The physical and chemical structure of TMC-1: The spatial distribution and excitation conditions of carbon, oxygen, and sulfur bearing species -- Depletion of CO and CS

We present a detailed study of the physical and chemical structure of TMC-1 from the observed spatial distribution of HC_3N, NNH+, CN, and their isotopologues; the O-bearing species C 18 O, C^17O, CH_3OH, CH_3CHO, and H_2CO; and the S-bearing species CS, HCS+, H₂CS, and H_2CCS. The well-known filamentary structure of TMC-1 can be interpreted as a single cloud with a velocity gradient perpendicular to the filament or less likely as being due to the alignment of three independent clouds in the line sight with velocities of 5.3-5.7, 5.7-5.9, and 5.9-6.3 ̨ms. The main contribution to the emission of all studied C-bearing species is at 5.83 ̨ms. We conclude that the three velocity components are dynamically connected and that the abundances of C-bearing molecules show a different behavior compared to O-bearing species, which have emission peaks at 5.7 and 6.25 ̨ms, and appear at different spatial locations than cyanopolyynes and C_nH and C_nN radicals. The detection of 13 C^ 18 O, with exactly the same line profile than that of C 18 O and C^ 17 O, excludes the possibility that the decrease of their emission at 5.83 ̨ms, could be due to self-absorption or absorption by a foreground cloud component. Hence, as indicated by the spatial distribution of C 18 O and C^ 17 O, CO is significantly depleted at the velocities and positions where the emission of the carbon chains and radicals is found. This suggests that the emission of O-bearing species comes from the external layers of the carbon-rich filaments. A comparison between S-bearing species and other molecules indicates that CS, CCS, HCS+$, and H$_2CS are significantly depleted in the zone of CO depletion. A clear increase of the deuteration of several molecular species was found in the zones where CO and CS are depleted. Radiative trapping effects modify the emission distribution of abundant species with a high dipole moment, and hence the determination of their abundances requires the observation of minor isotopologues. Our chemical models reveal that CO and CS depletion can be explained by adsorption onto grains, whereas such a process on carbon chains is slower than gas-phase conversion, thus explaining the segregation found by observations. Finally, the 12 C/^13C abundance ratio derived from CO, CH_3OH, and CN is 65±7, 45±5, and 72±13, respectively.