs. [100--170] $\mu$m 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, 9 Bright Class I ($L_{\rm bol}>10^4 L_\odot$), 20 Class II (14 Herbig Ae/Be stars and 6 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] vs. [100--170] two colours 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 the background emission or to the source confusion inside the LWS beam.