Open cluster ages and an apparent periodicity in the history of star formation in the solar neighborhood

The use of open cluster ages is introduced as a tool to investigate the Star Formation Rate (SFR) of the galactic disk in the solar neighborhood. The measurement of the SFR so obtained is shown to be in good agreement with that derived from chromospheric ages: a burst of star formation is clearly seen within the last 4x108 years and a second peak at 6x109 years. The use of open clusters gives a much firmer statistical basis from which to explore any fine structure in the SFR, and an examination of the age histogram obtained using data for 355 open clusters from the Lund Catalog of Open Cluster Data reveals a striking periodicity in the SFR of the Galactic disk on the order of 101 years. Analysis of the measured frequencies reveals the presence of two distinct modes of star formation in the disk, with a transition between the two modes clearly seen when the Magellanic cloud made its nearest approach ∼4x108 years ago. The most recent five measured frequencies of star formation are fit with the curve n=2.11x10-8˙e-1.67x10 -9˙Agey to within the measured errors.; An examination of the projection on the galactic plane of the location of the open clusters formed during each of the recent peaks of star formation finds clear evidence of physical correlation between the clusters formed during each starburst; further investigation suggests that the clusters were formed as the result of a spiral density wave traveling with the measured pattern speed Wp = 32.8--48.4 km/s, in good agreement with theoretical expectations of a density wave in the Galactic disk. A detailed atlas of open cluster age histograms by galactic sector is included, and a cursory analysis indicates that observed density wavefront development in the Galactic disk is primarily radial in nature.; As an appendix, new CCD photometries in the BVRI system are presented of the largely unstudied open clusters NGC 1857 and NGC 2355, located in the Galactic anti-center.