Chemical differentiation toward the Pipe nebula starless cores

We use the new IRAM 30-m FTS backend to perform an unbiased ~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 ≲ 15, have a poor chemistry with mainly simple species (e.g. CS and C₂H). The "oxo-sulfurated" cores, A_V ≃ 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 ≳ 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.