Astron. Astrophys. 327, L25-L28 (1997)

2. The DENIS brown dwarf mini-survey

DENIS observations are carried out on the ESO 1m telescope at La Silla (Chile), with a three channels infrared camera (Copet et al. 1997). Dichroic beam splitters separate the three channels, and focal reducing optics provides image scales of 3" on the 256 256 NICMOS3 arrays used for the two infrared channels and 1" on the 1024 1024 Tektronix CCD detector of the I channel. The instantaneous field of view is 12' for all three channels, and a focal plane microscanning mirror is used to obtain 1" sampling for the two infrared channels. The sky is scanned in a step and stare mode, along 30 degrees strips at constant right ascension which contitute the basic DENIS observing units.

The sources discussed in this letter were identified during analysis of 230 square degrees of DENIS data at high galactic latitude, which constitutes the "DENIS Brown Dwarf Mini-survey". The image data were obtained from the Paris processing center and had been processed with the standard pipeline software (Borsenberger 1997). The instrumental and sky background are derived from a local clipped mean along the strip. Flat-field corrections are derived from observation of the sunrise sky, which are analysed by a linear regression of the pixel values on the mean background level of each image. Source detection and photometry were performed in Grenoble, using the SExtractor package (Bertin & Arnouts 1996). The details of the selection process are described in Delfosse et al. (1997), who use the resulting sample to determine the luminosity function of extreme M dwarfs. In this letter we restrict discussion to the three reddest objects (DENIS-P J1228.2-1547, J1058.7-1548 and J0205.4-1159), whose parameters are summarised in Table 1. Figure 1 displays their positions in the I-J/J-K colour-colour diagram, along with some known very low mass stars. These three objects are clearly as red as (or redder than) GD165B, with I-J colours greater than 3.6.

Table 1. DENIS photometry and positions for the reddest Mini-survey objects. Positions are for epochs that range between 1995.9 - 1996.7 and for equinox J2000.0. They are based on the encoder readings of the ESO 1m telescope and only good to 10-20".

Fig. 1. The I-J/J-K diagram for the three redder DENIS objects. Symbols are: circle: previously known very low mass-stars, triangle: new DENIS objects

© European Southern Observatory (ESO) 1997

Online publication: April 6, 1998
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