All Tables

Table 1: Multiplicity among Hipparcos members of the three subgroups of Sco OB2. The columns show the subgroup name (Upper Scorpius; Upper Centaurus Lupus; Lower Centaurus Crux), the distance (see de Zeeuw et al. 1999), the age (Preibisch et al. 2002; de Geus et al. 1989 for US; Mamajek et al. 2002 for UCL and LCC), the number of known single stars, binary stars, triple systems and N>3 systems, and the binary statistics (see Sect. 7), after inclusion of the new results presented in this paper.
Table 2: We have obtained follow-up multi-color observations with NACO for 22 targets in the Sco OB2 association. We include in our analysis 9 targets with multi-color observations in the ADONIS sample. All targets listed above are known to have secondaries in the ADONIS survey. The table lists for each star the parallax and error (taken from de Bruijne 1999), the $K_{\rm S}$ magnitude, and the spectral type of the primary star. The $K_{\rm S}$ magnitudes are those derived in this paper for the 22 stars observed with NACO, and are taken from Kouwenhoven et al. (2005) for the other nine stars. The last column shows the subgroup membership of each star (US = Upper Scorpius; UCL = Upper Centaurus Lupus; LCC = Lower Centaurus Crux), taken from de Zeeuw et al. (1999).
Table 3: Criteria adopted to separate the secondaries with multi-color observations into confirmed companions, candidate companions, and background stars. The $\chi ^2$ values of 2.30 and 11.8 correspond to the $1\sigma$ and $3\sigma$ levels. This means that (statistically) 68.3% of the physical companions have $\chi ^2 < 2.30$ and 99.73% of the physical companions have $\chi ^2 < 11.8$ . A secondary with $\chi ^2 < 2.30$ is very likely a companion star. A secondary with $\chi ^2 > 11.8$ is almost certainly a background star. The secondaries with $2.30 < \chi ^2 < 11.8$ may be companions or background stars.
Table 4: Properties of the 25 confirmed companion stars found around the 22 members in our NACO survey and the 9 members with multi-color observations in the ADONIS survey. The columns show the secondary designation, the J, H, and $K_{\rm S}$ magnitudes, the angular separation, and the position angle (measured from North to East). Magnitude lower limits are given if a secondary is not detected in a filter. We list the absolute magnitude and corresponding mass in columns 7-10. The 11th column lists the status of the object (c = confirmed companion star, nc = new confirmed companion star). The last column shows additional remarks. A "J'', "H'', or "K'' means that the secondary flux in this filter was obtained from the image obtained without the NDF, using the PSF from the corresponding image that was obtained with NDF (see Sect. 2.4). Properties of the observed primaries, candidate companions, and background stars are not shown here; these are listed in Table A.1.
Table 5: The distribution of companion stars over angular separation $\rho$ and $K_{\rm S}$ magnitude for our sample of 31 targets with multi-color observations. Each entry lists the number of confirmed companion stars, i.e., the secondaries with $\chi ^2 < 2.30$ . Between brackets we list the number of candidate companion stars, which have $2.30 < \chi ^2 < 11.8$ . We have also included the candidate companion HIP 80142-2 ( $\rho =5.88''$ ), for which no $K_{\rm S}$ measurement is available. Several candidate companions are likely to be background stars, especially faint candidates at large separation. In the region $1'' \leq \rho \leq 4''$ and $12~{\rm mag} \leq K_{\rm S} \leq 14$ mag (which we will study in Sect. 6) we find one confirmed companion and no candidate companions.
Table 6: The number of background stars expected in our ADONIS and NACO fields of view, based on the background star study of Shatsky & Tokovinin (2002). Column 1 lists the four regions for which we study the background statistics. The targets with Galactic latitude $\vert b\vert\leq 5^\circ$ are included in the region GP. The other targets are grouped according to their membership of US, UCL, and LCC. For the 177 targets only observed with ADONIS and the 22 targets observed with NACO we list the expected number of background stars per field of view, and the number of targets observed in the four regions. In total we expect to find $70.75 \pm 4.90$ background stars in the ADONIS sample, and $18.96 \pm 3.96$ in the NACO sample.
Table 7: Accuracy of the $K_{\rm S}=12$ mag criterion to separate companions and background stars. This table contains 69 out of the 72 secondaries with multi-color observations in the ADONIS or NACO dataset. Three secondaries (1 candidate companion; 2 background stars) for which no $K_{\rm S}$ magnitudes are available, are not included. The first column shows the status of the secondary (c = confirmed companion, ? = candidate companion, b = background star). Columns 2 to 5 list the distribution over status for secondaries with $K_{\rm S}<12$ mag and $K_{\rm S}>12$ mag. Depending on the true nature of the candidate companion stars, the $K_{\rm S}=12$ mag criterion correctly classifies the secondaries in $\sim$ 80% of the cases.
Table 8: The observed and expected number of brown dwarfs with $1'' \leq \rho \leq 4''$ and $12 \leq K_{\rm S} \leq 14$ mag for the sample of 199 target stars. The left columns shows the various models for which we simulated observations. Each model has a semi-major axis distribution $f_a(a) \propto a^{-1}$ with $15~R_\odot \leq a \leq 5\times 10^6$ $R_\odot$ , and a multiplicity fraction of $F_{\rm M} =100\%$ . Columns 3 and 4 show for a survey of intermediate mass stars (late-B and A stars; $1.4~{M}_\odot < M < 7.7~{M}_\odot$ ) the expected number of brown dwarfs $N_{\rm\star ,BD,IM}$ and the substellar-to-stellar companion ratio $R_{\rm\star ,IM}$ , both with $1\sigma$ errors. By comparing the predicted values of $N_{\rm\star ,BD,IM}$ and $R_{\rm\star ,IM}$ with the observations, we can exclude models 1, 2, and 4. In Kouwenhoven et al. (2005) we exclude random pairing from the Preibisch mass distribution (models 1-3) since these models are inconsistent with the observed mass ratio distribution of stellar companions. We additionally list the values $N_{\rm\star ,BD,LM}$ and $R_{\rm\star ,LM}$ that are expected for a survey amongst 199 low-mass stars ( $0.3~{M}_\odot < M < 1.4~{M}_\odot$ ) in Cols. 5 and 6. For models with $F_{\rm M} < 100\%$ the expected number of brown dwarfs reduces to $F_{\rm M} \times N_{\rm\star ,BD}$ , while R remains unchanged. Models with a smaller semi-major axis range and models with the log-normal period distribution found by Duquennoy & Mayor (1991) have a larger expected value of $N_{\rm\star ,BD,IM}$ , $N_{\rm\star ,BD,LM}$ . Under the assumption that companion mass and semi-major axis are uncorrelated, the values of $R_{\rm\star ,IM}$ and $R_{\rm\star ,LM}$ are equal to those listed above, if the log-normal period distribution is chosen.
Table 9: The observed and expected number of brown dwarfs with $1'' \leq \rho \leq 4''$ for the sample of 199 target stars. In this table we show the results for the full brown dwarf mass range (0.02 $M_\odot \leq M \leq 0.08$ $M_\odot$ ), unlike in Table 8, where we show the results for the brown dwarfs restricted to $12 \leq K_{\rm S} \leq 14$ mag.
Table 10: An estimate of the number of primordial binaries in Sco OB2 with A and late-B primaries and brown dwarf companions (with 0.02 $M_\odot \leq M \leq 0.08$ $M_\odot$ ) in the angular separation range $1'' \leq \rho \leq 4''$ . Columns 1 and 2 list the model number (cf. Table 8), and the fraction of binaries with angular separation $1'' \leq \rho \leq 4''$ (assuming Öpik's law). Column 3 lists the fraction $F_{\rm IM}$ of primaries in the simulated association that is of type A or late-B. Column 4 shows the contribution of brown dwarf companions in Sco OB2 with A and late-B primaries in the angular separation range $1'' \leq \rho \leq 4''$ , inferred from the 28 free-floating brown dwarfs in Sco OB2 found by Martín et al. (2004), assuming that all brown dwarfs were formed as companions. Column 5 shows the total number of primordial brown dwarf companions with $1'' \leq \rho \leq 4''$ and A or late-B primaries, with the observed brown dwarf companions (corrected for unseen brown dwarfs with K>14 mag) included. The values of $N_{\rm BD,IM,i,total}$ are upper limits, as it is likely that not all free-floating brown dwarfs were formed as companions.
Table A.1: Results from our multi-color binarity study among 22 Sco OB2 member stars observed with NACO ( top part of the table) and the subset of 9 members with multi-color observations in in the ADONIS survey ( bottom part of the table). The columns show the Hipparcos number (for the targets) and the secondary designation, the J, H, and $K_{\rm S}$ magnitudes, the angular separation, and the position angle (measured from North to East). Lower limits to the magnitudes are given if an object is not detected in the NACO survey, unless the ADONIS measurement was available (marked with a $\star$ ). Entries marked with $\star \star$ have no available measurement, e.g., because the object is not in the field of view for that filter. For each primary and companion star we list the absolute $JHK_{\rm S}$ magnitudes and the mass in Cols. 7-10. We additionally provide absolute magnitudes and a mass estimate for the candidate companions, under the assumption that these are indeed companions. We stress that a significant number of the candidate companions may actually be background stars. The 11th column lists the status of the object (p = primary, c = confirmed companion star, $\rm nc = new$ confirmed companion star, ? = candidate companion star; b = background star). The last column provides additional remarks. A remark "J'', "H'', or "K'' means that the secondary flux in this filter was obtained from the image obtained without the NDF, using the PSF from the corresponding image that was obtained with NDF (see Sect. 2.4). If the secondary status was obtained without color information, an exclamation mark is placed in the last column. The results for the 9 targets with multi-color information in the ADONIS survey are marked with "ADO''.
Table A.2: Criteria used to determine whether a secondary is a companion star or a background star. Results are listed for secondaries found around the 22 targets observed with NACO ( top part of the table) and the 9 targets with multi-color observations in the ADONIS dataset ( bottom part of the table). Columns 1 and 2 show the secondary designation and the status of the component as determined in this paper (c = companion star; ? = candidate companion star; b = background star). Columns 3-5 show the compatibility of the location of the object in the color-magnitude diagrams with the isochrones in terms of $\chi ^2$ . Confirmed companions have $\chi ^2 < 2.30$ and (confirmed) background stars have $\chi ^2 > 11.8$ . The other secondaries have $2.30 < \chi ^2 < 11.8$ and are labeled "candidate companion''. A substantial fraction of these candidate companions may in fact be background stars. Several faint ( $K_{\rm S} > 14$ mag) secondaries are only detected in one filter (thus have no $\chi ^2$ ), and are all assumed to be background stars.
Table A.3: All companion stars identified in our ADONIS and NACO binarity surveys among A and late-B stars in Sco OB2 (Kouwenhoven et al. 2005, and this paper). The columns show the Hipparcos number of the primary star, the $JHK_{\rm S}$ magnitudes, the angular separation, the position angle, the current status of the companion, and the date of observation (dd/mm/yy). If measurements are performed in both the ADONIS and NACO surveys, the NACO data are provided. The wide companion of HIP 77315 at $\rho =37.37''$ is HIP 77317, another member of Sco OB2. These stars are found to be a common proper motion pair (Worley & Douglass 1997), and were both observed in our ADONIS survey. The confirmed and candidate companions for which the status is determined using their $JHK_{\rm S}$ photometry, are indicated with "confirmed'' and "inconclusive'', respectively. The candidate companions identified by Kouwenhoven et al. (2005), for which the status is determined using the $K_{\rm S}=12$ mag criterion, and indicated with "candidate'' here. Background stars are not listed here.