# H$_2$O distribution in the disc of HD 100546 and HD 163296: the role of dust dynamics and planet--disc interaction

**Authors:** L. M. Pirovano, D. Fedele, E. F. van Dishoeck, M. R. Hogerheijde, G. Lodato, S. Bruderer  
**arXiv:** [2207.10744](https://arxiv.org/abs/2207.10744)  
**Date:** 2022-07-21  
**Summary page:** https://www.summarizepaper.com/en/paper/2207.10744v1/

## Key points

- Study by Pirovano et al. (2022) on H$_2$O distribution in protoplanetary discs of two Herbig Ae stars using far-infrared observations from Herschel
- Low abundance of cold-water reservoirs in the discs, with few showing emission of hot water transitions above a few hundred Kelvin
- Intricate interplay between water distribution, dust dynamics, and planet-disc interactions within protoplanetary systems around Herbig Ae stars
- Opposite trends observed in the two systems: ground-state transitions detected in HD 100546 but high-J lines undetected; trend reversed in HD 163296
- HD 163296 has a water-rich hot inner disc within the snowline and a water-poor outer disc, possibly due to thermal desorption of icy grains migrating inward
- Size of H$_2$O emitting region in HD 163296 corresponds to a narrow dust gap seen at $r=10\,$au with ALMA
- Low-J lines detected in HD 100546 indicate an abundance of a few $10^{-9}$ in the cold outer disc ($>40$ au), spatial coincidence with previously observed H$_2$O ice in near-infrared
- ALMA observations reveal large dust gaps between approximately 40 and 150 au likely created by massive embedded protoplanets
- Warm molecular layer beyond the snow line highly depleted of water molecules, suggesting oxygen-poor chemical composition of gas
- Depletion indicates gas-phase oxygen readily consumed and closely linked to dust grain dynamics within the discs

## Summary

The study by Pirovano et al. (2022) investigates the distribution of H$_2$O in the protoplanetary discs of two Herbig Ae stars using far-infrared observations from Herschel. The researchers found a surprisingly low abundance of cold-water reservoirs in these discs, with only a few showing emission of hot water transitions at temperatures above a few hundred Kelvin. This suggests an intricate interplay between water distribution, dust dynamics, and planet-disc interactions within protoplanetary systems around Herbig Ae stars. Using the thermo-chemical model DALI and conducting spectral analysis, the team observed opposite trends in the two systems: while ground-state transitions were detected in HD 100546 and high-J lines were undetected, the trend was reversed in HD 163296. They determined that HD 163296 has a water-rich hot inner disc within the snowline (abundance $\gtrsim 10^{-5}$) and a water-poor outer disc ($< 10^{-10}$), possibly due to thermal desorption of icy grains migrating inward. Remarkably, they also found that the size of the H$_2$O emitting region in HD 163296 corresponds to a narrow dust gap seen at $r=10\,$au with ALMA. Similarly, low-J lines detected in HD 100546 indicate an abundance of a few $10^{-9}$ in the cold outer disc ($>40$ au), with spatial coincidence between cold H$_2$O transitions and previously observed H$_2$O ice in the near-infrared. Further supporting their findings are ALMA observations revealing large dust gaps between approximately 40 and 150 au in both systems - likely created by massive embedded protoplanets. The study also found that the warm molecular layer beyond the snow line is highly depleted of water molecules, suggesting an oxygen-poor chemical composition of gas. This depletion indicates that gas-phase oxygen is readily consumed and its distribution is closely linked to dust grain dynamics within the discs. In summary, this research provides valuable insights into the complex interplay between water distribution, dust dynamics, and planet-disc interactions within protoplanetary systems around Herbig Ae stars.