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arXiv · 1809.01705

H2CO ortho-to-para ratio in the protoplanetary disk HD 163296

V. V. Guzman·K. I. Oberg·J. Carpenter·R. Le Gal·C. Qi·J. Pagues
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Abstract

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 Sub-millimeter Array (SMA) combined with one highly resolved measurement of a single H2CO line with 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.

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V. V. Guzman, K. I. Oberg, J. Carpenter, R. Le Gal, C. Qi, J. Pagues. 2018-09-05. H2CO ortho-to-para ratio in the protoplanetary disk HD 163296. https://doi.org/10.3847/1538-4357/aad778

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