Nuclear spin conversion of formaldehyde in protostar environments induced by non reactive collisions

Context. Formaldehyde is an important diagnostic of the physical conditions in star forming regions. The temperature of formation is determined by measuring the relative abundance of para-H2CO and ortho-H2CO. The key hypothesis for this determination is that the ortho-para interconversion is strictly forbidden once the molecule is formed. However H2CO nuclear spin conversion mechanisms do exist in a gas phase either involving reactive or non-reactive collisions. This last process is governed by internal properties of the molecule, such as the mixing of energetically closed ortho-para level pairs, which are coupled through magnetic intramolecular interactions. This mixing is interrupted by a collision that makes the molecule leave the mixed state and puts it in a pure state, ortho or para, with a non zero probability for both isomeric forms. Aims. This model allows us to estimate the spin conversion induced by non reactive collisions in different conditions encountered in the interstellar medium. Methods. We calculated the ortho-para conversion rate in H2CO for different temperatures and H-2 abundances. Results. It is shown that the characteristic conversion time is always much longer than the H2CO lifetime. Conclusions. Consequently, the conversion probability is zero in gas-phase protostar environments for these non reactive collisions.