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Astron. Astrophys. 321, 189-201 (1997)

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1. Introduction

It has long been accepted that indirect evidence for circumstellar disks around low mass young stellar objects (YSOs) exists. Two of the primary arguments are observational: Firstly the inferred circumstellar masses of dust and gas surrounding classical T-Tauri Stars (cTTSs), measured by millimeter continuum observations, preclude a simple spherical distribution of the circumstellar material about such stars (Rydgren et al. 1982). The mass involved would, if spherically distributed, yield much higher optical extinctions than are actually observed (Lada & Adams 1992). Some alternate geometry, such as a disk, must exist to allow for both a substantial amount of circumstellar material and, at the same time, allow a relatively clear line of sight to the stellar surface. Secondly many cTTSs show blueshifted but not redshifted forbidden emission lines in their spectra, e.g. [OI] [FORMULA] 6300/6364, [NII] [FORMULA] 6548/6583, and [SII] [FORMULA] 6716/6731, indicative of low density gas envelopes or stellar winds. A large proportion of the observed [OI] [FORMULA] 6300 lines are highly blueshifted, with velocities in some cases of the order of -100 kms [FORMULA] (Edwards et al. 1987; Hartigan et al. 1995). One of the earliest explanations of the origin of these blueshifted profiles, from Appenzeller et al. (1983) and Appenzeller et al. (1984), invoked the presence of optically thick circumstellar disks which occlude the redshifted component of a stellar wind or other outflow close to the star. The subsequent success of disk models for cTTSs in explaining a variety of observational properties, such as the infrared excess and mass loss (see, for example, Edwards et al. 1993), supports the star+disk model as appropriate for low mass YSOs.

In the case of the more massive counterparts of the TTSs, the Herbig Ae/Be stars (HAEBESs), there remains considerable controversy in the literature about whether a significant fraction of these stars are surrounded by disks. For opposing views on this matter see Hillenbrand et al. (1992), Berrilli et al. (1992), Hartmann et al. (1993), Böhm & Catala (1994), Corcoran & Ray (1997a) and Ghandour et al. (1994). Certainly in a number of cases there is good evidence for disks: for example Grinin et al. (1994, 1996) and Grady et al. (1995) have observed polarimetric and photometric evidence, in a sample of HAEBESs, for an edge-on circumstellar disk-like envelope, associated with infall. It has been suggested that this subclass of HAEBESs, the UX Ori stars, may be the progenitors to stars similar to [FORMULA] Pictoris (Grinin et al. 1996). Here we take the approach that, as in the cTTSs, the forbidden lines can be used as a diagnostic for the presence of a disk.

Observations of forbidden lines, such as [OI] [FORMULA] 6300/6364 and [SII] [FORMULA] 6716/6731, however, in the case of the HAEBESS, are scarce. Finkenzeller (1985) published spectra of [OI] [FORMULA] 6300 forbidden line profiles of six HAEBESs and declared them to be "symmetrical to a good approximation." Nevertheless, deviations from symmetry were noted even by Finkenzeller (1985) in his small sample and, on close examination, all the profiles presented in Finkenzeller (1985) do show some degree of asymmetry. Böhm and Catala (1994) examined 33 HAEBESs for [OI] [FORMULA] 6300/6364 forbidden lines, using the interstellar absorption lines of neutral sodium (Na  D ) as an indirect measure of the systemic radial velocities of the stars. They found that roughly 50% (17) of the stars observed show the [OI] [FORMULA] 6300 line and only one of their sample (R Mon) shows strongly blueshifted emission in the fashion of young classical TTSs (Edwards et al. 1987, 1993). The "unshifted" lines of their sample show "symmetric profiles" with widths broadened to [FORMULA] 30-100 kms [FORMULA]. R Mon, the only exception, is believed to possess a circumstellar disk (Koresko et al. 1993) and is known to be the source of both an optical and a molecular outflow (Mundt et al. 1987; Fukui et al. 1993).

Böhm & Catala (1994) have proposed that the "unshifted" forbidden [OI] [FORMULA] 6300 does not support the presence of circumstellar disks around their sample HAEBESs and may instead arise from a spherically symmetric wind or the sum of spherically distributed streams forming at the star. Given, however, the high wind velocities (several 100 kms [FORMULA], Finkenzeller & Mundt 1984) inferred from the P-Cygni profiles of the permitted lines of these stars, it is somewhat difficult to see how such narrow forbidden line profiles (FWHM [FORMULA] 30-100 kms [FORMULA], Böhm & Catala 1994) could arise from a spherical wind.

In the case of the cTTSs, there is now considerable evidence that there are two distinct velocity components producing the forbidden emission lines (see, for example, Hamann 1994 and Hartigan et al. 1995). These components differ not only in velocity but in density as well (Hamann 1994). Moreover the high velocity component (hereafter the HVC) has been shown to be more spatially extended using high resolution long-slit spectroscopy (see, for example, Hirth et al. 1994a; Hirth 1994) than the low velocity component (or LVC). According to the model of Kwan and Tademaru (1988, 1995), the HVC and LVC are to be interpreted as the jet and disk wind respectively. To what extent is this picture relevant to the HAEBESs?

While a number of HAEBESs are known to possess clear LVC and HVC components in their forbidden emission lines (Hamann 1994; Böhm & Catala 1994), such stars are relatively rare. LkH [FORMULA] 233, PV Cep (Hamann 1994; Corcoran & Ray 1997b), V645 Cyg (Hamann & Persson 1989), and R Mon (Böhm & Catala 1994) all show blueshifted forbidden lines with typical velocities of a few hundred kms [FORMULA]. Moreover, in these cases, there is additional evidence that circumstellar disks exist for some of these stars, such as LkH [FORMULA] 233 and PV Cep which are both observed to have polarization patterns indicative of a disk-like geometry (Bastien & Ménard 1990; Gledhill et al. 1987). This paper presents the results of a survey of 56 Herbig Ae/Be stars, investigating their [OI] [FORMULA] 6300 forbidden emission lines (and in a few cases [SII] [FORMULA] 6717/6731 lines) and discusses the role of circumstellar disks as a possible means of explaining the results, with reference to the two-velocity outflow model of Kwan & Tademaru (1988, 1995).

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998
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