Astron. Astrophys. 327, 758-770 (1997)

1. Introduction

Mass accretion onto a protostar is generally accepted to take place through accretion from a disk rather than by direct infall. This concept, originally advocated from a theoretical point of view to solve the protostellar core's angular momentum problem, is supported by a variety of observations. In addition to accretion disks, stellar jets and molecular outflows have been identified observationally to be closely associated with young stellar objects (YSOs). Hence, a comprehensive picture of low mass star formation seems to emerge, where collapsing matter, accretion disk, protostar and mass outflow form constituents stipulating each other. Although a wealth of information about the properties of molecular cloud cores (hereafter called clouds) and the physical conditions in the environment of YSOs has been gathered (e.g., Myers 1991, André & Montmerle 1994) to observationally constrain the required boundary conditions, the physical processes relevant during the accretion-dominated star formation stages are still largely unknown.

The existence of disks around low mass YSOs has been inferred from comparison of computed and observed spectral energy distributions. Their long wavelength tails were modeled in terms of geometrically thin, and optically thick disks (e.g., Adams et al. 1988), but emission from the surrounding envelope outshines the contribution from the embedded star-disk system, besides the extinction of radiation originating near the envelope center.

Only low mass YSOs in rather late stages of their pre-main sequence evolution can be detected from the high frequency radio regime to the near infrared. Low angular resolution observations of T Tau stars (Beckwith et al. 1990) and interferometric observations of circumstellar disks (Sargent & Beckwith 1987, 1991, Dutrey et al. 1994) allow some insight into the disk properties. However, the derived disk masses, sizes, and mass accretion rates indicate that accretion has mainly ceased. Hence, the physical processes relevant in the T Tau stage might not be identical to those which are relevant in earlier stages.

Although there is evidence in favor of accretion disks around protostars in the collapse stage (Ladd et al. 1991, Terebey et al. 1993), extinction prevents escape of infrared radiation from the envelope interior. However, if physical conditions near a deeply embedded accretion disk are suitable for population inversion of some molecular energy levels, maser emission could eventually be strong enough to be detected. Especially in the radio regime, where extinction is very low, masers could be used as probes of the physical conditions in their immediate environment.

High angular resolution observations of the 22 GHz maser emission from L 1287 (H₂ O) were modeled in terms of a simple outflow motion but gave only unsatisfactory results. An alternative interpretation, that did not necessarily exclude the possibility of maser excitation in the outflow, was suggested in terms of a disk origin of the masers (Fiebig 1996a, Fiebig et al. 1996). Since the physical basis for this interpretation was not discussed before, this paper presents an investigation of the concept, that 22 GHz maser emission in L 1287 (H₂ O) arises from shock-compressed layers of individual clumps impinging onto the accretion disk around the deeply embedded YSO in L 1287.

© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998
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