Observed spectra (black) of the CH₃CCH (top) and the CH₃CCD (bottom) K=0 and K=1 transitions extracted towards the three molecular peaks in L1544 and the dust peak, using a circular aperture with diameter 31″. The extraction locations are indicated in Fig. 1. Shown in red are synthetic spectra produced with the LTE model generator of the python package pyspeckit. The input column densities are [4, 2, 5, 8](±0.3) × 10¹³ cm⁻² for CH₃CCH and [3, 1, 4, 5](±0.3) × 10¹² cm⁻² for CH₃CCD at the dust peak, CH₃OH peak, CH₃CCH peak, and c-C₃H₂ peak, respectively. The input excitation temperature is set to a constant value of 10 K for both molecules.