1 Introduction

A circumstellar (CS) disc is expected to be a natural byproduct of the star forming process (e.g. Shu et al. 1987).

Figure 1: ISO spectra of the 14 sample stars, superimposed on their spectral energy distributions. Crosses: observations; full line through the optical data: Kurucz model atmosphere; other full line: ISO-SWS/LWS observations; arrows indicate upper limits. The data are normalized to the V band

Figure 2: The ISO-SWS spectra of our programme stars. Together with HD 100546, we also show the spectrum of comet Hale-Bopp (Crovisier et al. 1997) for comparison

This theoretical expectation has obtained wide support from optical (e.g. McCaughrean & O'Dell 1996), infrared (e.g. Marsh et al. 1995) and millimetre observations of young stars (e.g. Mannings & Sargent 1997). The CS disc is expected and observed to gradually disappear, but remnants are still found around several Main-Sequence (MS) stars, such as Vega (Aumann et al. 1984). Earlier and recent modelling of T Tauri discs have shown that the most successful models are flaring passive discs (Kenyon & Hartmann 1987; Chiang & Goldreich 1997).

Herbig Ae/Be stars (hereafter HAEBEs), first described as a group by Herbig (1960), are believed to be the more massive analogues of T Tauri stars. They are seen as the progenitors of Vega-type stars (for recent reviews, see Waters & Waelkens 1998; Natta et al. 2000a). They are characterized by large IR excesses due to thermal re-emission of CS dust, show emission lines in their spectrum due to CS gas and have masses between 2 and 8 $M_{\odot}$ (Herbig 1960). Infrared spectroscopy offers a unique opportunity to scrutinize the composition and characteristics of their CS dust. Recent ISO (Kessler et al. 1996) studies have revealed a large variety in the properties of the dust around HAEBEs, from which it became clear that their dust is significantly different from that in the interstellar medium (Waelkens et al. 1996; Malfait et al. 1998a; Malfait et al. 1999b; van den Ancker et al. 1999).

   

Table 1: Parameters of the fourteen programme stars. The groups are defined below (see Sect. 2.1)

		(1)	(2)	(3)	(4)	(5)	(6)	(7)	(8)	(9)
Group	Object	Spectral	${T_{{\rm eff}}}$	log g	d	log (Age)	$E[B-V]_{{\rm CS}}$	$F_{800\,{\rm {\mu m}}} d^{2}$	${\rm\lambda_{onset}}$	$L_{{\rm IR}}/L_{*}$
		Type	(K)		(pc)	(yr)		(mJy ${\rm pc^{2}}$)	($\mu$m)
	AB Aur	B9/A0Ve	9750	5.0	144	6.3	0.07	11 106	1.1	0.48
Ia	HD 100546	B9Ve	11000	4.5	103	> 7.0	0.00	12 106	1.2	0.51
	HD 142527	F7IIIe	6250	4.0	200	5.0	0.08	166 106	1.1	1.06
	HD 179218	B9e	10000	5.0	240	5.0	0.06	11 106	1.7	0.62
	HD 100453	A9Ve	7500	4.5	-	-	0.06	-	1.2	0.54
Ib	HD 135344	F4Ve	6750	4.5	84	-	0.00	4 106	1.1	0.44
	HD 139614	A7Ve	8000	4.5	151	-	0.01	14 106	1.5	0.39
	HD 169142	A5Ve	10500	4.5	145	-	0.00	12 106	1.6	0.10
	HD 104237	A4Ve	10500	4.5	116	6.3	0.25	3 106	1.2	0.13
	HD 142666	A8Ve	8500	4.5	116	-	0.40	4 106	1.1	0.28
IIa	HD 144432	A9Ve	8000	4.5	> 200	-	0.05	> 4 106	1.1	0.26
	HD 150193	A1Ve	10000	4.0	150	> 6.3	0.30	3 106	1.2	0.15
	HD 163296	A3Ve	10500	4.0	122	6.6	0.02	34 106	1.2	0.16
	51 Oph	A0Ve	10000	4.0	131	5.5	0.03	<.8 106	2.3	< 0.024

References: (1): Malfait et al. (1998b), Dunkin et al. (1997), Gray & Corbally (1998); (2), (3) and (6): Malfait et al. (1998b); (4) and (5): van den Ancker et al. (1999); (7) and (9): this study, based upon sub-mm measurements by Sylvester et al. (1996), Mannings & Sargent (1997), Walker & Butner (1995), Henning et al. (1998) and Mannings & Sargent (2000); (8): this study.

This paper is one in a series of papers based upon ISO-SWS observations of HAEBE stars. In this study, we compiled a set of data which include, next to the ISO spectra, also UV, optical, IR and sub-mm photometry of a large sample of isolated HAEBE stars. A similar study was already presented by Sylvester et al. (1996) for a sample of Vega-like systems. Their ground-based observations in the IR with UKIRT are restricted to 2 ranges: 7.5-13.5 $\mu$m and 15.8-23.9 $\mu$m. Some of their sources (HD 135344, HD 139614, HD 142666, HD 144432, HD 169142 and 51 Oph) are also part of our HAEBE sample, and it is interesting to compare their results with ours. In this paper we give an overview of the IR features in our sample, together with a description of the Spectral Energy Distributions (SED) and we propose a global model to explain the SEDs. In Sect. 2, we describe our sample stars and their observations. We also present the SEDs (see Fig. 1) and indicate observational trends. ISO-SWS spectra and an inventory of solid state and PAH bands are shown in Sect. 3, where the individual sources are discussed as well. In Sect. 4 we propose a global model, and discuss grain processing. Our conclusions are summarized in Sect. 5. In a forthcoming paper, detailed radiative transfer models of some of the sources will be presented (Bouwman et al. in preparation).