Detection of a new interstellar molecular ion, H₂COH⁺ (protonated formaldehyde)

A new interstellar molecular ion, H₂COH⁺ (protonated formaldehyde), has been detected toward Sgr B2, Orion KL, W51, and possibly in NGC 7538 and DR21(OH). Six transitions were detected in Sgr B2(M). The 1_1,0-1_0,1 transition was detected in all sources listed above. Searches were also made toward the cold, dark clouds TMC-1 and L134N, Orion (3N, 1E), and a red giant, IRC +10216, without success. The excitation temperatures of H₂COH⁺ are calculated to be 60-110 K, and the column densities are on the order of 10¹²-10¹⁴ cm^-2 in Sgr B2, Orion KL, and W51. The fractional abundance of H₂COH⁺ is on the order of 10^-11 to 10^-9, and the ratio of H₂COH⁺ to H₂CO is in the range 0.001-0.5 in these objects. The values in Orion KL seem to be consistent with the "early time" values of recent model calculations by Lee, Bettens, & Herbst, but they appear to be higher than the model values in Sgr B2 and W51 even if we take the large uncertainties of column densities of H₂CO into account. We suggest production routes starting from CH₃ OH may play an important role in the formation of H₂COH⁺.