The millimeter-wave spectrum of methyl ketene and the astronomical search for it

The analysis of isomeric species of a compound observed in the interstellar medium (ISM) is a useful tool to understand the chemistry of complex organic molecules. It could, likewise, assist in the detection of new species.

Our goal consists in analyzing one of the two most stable species of the C₃H₄O family, methyl ketene, whose actual rotational parameters are not precise enough to allow its detection in the ISM. The obtained parameters will be used to search for it in the high-mass star-forming regions Orion KL and Sagittarius B2, as well as in the cold dark clouds TMC-1 in the Taurus Molecular Cloud and Barnard 1 (B1-b).

A millimeter-wave room-temperature rotational spectrum of methyl ketene was recorded from 50 to 330 GHz. The internal rotation analysis of its ground state and first torsional excited state was performed with the rho-axis method employing the RAM36 program.

More than 3000 transitions of the rotational spectrum of the ground state (K_amax = 18) and first torsional excited state (K_amax = 13) of methyl ketene were fitted using a Hamiltonian that contains 41 parameters with an RMS (root mean square) of 41 kHz. Column density limits were calculated but no lines were detected in the ISM belonging to methyl ketene.