The formaldehyde ortho para ratio as a probe of dark cloud chemistry and evolution

We present measurements of the H2CO ortho/para ratio toward four star-formi ng cores, L723, L1228, L1527, and L43, and one quiescent core, L1498. Combi ning these data with earlier results by Minh et al., three quiescent cores are found to have ortho/para ratios near 3, the ratio of statistical weight s expected for gas-phase formation processes. In contrast, ortho/para ratio s are 1.5-2.1 in five star-forming cores, suggesting thermalization at a ki netic temperature of 10 K. We attribute modification of the ortho/para rati o in the latter cores to formation and/or equilibration of H2CO on grains w ith subsequent release back into the gas phase due to the increased energy inputs from the forming star and outflow. We see accompanying enhancements in the H2CO abundance relative to H-2 to support this idea. The results sug gest that the formaldehyde ortho/para ratio can differentiate between quies cent cores and those in which low-mass star formation has occurred.