Ortho-para chemistry of formaldehyde reveals new clues in TW Hya’s protoplanetary disk

Star system: TW Hydrae | Distance from the Sun: 196 light years

The protoplanetary disk around TW Hya is a key site for studying how planets form and what materials they inherit. Understanding the distribution of chemical elements and molecules in such disks helps scientists learn about the building blocks of planets, especially those that could be habitable. One important molecule is formaldehyde (H2CO), which exists in two forms: ortho and para, depending on the spins of its hydrogen atoms.

Recent ALMA observations measured the amounts of ortho- and para-H2CO at different distances from the star, revealing how their ratio changes across the disk. This ratio, called the ortho-to-para ratio, can provide clues about the disk’s temperature and chemical history. Researchers developed a new model that includes detailed nuclear-spin chemistry to explain these observations. The model matches the observed H2CO amounts and the changing ortho-to-para ratio, especially the low values found in the outer disk. However, it cannot fully explain the high ratio seen closer to the star, suggesting that other processes, possibly involving ices, are at play. The study also shows that the amount of H2CO is very sensitive to the carbon-to-oxygen ratio and the rate of cosmic-ray ionization in the disk.

Key findings
- ALMA observations reveal how the ortho-to-para ratio of H2CO changes across TW Hya’s disk
- A new model including nuclear-spin chemistry matches most observed features
- The low ortho-to-para ratio in the outer disk is well explained, but the high ratio near the star remains puzzling
- The abundance of H2CO is highly sensitive to the carbon-to-oxygen ratio and cosmic-ray ionization rate
- More observations of ortho- and para-H2CO in other disks are needed to understand these processes

Scientific study

Ortho-Para Chemistry of H2CO in the Protoplanetary Disk TW Hya (external link)