1995A&A...298..213M

H₂¹³CO has been detected for the first time toward cold dark molecular clouds using the NRAO 12m telescope. The H₂¹³CO ortho/para abundance ratio R for B335, which we report as R∼1.7, suggests equilibrium at the local kinetic temperature and appears to be distinctly different from that for both TMC-1 and L134N, where R is close to or higher than the statistical value 3. Since only B335 among the observed positions includes an imbedded IR source, this difference may result from heating of the grain surfaces, providing the energy necessary for desorption of formaldehyde formed on the grains.