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Astron. Astrophys. 360, 213-226 (2000)

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1. Introduction

HAEBE stars represent the final stage of pre-main-sequence (PMS) evolution of intermediate-mass stars ([FORMULA] 2-10 [FORMULA]). As a consequence of the star formation process these stars are typically surrounded by a gas and dust envelope and/or disk. They may be the precursors of young main sequence [FORMULA]-Pictoris and Vega-type stars. These latter systems are surrounded by circumstellar debris disks, which perhaps contain planetary bodies. This would imply that the environment around HAEBE stars represents an early phase in the formation of planets. ISO spectroscopy of isolated HAEBE stars has strengthened this link between HAEBE stars and planet formation. Malfait et al. (1998b) showed that the ISO spectrum of the B9Ve star HD 100546 has a high abundance of crystalline silicates, and that the composition of the dust in this object is remarkably similar to that seen in the solar system comet Hale-Bopp (Crovisier et al. 1997). Moreover, the Fe star HD 142527 shows evidence for hydro-silicates, indicative of aquateous alteration of silicates in its cold circumstellar disk (Malfait et al. 1999), which is expected only to occur on the surfaces of larger bodies.

This paper is the second in a series in which we will study the circumstellar environment around the Herbig Ae/Be (HAEBE) stars AB Aurigae (A0Ve+sh) and HD 163296 (A1Ve). We have choosen to investigate these two stars because (i) their basic properties are well known, (ii) they are isolated and bright, making them ideal candidates to study their circumstellar material, (iii) they are nearby systems that have been spatially resolved in CO; HD 163296 has also been resolved in the continuum at 1.3 mm, and (iv) on the whole the two stars are very similar. The last point implies that differences in spectral energy distribution may be linked to differences in dust composition and morphology. This can provide important insights in the evolution of the circumstellar dust and may yield information on characteristic time scales of dust evolution (from comparison with stellar age) and/or on the importance of properties of the natal molecular cloud, such as initial cloud size, mass and angular momentum.

In the first paper (van den Ancker et al. 2000, hereafter Paper I) we presented new infrared spectra of these two well studied stars obtained with the Short- and Long Wavelength Spectrometers on board the Infrared Space Observatory (ISO) (Kessler et al. 1996). In this paper we present quantitative spectroscopic modelling with the aim to constrain dust properties such as composition, abundance and size- and shape-distribution. In a subsequent paper we intend to present a detailed multi-dimensional model for the dust distribution around these HAEBE systems.


[TABLE]

Table 1. Astrophysical parameters of the programme stars.
Notes:
[FORMULA]) Older photographic measurements (Gaposchkin et al. 1952) show values up to [FORMULA].
[FORMULA]) Corrected for revised distance (van den Ancker et al. 1997).
References:
(1) Hipparcos Catalogue; (2) van den Ancker et al. (1997); (3) Böhm & Catala (1993); (4) van den Ancker et al. (1998); (5) Paper I, but relevant parameters have been scaled such that they are conform spectral type A1Ve and not A3Ve; (6) Böhm & Catala (1995); (7) Mannings & Sargent (1997); (8) Houk & Smith-Moore (1988); (9) Halbedel (1996).


The geometry of the circumstellar environment around HAEBE stars remains a persistent problem. The key issue is that the observational evidence from spectral energy distributions (SEDs) and spectroscopy do not define the geometry of the circumstellar dust in a unique way (see Waters & Waelkens 1998 for a review). We will start out by summarizing this evidence immediately focusing on AB Aur and HD 163296. The second important problem addressed in this paper is the nature of the onset of near-IR emission. This will be discussed in Sect. 1.2.

1.1. Geometry of the circumstellar dust

In view of the similarity to their less massive counterparts the T Tauri stars, HAEBE stars are expected to have optically thick disks. Indeed, in the case of AB Aur high-resolution imaging together with a de-convolution method (Marsh et al. 1995), indicates a disk of size 36 and 72 AU at 11.7 and 17.9 µm respectively, adopting a distance of 144 pc. Direct observational evidence for a disk-like geometry comes from the kinematical properties of 13CO gas, observed in the [FORMULA] pure rotational transition in AB Aur and CO gas, observed in the [FORMULA] pure rotational transition in HD 163296 which seem consistent with a rotating Keplerian disk (Mannings & Sargent 1997, Mannings et al. 1997). The dimension of the CO emitting regimes has been estimated to be [FORMULA] AU, and [FORMULA] AU for AB Aur and HD 163296 respectively. The aspect ratio's imply that we see AB Aur almost edge-on at an inclination angle of [FORMULA] and HD 163296 at [FORMULA]. For this last star the continuum emission at 1.3 mm has been spatially resolved, showing an elongated structure with dimension [FORMULA] AU. Near-infrared interferometric observations of AB Aur (Millan-Gabet et al. 1999) have resolved the inner part of the dust distribution, determining the inner edge of the dust to be at 0.3 AU (see also Sect. 1.2). An upper limit on the inclination in this inner regime is determined to be [FORMULA], suggesting that the inner and outer regions of the disk do not have the same geometry.

The shape of the IR spectral energy distribution has often been used as a diagnostic for constraining the geometry of the HAEBE surroundings. In some cases, the SED may provide firm constraints especially when a lack in balance is found between energy absorbed in the UV and optical and energy re-emitted in the IR (Meeus et al. 1998). Such a discrepancy strongly points to a disk-like structure. One should, however, be very careful with conclusions relating to the spatial distribution of the dust derived from the SED only (e.g. Henning et al. 1998, Bouwman et al. 1999).

Evidence for the presence of a large optically thin medium comes from the strong 9.7 µm silicate emission (Cohen 1980, Sitko 1981, van den Ancker et al. 2000) observed in both stars. If the silicates are located in a geometrically thin disk, modelling shows one expects this disk to be optically thick at optical as well as IR wavelengths. However, it is substantially more difficult (if not impossible) to reconcile the presence of this emission adopting an optically thick "disk-only" model compared to assuming an optically thin emitting region. So, if a disk is expected on the basis of imaging, the silicate emission at least suggests the presence of an optically thin region of substantial size above the surface of this disk (e.g Chiang & Goldreich 1997). The forbidden [OI[FORMULA] 6300 emission (Corcoran & Ray 1997), which is narrow ([FORMULA]), symmetric and unshifted in AB Aur, could be formed in such an extended surface layer or disk atmosphere, and be broadened by Keplerian rotation.

The above arguments imply that the geometry of the CSM around these stars is likely to be complex, i.e. it cannot be explained by a "disk-only" model. Most likely only the inner part of the disk is optically thick and a substantial thin region, such as an extended surface layer or (flared) outer disk region - or both - is present as well.

1.2. The onset of near-IR emission

An important aspect of the geometry of the CSM of HAEBE stars is the innermost region where the hottest dust grains are present and which dominate the near-IR SED. The key question here is whether one is able to understand the onset of near-IR emission (from [FORMULA] 2 µm) in terms of a physically realistic scenario. The remarkable uniformity of the onset of near-IR emission in HAEBE systems (Meeus et al. in prep.) suggests a similar geometry and/or dust grain population at high ([FORMULA] K) temperatures at the inner regions of the CSM. In the following we will discuss several possible explanations for the near-IR onset.

Hillenbrand et al. (1992) suggested a model in which the observed near-IR flux is due to accretion luminosity from a geometrically thin active accretion disk. The emission in the NIR is explained in this model with (high temperature) accreting gas in the innermost part of the disk. Emission at longer wavelengths would be due to dust in an optically thick disk. The accretion rates Hillenbrand et al. (1992) derive are [FORMULA] and [FORMULA] [FORMULA] for AB Aur and HD 163296 respectively. Radio continuum observations (Skinner et al. 1993) point to much smaller accretion rates of [FORMULA] and [FORMULA] [FORMULA] for AB Aur and HD 163296 respectively, which are inconsistent with the high accretion rates needed in the Hillenbrand model. Also the lack of substantial veiling at optical wavelengths points towards small accretion rates. Böhm & Catala (1993) derived an upper limit for the accretion rate of AB Aur, from the veiling of photospheric lines, of [FORMULA] [FORMULA], consistent with Skinner et al. (1993). The strongest objection against the active accretion disk model comes from interferometric observations of AB Aur (Millan-Gabet et al. 1999) which can not be reproduced using this model.

A different explanation for the near-IR emission could be Polycyclic Aromatic Hydrocarbons (PAHs), of which the emission bands are located at 3.4, 6.2, 7.7, 8.6 and 11.3 µm. This makes PAHs a candidate for explaining the near-IR flux. Due to their small size, the characteristic temperature of PAH particles can be high as they are no longer expected to be in thermodynamic equilibrium. Quantum heating effects result in such high temperatures that their dominant emission is in the near-IR. To account for all observed near-IR emission however very high abundances are required (Natta et al. 1993) which seem unlikely. Even though their narrow characteristic features are present in AB Aur, it is not expected that PAHs are also responsible for the broad continuum contribution starting at [FORMULA] 2 µm. This conclusion is strengthened by the shape and strength of the near-IR emission in HD 163296, which is identical to AB Aur but shows no PAH bands (van den Ancker et al. 2000). A different explanation seems required.

One of the main points of this paper is that observed near-IR emission in the programme stars can be explained simply by dust in thermodynamic equilibrium. The dust species that have strong resonances in this spectral region ([FORMULA] 1-8 µm) are metallic iron, iron oxide, and carbonaceous dust grains, either graphitic or `amorphous'. The dust grains are heated by stellar radiation in the inner parts of the circumstellar disk up to the dust destruction temperatures of the individual dust species. We will show that in view of the high grain temperatures ([FORMULA] K) needed to produce the observed flux at near-IR wavelengths, metallic iron grains are the most likely candidates to explain the onset of the near-IR emission (see Sect. 3.1). With our model we can also reproduce the interferometric observations by Millan-Gabet et al. (1999) of AB Aur, showing an inner hole of 0.3 AU.

This paper is organized as follows: in Sect. 2 we discuss the chosen approach to model the ISO spectra and describes the model we used. Sect. 3 gives the results of the model fitting and a discussion over the implications of these results is presented in Sect. 4. We summarize our results in Sect. 5.

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Online publication: July 27, 2000
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