Research Highlights

The diversity of core-collapse supernova explosions and SN 1987A

The work of PhP, starting with Podsiadlowski et al. (1992, ApJ, 391, 246), has demonstrated that binary interactions play a major part in causing the observed diversity of core-collapse supernovae. These include binary mass stripping, probably producing the majority of stripped core-collapse supernovae (those containing no or just little hydrogen, e.g. SNe Ib, Ic, IIb), stars modified by accretion or even merging, producing supernova progenitors distinct from single stars. In particular, binary mergers can produce blue-supergiant progenitors rather than the standard red-supergiant progenitors expected from massive single stars (Podsiadlowski et al. 1990; Justham et al. 2014).

SN 1987A, the last naked-eye supernova since Kepler’s supernova in 1604, had such a blue-supergiant progenitor. PhP has been the principal driving force (jointly with some of his students, Natasha Ivanova and Thomas Morris, and other colleagues) to develop a consistent merger model for this unusual event with very anomalous properties (Podsiadlowski 2017). In particular, they showed that the triple-ring nebula, observed with the HST, which was dynamically ejected 20,000 yr before the explosion, provides a fingerprint for a merger of two massive stars at that time (Morris & Podsiadlowski 2007).

The final fate of massive stars and LIGO

There has been dramatic progress in recent years in understanding how the collapse of the core of a massive star that has run out of nuclear fuel can drive a supernova explosion, and the type of compact object (neutron star or black hole) this leaves behind. This for the first time makes it possible to link the evolution of massive stars to the supernovae they produce and the final remnants. In a series of papers, led by Fabian Schneider (Schneider et al. 2021; Schneider et al. 2024), they have been able to predict the remnant mass spectrum of massive stars, both for single and binary stars. LIGO, the gravitational wave detector, that has heralded a new epoch of gravitational-wave astronomy in 2015, can detect mergers of compact objects, in particular black holes, and should be able to test the very specific black-hole mass spectrum they have predicted in the near future (Schneider et al. 2023; Laplace et al. 2025).

The era of gravitational-wave astronomy

The first detection of gravitational waves with the Laser Interferometer Gravitational-Wave Observatory (LIGO) from the merger of two black holes in 2015 (Abbott et al. 2016) has initiated the new era of gravitational-wave astronomy. So far, LIGO has discovered the mergers of more than 100 compact objects (mostly black holes, but in some cases neutron stars). As LIGO is more sensitive to more massive objects, it particularly tests the fate of very massive stars. PhP is playing a major role in trying to understand the evolutionary origin of these objects in the context of standard binary evolution (e.g. Marchant et al. 2021) and a more exotic binary scenario which involves very rapidly rotating stars that evolve homogeneously (e.g. Marchant et al. 2016; du Buisson et al. 2020).

Binary evolution theory

PhP is an expert on all aspects of binary evolution, whether it involves the modeling of the various types of binary interactions, such as stable mass transfer, unstable mass transfer and common evolution or merging (including hydrodynamical mergers simulations; e.g. Fabian, Ohlmann et al. 2019). In particular, the conditions for unstable mass transfer and the resulting common-envelope phase has been a major effort in recent years (e.g. Podsiadlowski 2001; Ivanova et al. 2012; Clayton et al. 2017; Bronner et al. 2024). With Shazrene Mohamed he developed the concept of “wind Roche-lobe overflow”, where the slow wind off a giant or Mira variable effectively fills its Roche lobe (e.g. , Podsiadlowski & Mohamed 2007; Mohamed & Podsiadlowski 2007) and demonstrated how mass loss in symbiotic binaries shapes their circumstellar medium, providing a potential signature in Type Ia supernovae (Patat et al. 2007; Booth et al. 2016).

Stellar hydrodynamics: magnetic stars, eta Car, supernova collisions and unstable envelopes

Binary mergers are one of the most dramatic types of binary, but are still poorly studied in the literature. With Fabian Schneider and Sebastian Ohlmann, PhP has systematically studied mergers as a possible origin of magnetic stars, combining 3d MHD simulations of mergers with stellar-evolution modeling of the merger product (Schneider, Ohlmann et al. 2019; 2020). Arguably the most dramatic observed merger event was the Great Eruption of eta Carinae in the 1840s. With Ryosuke Hirai, Stan Owocki, Fabian Schneider and Nathan Smith, they have been able to not only reproduce the Great Eruption, but also many other major historical features of this extraordinary event, involving initially a triple-star system that ultimately became unstable (Hirai et al. 2021). Ryosuke Hirai and PhP have systematically been exploring the effects of a supernova on a companion and possible detectable signatures in the supernova (Hirai et al. 2018) and, most dramatically, direct dynamical interactions of a newborn neutron star with a companion (Hirai et al. 2022) , as likely has now been detected in the recent SN 2022jli (Hirai et al. 2025). Matthew Clayton and PhP have demonstrated, using realistic 1d stellar hydrodynamics calculations, that stars with very extended envelopes are dynamically unstable and may experience episodic mass ejection events (e.g. Clayton 2017). This may explain the circumstellar medium inferred in some SNe II-P from red supergiants (Clayton 2018).

Binaries with compact components and millisecond pulsars

PhP has worked on most types of binaries that include compact companions, whether these are white dwarfs (Podsiadlowski et al. 2003), neutron stars (Podsiadlowski et al. 2002, Pfahl et al. 2003) or black holes (Podsiadlowski et al. 2003; in particular, with Saul Rappaport, Stephen Justham, Eric Pfahl, Zhanwen Han). Of special interest have been ultraluminous X-ray binaries (e.g. Rappaport et al. 2005), that appear to significantly radiate above the Eddington limit. He has also demonstrated how irradiation effects can drive mass transfer in X-ray binaries, possibly explaining the statistically large observed mass-accretion rates in these systems (Podsiadlowski 1991). He has pioneered binary population synthesis studies (where whole populations of binaries are simulated) that, for the first time, instead of using simplified prescriptions, use realistic binary evolution calculations and demonstrated the importance of intermediate-mass stars in the context of low-mass X-ray binaries. With Thomas Tauris, Norbert Langer, Saul Rappaport, PhP has worked extensively on the origin of millisecond pulsars, including double-neutron star systems (e.g. Tauris et al. 2017). He has been particularly interested in the role natal neutron star kicks play in the formation of binary pulsar systems (Brandt & Podsiadlowski 1995). With Norbert Langer, he has argued that, based on the nature of the supernova explosion, there should both be a high-velocity and a low-velocity kick component (Podsiadlowski, Langer, et al. 2004), a proposal now strongly supported by both actual supernova calculations and observational evidence.

Hot subdwarfs as tests of binary evolution

Hot subdwarfs are compact stars burning helium in the centre that have lost their hydrogen envelopes most likely by a binary interaction. This generally requires special circumstances and makes them particularly suitable to study binary evolution physics (in particular, the efficiency of common-envelope evolution, the conservativeness of mass transfer, etc.). Zhanwen Han and PhP have performed a pioneering study on the origin of hot subdwarfs that has become the standard reference in the field (published in Han et al. 2002; Han et al. 2003). They also applied their work to the UV upturn in elliptical galaxies which do not contain other UV-bright stars (Han et al. 2007). In recent years there has been an enormous wealth of new data that show many new effects, suggesting even new channels for forming hot subdwarfs. PhP is leading a major project to update these earlier studies with Zhanwen Han and Stephen Justham and many of the principal observers in the field (e.g. Stephan Geier, Ulrich Heber, Veronika Schaffenroth for an incomplete list).

Type Ia supernovae

Type Ia supernovae (SNe Ia) are believed to be caused by thermonuclear explosions of CO white dwarfs, but it has remained a major decades-old unsolved problem what could cause these explosions. PhP has worked on both of the two most popular models, involving either accretion in a binary from a stellar companion (the so-called single-degenerate model; e.g. Han et al. 2004) or the merger of two white dwarfs (the so-called double-degenerate model; e.g. Yoon et al. 2007). With Xiangcun Meng, he has recently developed a new version of the single-degenerate model, which they named the common-envelope wind model (Meng & Podsiadlowski 2017). With Peter Höflich, he is currently reassessing the progenitor controversy using the latest observational data and theoretical developments, which already rules out many recent progenitor models. This will undoubtedly become a major effort at the Centre.

Planets around pulsars

The first planets were not discovered around a normal star like the Sun but around a pulsar, PSR 1257+12 (Wolszczan & Frail 1992, Nat, 355, 145). PhP has developed a whole number of possible models (see the review Podsiadlowski 1993) to explain such planets, including the formation of the system from a collision of a neutron star with a solar-type system in a dense cluster, formation in a circumbinary disk, etc. The probably most likely scenario is that they formed as second-generation planets out of the debris of a destroyed companion star. In this case, the planet formation could be very similar to planet formation in our own solar system.