Periodic Reporting for period 3 - ASTROFLOW (The influence of stellar outflows on exoplanetary mass loss)
Reporting period: 2021-09-01 to 2023-02-28
The main goal of ASTROFLOW is to understand exoplanetary mass loss, by quantifying the influence of stellar outflows and irradiation on atmospheric escape of close-in exoplanets orbiting low-mass stars. Escape plays a key role in planetary evolution, population, and potential to develop life. Stellar irradiation and outflow affect planetary mass loss: irradiation heats planetary atmospheres, which inflate and more likely escape; outflow causes pressure confinement around otherwise freely escaping atmospheres. This external pressure can increase, reduce or even suppress escape rates; its effects on exoplanetary mass loss remain largely unexplored due to the complexity of such interactions.
ASTROFLOW is filling this knowledge gap by developing a novel modelling framework of atmospheric escape that considers the effects of realistic stellar outflows on exoplanetary mass loss. Our scientific objectives are: 1) Realistically characterise stellar outflows (winds and coronal mass ejections) to derive their physical properties at the orbits of exoplanets. 2) Characterise the physical conditions of atmospheric escape of close-in exoplanets, including how escape is affected by realistic conditions of stellar outflows.
Our modelling framework consists of state-of-the-art, time-dependent, 3D simulations of stellar outflows, which are being/will be coupled to novel 3D simulations of atmospheric escape. Our models account for the major underlying physical processes of mass loss. With this, we can determine the response of planetary mass loss to realistic stellar particle, magnetic and radiation environments and we can then characterise the physical conditions of the escaping material. To compare with spectroscopic transit observations, we produce synthetic line profiles of atmospheric escape observations. In this way, our models are used to characterise exoplanetary systems.
ASTROFLOW is filling this knowledge gap by developing a novel modelling framework of atmospheric escape that considers the effects of realistic stellar outflows on exoplanetary mass loss. Our scientific objectives are: 1) Realistically characterise stellar outflows (winds and coronal mass ejections) to derive their physical properties at the orbits of exoplanets. 2) Characterise the physical conditions of atmospheric escape of close-in exoplanets, including how escape is affected by realistic conditions of stellar outflows.
Our modelling framework consists of state-of-the-art, time-dependent, 3D simulations of stellar outflows, which are being/will be coupled to novel 3D simulations of atmospheric escape. Our models account for the major underlying physical processes of mass loss. With this, we can determine the response of planetary mass loss to realistic stellar particle, magnetic and radiation environments and we can then characterise the physical conditions of the escaping material. To compare with spectroscopic transit observations, we produce synthetic line profiles of atmospheric escape observations. In this way, our models are used to characterise exoplanetary systems.
In the first half of this project, the ASTROFLOW group worked in two directions. One part of the team developed models of winds of cool stars and studied the propagation of energetic particles through these winds. More recently, we have also modelled the propagation and ejection of bursty events, known as coronal mass ejections. Our models allow us to characterise the particle and magnetic environment surrounding exoplanets. Additionally, they can be compared to radio observations, allowing us to better understand specific systems.
The other part of the team worked on developing 3D simulations of atmospheric escape of exoplanets. The escape process occurs when stellar high-energy radiation is deposited at the lower atmosphere of the planet, heating the atmosphere, which expands and more easily evaporate.
The evaporation process however does not occur in vacuum. On the contrary, the stellar wind that surrounds the escaping atmosphere can shape and alter escape. Therefore, both parts of the team work together to provide a more realistic physical characterisation of atmospheric escape in close-in exoplanets.
The stellar wind is able to shape atmospheric escape — combining the effects of stellar winds with the orbital motion, the escaping atmosphere takes the form of a “comet-like tail”. These elongated tails can be detected in spectroscopic transits. One recent highlight of our modelling research was that we showed that in the presence of a planetary magnetic field, the “comet-like tail” can take a different form — escape in this case occurs through a “double tail structure”, in which evaporation takes place mostly through the poles of the planet.
The other part of the team worked on developing 3D simulations of atmospheric escape of exoplanets. The escape process occurs when stellar high-energy radiation is deposited at the lower atmosphere of the planet, heating the atmosphere, which expands and more easily evaporate.
The evaporation process however does not occur in vacuum. On the contrary, the stellar wind that surrounds the escaping atmosphere can shape and alter escape. Therefore, both parts of the team work together to provide a more realistic physical characterisation of atmospheric escape in close-in exoplanets.
The stellar wind is able to shape atmospheric escape — combining the effects of stellar winds with the orbital motion, the escaping atmosphere takes the form of a “comet-like tail”. These elongated tails can be detected in spectroscopic transits. One recent highlight of our modelling research was that we showed that in the presence of a planetary magnetic field, the “comet-like tail” can take a different form — escape in this case occurs through a “double tail structure”, in which evaporation takes place mostly through the poles of the planet.
After identifying this double tail structure, there are still many questions that remain open. Which planets show this type of structure? How does the stellar wind alter this structure? What happens to planet evaporation when a coronal mass ejection impacts the planet? In the second half of our project, we will investigate these open questions.