H 2 CO Ortho-to-para Ratio in the Protoplanetary Disk HD 163296

• Published in: The Astrophysical journal Vol. 864; no. 2; p. 170
• Main Authors: Guzmán, V. V., Öberg, K. I., Carpenter, J., Gal, R. Le, Qi, C., Pagues, J.
• Format: Journal Article
• Language: English
• Published: 10-09-2018
• ISSN: 0004-637X; 1538-4357
• DOI: 10.3847/1538-4357/aad778

Ortho-to-para (o/p) ratios of species like water, ammonia, and formaldehyde (
) 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
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
line with the Atacama Large Millimeter/submillimeter Array (ALMA) toward the disk around Herbig Ae star HD 163296. We find a disk-averaged
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
in this disk.