Schmidt plate survey in the galactic centre direction. 1. Investigation of open clusters in the Sagittarius‐Carina spiral arm

Abstract

AbstractA study of four open clusters on Tautenburg Schmidt plates in the direction to the Galactic Centre (l = 17.0°, b = +0.8°) is presented. In a field of 8.95 square degrees proper motions and B, V magnitudes were determined for about 36 500 stars up to the limiting magnitudes B = 17m.9, V = 16m.8. For stars brighter than B = 16m an accuracy of about 3 mas/year has been estimated for proper motions. The rms errors of stellar magnitudes and (B—V) colours is about 0.1 mag.There are two open clusters already known in this sky area: Trumpler 32 (C 1814‐133) and NGC 6611 (C 1816‐138). By the inspection of the plates two additional anonymous clusters were identified near the double star ADS 11285 or BD −14°5014 (C 1819‐146) and near the star BD −14°5016 (C 1820‐146). The cluster membership determination was carried out using information on spatial and proper motion distributions of stars in the field. The colour‐magnitude diagrams up to the limiting stellar magnitude were constructed. For each cluster interstellar extinction AV, diameters of the core and corona, ages and spatial velocity components (V, W) relative to the LSR in the Y, Z‐galactic directions were derived.The distances to the clusters were obtained to 1720 pc, 2260 pc, 2130 pc and 2130 pc for Trumpler 32, NGC 6611, C 1819‐146 and C 1820‐146, respectively. All clusters are situated in the Sagittarius‐Carina spiral arm. Their spatial location confirms the assumption that galactic spiral arms are close to logarithmic spirals with a characteristic angle of about 20 degrees.