Research projects

Here you can find an overview of our main current research projects. However, we are also interested in other topics. Please have a look at our publications and contact us, if you are interested in a collaboration.

Hunting and gathering hot subdwarf stars

Geier et al. 2019, A&A, 621, 38

Hot subdwarfs play an important role in our understanding of binary evolution, stellar atmospheres and interiors and are key objects to study peculiar events, ranging from star-planet interactions to type Ia supernova progenitors. Analyses of large, complete, and volume-limited samples can therefore constrain a valuable variety of astrophysical topics.
We are working on compiling all-sky catalogues of hot subdwarfs, among them the first volume-limited samples of spectroscopically confirmed hot subdwarfs. Candidate samples are also compiled to provide target lists for large photometric (TESS, BlackGEM, LSST, PLATO) and spectroscopic surveys.
All our observations are uploaded to our database AOTS , which is accessible to the public.
As members of the 4MOST collaboration, we will conduct an all-sky survey for hot subluminous stars in the Southern hemisphere.
In addition to that, we also compile and follow-up all-sky samples of related objects such as extremely low-mass white dwarfs, blue horizontal branch stars and progenitors in the red-giant stage.

Pelisoli, I., & Vos, J.: Gaia Data Release 2 catalogue of extremely low-mass white dwarf candidates, 2019, MNRAS, 488, 2892

Kepler, S. O., Pelisoli, I., Koester, D., et al.: White dwarf and subdwarf stars in the Sloan Digital Sky Survey Data Release 14, 2019, MNRAS, 486, 2169

Geier, S., Raddi, R., Gentile Fusillo, N. P., Marsh, T. R.: The population of hot subdwarf stars studied with Gaia. II. The Gaia DR2 catalogue of hot subluminous stars, 2019, A&A, 621, 38

Observing and studying close binary stars

Erebos elm_picture
V. Schaffenroth/Artist's conception of extremely low mass detached double white dwarf binary. Credit: Melissa Weiss

Hot subdwarfs, extremely low-mass white dwarfs, as well as central stars of planetary nebulae are frequently found in close binary systems. Studying these systems allows us to understand the common envelope phase and the formation of asymmetrical planetary nebulae. We are also searching for the progenitor systems of supernovae Ia and verification binaries for gravitational wave detectors such as LISA.
Furthermore, these systems allow us to study the influence of substellar companions on late stellar evolution. For that we formed an international collaboration and were granted a Large Programme at the ESO-VLT (Eclipsing Reflection Effect Binaries from Optical Surveys, EREBOS). Our goal is to observe a newly discovered sample of eclipsing hot subdwarf binaries with low-mass companions, determine the fundamental parameters of those systems and search for substellar companions like brown dwarfs and hot Jupiter planets. In addition to the spectroscopic follow-up with the FORS2 instrument at the VLT, we use smaller telescopes in the to obtain follow-up photometry and spectroscopy.
Currently we also make use of the Transiting Exoplanet Survey Satellite TESS, which is gathering light curves for thousands of nearby stars (including hundreds of hot subdwarfs and white dwarfs) with precision better than 1%. Although its primary goal is to find planets, the 2-minute cadence light curves obtained by TESS for pre-selected stars, are also ideal to search for variability effects caused by a binary companion, such as reflection and eclipses. We are members of the TESS Asteroseismic Science Consortium TASC and coordinate the efforts to observe compact close binaries. The brightness of TESS pre-selected targets also makes them ideal for ground-based follow-up, allowing for thorough characterisation of the observed systems and providing constraints for binary evolution models. As members of the BlackGEM collaboration, we will also study fainter close binaries based on multi-band light curves.

Schaffenroth, V., Barlow, B. N., Geier, S., et al.: The EREBOS project: Investigating the effect of substellar and low-mass stellar companions on late stellar evolution. Survey, target selection, and atmospheric parameters, 2019, A&A, 630, 80

Schaffenroth, V., Geier, S. Heber, U., et al.: The MUCHFUSS photometric campaign, 2018, A&A, 614, 77

Hypervelocity stars


Hypervelocity stars (HVSs) travel with such extreme velocities that they are unbound to the gravitational potential of our galaxy. Dynamical ejection via gravitational interaction with a massive black hole is their most likely origin, but also ejections from an extremely tight binary by a thermonuclear supernova explosion are possible.
US 708, which travels with a velocity of about 1200 kilometers per second, is in contrast to the other hypervelocity stars a fast rotating, compact helium star likely formed by interaction with a close companion, fits well into this alternative acceleration scenario. US 708 originally resided in an ultracompact binary system and transferred helium to a massive white dwarf companion. An ignition of the helium on the surface of the white dwarf triggered its explosion as thermonuclear supernova of type Ia. The surviving companion was then ejected and is now travelling with extreme velocity. These results provide observational evidence that there is a link between hypervelocity helium stars and supernova Ia, which might help us to understand the progenitors of those explosions better.
We perform an extensive spectroscopic survey to find more fast hot subdwarfs like US 708.

Irrgang, A., Geier, S., Heber, U., et al.: PG 1610+062: a runaway B star challenging classical ejection mechanisms, 2019, A&A, 628, 5
Press releases: Keck Observatory, Scientific American

Geier, S., Fuerst, F., Ziegerer, E., et al.: The fastest unbound star in our Galaxy ejected by a thermonuclear supernova, 2015, Science, 347, 1126
Press releases: IfA Hawaii, University Erlangen Radboud University Nijmegen Yunnan Observatory

Quantitative spectroscopic analyses of hot compact stars


Quantitative spectroscopy is not only the key to understand the evolutionary of stars but also allows us to use hot subdwarfs and white dwarfs as laboratories for fundamental physics or powerful tools for Galactic archaeology and cosmology. We employ state-of-the art model atmosphere codes to analyse spectroscopic observations obtained from both ground-based and space-based observatories. Deriving the atmospheric parameters of hot subdwarfs, central stars of planetary nebulae, and hot white dwarfs, we are able to disentangle the various evolutionary paths of these objects, and in some cases even to study stellar evolution in real time.

Irrgang, A., Geier, S., Kreuzer, S., et al.: A stripped helium star in the potential black hole binary LB-1, 2020, A&A, 633, 5L
Press releases: Potsdam University

Dorsch, M., Latour, M., Heber, U.: Heavy metals in intermediate He-rich hot subdwarfs: the chemical composition of HZ 44 and HD 127493, 2019, A&A, 630, 130

Reindl, N., Geier, S., Ostensen, R. H..: Discovery of two bright DO-type white dwarfs, 2018, MNRAS, 480, 1211

Extremely-low mass white dwarf stars

Pelisoli & Vos 2019, MNRAS, 488, 2892

Extremely-low mass white dwarf stars (ELMs) are degenerate helium-core stars with mass below 0.3 solar masses, born either as a result of a common-envelope phase or after a stable Roche-lobe overflow episode in a multiple system. The Universe is not old enough for ELMs to have formed through single-star evolution channels. As remnants of binary evolution, ELMs can shed light onto the poorly understood phase of common-envelope evolution and provide constraints to the physics of mass accretion. Most known ELMs will merge in less than a Hubble time, providing an important contribution to the signal to be detected by upcoming space-based gravitational wave detectors. We have compiled the largest catalogue of ELM candidates from the Gaia DR2 data, and are currently carrying out observational follow-up with the goal of compiling the first volume-limited sample of ELMs. This pioneer sample will serve as benchmark to test and improve binary evolution models, as well as contribute to interpret gravitational wave signals detected from space.

Pelisoli, I., Bell, K. J., Kepler, S. O., Koester, D.: The sdA problem - III. New extremely low-mass white dwarfs and their precursors from Gaia astrometry, 2019, MNRAS, 482, 3831

Wide binary hot subdwarf stars

rlof Pq

Hot subdwarf binaries are ideal systems to constrain binary interaction mechanisms. The majority of the hot subdwarf binaries are formed from low mass progenitors that ignite He under degenerate conditions. This places strong constraints on the mass of those stars. Furthermore, the only formation channels for sdB stars are through binary interactions. Every sdB star that is observed is thus an extra datapoint to match to the theoretical models. Wide sdB binaries with main sequence companions have the added advantage that they are double-lined systems allowing to constrain both components based on spectroscopic observations. Even though it takes years to solve the orbits, we currently have a reasonable sample that can be used as a test ground for binary interactions.
Interesting correlations found in the orbital properties of wide sdB+MS binaries are the increasing eccentricity with increasing orbital period and a strong relation between the mass ratio and the orbital period. The eccentricity is unexpected as the sdB progenitors ascend the RGB and the binaries are expected to circularise long before the mass-loss phase starts. The observed properties indicate that eccentricity pumping mechanisms are at play during the mass-loss phase, including the formation of a circumbinary disk. The correlation between orbital period and mass ratio is potentially even more surprising as there is no a priori reason for such a correlation to exist. While it was originally attributed to the stability of RLOF, binary population synthesis studies have shown that Galactic evolution, and specifically the metallicity gradient with time, is to blame for this relation.
There are still many uncertainties in the interaction between the loosely bound envelope of a giant and it's main sequence companion. Our current work is focused on using machine learning methods to improve the current population synthesis methods and constrain some of the unknown parameters of the interaction mechanisms.

Vos, J., Vuckovic, M., Chen, X., et al.: The orbital period-mass ratio relation of wide sdB+MS binaries and its application to the stability of RLOF, 2019, MNRAS, 482, 4592