Chemical differentiation toward the Pipe nebula starless cores

We use the new IRAM 30-m FTS backend to perform an unbiased similar to 15 GHz wide survey at 3 mm toward the Pipe nebula young diffuse starless cores. We discover an unexpectedly rich chemistry. We propose a new observational classification based on the 3 mm molecular line emission normalized by the core visual extinction (A(V)). On the basis of this classification, we report a clear differentiation between cores in terms of chemical composition and line emission properties, which enables us to define three molecular core groups. The diffuse cores, A(V) less than or similar to 15, have a poor chemistry with mainly simple species (e.g. CS and C2H). The oxo-sulfurated cores, A(V) similar or equal to 15-22, appear to be abundant in species such as SO and SO2, but also in HCO, which seem to disappear at higher densities. Finally, the deuterated cores, A(V) greater than or similar to 22, have a typical evolved chemistry prior to the onset of the star formation process, with nitrogenated and deuterated species, as well as carbon chain molecules. On the basis of these categories, one of the diffuse cores (core 47) has the spectral line properties of the oxo-sulfurated ones, which suggests that it is a failed core.