Emmy Noether Research Group on Stellar Atmospheres and Mass Loss

Analyzing Massive Stars

For most stars, light is the only piece of information available. By dispersing their light into a spectrum, we can learn more about the properties and the status of the stars.

A profound interpretation of the spectra for hot and massive stars is particularly challenging. The extreme conditions and the strong winds make common and handy techniques inapplicable. One example is that we use the concept of a `Local Thermodynamic Equilibrium` in their outer layers, exactly where the spectrum of a star is formed. Instead, complex numerical models are necessary to simulate the so-called ‘stellar atmosphere’ and understand the formation of the emergent spectrum.

By comparing synthetic spectra from stellar atmospheres to observations, we deduce the fundamental parameters of the highly influential class of hot stars, ranging from basic parameters such as their temperature, over their chemical composition to the strength of their wind and their ionizing flux.


Predicting Stellar Feedback

The interplay between radiation and matter is one of the fundamental processes in astrophysics. With mass-loss rates a billion times stronger than our sun, hot and massive stars dominate not only their immediate surrounding, but can influence whole clusters and drive the chemical evolution of their host galaxy.

Given their key role, the proper treatment of ionizing and mechanical feedback of hot and massive stars is of major importance on all astrophysical scales. Unfortunately, providing robust predictions has turned out to be a difficult task over the past decades and existing recipes are flawed by often being limited to narrow parameter regimes. From the interpretation of ionizing fluxes to the mass spectrum of black holes, a range of major astrophysical problems is inherently tied to the uncertainties in the underlying feedback prescriptions and stellar evolution models.

Next-generation stellar atmospheres that include hydrodynamics are a key tool in overcoming present limitations as they are able to obtain predictions and unique insights into stellar feedback at any metallicity, where currently no robust description is available.


Developing Stellar Atmospheres

As outlined above with their vital role for analyses and predictions, stellar atmospheres are a fundamental ingredient in order to interpret observations and derive fundamental insights into how stars interact with their environment. While the first model efforts go back to a time where computer code was written on punched cards, the field of developing stellar atmospheres is far from being complete. While modern computers help to decrease computation times for simple models, high-resolution spectra and multi-wavelength astronomy provide a constant demand for a more detailed treatment, increasing the computational effort.

Essentially a laboratory for astrophysics, a stellar atmosphere code must constantly be maintained, improved and extended. This does not only include additional physics and performance improvements, but also the documentation and accessibility to the user. As an active developer of PoWR, it is one of my personal aims to supply the community with a tool that is powerful, but also easy enough to handle despite all the necessary complexity when combining all the different physics and numerics. This effort will help to pave the way to properly interpret observations with next-generation telescopes and reach theoretical breakthroughs in understanding stellar winds across cosmic times.

Emmy Noether Research Group on Stellar Atmospheres and Mass Loss at ARI/ZAH
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