2018ApJ...864..170G

Ortho-to-para (o/p) ratios of species like water, ammonia, and formaldehyde (H₂CO) are believed to encode information about the formation history of the molecule. Measurements of o/p ratios in protoplanetary disks could thus be used to constrain their physical and chemical histories. We present the first measurement of the H₂CO o/p ratio in a protoplanetary disk, using three ortho and two para lines observed with the Submillimeter Array (SMA) combined with one highly resolved measurement of a single H₂CO line with the Atacama Large Millimeter/submillimeter Array (ALMA) toward the disk around Herbig Ae star HD 163296. We find a disk-averaged H₂CO o/p ratio of 1.8-2.8 (depending on the assumed disk structure), corresponding to a spin temperature of 11-22 K. We also derive a rotational temperature of 24 K from the flux ratio of the three ortho lines. The observed spatial distribution, as seen by ALMA, as well as the rotational temperature and the o/p ratio, at the large scales the SMA is most sensitive to, are consistent with a low-temperature formation pathway, most likely grain surface chemistry, of H₂CO in this disk.