Remote young stellar groups and the rotation of the outer Galactic disk

Abstract

We have obtained UBV CCD‐magnitudes for stars in eight distant, young groups in the third Galactic quadrant. Comparison with theoretical isochrones yields ages and distances of significantly improved precision compared to previous studies. One of the best observed groups (in the faint H ii region S 289) is located twice as far from the Galactic center as the Sun, at the outer limit of active star formation in the Galactic disk. Together with published radial velocities and allowing for a metallicity gradient, the new distances harden the evidence for a flat Galactic rotation curve out to R≂16 kpc.