A massive protostellar core with an infalling envelope

Context. Due to the short timescales involved and observational difficulties, our knowledge of the earliest phases of massive star formation remains incomplete. Aims. We aim to explore the physical conditions during the initial phases of high-mass star formation and to detect a genuine massive (mass M > 8 M-circle dot) protostar at an early evolutionary stage. Methods. We have launched a multi- wavelength study of young and massive star- forming regions that were identified by the ISOPHOT Serendipity Survey ( ISOSS) performed with the ISO space telescope. The follow-up observations include ground-based near-infrared imaging and (sub) mm continuum and molecular line measurements (both single-dish and interferometric), as well as mid-to far-infrared measurements with the Spitzer Space Telescope. The combined spectrophotometric data are used to determine source temperatures T and masses M. Results. ISOSS J23053+5953 is a massive (M similar to 900 M-circle dot, luminosity L similar to 2100 L-circle dot) and cold (T similar to 17 K) star- forming region with two protostellar/ protocluster candidates (T less than or similar to 20 K and T similar to 17.5 K, M similar to 200 M circle dot each). The low temperatures are strongly confined by the spectrophotometric Spitzer data in the FIR. Interferometric observations reveal that the colder core (SMM2) has a mass of M = 26 M-circle dot within a region of 8700 x 5600 AU and drives an outflow. It also shows signs of infall in both single-dish and interferometric measurements, and its luminosity can be explained by accretion. We also detect a large-scale jet that is traced by H-2 emission. Conclusions. The cold mm-core ISOSS J23053+ 5953 SMM2 is a promising candidate for a high-mass protostar in an early evolutionary stage and one of the few objects showing both infall signatures and jets as a sign of accretion.