A search for interstellar ch₃d: limits to the methane abundance in orion-kl

A search has been performed for interstellar CH3D via its J_K = 1₀-0₀ transition at 232 GHz and its J_K = 2₀-1₀ and J_K = 2₁-1₁ lines at 465 GHz using the NRAO 12 m and CSO 10 m telescopes toward Orion-KL. This search was done in conjunction with laboratory measurements of all three transitions of CH₃D using millimeter/submillimeter direct absorption spectroscopy. The molecule was not detected down to a 3 σ level of T_A< 0.05 K for the J_K = 2₁-1₁ line toward Orion, which indicates an upper limit to the CH₃D column density of N < 6 × 10¹⁸ cm^-2 in the hot core region and a fractional abundance (with respect to H₂) of <6 × 10^-6. These measurements suggest that the methane abundance in the Orion hot core is f < 6 × 10^-4, assuming D/H ∼ 0.01. Such findings are in agreement with recent hot core chemical models, which predict CH₄/H₂ ∼ 10^-4.