1 Institute for Astronomy, K.U. Leuven, Belgium
Recent ISO-results have shown that, besides the so-called 'astronomical
silicate' (which is amorphous), several young stars also show crystalline
silicates. Surprisingly, also meteors, comets and interplanetary dust
particles show crystalline silicates.
Here we present ISO-SWS observations of a large sample of
isolated Herbig Ae/Be stars, covering a wide range in spectral type
(B9-K4).
These spectra reveal an amazing variety in spectral features, such as
PAH's, water ice and silicates.
Amorphous silicates are seen at 9.7 and 18
m, while crystalline
silicates, such as forsterite (20, 24 and 34m), olivines (28m)
and pyroxenes (43m) are seen aswell. An intruiging fact is that
some stars show amorphous silicate (9.7m) in emission, while
others show it in absorption. This was already revealed by IRAS (Bhatt.
H.C. and Gorti U., 1993, BASI 21,541).
We measured and identified features for a group of 34 young stellar
objects,
from which most are Herbig Ae/Be stars.
We have already studied a large group of our sources through optical
spectra and UV-IR photometry. These data reveal information about the
abundances
of the stellar atmosphere, outflow/accretion, CS extinction, inclination
of the CS disk and age. Our findings are now confronted with properties
of the CS dust. Correlation between strength, structure and presence of
features on one hand and spectral type, E(B-V), presence of
accretion/outflow,
chemical composition and evolutionary stadium on the other hand, are
looked for. From the ISO-SWS spectra, we can distinguish five groups, which may
also be a distinction in evolutionary status. Preliminary results from
our program will be presented.