Dr. Andreas Sander

Armagh Planetarium and Observatory
  • Armagh Observatory and Planetarium

About Dr. Andreas Sander

Research Interests: Massive Stars and their role in shaping our Universe, Expanding Stellar Atmospheres, Radiative Transfer, Stellar Feedback, High Mass X-ray Binaries, Gravitational Wave Progenitors.

I am a post-doctoral research fellow with scientific expertise in stellar atmospheres, in particular for hot and massive stars. These stars are shaping their environment via their ionizing radiation and strong stellar wind, which is typically a billion times stronger than our solar wind.

To get a physical insight of these highly impactful stars, we need to understand the light they are sending us. Therefore, I am actively developing PoWR, a leading code in simulating the complex atmospheres of hot stars with radiatively driven winds.

Located at the conjunction of theory and observations, I use my atmosphere models to quantify the physical properties of massive stars and their winds. This includes the analysis of observations from modern, large-scale telescopes as well as providing predictions for mass-loss rates, stellar spectra and feedback, crucial ingredients to better understand stellar populations and the origin of massive black holes and gravitational waves.

Research

Armagh Planetarium and Observatory

Analzying 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.

Armagh Planetarium and Observatory

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.

Armagh Planetarium and Observatory

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.

Mass-loss recipes for massive He ZAMS stars

This page provides a python script to calculate the mass-loss rate for hydrogen-free, massive helium stars on the He Zero Age Main Sequence (He ZAMS). The calculations are based on a set of hydrodynamically consistent stellar atmosphere models with winds driven by the hot iron opacity bump. The latter limits the validity towards lower mass He stars, for which the mass-loss rates will be underestimated. We expect reasonable values for the mass-loss rates for He stars above 8-10 solar masses. The metallicity range of the calculations ranges from 2.0 solar to 0.02 solar. While the recipe has been designed for possible extrapolations, it is important to consider the discussions in the corresponding paper when employing the resulting mass-loss rates beyond the luminosity or metallicity range of the underlying model set.

The script can either be called individually via the command line or be imported as a module in other Python scripts. It contains formulae for the mass loss as a function of luminosity as well as the mass loss as a function of Γ or L/M.
Generally, the use of the Γ-dependent formula is recommended as it reflects the nature of WR-type mass loss as an L/M-dependent quantity.

More information is given in the corresponding paper: Sander & Vink (2020, MNRAS, in press, preprint on arXiv:2009.01849)

Please contact Andreas Sander for further questions and discussions.

Download mdot-hezams-massive.py

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