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Astron. Astrophys. 336, 565-586 (1998)

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1. Introduction

Herbig Ae/Be (HAEBE) stars are young intermediate-mass pre-main sequence stars. As the more massive (M = 2-8 [FORMULA]) counterparts of the T Tauri stars, they represent the connecting link between low- and high-mass young stellar objects. Since their first description by Herbig (1960), the number of known HAEBE stars and candidates increased continuously with the catalogues of Strom et al. (1972), Finkenzeller & Mundt (1984), and Thé et al. (1994) from originally 26 sources up to nearly 300 objects today. FU Orionis stars probably represent a transition stage in the evolution of solar-like pre-main sequence stars and attracted a lot of attention because of their expected association with highly active circumstellar disks (Hartmann & Kenyon 1996).

Both HAEBE and T Tauri stars share a lot of peculiarities like infrared excess emission, mass loss, association with molecular gas and optical nebulosities as well as variable metallic and Balmer emission lines. Despite these similarities, the group of HAEBE stars spans a much wider range in mass and luminosity than the T Tauri stars. We should also note that not all classification criteria always apply to an individual source.

Because of their youth, young stellar objects are usually located close to or in the region where they have been formed and their evolutionary status is directly related to the structure of the molecular cloud/circumstellar environment. This is the reason why the investigation of these regions is of special interest. To explain the spectral energy distributions of the HAEBE stars at infrared/millimetre wavelengths, circumstellar disks, spherically symmetric envelopes or a combination of both were used (Hillenbrand et al. 1992, Hartmann et al. 1993, Natta et al. 1993, Henning et al. 1994). In contrast to the classical T Tauri stars, the general presence of accretion disks around the HAEBE stars is a much less observationally proven fact. The relative contribution of the disk/envelope emission will change with luminosity and age of the objects as well as with wavelength, beam size of the observations, and possibly with the inclination of the system. Di Francesco et al. (1997) tried to constrain the presence of disk material around the HAEBE stars by millimetre interferometry measurements. They could only detect Elias 1 and two deeply embedded millimetre sources near the target stars, LkH[FORMULA] 198 MM and LkH[FORMULA] 225 S (= V 1318 Cyg S), with certainty. Meanwhile, Mannings & Sargent (1997) unambiguously demonstrated the presence of disks around AB Aur and HD 163296. HAEBE stars with strong forbidden line emission show evidence for asymmetric line profiles (obscuration of redshifted forbidden emission), attributed to the occulting effects of a disk (Corcoran & Ray 1997). In addition, the relative strength of outflows scales with the reddening of the star in the same way as for classical T Tauri stars (Corcoran & Ray 1998).

We should also note that binary and even multiple systems can be associated with a millimetre emission peak and it may be difficult to attribute a certain millimetre flux density to an individual optical/infrared source. Furthermore, there remains the question as to how clumpy the dust distribution in the envelopes is (Grinin et al. 1991, Friedemann et al. 1992).

Previous studies did not demonstrate that single-dish On-On measurements give sufficient information for a detailed radiative transfer modelling. By mapping the R Cr A and T Cr A region, Henning et al. (1994) already showed that the 1.3 mm emission was not associated with the two HAEBE stars, but with the deeply embedded source IRS 7. Another example is LkH[FORMULA] 198 MM which is a deeply embedded millimetre source close to the HAEBE stars in this region. The problem of the flux density assignment was a strong motivation for this millimetre continuum study. To give answers to some of the previous questions, we extended our 1.3 mm On-On continuum survey of southern HAEBE stars (Henning et al. 1994) and mapped 20 northern and southern HAEBE stars as well as 5 FU Orionis stars with the IRAM and SEST telescopes. The 1.3 mm dust continuum emission is an excellent tracer of dense and cold circumstellar envelopes and provides good column density and mass estimates, whereas easily observable molecular lines are going to be optically thick in high-density regions ([FORMULA]). The contribution of free-free emission at 1.3 mm wavelength should be in general less than 10% in the case of HAEBE stars (see, e.g., Mannings & Sargent 1997). To compare the 1.3 mm emission with thermal emission and scattered light in the near infrared, we compare our millimetre continuum maps with NIR images of the regions.

In the following, we describe the observations (Sect. 2), discuss the derived physical properties and individual objects in Sect. 3, and present the results of the radiative transfer calculations for four characteristic sources in Sect. 4.

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© European Southern Observatory (ESO) 1998

Online publication: July 20, 1998
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