arXiv subjects

Marco Vetter

Publications and source records attributed to Marco Vetter.

4 recordsLinked to original sources

arXiv2026

Localized $^{18}$O production in white dwarf mergers

The merger of a He white dwarf (WD) and a CO WD is the favored formation channel for R Coronae Borealis (RCB) stars. These stars exhibit ${^{16}}\mathrm{O}/{^{18}}\mathrm{O}$ ratios that are orders of magnitude lower than the solar value. However, it is not fully understood whether such low ${^{16}}\mathrm{O}/{^{18}}\mathrm{O}$ ratios can be achieved in WD merger remnants for the predicted lifetime of RCB stars of around $10^4\,\mathrm{years}$. In this work, we perform detailed nucleosynthesis calculations of a 3D magnetohydrodynamical simulation of a merger of a $0.3\,M_\odot$ He WD and a $0.6\,M_\odot$ CO WD for $4000\,\mathrm{s}$ at which point a steady state in temperature and density is reached. From this point, we follow several radial zones to study the long-term production of ${^{18}}\mathrm{O}$ and its variability throughout the burning region. We find that the asymmetric merger process leaves an imprint on the distribution of the abundances at the end of our hydrodynamic simulation. During the long-term evolution up to $100\,\mathrm{years}$, we observe ${^{16}}\mathrm{O}/{^{18}}\mathrm{O}$ ratios of order of unity, although the timescale on which ${^{18}}\mathrm{O}$ is destroyed again is highly location dependent. Importantly, our calculations suggest that in the outer layers of the burning shell, the dominant production channel is $^{14}\mathrm{C}(α,γ)^{18}\mathrm{O}$ instead of the commonly considered $^{14}\mathrm{N}(α,γ)^{18}\mathrm{F}(β^+)^{18}\mathrm{O}$ reaction, whereby the former can be sustained for longer periods of time. Furthermore, these outer regions do not reach the conditions necessary for fast $α$-captures in ${^{18}}\mathrm{O}$ to ${^{22}}\mathrm{Ne}$, thus being favorable to maintaining a low ${^{16}}\mathrm{O}/{^{18}}\mathrm{O}$ ratio.

astro-ph.SR↗

arXiv2025

Magnetically driven outflows in 3D common-envelope evolution of massive stars

Recent three-dimensional magnetohydrodynamical simulations of the common-envelope interaction revealed the self-consistent formation of bipolar magnetically driven outflows launched from a toroidal structure resembling a circumbinary disk. So far, the dynamical impact of bipolar outflows on the common-envelope phase remains uncertain and we aim to quantify its importance. We illustrate the impact on common-envelope evolution by comparing two simulations -- one with magnetic fields and one without -- using the three-dimensional moving-mesh hydrodynamics code AREPO. We focus on the specific case of a $10 M_\odot$ red supergiant star with a $5 M_\odot$ black hole companion. By the end of the magnetohydrodynamic simulations (after $\sim 1220$ orbits of the core binary system), about $6.4 \%$ of the envelope mass is ejected via the bipolar outflow, contributing to angular momentum extraction from the disk structure and core binary. The resulting enhanced torques reduce the final orbital separation by about $24 \%$ compared to the hydrodynamical scenario, while the overall envelope ejection remains dominated by recombination-driven equatorial winds. We analyze field amplification and outflow launching mechanisms, confirming consistency with earlier studies: magnetic fields are amplified by shear flows, and outflows are launched by a magneto-centrifugal process, supported by local shocks and magnetic pressure gradients. These outflows originate from $\sim 1.1$ times the orbital separation. We conclude that the magnetically driven outflows and their role in the dynamical interaction are a universal aspect, and we further propose an adaptation of the $α_\mathrm{CE}$-formalism by adjusting the final orbital energy with a factor of $1+ M_\mathrm{out}/μ$, where $M_\mathrm{out}$ is the mass ejected through the outflows and $μ$ the reduced mass of the core binary. (abridged)

astro-ph.SR↗

arXiv2024

Large-scale ordered magnetic fields generated in mergers of helium white dwarfs

Stellar mergers are one important path to highly magnetised stars. Mergers of two low-mass white dwarfs may create up to every third hot subdwarf star. The merging process is usually assumed to dramatically amplify magnetic fields. However, so far only four highly magnetised hot subdwarf stars have been found, suggesting a fraction of less than $1\%$. We present two high-resolution magnetohydrodynamical (MHD) simulations of the merger of two helium white dwarfs in a binary system with the same total mass of $0.6\,M_\odot$. We analysed an equal-mass merger with two $0.3\,M_\odot$ white dwarfs, and an unequal-mass merger with white dwarfs of $0.25\,M_\odot$ and $0.35\,M_\odot$. We simulated the inspiral, merger, and further evolution of the merger remnant for about $50$ rotations. We found efficient magnetic field amplification in both mergers via a small-scale dynamo, reproducing previous results of stellar merger simulations. The magnetic field saturates at a similar strength for both simulations. We then identified a second phase of magnetic field amplification in both merger remnants that happens on a timescale of several tens of rotational periods of the merger remnant. This phase generates a large-scale ordered azimuthal field via a large-scale dynamo driven by the magneto-rotational instability. Finally, we speculate that in the unequal-mass merger remnant, helium burning will initially start in a shell around a cold core, rather than in the centre. This forms a convection zone that coincides with the region that contains most of the magnetic energy, and likely destroys the strong, ordered field. Ohmic resistivity might then quickly erase the remaining small-scale field. Therefore, the mass ratio of the initial merger could be the selecting factor that decides if a merger remnant will stay highly magnetised long after the merger.

astro-ph.SR↗

arXiv2024

Evolution and final fate of massive post-common-envelope binaries

Mergers of neutron stars (NSs) and black holes (BHs) are nowadays observed routinely thanks to gravitational-wave (GW) astronomy. In the isolated binary-evolution channel, a common-envelope (CE) phase of a red supergiant (RSG) and a compact object is crucial to sufficiently shrink the orbit and thereby enable a merger via GW emission. Here, we use the outcomes of two three-dimensional (3D) magneto-hydrodynamic CE simulations of an initially 10.0 solar-mass RSG with a 5.0 solar-mass BH and a 1.4 solar-mass NS, respectively, to explore the further evolution and final fate of the post-CE binaries. Notably, the 3D simulations reveal that the post-CE binaries are likely surrounded by circumbinary disks (CBDs), which contain substantial mass and angular momentum to influence the subsequent evolution. The binary systems in MESA modelling undergo another phase of mass transfer (MT) and we find that most donor stars do not explode in ultra-stripped supernovae (SNe), but rather in Type Ib/c SNe. The final orbits of our models with the BH companion are too wide, and NS kicks are actually required to sufficiently perturb the orbit and thus facilitate a merger via GW emission. Moreover, by exploring the influence of CBDs, we find that mass accretion from the disk widens the binary orbit, while CBD-binary resonant interactions can shrink the separation and increase the eccentricity depending on the disk mass and lifetime. Efficient resonant contractions may even enable a BH or NS to merge with the remnant He stars before a second SN explosion, which may be observed as gamma-ray burst-like transients, luminous fast blue optical transients and Thorne-Żytkow objects. For the surviving post-CE binaries, the CBD-binary interactions may significantly increase the GW-induced double compact merger fraction. We conclude that accounting for CBD may be crucial to better understand observed GW mergers.

astro-ph.HE↗