All Figures

$\begin{figure} \par\includegraphics[width=8.8cm]{6993f01.ps}\end{figure}$ Figure 1: Mosaic of the I-band images taken with WFC/INT (WFC03). North is up and east to the left. The size of each WFC chip is about 11 $\times$ 22 arcmin². The glare of the OB star system $\sigma$ Ori is clearly visible to the northeast of the vertical detector (CCD#2). The mosaic on to a DSS-2-IR image centred on $\sigma$ Ori can be found in Fig. 4 of Caballero (2005). See also the false-colour composite image in Fig. B.1, available on-line.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm]{6993f02.eps}\end{figure}$ Figure 2: I vs. I-J colour-magnitude diagram from our WFC03/INT-ISAAC/ Antu survey. The different symbols indicate: probable fore- and background sources (small dots, " $\cdot$ ''); spectroscopically confirmed cluster members from Table 2 (filled stars, " $\star$ ''); cluster member candidates with spectral type determination (encircled dots, " $\odot$ ''); cluster member candidates without spectral type determination (open circles, " $\circ$ ''); the four new cluster member candidates (filled circles, " $\bullet$ ''); S Ori J053948.1-022914 (the open triangle, " $\triangle$ ''); the three detected cluster non-members (asterisks, "*''). Dash-dotted lines indicate the completeness and detection limits of our survey. The dashed line is the lower envelope of confirmed cluster members with youth indicators. The solid line is the boundary considered as the selection criterion. Colour versions of all our figures are available in the electronic publication.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm]{6993f03.eps}\end{figure}$ Figure 3: The IJ survey region. The incomplete coverage of the J survey is indicated with dashed lines. Code: filled circles, our cluster members and candidates; big open stars, $\sigma$ Ori AB and E (at the centre of the cluster); small open stars, other young stars in the survey area more massive than 1.2 $M_\odot$ from Caballero (2007). Note the increasing density of objects towards the centre of the cluster.
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In the text

$\begin{figure} \par\mbox{\includegraphics[width=8.8cm,clip]{6993f04a.eps}\includ... ...ip]{6993f04c.eps}\includegraphics[width=8.7cm,clip]{6993f04d.eps} } \end{figure}$ Figure 4: I vs. I-J ( top left), I vs. $I-K_{\rm s}$ ( bottom left), and Jvs. $J-K_{\rm s}$ ( top right) colour-magnitude diagrams, and $J-K_{\rm s}$ vs. I-J colour-colour diagram ( bottom right). I is from WFC03, J from ISAAC and $HK_{\rm s}$ from the ground-based follow-up data. The symbols are as in Fig. 2. Objects with discs from IRAC excess flux are marked with big circles. Solid and dash-dotted lines are the Dusty00 and Cond03 3 Ma-old isochrones at 360 pc of the Lyon group. Dotted lines in the colour-colour diagram mark the approximate boundaries of the sequence of field ultracool dwarfs (Dahn et al. 2002). The very red object with $I-J = 1.75\pm 0.11$ mag and $J-K_{\rm s} = 2.08\pm 0.11$ mag is S Ori J053902.1-023501, whose spectral energy distribution (Fig. 6) shows evidence for a circumstellar disc (Sect. 4.2).
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm]{6993f05.eps}\end{figure}$ Figure 5: [3.6]-[8.0] vs. $J-K_{\rm s}$ colour-colour diagram. The symbols are as in Figs. 2 and 4. The dotted line marks the boundary between objects with and without discs. Field ultracool dwarfs with spectral types in the range M3.0-L5.0 from Patten et al. (2006) are shown with open (black) squares.
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In the text

$\begin{figure} \par\includegraphics[width=18cm]{6993f06.eps}\end{figure}$ Figure 6: Spectral energy distributions from the I band to the [8.0] band of nine representative substellar objects in $\sigma$ Orionis (filled stars). The open squares indicate the SEDs of ultracool field dwarfs of similar spectral type, normalised at the J band.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm]{6993f07.eps}\end{figure}$ Figure 7: Luminosity function in the J-band. Note the scarcity of objects at $J \sim$ 16.0 mag.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm]{6993f08.eps}\end{figure}$ Figure 8: Number of objects within each mass interval ( top panel) and mass spectrum ( bottom panel) from 0.110 to 0.006 $M_\odot$ . Dotted vertical lines denote the hydrogen ( left) and deuterium ( right) burning mass limits. The values not corrected for contamination are shown with thin lines ( top panel) and open circles ( bottom panel), while the values corrected for contamination are shown with thick lines ( top panel) and filled circles ( bottom panel). The fits of the mass spectrum in the three substellar bins (thin dashed line) and the four bins (thick dashed line) are also shown.
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In the text

$\begin{figure} \par\includegraphics[width=8.8cm]{6993f09.eps}\end{figure}$ Figure 9: Normalised cumulative number of objects ( $\sum _i i/N$ ) vs. mass. Filled circles indicate the most probable masses for 3 Ma and 360 pc. The error bars indicate the masses for the extreme cases of a younger closer cluster (1 Ma and 300 pc) and of an older farther cluster (5 Ma and 430 pc). Dotted vertical lines denote the hydrogen (left) and deuterium (right) burning mass limits.
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In the text

$\begin{figure} \par\includegraphics[width=17cm]{6993fb1.ps}\end{figure}$ Figure B.1: False-colour composite image of the northern part of survey area corresponding to the detector CCD#2 of the WFC (11 arcmin wide; north is up, east is left). Blue is for photographic I (from the Canadian Astronomy Data Centre), green is for IRAC 3.6 $\mu$ m and red is for IRAC 5.8 $\mu$ m. The two very low-mass stars and the eleven brown dwarfs in the area are marked with white circles. The brightest blueish star is HD 294278, a K2-type foreground dwarf. The remaining bright greenish and orange stars are known classical T Tauri stars in $\sigma$ Orionis, like TX Ori and V505 Ori.

In the text

$\begin{figure} \par\includegraphics[width=8.8cm]{6993f05.eps}\end{figure}$	Figure 5: [3.6]-[8.0] vs. $J-K_{\rm s}$ colour-colour diagram. The symbols are as in Figs. 2 and 4. The dotted line marks the boundary between objects with and without discs. Field ultracool dwarfs with spectral types in the range M3.0-L5.0 from Patten et al. (2006) are shown with open (black) squares.
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$\begin{figure} \par\includegraphics[width=18cm]{6993f06.eps}\end{figure}$	Figure 6: Spectral energy distributions from the I band to the [8.0] band of nine representative substellar objects in $\sigma$ Orionis (filled stars). The open squares indicate the SEDs of ultracool field dwarfs of similar spectral type, normalised at the J band.
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$\begin{figure} \par\includegraphics[width=8.8cm]{6993f07.eps}\end{figure}$	Figure 7: Luminosity function in the J-band. Note the scarcity of objects at $J \sim$ 16.0 mag.
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$\begin{figure} \par\includegraphics[width=8.8cm]{6993f09.eps}\end{figure}$	Figure 9: Normalised cumulative number of objects ( $\sum _i i/N$ ) vs. mass. Filled circles indicate the most probable masses for 3 Ma and 360 pc. The error bars indicate the masses for the extreme cases of a younger closer cluster (1 Ma and 300 pc) and of an older farther cluster (5 Ma and 430 pc). Dotted vertical lines denote the hydrogen (left) and deuterium (right) burning mass limits.
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