All Figures

$\begin{figure} \par\includegraphics[width=7.9cm,clip]{ps/1124_f1.ps} \end{figure}$ Figure 1: The radial profile of the PSF for the target star HIP53701. The observations are obtained using the ADONIS/SHARPII+ system on June 7, 2001 in J, H, and $K_{\rm S}$ . The corresponding Strehl ratios for these observations are 18% in J, 26% in H and 31% in $K_{\rm S}$ .
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In the text

$\begin{figure} \par\includegraphics[width=8.2cm,clip]{ps/1124_f2.ps} \end{figure}$ Figure 2: An example illustrating the observing strategy. Observations are obtained with ADONIS/SHARPII+ system in J, H, and $K_{\rm S}$ . Four observations of each star are made, each observed with a different offset, in order to maximize the field of view. The frame size of the individual quadrants is $12.76'' \times 12.76''$ . Combination of the quadrants results in an effective field of view of $19.1'' \times 19.1''$ . This figure shows $K_{\rm S}$ band observations of the A0 V star HIP75957. Two stellar components at angular separations of 5.6'' and 9.2'' (indicated with the black circles) are detected. These components are visible to the left (East) and bottom-right (South-West) of HIP75957 and are probably background stars. All other features are artifacts.
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In the text

$\begin{figure} \par\includegraphics[width=7.1cm,clip]{ps/1124_f3.ps} \end{figure}$ Figure 3: Angular separation between the target stars and observed components as a function of the component's $K_{\rm S}$ magnitude ( top) and component's $K_{\rm S}$ magnitude relative to that of the target star ( bottom). The figures show the 41 new (dots) and 33 previously known (plusses) companion stars ( $K_{\rm S} < 12~{\rm mag}$ ), respectively. The triangles are the 77 background stars ( $K_{\rm S} > 12~{\rm mag}$ ; Sect. 3). The solid curves show the estimated detection limit as a function of angular separation. These figures clearly demonstrate that faint close companions are not detected because of the extended PSF halo of the target star. The effective field of view is $19.1'' \times 19.1''$ . Companion star stars with angular separations larger than $\frac{1}{2}\sqrt{2} \cdot 19.1'' = 13.5''$ cannot be detected. For small angular separation ( $\rho < 3.5''$ ) no faint ( $K_{\rm S} > 12~{\rm mag}$ ) components are found. This could indicate a gap in the companion mass function, or a lower mass limit for companion stars at small separation (see Sect. 4.3).
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In the text

$\begin{figure} \par\includegraphics[width=7cm,clip]{ps/1124_f4.ps} \end{figure}$ Figure 4: The number of objects per angular separation bin versus angular separation (bin size 2''). The distribution of presumed background stars (dotted histogram) is clearly different from that of the companion stars (solid histogram). The expected angular separation distribution for background stars (in units of stars per bin size) is represented with the solid curve. The curve is normalized such that the number of stars corresponds to the expected number of background stars with $12~{\rm mag} < K_{\rm S} < 16~{\rm mag}$ (cf. Fig. 5). The excess of close ( $\rho \lesssim 6''$ ) companions could be caused by faint companion stars that are misclassified as background stars. Objects with $\rho < 0.1''$ are too deep in the PSF halo of the primary to be detectable. Objects with $\rho > 15''$ cannot be measured due to the limited field of view of the ADONIS/SHARPII+ system.
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In the text

$\begin{figure} \par\includegraphics[width=7.1cm,clip]{ps/1124_f5.ps} \end{figure}$ Figure 5: The expected background star distribution as a function of angular separation and the $K_{\rm S}$ magnitude distribution of background stars as observed by Shatsky & Tokovinin (2002). The distribution of background stars over the field of view is assumed to be homogeneous. The triangles represent the observed stellar objects that we classify as background stars. The contours indicate the background star density in units of arcsec^-1mag^-1. The distribution is normalized, so that for $14~{\rm mag} \leq K_{\rm S} \leq 15~{\rm mag}$ and $5'' \leq \rho \leq 13''$ the number of background stars equals 19, i.e. the observed number of background stars with these properties. Observational biases are not considered. The dashed line represents our criterion to separate companion stars ( $K_{\rm S} \leq 12~{\rm mag}$ ) and background stars ( $K_{\rm S} > 12~{\rm mag}$ ). The excess of background stars at small angular separations could be caused by faint companion stars that are misclassified as background stars.
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In the text

$\begin{figure} \par\includegraphics[width=7.3cm,clip]{ps/1124_f6.ps} \end{figure}$ Figure 6: Top: the distribution of presumed companion stars over angular separation and position angle. Previously known companions and new companions are indicated with the plusses and dots, respectively. Bottom: the distribution of background stars over angular separation and position angle. The distribution of companion stars and background stars over position angle is random for angular separation smaller than 9.6'' (see text). The companion stars are more centrally concentrated than the background stars.
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In the text

$\begin{figure} \par\includegraphics[width=15cm,clip]{ps/1124_f7.ps} \end{figure}$ Figure 7: The distribution of 74 detected candidate companions ( top) and the 77 presumed background stars ( bottom) over Galactic coordinates. The number of companions and background stars found per target star is indicated by the size of the diamonds and triangles, respectively. Hipparcos member stars that were not observed are shown as dots. The borders of the three subgroups of Sco OB2 (Upper Scorpius, Upper Centaurus Lupus, and Lower Centaurus Crux) are indicated by the dashed lines. The background stars are concentrated towards the location of the Galactic plane (see also Fig. 8).
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In the text

$\begin{figure} \par\includegraphics[width=7.2cm,clip]{ps/1124_f8.ps} \end{figure}$ Figure 8: The distribution of the 74 detected candidate companion stars (solid histogram) and 77 presumed background stars (dotted histogram) as a function of Galactic latitude. The dashed histogram represents the distribution of Hipparcos member stars over Galactic latitude. The bin size is $5^\circ$ for the three histograms. The distributions are clearly different: the companion star and Hipparcos member star distribution peak at about the central Sco OB2 latitude. As expected, the background star distribution peaks at the location of the Galactic plane.
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In the text

$\begin{figure} \par\includegraphics[width=3.7cm,clip]{ps/1124_f9a.ps}\hspace*{3mm} \includegraphics[width=3.7cm,clip]{ps/1124_f9b.ps} \end{figure}$ Figure 9: Left: the hierarchical triple HIP68532. Both companion stars (indicated with the white dots) were previously undocumented. Right: HIP69113, another multiple system, where both close companion stars (white dots) were previously undocumented. A third known companion star with angular separation of 28'' is not detected in our survey because of the large angular separation.
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In the text

$\begin{figure} \par\includegraphics[width=16.4cm,clip]{ps/1124_f10.ps} \end{figure}$ Figure 10: The color-magnitude diagram of the primaries, companion stars and background stars for which we obtained ADONIS/SHARPII+ multi-color AO observations. The $M_{K_{\rm S}}$ magnitudes are corrected for distance and extinction. Companion stars in the three different subgroups are indicated with plusses (US), asterisks (UCL) and crosses (LCC). Target stars and background stars are indicated with diamonds and triangles, respectively (see Sect. 3). The curves represent isochrones of 5 Myr (solid curve) and 20 Myr (dotted curve). 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. The mass scale is indicated for the 20 Myr isochrone ( left) and the 5 Myr isochrone ( right). The brown dwarf candidates identified by Martín et al. (2004) are indicated as dots. The median formal errors are indicated as error bars in the top-right corner of each plot. The locations of the target stars and companion stars are consistent with the isochrones (within the error bars). The presumed background stars are located far away from the isochrones, implying that our criterion to separate companion stars and background stars is accurate.
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In the text

$\begin{figure} \par\includegraphics[width=7.4cm,clip]{ps/1124_f11.ps} \end{figure}$ Figure 11: Top: the relation between mass and $M_{K_{\rm S}}$ magnitude. The 5 Myr isochrone (solid curve) is used to find masses of the companion stars in US, while the 20 Myr isochrone (dotted curve) is used for theUCL and LCC subgroups. The gray-shaded area is enclosed by the 15 Myr and 23 Myr isochrones, and represents the uncertainty inthe age of the UCL and LCC subgroups. For objects at the mean Sco OB2 distance of 130 pc, $M_{K_{\rm S}}=6$ corresponds to $K_{\rm S}=11.6$ . Bottom: the derivative of the mass-magnitude relation. The conversion from $M_{K_{\rm S}}$ to mass is most accurate where the absolute value of the derivative ${\rm d}M/ {\rm d}M_{K_{\rm S}}$ is small.
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In the text

$\begin{figure} \par\includegraphics[width=7.4cm,clip]{ps/1124_f12.ps} \end{figure}$ Figure 12: Top: the mass distribution of the 74 companion stars (solid histogram) and corresponding primaries (dashed histogram) observed in our AO survey. The bin size is $0.5~{M}_\odot$ . Masses are derived from their $K_{\rm S}$ magnitude and age (see Sect. 4.3). The median errors in the mass are $0.4~{M}_\odot$ for the primaries and $0.1~{M}_\odot$ for the companions. Bottom: companion star mass versus angular separation. New and previously known companion stars are indicated as dots and plusses, respectively. Low-mass close companion stars are not detected due to the PSF wings of the corresponding primary star.
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In the text

$\begin{figure} \par\includegraphics[width=7.2cm,clip]{ps/1124_f13.ps} \end{figure}$ Figure 13: Companion star mass versus primary star mass. The dashed lines represents the q=0.25, q=0.5, and q=1 binaries and are shown to guide the eye. The 41 new companion stars and the 33 previously known companion stars are indicated with dots and plusses, respectively. The observed companion stars have masses lower than their associated primaries. Most systems with previously undocumented companion stars have q < 0.25.
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In the text

$\begin{figure} \par\includegraphics[width=7.5cm,clip]{ps/1124_f14.ps}\hspace{0cm} \end{figure}$ Figure 14: Top: the mass ratio distribution for the 74 systems for which we observe companion stars in our near-infrared AO survey (histogram). The mass ratio is defined as $q = M_{\rm c}/M_{\rm p}$ where $M_{\rm c}$ is the companion star mass and $M_{\rm p}$ the mass of the corresponding primary star. All primaries are Hipparcos member stars with A and late-B spectral type. The q distribution does not change significantly if ages of 15 Myr or 23 Myr for UCL and LCC are assumed (gray-shaded areas). The mass ratio distributions $f(q) = q^{-\Gamma }$ are represented by the curves. For our observations we find $\Gamma = 0.33$ (dotted curve). Shatsky & Tokovinin (2002) find $\Gamma = 0.5$ (dashed curve). The distribution which follows from random pairing (dash-dotted curve) is clearly excluded. The curves are normalized using the observed mass ratio distribution for the companion stars in the range 0.5 < q < 1.0. Bottom: the observed cumulative mass ratio distribution (histogram). The dotted and dashed curve are cumulative distribution functions corresponding to $\Gamma = 0.33$ and $\Gamma = 0.5$ , respectively. The fitted function described in Eq. (4) is shown as the thin solid curve. The dash-dotted curve represents the cumulative q distribution for random pairing.
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In the text

$\begin{figure} \par\includegraphics[width=7.6cm,clip]{ps/1124_f15.eps} \end{figure}$ Figure 15: The fraction of stellar systems which is multiple versus the spectral type of the primary, for the three subgroups of Sco OB2. Only confirmed Hipparcos member primaries are considered here. The binarity dataset consists of literature data and our near-infrared AO survey. The light and dark grey parts of the bars correspond to literature data and the new data presented in this article, respectively. The spectral types of the companion stars (not included in this plot) are always later than those of the primary stars. Apparently, the multiplicity is a function of spectral type, but this conclusion may well be premature when observational biases are not properly taken into account. That this is at least partly true, is supported by our detection of 41 new close companion stars at later spectral types.
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In the text

$\begin{figure} \par\includegraphics[width=7.9cm,clip]{ps/1124_f1.ps} \end{figure}$	Figure 1: The radial profile of the PSF for the target star HIP53701. The observations are obtained using the ADONIS/SHARPII+ system on June 7, 2001 in J, H, and $K_{\rm S}$ . The corresponding Strehl ratios for these observations are 18% in J, 26% in H and 31% in $K_{\rm S}$ .
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$\begin{figure} \par\includegraphics[width=3.7cm,clip]{ps/1124_f9a.ps}\hspace*{3mm} \includegraphics[width=3.7cm,clip]{ps/1124_f9b.ps} \end{figure}$	Figure 9: Left: the hierarchical triple HIP68532. Both companion stars (indicated with the white dots) were previously undocumented. Right: HIP69113, another multiple system, where both close companion stars (white dots) were previously undocumented. A third known companion star with angular separation of 28'' is not detected in our survey because of the large angular separation.
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