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

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.

arXiv comment9 pages, 7 figures, 4 tables; Accepted for publication in ApJ

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• Molecules with ALMA at Planet-forming Scales (MAPS) VI: Distribution of the small organics HCN, C2H, and H2CO
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• An ALMA Survey of H${}_2$CO in Protoplanetary Disks
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• ALMA chemical survey of disk-outflow sources in Taurus (ALMA-DOT) V: Sample, overview, and demography of disk molecular emission
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• Organic molecules in the protoplanetary disk of DG Tau revealed by ALMA
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• The centimeter emission from planet-forming disks in Taurus
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• Vacuum ultraviolet photodesorption and photofragmentation of formaldehyde-containing ices
• The First Detection of CH${}_2$CN in a Protoplanetary Disk
• Molecules with ALMA at Planet-forming Scales (MAPS) III: Characteristics of Radial Chemical Substructures
• Ortho-Para Chemistry of H2CO in the Protoplanetary Disk TW Hya