The seeds of star formation in the filamentary infrared-dark cloud G011.11-0.12

Context. Infrared-dark clouds (IRDCs) are the precursors to massive stars and stellar clusters. G011.11-0.12 is a well-studied filamentary IRDC, though, to date, the absence of far-infrared data with sufficient spatial resolution has limited the understanding of the structure and star-formation activity. Aims. We use Herschel to study the embedded population of young pre- and protostellar cores in this IRDC. Methods. We examine the cloud structure, which appears in absorption at short wavelength and in emission at longer wavelength. We derive the properties of the massive cores from the spectral energy distributions of bright far-infrared point sources detected with the PACS instrument aboard Herschel. Results. We report on the detection and characterization of pre- and protostellar cores in a massive filamentary infrared-dark cloud G011.11-0.12 using PACS. We characterize 18 cores directly associated with the filament, two of which have masses over 50 M-circle dot, making them the best candidates to become massive stars in G011.11-0.12. These cores are likely at various stages of protostar formation, showing elevated temperature (< T > similar to 22 K) with respect to the ambient gas reservoir. The core masses (< M > similar to 24 M-circle dot) are small compared to that in the cold filament. The mean core separation is 0.9 pc, well in excess of the Jeans length in the filament. Conclusions. We confirm that star formation in IRDCs is underway and diverse, and IRDCs have the capability of forming massive stars and clusters.