THE LOCATION OF STELLAR CLUSTERS IN THE BIG DENT - AN AGE GRADIENT ALONG THE Z-AXIS

The ages and locations of stellar clusters and individual Cepheid star s in the depression called the ''Big Dent'' are analyzed. The Z-locati ons of the cluster sample display a well-defined Z-age stratification. The Cepheid sample has ages between 3 and 8 x 10(7) yr and shows a de parture from the midplane similar to the one observed for clusters of the same age range. The existence of such an age gradient seems to cor roborate previous hypotheses suggesting that the star formation activi ty was probably triggered by the same strong perturbation which origin ated the depression. A model in which the Big Dent was created by a co llision of a high-velocity cloud with the Galactic disk is able to rep roduce the observed gradient.