ISO spectroscopy of circumstellar dust in 14 Herbig Ae/Be systems: Towardsan understanding of dust processing

We present Infrared Space Observatory (ISO) spectra of fourteen isolated He rbig; Ae/Be (HAEBE) stars, to study the characteristics of their circumstel lar dust. These spectra show large star-to-star differences, in the emissio n features of both carbon-rich and oxygen-rich dust grains. The IR spectra were combined with photometric data ranging from the UV through the optical into the sub-mm region. We defined two key groups, based upon the spectral shape of the infrared region. The derived results can be summarized as fol lows: (1) the continuum of the IR to sub-mm region of all stars can be reco nstructed by the sum of a power-law and a cool component, which can be repr esented by a black body. Possible locations for these components are an opt ically thick, geometrically thin disc (power-law component) and an opticall y thin hared region (black body); (2) all stars have a substantial amount o f cold dust around them, independent of the amount of mid-IR excess they sh ow; (3) also the near-IR excess is unrelated to the mid-IR excess, indicati ng different composition/location of the emitting material; (4) remarkably, some sources lack the silicate bands; (5) apart from amorphous silicates, we find evidence for crystalline silicates in several stars, some of which are new detections; (6) PAH bands are present in at least 50% of our sample , and their appearance is slightly different from PAHs in the ISM; (7) PAH bands are, with one exception, not present in sources which only show a pow er-law continuum in the IR; their presence is unrelated to the presence of the silicate bands; (8) the dust in HAEBE stars shows strong evidence for c oagulation; this dust processing is unrelated to any of the central star pr operties (such as age, spectral type and activity).