H2CO Ortho-to-para Ratio in the Protoplanetary Disk HD 163296

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 fir...

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Bibliographic Details
Published in:The Astrophysical journal 2018-09, Vol.864 (2)
Main Authors: Guzmán, V. V., Öberg, K. I., Carpenter, J., Gal, R. Le, Qi, C., Pagues, J.
Format: Article
Language:English
Subjects:
Ammonia
Arrays
astrochemistry
Astrophysics
Disks
ISM: abundances
ISM: molecules
Low temperature
molecular processes
Organic chemistry
Planet formation
Protoplanetary disks
Protoplanets
Radio telescopes
Spatial distribution
Spin temperature
Surface chemistry
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Summary: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.
ISSN:0004-637X
1538-4357
DOI:10.3847/1538-4357/aad778