Deuteration of HC

Context. Deuterated molecules are a useful diagnostic tool to probe the evolution and the kinematics in the earliest stages of star formation. Due to the low temperatures and high densities in the centre of pre-stellar cores, the deuterium fraction is enhanced by several orders of magnitude with respect to the cosmic D/H abundance ratio. Aims. We study the distribution of the emission and the deuteration of the two carbon chains HC3N and CH3CCH throughout the prototypical pre-stellar core L1544. Methods. We analyse emission maps of CH3CCH, CH2DCCH, CH3CCD, HC3N, HCC13CN, and DC3N, observed towards L1544 with the IRAM 30 m single-dish radio telescope. We use non-local thermodynamic equilibrium radiative transfer calculations, combined with chemical modelling of the molecular abundances, to constrain physical parameters of the observed species. Following this, we derive the corresponding column density and deuteration maps and analyse the chemical processes influencing the different molecular distributions. Results. We find levels of deuteration of N(DC3N)/N(HC3N) = 0.04-0.07, N(CH2DCCH)/N(CH3CCH) = 0.09-0.15, and N(CH3CCD)/N(CH3CCH) = 0.07-0.09. The deuteration of HC3N appears homogeneous across the core, with widespread D-fraction values above 0.06, tracing intermediate-density gas in the outer layers of the core, at densities less than 105 cm−3. CH3CCD is most efficiently formed in the higher-density regions towards the core centre, while the deuteration fraction of CH2DCCH traces a local density enhancement in the north-east of the core, coinciding with the CH3OH emission peak. Conclusions. The results suggest that gas-phase reactions dominate the formation and deuteration of both HC3N and CH3CCH in L1544, with spatial variations driven by physical structure, density and external radiation. The significantly higher deuteration fraction of CH2DCCH compared to CH3CCD and a tentative gradient with higher values in the north suggest that there are different deuteration mechanisms for the two functional groups, with varying efficiency across the core. Similarities between the CH2DCCH emission and CH2DOH might indicate an additional deuteration pathway of CH3CCH on the surfaces of dust grains, as observed for H2CO.