All Figures

$\begin{figure} \begin{tabular}{ccc} \includegraphics[width=4cm,clip]{ps/4396_f1... ...\includegraphics[width=4cm,clip]{ps/4396_f1c.ps}\\ \end{tabular} \end{figure}$ Figure 1: With our NACO survey we find three close companions (shown in the figure), which were not detected in our ADONIS survey. The panels ( $6.6'' \times 6.6'')$ are centered on the primary stars. Left: the binary HIP 63204 in $K_{\rm S}$ , with a companion at angular separation 0.15'' and a background star at angular separation 1.87''. Middle: the binary HIP 73937 in $K_{\rm S}$ , with a close companion at $\rho =0.24''$ and a background star at $\rho =3.56''$ . Right: HIP 79771 in $K_{\rm S}$ , with two companion stars at $\rho =0.44''$ and $\rho =3.67''$ . Companions are indicated with white arrows and background stars with black arrows. Several artifacts are visible in the fields of HIP 63204 and HIP 79771, which can easily be recognized as such. The panels show a subset of the total field of view for each observation, which is $14'' \times 14''$ . For these three targets we observe no stellar components other than those shown in the panels.
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In the text

$\begin{figure} \par\includegraphics[width=6.5cm,clip]{ps/4396_f2.ps} \end{figure}$ Figure 2: The radial profile of the PSF for the target star HIP 78968. The observations are obtained using the NACO system in the night of May 5, 2004 in J, H, and $K_{\rm S}$ . The corresponding Strehl ratios for these observations are 6.5% in J, 15.6% in H, and 23.6% in $K_{\rm S}$ . Observations are obtained with neutral density filter (NDF; solid curves) and without NDF (dotted curves). The profiles are normalized such that the peak flux for the images obtained with NDF is 100 in J, 10 in H, and 1 in $K_{\rm S}$ . The observations with NDF allow us to extract the PSF and measure the flux of the primary star. The images obtained without NDF are much deeper, but the primary is usually saturated. Assuming the PSF is similar, we use the non-saturated PSF to analyze the secondaries in the saturated image.
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In the text

$\begin{figure} \par\includegraphics[width=7.1cm,clip]{ps/4396_f3.ps} \end{figure}$ Figure 3: The detection limit and completeness limit for several targets in our ADONIS survey (Kouwenhoven et al. 2005) and NACO survey (this paper). The target star, Strehl ratio, and instrument are indicated in each panel. The lower and upper curve show the detection limit and the completeness limit, respectively. The detection and completeness limits shown above are representative for the ADONIS and NACO observations. The target stars have $K_{\rm S}$ magnitudes of 7.33 mag (HIP 81949), 7.42 mag (HIP 78968), 7.45 mag (HIP 80799), 6.52 mag (HIP 58859), 6.26 mag (HIP 76048), and 6.71 mag (HIP 65178). The completeness limit is generally $\sim$ 0.3 mag brighter than the detection limit. At close angular separation a higher Strehl ratio results in a fainter detection limit. The dashed rectangle encloses the region with $12~\mbox{mag} \leq K_{\rm S} \leq 14~\mbox{mag}$ and $1'' \leq \rho \leq 4''$ , which is relevant for our analysis of the substellar population in Sco OB2 (see Sect. 6). In this region the NACO observations are complete and ADONIS observations are more than 95% complete.
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In the text

$\begin{figure} \par\begin{tabular}{cc} \includegraphics[width=7.1cm,clip]{ps/4... ...ncludegraphics[width=7.1cm,clip]{ps/4396_f4b.ps}\\ \end{tabular} \end{figure}$ Figure 4: The color-color diagram ( left) and color-magnitude diagram ( right) of the objects in our sample. Measurements are shown for the 22 targets observed with NACO and for the 9 targets with multi-color observations in the ADONIS sample. Both panels show target stars (circles), confirmed and candidate companions (squares), and background stars (triangles). The target stars and secondaries in the US subgroup are indicated with filled symbols; those from UCL and LCC are indicated with open symbols. The $1\sigma$ error bars are indicated for all data points. Lower limits are given for objects that are not detected in all three filters. Several detected objects are outside the ranges of the figures; these are all background stars. The status (companion or background star) of the secondaries is discussed in Sect. 4.2. The 5 Myr isochrone for US and the 20 Myr isochrone for UCL and LCC are indicated with the solid and dotted curves, respectively.
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In the text

$\begin{figure} \par\includegraphics[width=14cm,clip]{ps/4396_f5.ps} \end{figure}$ Figure 5: The (absolute) color-magnitude diagrams for the 22 targets in our NACO sample and the 9 targets with multi-color observations in the ADONIS sample. The results are split into the three subgroups US ( top), UCL ( middle), and LCC ( bottom). The primary stars are indicated with circles; the confirmed companions with large squares, and the candidate companions with small squares. The $M_{K_{\rm S}}$ magnitude is derived from the $K_{\rm S}$ magnitude by correcting for distance and extinction for each target individually. The solid curves represent isochrones of 5 Myr (for US) and 20 Myr (for UCL and LCC). The 15 Myr and 23 Myr isochrones enclose the gray-shaded area and represent the uncertainty in the age of the UCL and LCC subgroups. For each data point we indicate the $1\sigma$ errors. The photometry of the observed objects cannot be used to distinguish between the subgroups of Sco OB2, due to the errors and the small sample. The free-floating brown dwarfs in US identified by Martín et al. (2004) are indicated with triangles, adopting a distance of 145 pc.
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In the text

$\begin{figure} \par\includegraphics[width=6.8cm,clip]{ps/4396_f6.ps} \end{figure}$ Figure 6: The expected number of background stars brighter than $K_{\rm S}$ at angular separation smaller than $\rho$ for the combined sample of 199 targets (solid contours). Overplotted are the background stars detected around the 22 NACO targets (filled triangles), and those found around the 177 targets observed with ADONIS only (open triangles). The observed cumulative number of background stars is indicated with the dotted contours, for values of 10 and 50 background stars, respectively. In Kouwenhoven et al. (2005) we classify secondaries brighter than $K_{\rm S}=12$ mag (upper horizontal line) as candidate companions, and those fainter than $K_{\rm S}=12$ mag as probable background stars. The faintest objects we detect in our ADONIS and NACO surveys have $K_{\rm S}$ magnitude of approximately 15 and 17 magnitudes, respectively (lower horizontal lines).
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In the text

$\begin{figure} \par\includegraphics[width=6.8cm,clip]{ps/4396_f7.ps} \end{figure}$ Figure 7: The $\chi ^2$ distance to the isochrone in the $(J-K_{\rm S},M_{K_{\rm S}})$ diagram versus $N(K_{\rm S},\rho )$ , where $N(K_{\rm S},\rho )$ is the expected number of background stars brighter than $K_{\rm S}$ and angular separation smaller than $\rho$ . The vertical dashed lines correspond to $\chi ^2=2.30$ ( $1\sigma$ ; left) and $\chi ^2=11.8$ ( $3\sigma$ ; right). The symbols represent the confirmed companions (filled squares), the candidate companions (open squares), and background stars (triangles). Note that the classification of a secondary is based on the $\chi ^2$ values for the different color-magnitude diagrams; not only for the $\chi ^2$ of the $(J-K_{\rm S},M_{K_{\rm S}})$ -diagram, which is shown above. The expected number of background stars in a field (vertical axis) is used as a consistency check. The horizontal dotted line represents the 1% filter used by Poveda et al. (1982) to separate companion stars (below the line) and background stars (above the line). The 1% filter is a reasonable method when multi-color observations are not available, but is not used in our study. The triangle in the lower-right quadrant represents HIP 63204-1 (see Sect. 4.4).
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In the text

$\begin{figure} \par\includegraphics[width=6.8cm,clip]{ps/4396_f8.ps} \end{figure}$ Figure 8: Companion star magnitude $K_{\rm S}$ ( top) and magnitude difference $\Delta K_{\rm S}$ ( bottom) versus angular separation for the combined ADONIS and NACO datasets. The symbols represent confirmed companions (filled squares), candidate companions (open squares), and background stars (triangles); see Sect. 4.2 for further information on this classification. The horizontal line corresponds to $K_{\rm S}=12$ mag, the criterion used by Kouwenhoven et al. (2005) to separate companion stars and background stars. Typical detection and completeness limits corresponding to the observations are shown in Fig. 3. For a given $K_{\rm S}$ magnitude, the number of background stars closer than angular separation $\rho$ is given by Eq. (3). This figure clearly shows the dearth of brown dwarf companions with $1'' < \rho < 4''$ around A and late-B stars in Sco OB2 (region indicated with the dashed rectangle).
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In the text

$\begin{figure} \includegraphics[width=12cm,clip]{ps/4396_f9.ps} \end{figure}$ Figure 9: The companion mass distribution f_M₂(M₂) for a simulated association. In each panel we show the distribution f_M₂(M₂) of an association consisting of 50 000 binaries for which the primary mass distribution is given by Eq. (7). From left to right, the panels show the distribution of companion mass in all binaries with stellar primaries, for those of G and K primaries, and for those of A and B primaries, respectively. In the top panels we adopted a mass ratio distribution $f_q(q) \propto q^{-0.33}$ with $0 < q \leq 1$ . In the bottom panels we adopt the same distribution, but with the additional constraint that $M_2 \geq 0.02$ $M_\odot$ . The brown dwarf regime is indicated with the dashed lines. This figure shows that, even though the mass ratio distribution is strongly peaked to low values of q, the substellar-to-stellar companion ratio among intermediate- and high-mass stars is very low.
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In the text