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Astron. Astrophys. 364, 741-762 (2000)

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1. Introduction

Lynds 1551 is one of the most intensively studied molecular outflow sources. Lying at a distance of [FORMULA] 150 pc in the Taurus-Auriga dark cloud, it is associated with a 30 [FORMULA] Class I protostar, L1551 IRS 5. This is presumed to be in a pre-T Tauri phase and the driving source of a molecular outflow (Snell et al. 1980; Rainey et al. 1987; Fridlund & White 1989a,b; White et al. 1991), and an optical jet (Mundt & Fried 1983; Fridlund & Liseau 1988). L 1551 IRS5 has long been believed to represent arche-typically the very early stages of low mass star formation. It is special in that it is closest to the Sun and that it displays its outflow at only a slight inclination angle. In addition, IRS5 is relatively isolated, thus largely reducing source confusion and signal contamination problems, which has made possible observational studies at high resolution and accuracy not obtainable elsewhere. These high resolution studies have revealed that protostars are seemingly far more complex systems than commonly believed. The extinction, [FORMULA], towards IRS 5 has been estimated to be [FORMULA] 150[FORMULA] (Smith et al. 1987; Stocke et al. 1988). Continuum maps reveal that the dense central core is surrounded by an extended cloud (Woody et al. 1989; Keene & Masson 1990; Lay et al. 1994). The spectral energy distribution and intensity maps of L1551 IRS 5 have been modelled in detail using radiative transfer methods in spherical (Butner et al. 1991, 1994) and axially-symmetric (Men'shchikov & Henning 1997, hereafter MH97) geometries. These suggest that a flat accretion disc (Butner et al. 1994) or a geometrically thick torus (MH97) lies inside the extended cloud.

High resolution radio observations have shown evidence for a double source located at IRS 5 (Rodríguez et al. 1986, 1999; Campbell et al. 1988; Bieging & Cohen 1985). Other interpretations of the available data suggest a different morphology, with a binary system lying at the centre of IRS 5 whose components are separated by [FORMULA] 50 AU (Looney et al. 1997), which is in turn surrounded by a dusty disc. Hubble Space Telescope (HST) observations by Fridlund & Liseau (1988) suggest that there are two distinct optical jets, supporting this circumbinary interpretation, and that the central region is surrounded by a torus, with a mass [FORMULA] 0.1-0.3 [FORMULA], and a radius of [FORMULA] 700 AU. This in turn is surrounded by an [FORMULA] 70 AU central cavity, which contains the double radio source. There appears to be an evacuated cavity in the torus, with a half-opening angle of about 50-55o. The axis of the molecular outflow is inclined at about 30-35o to the line of sight.

In this paper, we report spectroscopic observations obtained with the ISO Long and Short Wavelength spectrometers (LWS, SWS) towards IRS 5, L1551 NE, HH 29 and at a number of locations along the molecular outflow. Archival data from the HST/NICMOS camera and using an infrared spectrometer at UKIRT are also examined to provide constraints to the modelling. We then present a detailed self-consistent two-dimensional continuum radiative transfer model for IRS 5 which is consistent with the available data.

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© European Southern Observatory (ESO) 2000

Online publication: January 29, 2001