The Young Cluster IC 5146

The B0 V star BD +46°3474 lies near the front surface of a dense molecular cloud and illuminates the emission/reflection nebula IC 5146. The HAeBe variable BD +46°3471 is embedded in the same cloud, about 10' (3.5 pc) away. CCD photometry in BVRI (to V=22) and in JHK (to about K=16.5) has been obtained for the young clusters surrounding each of these two bright stars. Some 100 emission-Hα stars brighter than R=20.5 have been found in the area, most of them in IC 5146. (Among these are two that have spectra resembling a high-excitation Herbig-Haro [HH] object plus a stellar continuum.) A distance of 1.2 kpc follows from the photometry of several late-type IC 5146 cluster members; the average extinction from 38 stars classified spectroscopically is A_V=3.0+/-0.2 mag. Although optical photometry is available for 700 stars in the IC 5146 field, only about half (including all the Hα emitters) lie above the main sequence, while a substantial fraction of these are estimated to be foreground. A number of such interlopers have been identified on the basis of proper motion or abnormally low A_V. The age distribution of the Hα emitters has been estimated by reference to several sets of theoretical isochrones. There is substantial disagreement, but the median age does appear to be near 1 Myr. The spectrum of +46°3474 is unexceptional except for an unusually low vsini (10 km s^-1), but +46°3471 has a complex emission plus absorption spectrum. Our interpretation of the structure of IC 5146 on the basis of optical and radio radial velocities follows a proposal by Roger & Irwin in 1982, namely, that +46°3474 formed near the near surface of the present cloud and evacuated a blister cavity out of which gas and dust are now flowing through a funnel-shaped volume in the approximate direction of the Sun. It is suggested that the IC 5146 cluster stars formed in a dense foreground section of the molecular cloud that was dissipated following the appearance of +46°3474.