First detection of doubly deuterated methyl acetylene (CHD₂CCH and CH₂DCCD)^★

We report the first detection in space of the two doubly deuterated isotopologues of methyl acetylene. The species CHD₂CCH and CH₂DCCD were identified in the dense core L483 through nine and eight, respectively, rotational lines in the 72-116 GHz range using the IRAM 30m telescope. The astronomical frequencies observed here were combined with laboratory frequencies from the literature measured in the 29-47 GHz range to derive more accurate spectroscopic parameters for the two isotopologues. We derive beam-averaged column densities of (2.7 ± 0.5) × 10¹² cm^-2 for CHD₂CCH and (2.2 ± 0.4) × 10¹² cm^-2 for CH₂DCCD, which translate to abundance ratios CH₃CCH/CHD₂CCH = 34 ± 10 and CH₃CCH/CH₂DCCD = 42 ± 13. The doubly deuterated isotopologues of methyl acetylene are only a few times less abundant than the singly deuterated ones, concretely around 2.4 times less abundant than CH₃CCD. The abundances of the different deuterated isotopologues with respect to CH₃CCH are reasonably accounted for by a gas-phase chemical model in which deuteration occurs from the precursor ions C₃H₆D⁺ and C₃H₅D⁺, when the ortho-to-para ratio of molecular hydrogen is sufficiently low. This points to gas-phase chemical reactions, rather than grain-surface processes, as responsible for the formation and deuterium fractionation of CH₃CCH in L483. The abundance ratios CH₂DCCH/CH₃CCD = 3.0 ± 0.9 and CHD₂CCH/CH₂DCCD = 1.25 ± 0.37 observed in L483 are consistent with the statistically expected values of three and one, respectively, with the slight overabundance of CHD₂CCH compared to CH₂DCCD being well explained by the chemical model.