We sought to determine the maximum age of Trapezium systems by studying possible trapezium systems that were selected independently of their occurrence in H II regions. We started with the unpublished catalog by Allen, Tapia, & Parrao of all the known visual systems having three or more stars in which the maximum separation is less than 3.0 times the minimum separation. Their catalog has 968 such systems whose most frequent primary type is F, which does not describe young systems. With a CCD on the Kitt Peak 0.9 m telescope we obtained UBV frames for 265 systems accessible with our equipment on Kitt Peak. The frames were used to obtain UBV photometry for about 1500 stars with an accuracy of ±0.04 mag between V=7 and 14 mag. Also these frames were used to obtain astrometry with an accuracy of ±0°.015 in position angle and ±0".01 in separation. For the brightest star in each system we obtained a spectral type to determine the distance and reddening to the system. The measures were used to determine physical membership from stars that (1) fit a single color-magnitude diagram, (2) fit a common color-color diagram, and (3) show no astrometric motion compared to visual measures made (mostly) a century ago. Combining the results with spectroscopic data for 20 additional Allen et al. systems by Abt, we found that 126 systems had only optical companions to the primaries, 116 systems contained only a single physical pair, 13 were hierarchical systems with 3-6 members and having separation ratios of more than a factor of 10, two were small clusters, and only 28 fitted the criteria of Trapezium systems. However, as shown by Ambartsumian, about 9% of the hierarchical systems should appear to be Trapezium systems in projection. Those, like other hierarchical systems, have a broad distribution of primary spectral types. We isolated 14 systems that seem to be true Trapezium systems. They have primary types of B3 or earlier, indicating a maximum age of about 5x107 yr. This upper limit is consistent with the estimate made by Allen & Poveda for an age of several million years for these dynamically unstable systems. These Trapezia are also large with a median radius of 0.2 pc and a maximum radius of 2.6 pc. We asked why the sample of 285 possible Trapezium systems yielded only 14 true ones, despite the attempt made by Allen et al. to eliminate optical companions with a ``1% filter'', i.e., demanding that each companion have less than a 1% chance of being a field star of that magnitude within a circle of its radius from the primary. The explanation seems to be that the double star catalogs are based mostly on BD magnitudes that, fainter than V=12 mag, are systematically too faint by 1 mag.
Stars: Binaries: Visual - Galaxy: Open Clusters and Associations: General - Stars: Fundamental Parameters
VizieR on-line data:
<Available at CDS (J/ApJ/541/841): table3a.dat table3b.dat>