# 3D model of hydrogen atmospheric escape from HD209458b and HD189733b: radiative blow-out and stellar wind interactions

**Authors:** Vincent Bourrier, Alain Lecavelier des Etangs  
**arXiv:** [1308.0561](https://arxiv.org/abs/1308.0561)  
**Date:** 2013-08-01  
**Summary page:** https://www.summarizepaper.com/en/paper/1308.0561v1/

## Key points

- Transit observations of exoplanets HD209458b and HD189733b reveal evidence of neutral hydrogen escaping from these planets
- A 3D particle model is presented to understand the dynamics of this phenomenon
- For HD209458b, observed velocities (-130km/s) can be explained by radiation-pressure acceleration
- Ionizing flux approximately 3-4 times solar and a hydrogen escape rate ranging from 10^9-10^11g/s are consistent with data and predictions for HD209458b
- For HD189733b, absorption by neutral hydrogen was observed at higher velocities (-230 to -140km/s), suggesting an additional acceleration mechanism possibly due to interactions with stellar wind protons
- Stellar wind properties include temperature ~3x10^4K and velocity 200+-20km/s
- Escape rate for HD189733b ranges from 4x10^8-10^11g/s and ionizing flux is 6-23 times solar
- An "escape-limited" saturation regime exists where most escaping gas interacts with stellar protons at densities above ~3x10^5/cm3
- Non-detection of escaping hydrogen in earlier observations may be attributed to suppression of stellar wind or lower escape rates compared to later observations
- Best-fit simulations show that the shape of the escaping atmosphere resembles a cometary tail for both planets

## Summary

The study of transit observations in Ly-alpha of exoplanets HD209458b and HD189733b has revealed evidence of neutral hydrogen escaping from these planets. To better understand the dynamics of this phenomenon, the authors present a 3D particle model and calculate theoretical absorption line profiles in the Ly-alpha region. For HD209458b, the observed velocities of escaping atoms (-130km/s) can be explained by radiation-pressure acceleration. The authors also find that an ionizing flux approximately 3-4 times solar and a hydrogen escape rate ranging from 10^9-10^11g/s are consistent with both data and predictions. In contrast, for HD189733b, absorption by neutral hydrogen was observed at higher velocities (-230 to -140km/s), suggesting an additional acceleration mechanism possibly due to interactions with stellar wind protons. The study further reveals properties of the stellar wind (temperature ~3x10^4K, velocity 200+-20km/s) as well as the escape rate (4x10^8-10^11g/s) and ionizing flux (6-23 times solar). It also highlights an "escape-limited" saturation regime where most escaping gas interacts with stellar protons at densities above ~3x10^5/cm3. The non-detection of escaping hydrogen in earlier observations can be attributed to either a suppression of stellar wind or a lower escape rate compared to later observations. Overall, best-fit simulations show that the shape of the escaping atmosphere resembles a cometary tail for both planets. This comprehensive analysis provides valuable insights into atmospheric escape processes in exoplanets and highlights different mechanisms at play in different systems.