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Astron. Astrophys. 338, 977-987 (1998)

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1. Introduction

The Chamaeleon Molecular Cloud is one of the most interesting targets to investigate the formation of low mass stars and the low mass end of the initial mass function (IMF), because of its nearby location and its position far away of the galactic plane ([FORMULA]) where the density of background stars is relatively small. A complete census of young objects requires to survey deeply wide areas in a spectral range that is not much hampered by dust extinction, namely the near and mid-infrared. This has been made possible only recently thanks to the release of data provided by large scale sky surveys in the near-infrared such as DENIS (Epchtein, 1997).

Low mass young stellar objects (YSOs) are characterised by [FORMULA] line emission (Hartigan, 1993) and an infrared excess that reveals the presence of circumstellar material in the near-infrared (Whittet et al., 1987; Whittet et al., 1991), or in the mid and far-infrared as shown by the IRAS (Rydgren, 1980; Baud et al., 1984; Assendorp et al., 1990; Prusti et al., 1991) and ISO (Nordh et al., 1996) data. X-ray emission surveys provided by the Einstein and ROSAT missions, have also contributed to identifying young objects (Lawson et al., 1996; Alcala et al., 1997). Finally, millimetre observations (Mattila et al., 1989; Henning et al., 1993), mostly in CO lines, allow the identification through the measurement of the gas emission and outflows that often characterise YSOs.

Known pre-main-sequence stars in the Chamaeleon cloud consist essentially of T Tauri stars (TTS) (Appenzeller & Mundt, 1989) of spectral type ranging from K5 to M5 (Appenzeller et al., 1983). Their mass ranges from 0.2 to [FORMULA] with a distribution peaking at [FORMULA], and their current age estimate is greater than [FORMULA] years, according to Lawson et al. (1996).

The Chamaeleon I (Cha I) cloud has been surveyed in various spectral ranges from millimetre to X-ray, but investigation are yet limited to small areas or low sensitivity.

During its first year of operations, DENIS has covered the whole surface of the Cha I cloud in the [FORMULA] bands with a good sensitivity ([FORMULA]). Thanks to its wide surface coverage, stars far away from the known cores of the cloud can be detected. An extinction map of the cloud was recently drawn out using J star counts (Cambresy et al., 1997), and the aim of the present work is to pursue the exploitation of these data in order to pick up already catalogued TTS and to provide an homogeneous set of data and to try to single out new YSO candidates, especially toward the low luminosity end.

Up to now, 126 pre-main-sequence stars have been recognised in the Cha I cloud by various authors. Sect. 2 presents the DENIS photometric data of these 126 stars and of 54 new candidates that have been selected using the method described in Sect. 3. The nature of the sources is discussed in Sect. 4.1 and some constraints on the circumstellar environments are derived in Sect. 4.2. Finally, in Sect. 4.3, we discuss the luminosity function, and we estimate the age of the period of star formation in the cloud.

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© European Southern Observatory (ESO) 1998

Online publication: September 17, 1998