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Astron. Astrophys. 325, 685-692 (1997)

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1. Introduction

Young stellar objects (YSOs) are characterized by being deeply embedded in the gas and dust of their parent clouds. During the protostellar phase, the mass of the circumstellar material decreases in part due to accretion onto the central star, in part because of outflow. Because the submillimetre range is sensitive to the cold circumstellar material, it is an ideal domain for studying the first steps of stellar evolution (Cabrit & André 1991; Reipurth et al. 1993; André & Montmerle 1994; Henning et al. 1994).

André et al. (1993) and Saraceno et al. (1996) suggested that the bolometric ([FORMULA]) and the millimetre ([FORMULA]) luminosities can provide a very useful tool for tracing the different phases of protostellar evolution. This evolution is, in fact, determined by two factors: the central object's mass which defines [FORMULA] and the circumstellar mass which is proportional to the millimetre flux since the dust can be assumed to be optically thin in this wavelength range. André et al. (1993) and Saraceno et al. (1996) assumed that the dust characteristics are constant during the pre-main-sequence evolution. Nevertheless, dust properties may change during protostellar evolution (Seab 1987; Cardelli et al. 1989). Starting from these considerations, Dent et al. (1995), using a submillimetre colour-colour diagram on a sample of young sources in different evolutionary stages, found that [FORMULA], the exponent of the dust opacity law, seems to change with time, suggesting an evolution of dust properties.

In this work, we discuss the possibility and the impact of a change in dust properties during the evolution. We present 350 µm to 1.3 mm continuum observations of the ten most luminous ([FORMULA] 200  [FORMULA]) Class I sources (Lada 1987) in L1641, selected from the work of Strom et al. (1989, hereafter SNS). These sources are highly embedded, associated with dense cores and outflow activity. They cover a large range of ([FORMULA] / [FORMULA]) values and are well suited to the purpose of this work. For these objects, we determine dust properties and masses and we attempt to identify a possible evolution of dust characteristics.

The giant molecular cloud L1641 is located in the southern part of the Ori A molecular cloud at a distance of 480 pc (SNS). This cloud and its associated IR sources have been extensively studied (SNS; Chen et al. 1993a, b; Chen & Tokunaga 1994). The outflow activity in this cloud has been studied by Morgan & Bally (1991) and by Morgan et al. (1991). Individual YSOs in this cloud have been studied by Davis & Eislöffel (1995), McMullin et al. (1994), Morgan et al. (1990) and Myers et al. (1987).

Sect. 2 presents the observations and Sect. 3 deals with the data analysis and results. The possible change of [FORMULA] with time is discussed in Sect. 4. Finally, the main conclusions are drawn in Sect. 5. A discussion of individual sources is presented in the Appendix.

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© European Southern Observatory (ESO) 1997

Online publication: April 28, 1998