A Spitzer census of the IC 348 nebula

Spitzer mid-infrared surveys enable an accurate census of young stellar objects by sampling large spatial scales, revealing very embedded protostars, and detecting low-luminosity objects. Taking advantage of these capabilities, we present a Spitzer-based census of the IC 348 nebula and embedded star cluster, covering a 2.5 pc region and comparable in extent to the Orion Nebula. Our Spitzer census supplemented with ground-based spectra has added 42 Class II T Tauri sources to the cluster membership and identified similar to 20 Class 0/I protostars. The population of IC 348 likely exceeds 400 sources after accounting statistically for unidentified diskless members. Our Spitzer census of IC 348 reveals a population of Class I protostars that is anticorrelated spatially with the Class II / III T Tauri members, which comprise the centrally condensed cluster around a B star. The protostars are instead found mostly at the cluster periphery about similar to 1 pc from the B star and spread out along a filamentary ridge. We further find that the star formation rate in this protostellar ridge is consistent with that rate which built the older exposed cluster, while the presence of 15 cold, starless, millimeter cores intermingled with this protostellar population indicates that the IC 348 nebula has yet to finish forming stars. Moreover, we show that the IC 348 cluster is of order 3 - 5 crossing times old, and, as evidenced by its smooth radial profile and confirmed mass segregation, is likely relaxed. While it seems apparent that the current cluster configuration is the result of dynamical evolution and its primordial structure has been erased, our finding of a filamentary ridge of Class I protostars supports a model in which embedded clusters are built up from numerous smaller subclusters. Finally, the results of our Spitzer census indicate that the supposition that star formation must progress rapidly in a dark cloud should not preclude these observations that show it can be relatively long lived.