Astron. Astrophys. 329, 131-136 (1998)

1. Introduction

The Herbig Ae/Be stars (hereafter Haebe) are pre-main-sequence stars of intermediate mass, and so are somewhat more massive analogs of the T Tauri stars. A characteristic of all known Haebe stars is the presence of an IR excess due to thermal re-radiation of a circumstellar (CS) dust shell. The spatial distribution of the CS material causing this IR excess is still questioned: some authors argue for a disk-like distribution of the CS dust (Hillenbrand et al. 1992; Corcoran & Ray 1994; Marsh et al. 1995), while others favour a spherical symmetric model (Berilli et al. 1992; Lorenzetti et al. 1996). It appears that the observation of the IR energy distribution alone is not sufficiently constraining for a detailed geometrical model.

Hillenbrand et al. (1992) have elaborated a classification of Haebe stars in terms of the IR excess, and thus define three groups. Group I contains stars with a large IR excess, showing a slope ; they interpret the objects of this group as being surrounded by a geometrically flat, optically thick CS accretion disk. Group II shows flat or rising IR spectra; these are interpreted as still younger objects where a major fraction of the gas and dust is not confined to a disk. The last group, G III, contains stars with small IR excesses, due only to gas. The scarce spatial information that is available confirms a picture in which the Group II sources, which are often resolved in the near-IR, are embedded in a loose environment that is hardly structured (Li et al. 1994). Group I sources are rarely spatially resolved, but Marsh et al. (1995) report a disk-like structure for the inner regions around AB Aurigae. These results suggest that the outer circumstellar regions, which are not gravitationally bound to the star, are close to spherically symmetric and that once this outer shell has disappeared a more disk-like circumstellar environment dominates the IR radiation. On the other hand, other authors, such as Lorenzetti et al. (1996), claim that a spherically symmetric dust model can account for most observations. Also Miroshnichenko (1997) successfully models the spectral shape of IR emission around Herbig stars with a spherical dust envelope. However, Sylvester et al. (1996), modeling the excess IR emission of Vega-like systems, some of which are known Herbig stars, conclude that the data are not in agreement with the hypothesis of a spherical envelope.

The conflicting results of these studies suggest that more and different observations are required to further constrain the models. Ideally, one would aim at high spatial resolution, but this is only attainable for a few nearby objects. The strategy developed in this paper is based on the spatial information that can be obtained from spectroscopy of the circumstellar lines and from observations of the photometric variability, both of which probe the line of sight in the vicinity of the star. For this purpose, three known Herbig Ae stars with comparable spectral types (A7-A9) and very similar IR excesses were selected and observed.

The contents of this paper are as follows. In the next Section the existing literature about the selected objects, HD 139614, HD 142666, and HD 144432, is discussed and the observations carried out in preparing this paper are summarised. The extinction and energy distribution are presented in Sect. 3, where the optical photometry is also discussed. Sect. 4 is devoted to the study of several important CS spectral features. The discussion of all these observations in Sect. 5 strongly argues for a disk-like distribution of the circumstellar dust around these stars.

© European Southern Observatory (ESO) 1998

Online publication: November 24, 1997
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