Gravitational-Wave Signals for Supernova Explosions of Three-Dimensional Progenitors

Core-collapse supernovae (SNe) are sources of gravitational waves (GWs) produced by hydrodynamical instabilities and highly time-dependent anisotropies of the neutrino radiation. In this work we analyze both contributions to the GW signal for two state-of-the-art three-dimensional (3D) SN models computed with the Prometheus-Vertex neutrino-hydrodynamics code. In contrast to the far majority of models analyzed for GWs so far, our core-collapse simulations were started with 12.28 M_sun (18.88 M_sun) progenitors, whose final hour (7 min) of convective oxygen-shell burning was computed in 3D and featured a vigorous oxygen-neon shell merger. The corresponding large-scale asymmetries in the oxygen layer are conducive to buoyancy-aided neutrino-driven explosions. The models were continuously evolved in 3D from the pre-collapse evolution until 5.11 s (1.68 s) after the core bounce. The GW signals result from the well-known dynamical phenomena in the SN core such as prompt postshock convection, neutrino-driven convection, the standing accretion shock instability, proto-neutron star oscillations, and anisotropic ejecta expansion. They do not exhibit any new or specific features that can be unambiguously connected to the powerful pre-collapse activity in the progenitors, but we identify interesting differences compared to results in the literature. We also discuss measurement prospects by interferometers, confirming that GW signals from future Galactic SNe will be detectable with existing and next-generation experiments working in the frequency range f ~ 1-2000 Hz.

💡 Research Summary

This paper presents a comprehensive analysis of gravitational‑wave (GW) emission from core‑collapse supernovae (CCSNe) using two state‑of‑the‑art three‑dimensional (3D) simulations that include fully 3D pre‑collapse progenitor evolution. The authors selected non‑rotating progenitors of 12.28 M⊙ and 18.88 M⊙, each of which was evolved in 3D for the final hour (for the lower‑mass star) or seven minutes (for the higher‑mass star) of convective oxygen‑shell burning. During this phase a vigorous oxygen‑neon shell merger develops, creating large‑scale asymmetries in density, velocity, and composition. These asymmetries are carried into the collapse phase and are shown to accelerate the growth of post‑shock convection and the standing accretion‑shock instability (SASI), thereby facilitating earlier neutrino‑driven explosions.

The collapse and explosion phases were simulated with the Prometheus‑Vertex code, which couples a Newtonian hydrodynamics solver (Prometheus) with a multi‑group, O(v/c) neutrino‑transport module (Vertex). Neutrino transport is treated with the ray‑by‑ray‑plus (RbR+) approximation, and general‑relativistic corrections are applied to the gravitational potential (Case A) and to the transport. The computational grid employs a Yin‑Yang topology, providing uniform angular resolution of 3.5° (12.28 M⊙ model) and 2° (18.88 M⊙ model) over the full 4π sphere. The 12.28 M⊙ model uses the SFHo equation of state, while the 18.88 M⊙ model adopts LS220. The simulations were continued for 5.11 s and 1.68 s after bounce, respectively, with explosion times (defined by the shock radius exceeding 400 km) of 0.292 s and 0.469 s.

Gravitational‑wave extraction follows the standard Newtonian quadrupole formula for matter motions, supplemented by a separate term accounting for anisotropic neutrino emission. The resulting GW signal exhibits the well‑known components observed in previous CCSN studies:

• Prompt post‑shock convection (∼10 ms after bounce) produces a short burst at 100–200 Hz.
• Neutrino‑driven convection and SASI generate quasi‑periodic emission in the 100–250 Hz band lasting several hundred milliseconds.
• Proto‑neutron‑star (PNS) surface g‑ and f‑mode oscillations dominate the 500 Hz–>1 kHz region, with frequencies rising as the PNS contracts.
• Asymmetric ejecta expansion yields a very low‑frequency (<10 Hz) “memory” component.
• Anisotropic neutrino emission contributes a persistent low‑frequency signal (0.1–10 Hz).

The total GW energy radiated by matter motions is 6.4 × 10⁻¹⁰ M⊙c² (12.28 M⊙) and 1.0 × 10⁻⁹ M⊙c² (18.88 M⊙); the corresponding neutrino‑induced GW energies are 4.2 × 10⁻¹¹ M⊙c² and 7.0 × 10⁻¹¹ M⊙c², i.e., roughly 5–7 % of the matter contribution. The compactness parameters ξ₁.₅ are 0.515 and 0.992, indicating relatively high core compactness that aids explosion.

A key finding is that, despite the pronounced pre‑collapse asymmetries caused by the oxygen‑neon shell merger, the GW waveforms do not display any unmistakable signatures that could be uniquely traced back to those progenitor features. The signals remain qualitatively similar to those obtained from models that start from spherically symmetric 1D progenitors, although subtle quantitative differences (e.g., slightly larger amplitudes of the PNS f‑mode) are noted.

Detection prospects were evaluated for current ground‑based interferometers (Advanced LIGO, Virgo, KAGRA) and for planned third‑generation detectors (Einstein Telescope, Cosmic Explorer). Assuming a Galactic source at 10 kpc, the 18.88 M⊙ model yields a signal‑to‑noise ratio of ≈8–12 in the 1–2000 Hz band for Advanced LIGO‑design sensitivity, making detection plausible. The most promising band for extraction of physical information is 500–1000 Hz, where PNS oscillations dominate. Low‑frequency memory components are below the sensitivity of present ground‑based detectors but could be observable with future space‑based deci‑Hertz missions (e.g., DECIGO, BBO).

In summary, the paper demonstrates that incorporating realistic 3D pre‑collapse progenitor evolution influences the timing and vigor of the explosion but does not imprint distinctive new features on the GW signal. The study reinforces the view that GW observations of a Galactic CCSN will be dominated by the well‑understood hydrodynamic phases (prompt convection, SASI, PNS modes) and that upcoming GW observatories will be capable of detecting such signals, thereby providing a powerful probe of the core‑collapse mechanism, the nuclear equation of state, and the dynamics of the nascent proto‑neutron star.