— academic career —

Hi, I'm Daniel! I am a researcher at KU Leuven specializing in the study of massive stars. Massive stars are several times more massive than our Sun, they are millions of times brighter, and have strong UV-radiation driven matter outflows, called stellar winds. Since they are hot, they emitt copious amounts of ionizing radiation and deposit heavy elements via their stellar winds and when they explode as a supernova at the end of their live. All of this makes massive stars the enigmatic cosmic engines of the Universe.

I began my academic journey with a Bachelor's degree in Physics at the University of Bonn in 2018. Following that, I continued my studies at the same university and obtained my Master's degree in Astrophysics in 2020, under the guidance of Prof. Dr. Norbert Langer. During my time in Bonn, my research focused on both single and binary stellar evolution models. In my thesis, we created a synthetic stellar population that we can compare to observations, allowing us to improve our understanding of massive star evolution by linking the early stages of massive stars to their later evolutionary stages. I obtained my PhD at the University of Potsdam in 2024. During that time I analyzed the spectra of the most massive stars hosted by the Small Magellanic Cloud, a low-metallicity satellite dwarf galaxy of the Milky Way. In my current research, I combine my skills in theoretical and observational astrophysics and the newly obtained insights into massive star evolution at low metallicity to create a new tool that fully consistently models the populations and spectra of distant galaxies. This will allow us to better understand the spectra of redshifted galaxies, as obtained by the James Webb space telescope, and, hence, the conditions in the Early Universe.

My scientific curriculum vitae can be downloaded here (state: 3th Feb 2025)

— Summary —

In the Universe there are stars, which are several times heavier than our Sun. New born stars with more than eight times the mass of our Sun are called in astrophysics massive stars. These stars have very high surface temperatures (more than 25,000 K) and consequently emit most of their light as ultraviolet (UV) radiation. Hence, they appear blue to the human eye. Blue massive stars are also much brighter than our Sun. In fact, they are so bright that most of the light of the stars we can see by eye on the night sky is originating from massive stars. Moreover, with powerful telescopes it is actually possible to measure the light of such stars in other nearby galaxies. My current research focuses on the study of massive stars located in the nearby statelite galaxies called the Small- and Large Magellanic Cloud (SMC and LMC). With my studies I aim to improve our understanding of the evolution of massive stars by comparing the observed properties with theoretical models. Studying these stars in the SMC and LMC are proxies that help us to understand complete populations of stars and galaxies in the Early Universe.

Massive stars strongly influence their surroundings during their entire evolution. When a massive star is still core hydrogen burning it is very hot and emits copious amounts of UV radiation. However, not all of this radiation can escape, as some fraction is absorped by heavier elemets — such as iron — in the stars atmosphere and is converted to kinetic energy in form of a stellar wind. These stellar winds can become very powerful and are thought to remove noticable amounts of material from the stellar surface, enriching its surrounding material. When the hydrogen is exhausted in the core of a star, it will start burning helium in its core while having a hydrogen rich envelope. In the case the star has still a large hydrogen rich envelope it will expand to several thousand times the radius of our Sun and because of that, the surface of the star will appear cooler (about 3500 K). On the night sky the extended star would appear red to the human eye. These stars are called "red supergiants" (RSG). However, if the star can lose its hydrogen rich envelope, for example it is blown away by a stellar wind, the processed material of the previously hydrogen burning core will be exposed at the surface and the star will appear even hotter (about 100,000 K). As a consequence it will have more UV radiation which can be used to power an even stronger wind. These partially or even fully stripped stars are typically classified as so-called "Wolf-Rayet" (WR) stars.

However, massive stars are rarely born alone and most of them can be found in binary or even higher multiplicity systems. All stars expand during their evolution and in a close binary system at some point the expanding material will feel the gravitiational attraction from the second star. When this happens, it is energetically easier for the material to be transfered to the secondary than being bound to the expanding star. This will start a phase of mass transfer during which the hydrogen rich envelope of the expanding star will be removed. After a phase of stable mass transfer the binary will consist of a partially stripped star with a core hydorgen burning companion. However, the phase of mass transfer is short, compared to the lifetime of a star and observational counterparts are hard to detect. Hence, there are still many unknowns in the physics of mass transfer and the accretion efficiency of the secondary companion.

In my current work I am in the unique position to be working on both, the observational as well as the theoretical side to improve our understanding of massive stars. On the observational side, I have obtained multi-epoch optical observations of the most massive stars known in the SMC and combined with UV spectra from the Hubble Space Telescope (HST) I am searching for pre- and post interaction massive binaries with the aim of getting valuable information on their stellar and wind properties, as well as their orbital configuration. This is a pioneering work, which lays the foundation for a novel understand of the efficiency of mass transfer in low metallicity environments as well as it helps us to understand how stellar winds are formed in pre- and post interaction massive binaries. On the theoretical side I am working at the forefront and calculate large grids of detailed binary evolution models and compare their predictions to observed stellar populations — such as the WR population in the SMC and LMC — trialing our current physical understanding or massive star evolution.

— Noticable Works —

— A synthetic population of Wolf-Rayet stars in the LMC —

More than 50% of all massive stars are borne in binaries and will interact with each other during their life. However, it is not fully clear how these binary interactions influence our picture of stellar evolution. Are there dominant formation channels for specific kinds of stars that we observe? As already described above, Wolf-Rayet stars are stars that have lost most of their hydrogen rich envelope, either by removal of a stellar wind or by removal during an interaction with a companion star. Wolf-Rayet stars come in different flavours and can be subdivided in four classes: WN type stars with hydrogen that show strong emission lines from nitrogen, helium and hydrogen; WN type stars without hydrogen, showing strong emission lines from nitrogen and helium; WC type stars, showing strong emission lines of carbon and helium (and maybe oxygen); And WO type stars showing strong emission lines of oxygen, carbon and helium. The total numbers of these subclasses in a complete statistical representative population of stars strongly depends on the assumed physics parameters, such as the mass-loss rates as well as the mass transfer efficiency.

In our study we wanted to benefit of this sensitivity to test our understand of massive single and binary star evolution and to explore if there is a dominant channel in forming Wolf-Rayet stars. As the Wolf-Rayet population of our Galaxy is not completely known and the population of the SMC is too small to be statistical relevant, we decided to use the observed Wolf-Rayet population of the LMC as our testbed. Hence, we create a synthetic Wolf-Rayet population at LMC metallicity based on more than 10,000 detailed evolutionary models and compared the observed and predicted luminosity distributions of the individual Wolf-Rayet subclasses.

We were able to match and explain the observed luminosity distribution of the observed H-free WN and WC population with our model calculations. From our model calculations we propose that binary evolution is the dominant channel to form Wolf-Rayet stars in low metallicity environments and that only the brightest Wolf-Rayet stars can originate from single stars. Our models have some problems in explaining the observed population of WN stars with hydrogen at the surface, which might be linked to missing physics during and/or after mass transfer. If you are interested in more details, please have a look here.

— A bimodal temperature distribution in low metallicity Wolf-Rayet populations? —

Measuring the stellar properties, such as the temperature of a Wolf-Rayet stars is not easy. A Wolf-Rayet star is covered by a strong stellar wind, which is optically thick and hides the stellar surface from the observer. Stellar temperatures can only be inferred from detailed stellar atmosphere models. However there is a difference between the stellar temperature predicted by stellar evolution models and the ones which are observed. Previous studies, explained these discrepancies by an inflation of the envelope of these stars, making them appear larger and hence cooler. At low metallicity, the stellar winds get weaker and one can see, at least for the least luminous Wolf-Rayet stars, parts of the surface of the star. Also the effect of envelope inflation is expected to be less pronounced or even absent, simplifying this picture. Nonetheless, the SMC hosts Wolf-Rayet stars, that appear to be cooler (50,000 K) than the rest of the population (typical temperatures are around 100,000 K). Hence, the question about the origin of these stars seems to be unanswered.

In most binary evolutionary calculations the evolution of the secondary component is not included, only approximated or just neglected. In our recent work we remedied this situation and calculated the full evolution of the secondary component, with the option for it to have a second mass transfer phase back on a compact companion (the previos primary component). The models showed something surprising, the secondaries which were able to accrete a significant amount of material during the first mass transfer event changed there structure and, hence, will lose less mass during their mass transfer phase. As a consequence they can stay at cooler temperatures and might be the explanation of the observed Wolf-Rayet stars with lower surface temperatures.

Based on these calculations we propose that there must be a bimodal temperature distribution of Wolf-Rayet stars in low metallicity environments in which the hotter of the Wolf-Rayet stars originate from the primary components as well as the secondary components that did not accrete a noticable amount of material, while the cooler of the Wolf-Rayet stars originate from secondary components that have accreted several solar masses of material. If you are interested in more details, please have a look here.

— Cl* NGC 346 SSN 7: A binary undergoing mass transfer —

Massive stars spend about 90% of their time fusing hydrogen in the core to helium. This process can take typically a few million years. A phase of mass transfer is — in a astronomical sence — much shorter, as it typically happens on a timescale of 10,000 to 100,000 years. Hence, catching such a system during a mass transfer phase is like finding a neadle in the haysack. However, in our recent work, Matthew Rickard and I, have discovered the most massive interacting binary so far. It is called Cl* NGC 346 SSN 7 and hosts a partially stripped star which has a mass of 32 times the solar mass and the accretor, which has 55 times the mass of our Sun. The two stars orbit each other every 3 days while exchanging mass.

The system is a unique example that can help us to constrain the efficiency and physics of mass transfer in such massive systems. With state-of-the-art binary stellar evolution models we were able to understand the current state of the binary system and make preditions about its future evolution. In the (astronomically seen) near future, the two stars will stop transferring mass and the primary will evolve into a Wolf-Rayet star, while the secondary will continue burning hydrogen in the core. After a few hundred thousand years, the primary will "die" and leave behind a black hole, that will continue orbiting the secondary star. A few million years later, the secondary will expand enough, such that it will start tranfering mass back onto the compact companion. When the mass transfer event is finished, the secondary will also evolve into a Wolf-Rayet star, but will have a lower temperature (see above). Again after a few hundred thousand years also the secondary will "die" and form a black hole. These two black holes will orbit each other for about 18 billion years, emitting gravitiational waves, until they come close enought and finally will merge. This mergere would be detecable with current gravitiational wave detectors, such as Ligo and Virgo.

— first-authored publications —

— co-authored publications —

— Theses —