Astronomy & Astrophysics

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$\begin{figure} \par\includegraphics[width=7.6cm,clip]{2164fg01.eps}\end{figure}$ Figure 1: I, I-zcolour-magnitude diagram built using long (3 $\times$ 360 s) exposures of NGC 2264. The solid isochrones are DUSTY models for ages of 2 and 5 Myr; the dashed isochrones are the NextGen models for the same ages. All isochrones are for distance modulus 9.4; 760 pc. 236 BD candidates (squares) were selected to be redward of the sloping straight line (long dash). Small dots redward of this line were rejected from the candidacy on visual inspection. Typical error bars for I=22 of $\sim$ 0.1 mag are shown. The mass scale (in $M_{\odot }$ ) is for the 2 Myr models; it and the estimated completeness limit are indicated.
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In the text

$\begin{figure} \par\includegraphics[width=7.6cm,clip]{2164fg02.eps}\end{figure}$ Figure 2: All 79 $IzJHK_{\rm s}$ candidates (open circles), plotted in the I-J, $J-K_{\rm s}$ diagram. The errorbar assumes $JHK_{\rm s}$ accurate to $\pm$ 0.3 mag and I to $\pm$ 0.1 mag The reddening vector is shown. The solid lines are observed loci of dwarfs taken from a compilation of sources (see text). The dash-dot line is a the locus of G0-M 5 giants from Bessell & Brett (1988). $A_{\rm v}$ has been computed by dereddening all points to the extrapolated (dotted) locus of dwarfs. After dereddening, all objects are potentially either cluster or field M-dwarfs.
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In the text

$\begin{figure} \par\includegraphics[width=7.6cm,clip]{2164fg03.eps}\end{figure}$ Figure 3: Similar to Fig. 2, but for $JHK_{\rm s}$ . Since the colours plotted here vary relatively less among the sample, the errorbars appear larger. Therefore, a distinction has been made between the best 2MASS photometry (all magnitudes flagged A, filled points) and those with any magnitude flagged B or C. For the former, $JHK_{\rm s}$ are taken accurate to $\pm$ 0.1 mag, for the latter, $\pm$ 0.3 mag, hence the errorbars plotted. Again, $A_{\rm v}$ has been calculated by dereddening to the extrapolated (dotted) locus of observed late-type dwarf colours. The locus of giant colours is also plotted (dash-dot line).
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In the text

$\begin{figure} \par\includegraphics[width=7.95cm,clip]{2164fg04.eps}\end{figure}$ Figure 4: To check for obvious $K_{\rm s}$ -band excesses which may affect our derivation of $A_{\rm v}$ , we have plotted all 79 candidates in the $I-J,I-K_{\rm s}$ diagram. Clearly, no objects have significant excesses.
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In the text

$\begin{figure} \par\includegraphics[width=7.9cm,clip]{2164fg05.eps}\end{figure}$ Figure 5: Comparison of $A_{\rm v}$ derived by the two different methods indicated by the axis labels and detailed in the text. Both methods yield very similar values. The dotted line denotes equal $A_{\rm v}$ from each method. We have singled out eight objects with low reddening, lying to the left of the sloping line, plus two more objects highlighted by open squares, for particular discussion.
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In the text

$\begin{figure} \par\includegraphics[width=7.9cm,clip]{2164fg06.eps}\end{figure}$ Figure 6: The $I-J,I-K_{\rm s}$ diagram, after dereddening. All objects lie close to the theoretical DUSTY and/or NextGen isochrones (dotted and dashed lines, respectively). The substellar limit for suggested by each set of models is indicated; with reference to DUSTY models, many objects are potentially substellar. The typical errors are the same as in Fig. 4.
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In the text

$\begin{figure} \par\includegraphics[width=7.8cm,clip]{2164fg07.eps}\end{figure}$ Figure 7: The sample, plotted in the $I-K_{\rm s},J-H$ diagram, after dereddening. The dotted lines represent DUSTY isochrones for ages 1 and 5 Myr; the dashed lines are NextGen models for 2 and 5 Myr. The dash-dot line is a locus of observed dwarf colours. The errorbar in J-H is large, but mass scales for the mean of each pair of isochrones are derived from the much more diagnostic $I-K_{\rm s}$ colour, and are shown for DUSTY models (at left) and NextGen (at right). Note that the errorbars do not reflect uncertainties in $A_{\rm v}$ itself in this or subsequent figures (until Fig. 11, see Sect. 4.3). The whole sample is represented by open circles; points with the best 2MASS photometry (all magnitudes flagged A) are doubly overplotted open symbols. The small filled squares are 65 candidate late-type cluster members (Lamm et al. 2004) recovered in our sample (see text); bold triangles are confirmed variable PMS objects drawn from the same sample by these authors.
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In the text

$\begin{figure} \par\includegraphics[width=7.9cm,clip]{2164fg08.eps}\end{figure}$ Figure 8: As Fig. 7, but for dereddened $I-K_{\rm s},J-K_{\rm s}$ . Again, mass scales are derived using the $I-K_{\rm s}$ colour, with typical errobar shown. The error in $J-K_{\rm s}$ is large, but it is seen that most candidates lie in a space quite consistent with either NextGen or DUSTY models for the cluster (assumed distance modulus 9.4).
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In the text

$\begin{figure} \par\includegraphics[width=7.8cm,clip]{2164fg09.eps}\end{figure}$ Figure 9: The dereddened $I,I-K_{\rm s}$ diagram. The 65 objects we have in common with the sample of Lamm et al. (2004) are plotted as small filled squares; objects with 2MASS photometry flagged AAA are overplotted as open octagons. The errorbar for I is $\pm$ 0.1 mag which is an overestimate for the brighter part of the sample. Note that the errorbars do not take into account uncertainties in $A_{\rm v}$ ; i.e. this quantity is treated as exact. Large open triangles indicate the four previously identified NGC 2264 members from Fig. 7. Mass scales are shown for both NextGen and DUSTY models (dashed and dotted lines, respectively). The long/short dashed line is the zero-age main sequence (see text). The dash-dot line is a 5 Gyr field model, chosen for distance modulus 4 (63 pc), chosen to fit the 2 objects with $I-K_{\rm s}>3.8$ which are discussed in the text. All other objects have locations with respect to the isochrones consistent with them being cluster members with masses above 0.1 $M_{\odot }$ (NextGen) but the existence of a few objects very close to the substellar limit is not ruled out by the DUSTY models. The most likely such objects are overplotted by bold squares, and will be discussed in the text. Bold hexagons highlight two further objects whose candidacy will be discussed.
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In the text

$\begin{figure} \par\includegraphics[width=7.8cm,clip]{2164fg10.eps}\end{figure}$ Figure 10: As for Fig. 9, for dereddened I,I-J.
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In the text

$\begin{figure} \par\includegraphics[width=7.9cm,clip]{2164fg11.eps}\end{figure}$ Figure 11: Dereddened I,I-J diagram illustrating the likely uncertainties on masses once 2MASS photometric errors are propagated through the dereddening process. The bold squares have J-mag flag A for which we adopt $\Delta$ $A_{\rm v}=\pm 1$ mag, hence $\Delta$ I is $\pm$ 0.5 mag, $\Delta$ $J\pm 0.3$ mag and $\Delta$ $I-J \pm 0.8$ mag maximum. For the other objects (triangles; J flag B, hexagons; J flag C), we adopt $\Delta$ $A_{\rm v}=\pm 2$ and 3 mag respectively, with correspondingly larger errorbars in I and I-J. Dash-dot lines are 5 Gyr NextGen isochrones for distance moduli 4 (as in Fig. 10) and 6. Other isochrones are as Fig. 10.
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In the text

$\begin{figure} \par\includegraphics[width=8.3cm,clip]{2164fg12.ps}\end{figure}$ Figure 12: Spatial location of all 79 candidate objects plotted on the optical (R-band) DSS image which spans $\sim$ $40\hbox {$^\prime $ }\times 40\hbox {$^\prime $ }$ . The 10 objects discussed in Sects. 4.1 and 4.2 are overplotted as larger 20 $^{\prime \prime }$ circles. The fields of view of the two 12K fields observed are shown, and extend in RA beyond the DSS image. North is up and East to the left.
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In the text

$\begin{figure} \par\includegraphics[width=7.95cm,clip]{2164fg04.eps}\end{figure}$	Figure 4: To check for obvious $K_{\rm s}$ -band excesses which may affect our derivation of $A_{\rm v}$ , we have plotted all 79 candidates in the $I-J,I-K_{\rm s}$ diagram. Clearly, no objects have significant excesses.
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$\begin{figure} \par\includegraphics[width=7.9cm,clip]{2164fg06.eps}\end{figure}$	Figure 6: The $I-J,I-K_{\rm s}$ diagram, after dereddening. All objects lie close to the theoretical DUSTY and/or NextGen isochrones (dotted and dashed lines, respectively). The substellar limit for suggested by each set of models is indicated; with reference to DUSTY models, many objects are potentially substellar. The typical errors are the same as in Fig. 4.
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$\begin{figure} \par\includegraphics[width=7.9cm,clip]{2164fg08.eps}\end{figure}$	Figure 8: As Fig. 7, but for dereddened $I-K_{\rm s},J-K_{\rm s}$ . Again, mass scales are derived using the $I-K_{\rm s}$ colour, with typical errobar shown. The error in $J-K_{\rm s}$ is large, but it is seen that most candidates lie in a space quite consistent with either NextGen or DUSTY models for the cluster (assumed distance modulus 9.4).
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$\begin{figure} \par\includegraphics[width=7.8cm,clip]{2164fg10.eps}\end{figure}$	Figure 10: As for Fig. 9, for dereddened I,I-J.
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