An ISO-LWS two-colour diagram of Herbig Ae/Be stars

In this paper, we present and discuss an infrared two-colour diagram built with the 60, 100, and 170 μm photometry of the whole sample of Herbig Ae/Be stars observed by the spectrometers on board of ISO satellite. An overview of this diagram reveals a certain degree of homogeneity in the behaviour of these stars and their IR-emitting environments, with some exceptions. In particular, we account for the objects located to the left of the blackbody line. In addition, the infrared colours obtained with ISO gave us the opportunity to compare with the IRAS measurements, which generally appear in good agreement. Finally, a simple spherically symmetric model of pre-ZAMS circumstellar environment is used to obtain the two-colour diagram, as a diagnostic tool complementary to the best-fit of the spectra in investigating the distribution of matter around these stars.