H₂ in the circumstellar environments of herbig Ae/Be stars

We analyzed the circumstellar (CS) H₂ observed in the FUSE spectra of 18 Herbig Ae/Be stars (HAeBes) covering a broad spectral range. To better diagnose the origin of the detected H2 and its excitation conditions, we used a model photodissociation region (PDR). Our analysis demonstrates that the excitation of H2 is clearly different around most of the HAeBes compared to the interstellar medium. Moreover, the characteristics of H2 around these stars give evidence for different excitation mechanisms. For the most massive Be stars, the excitation diagrams are reproduced well by a PDR model. Our results favor an interpretation in terms of large CS envelopes, remnants of the molecular clouds in which the stars were formed. On the other hand, the group of Ae stars known to possess disks is more inhomogeneous. In most cases, when CS H₂ is detected, the lines of sight do not pass through the disks. The excitation conditions of H₂ around Ae stars cannot be reproduced by PDR models and correspond to warm and/or hot excited media very close to the stars. Our results suggest structural differences between Herbig Ae and Be star environments. Herbig Be stars do evolve faster than Ae stars, and consequently, most Herbig Be stars are younger than Ae ones at the time we observe them. It is thus more likely to find remnants of their parent cloud around them.