Dissecting an intermediate - mass ( IM ) protostar Chemical differentiation in IC 1396

Aims. Our aim is to unveil the physical conditions and structure of the intermediate mass (IM) protostar IRAS 21391+5802 (IC 1396 N) at scales of ∼1000 AU. Methods. We have carried out high-angular resolution (1. ′′ 4) observations in the continuum at 3.1mm and in the N 2 H + 1→0, CH 3 CN 5 k →4 k and 13 CS 2→1 lines using the Plateau de Bure Interferometer (PdBI). In addition, we have merged the PdBI images with previous BIMA (continuum data at 1.2mm and 3.1mm) and single-dish (N 2 H + 1→0) data to have a comprehensive description of the region. Results. The combination of our data with BIMA and 30m data show that the bipolar outflow associated has completely eroded the initial molecular globule. The 1.2mm and 3.1mm continuum emissions are extended along the outflow axis tracing the warm walls of the biconical cavity. Most of the molecular gas, however, is located in an elongated feature in the direction perpendicular to the outflow. A strong chemical differentiation is detected across the molecular toroid, with the N 2 H + 1→0 emission absent in the inner region. Conclusions. Our PdBI data show two different regions in IC1396 N: (i) the young stellar objects (YSO) BIMA 3 and the protoclus-ter BIMA 2, detected in dust continuum emission and one of the individual cores, IRAM 2A, in the CH 3 CN 5 k →4 k line, and (ii) the clumps and filaments that have only been detected in the N 2 H + 1→0 line. The clumps belonging to this second group are located in the molecular toroid perpendicular to the outflow, mainly along the walls of the biconical cavity. This chemical differentiation can be understood in terms of the different gas kinetic temperature. The [CH 3 CN]/[N 2 H + ] ratio increases by 5 orders of magnitude with gas temperature, for temperatures between 20 K and 100 K. The CH 3 CN abundance towards IRAM 2A is similar to that found in hot corinos and lower than that expected towards IM and high mass hot cores. This could indicate that IRAM 2A is a low mass or at most Herbig Ae star (IRAM 2A) instead of the precursor of a massive Be star. Alternatively, the low CH 3 CN abundance could also be the consequence of IRAM 2A being a Class 0/I transition object which has already formed …