that the accumulation of atmospheric O$_2$ is unlikely unless water loss can take place after the star’s most active phase.'
author:
- 'C. P. Johnstone'
bibliography:
- 'mybib.bib'
title: Hydrodynamic escape of water vapor atmospheres near very active stars
---

Introduction
============

Water is one of the most important molecules for planets and can influence significantly their surfaces, climates, and potential for sustaining life. In addition to being present in the atmospheres of solar system planets, water has been detected in the atmospheres of many exoplanets (e.g. @Tinetti07; @Wakeford13; @Brogi14; @Tsiaras19). A large number of physical processes take place in the upper atmospheres and exospheres of planets that cause water to be lost to space, most of which are driven by interactions with the central star. For example, non-thermal escape processes include ion pick-up by stellar winds (@Kislyakova14a), cold ion outflows (@Glocer07), and the loss of photochemically produced high-energy particles (@Amerstorfer17). Thermal escape mechanisms include hydrodynamic escape, which takes place when the upper atmosphere is heated to such a temperature that the thermal pressure causes it to accelerate away from the planet at speeds exceeding the escape velocity. This can happen without any outside influences for very low-mass planets (@Stoekl15), 