  We observed molecular hydrogen around a sample of pre-main sequence stars in
order to better characterize their gaseous CS environments. We analyzed the
FUSE (Far Ultraviolet Spectroscopic Explorer) spectra of a sample of Herbig
Ae/Be stars (HAeBes) covering a broad spectral range, including the
main-sequence A5 star Beta-Pictoris. To better diagnose the origin of the
detected H2 and its excitation conditions, we used a model of a
photodissociation region. 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 Herbig Ae and Be stars give
evidence for different excitation mechanisms. For the most massive stars of our
sample (B8 to B2 type), the excitation diagrams are reproduced well by a model
of photodissociation regions (PDR). 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 (later than B9
type) 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. In
addition, no clear correlation has been found between the ages of the stars and
the amount of circumstellar H2. 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.
