1 Introduction

The Chamaeleon complex is one of the nearest star-formation regions to the Sun (between 160 and 180 pc, Whittet et al. 1997). It consists of three main dark clouds, designated Chamaeleon (Cha) I, II and III (Schwartz 1977). Its relative proximity, high Galactic latitude ( $b \approx -17 \hbox{$^\circ$ }$) and young age ($\sim$10⁶ yrs) make it an ideal location to search for low-mass T Tauri stars (TTS) and young brown dwarfs. The low-mass population can provide constraints on the shape of the initial mass function (by investigating the slope), which is still poorly determined at low masses (Tinney 1993; Méra et al. 1996).

Young stellar objects are characterised by their H$\alpha$ emission line (Hartigan 1993) and an infrared excess that reveals the presence of circumstellar material in the near infrared (Larson et al. 1998; Oasa et al. 1999; Gómez & Kenyon 2001), the mid-infrared (ISO, Persi et al. 2000) and the far-infrared (IRAS, Prusti et al. 1992). X-ray observations (ASCA, ROSAT) allow the identification of young stellar objects (Yamauchi et al. 1998; Alcalá et al. 2000). The origin of the X-ray emission from the low-mass pre-main sequence stars may be due to an enhanced dynamo activity with a coupled process of surface convection and differential rotation of a star or between a star and the inner disks (e.g. in $\rho$ Oph, Montmerle et al. 2000). Millimeter observations such as the detection of CO and CS lines that reveal gas emission and outflows often allow one to characterise young stellar objects (Olmi et al. 1994).

Weak line TTS are known as pre-main sequence objects without a disk and are usually found to be associated with X-ray sources. In contrast, classical TTS with a circumstellar disk show strong H$\alpha$ emission and excess IR emission. Brown dwarfs are low-mass stars (<0.08 $M_{\odot}$) which never reach the reaction of hydrogen fusion. They are mainly powered by gravitational energy. Therefore they are relatively faint objects and can only be detected at nearby distances ( $\mathrel{\hbox to 0pt{\lower 3pt\hbox{$\mathchar''218$ }\hss} \raise 2.0pt\hbox{$\mathchar''13C$ }}$30 pc). However, young brown dwarfs are much more luminous ($\sim$100 times more than field brown dwarfs). Such young brown dwarfs can be found in star-forming regions out to a few 100 pc (Neuhäuser & Comerón 1999).

The Cha II has not been mined as thoroughly as the Cha I cloud. In the Chamaeleon clouds, the pre-main-sequence stars, mainly TTS, were first discovered by Schwartz (1977). No brown dwarfs have yet been found in the Cha II. Recently, Comerón et al. (2000) found eleven young brown dwarfs in the Cha I cloud from pointed IR observations. With the advent of large near-IR surveys (DENIS, 2MASS), it is now possible to easily identify such samples across large regions of the sky. Cambrésy et al. (1998) have used DENIS data to find young stellar objects in the Cha I cloud.

In this paper, we introduce a low-mass star sample in the Cha II dark cloud, extracted from the DENIS survey. Section 2 describes the observations and the data reduction. In Sect. 3, we construct an extinction map of the Cha II cloud, and present the color-color I-J/ $J-K_{\rm s}$ diagram used to search for new low-mass members. Finally, Sect. 4 discusses the position of our sources on the color-magnitude diagram using the evolutionary tracks for low-mass stars as modeled by Baraffe et al. (1998).