All Figures

$\begin{figure} \par\includegraphics[width=6.15cm,clip]{8116f1.ps}\end{figure}$ Figure 1: Colour-magnitude diagram of low-mass $\sigma$ Orionis member candidates. Our objects with flux excesses at 5.8 and 8.0 $\mu$ m are plotted as large filled circles (some are labelled). Cluster members with flux excesses reported in Caballero et al. (2007) are indicated with filled circles of intermediate size. Masses in Jovian units are given on the right hand side. Overplotted with a solid line is the 3-Myr isochrone from Baraffe et al. (2002, see text). Current IRAC/ Spitzer data cannot provide strong constraints on the presence of flux excesses below J = 18.5 mag.
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In the text

$\begin{figure} \par\includegraphics[width=6.15cm,clip]{8116f2.ps}\end{figure}$ Figure 2: Colour-colour diagram of $\sigma$ Orionis low-mass member candidates. Objects with flux excesses at 5.8 and 8.0 $\mu$ m are indicated with filled circles; objects with no IRAC/ Spitzer flux excesses are depicted with open circles. The planetary-mass objects of Table 1 are shown with large filled circles (some are labeled). Arrows (upper limits) stand for cluster planetary-mass candidates that remain undetected in the [8.0]-band. Overplotted (solid line) is the field sequence of discless late-M and L dwarfs. Except for our objects, error bars are avoided for clarity.
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In the text

$\begin{figure} \par\includegraphics[width=4.2cm,clip]{8116f3a.ps}\includegraphic... ...f.ps}\includegraphics[width=4.2cm,clip]{8116f3g.ps}\hspace*{2.1cm}} \end{figure}$ Figure 3: Spectral energy distribution ( $IJHK_{\rm s}$ , [3.6], [4.5], [5.8], and [8.0], solid line, filled circles) of the seven $\sigma$ Orionis isolated planetary-mass objects for which IRAC/ Spitzer flux excesses have been detected. Upper limits are indicated with arrows. All data are normalized to 1.0 in the J-band. For comparison purposes, the average energy distributions of discless M8 to L4 field dwarfs are overplotted with dotted lines.
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In the text

$\begin{figure} \par\includegraphics[width=6.15cm,clip]{8116f4.ps}\end{figure}$ Figure 4: Inner disc frequency of $\sigma$ Orionis stars, brown dwarfs and planetary-mass objects as a function of mass. Measurements from Hernández et al. (2007) and Caballero et al. (2007) are plotted as squares and triangles, respectively. Our determination for the planetary-mass regime of the cluster is indicated with an arrow (lower limit, see text). Note that all measurements are obtained from the analysis of IRAC/ Spitzer data. Horizontal error bars account for mass intervals.
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In the text