PRECISION PHOTOMETRY OF YOUNG STELLAR GROUPS TOWARDS THE OUTER GALACTIC DISK AND THE GALACTIC ROTATION CURVE

We have used a CCD to obtain stellar UBV magnitudes to V congruent-to 18 towards eight distant, young stellar groups in the third Galactic quadrant where interstellar extinction is low (l=210-degrees-242-degrees). Comparison with theoretical isochrones yields ages and distances of significantly improved precision compared to previous studies based on photoelectric diaphragm photometry. One of the groups (in the faint H II region S 289) is located twice as far from the Galactic center as the Sun and is likely located near 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 congruent-to 16 kpc.