Why This Matters
When the most massive stars reach the end of their lives, they don't always go out with the spectacular bang we associate with supernovae. Some undergo "failed supernovae"—core collapses that produce black holes rather than neutron stars, with little to no observable explosion. Yet even these quiet deaths aren't entirely silent. New research from Paradiso, Vallejo, and Coughlin reveals that neutrinos released during the collapse generate a weak shockwave that propagates through the star's outer layers, potentially creating a detectable transient signal before the star disappears entirely. By analyzing the stability and evolution of these shockwaves, the authors provide a framework for understanding when failed supernovae actually eject material—and how much.
What They Found
The authors built on recent work identifying two self-similar solutions describing how neutrino-driven shockwaves propagate through stellar envelopes in failed supernovae. Their key contribution is demonstrating that these solutions are not equally viable: the larger Mach number solutions are unstable, with the shock's Mach number growing over time as α ≲ 0.1, deviating from self-similar predictions. By contrast, the smaller Mach number solutions remain stable.
More significantly, they show that above a critical mass loss rate—one readily achievable in realistic core-collapse scenarios—the shock asymptotically strengthens and approaches the strong-shock limit. This critical threshold depends on the ratio of mass lost to neutrinos relative to the mass enclosed by the shockwave, as well as the stellar density gradient at the shock's location.
!Shock Mach number evolution over time, showing unstable growth in higher Mach number solutions
These findings have immediate implications for progenitor dependence. Red supergiants, which possess extended envelopes and experience relative mass losses well exceeding the critical value at shock formation, are predicted to more readily eject material and produce more luminous transients. Conversely, more compact progenitors—which have lower relative mass losses—should produce fainter events with less ejected material.
Why It Matters
Failed supernovae occupy a unique niche in multi-messenger astronomy. Unlike successful supernovae, which are bright across the electromagnetic spectrum, or neutron star mergers, which produce gravitational waves, failed supernovae are inherently dim yet potentially detectable through their neutrino-driven breakout transients. Understanding the physics governing mass ejection in these events is essential for interpreting observations from wide-field transient surveys and for predicting the rates at which such events should appear.
The authors' results suggest that progenitor properties—particularly envelope structure and pre-collapse mass loss—directly control the observability of failed supernovae. This provides a testable prediction linking stellar evolution to transient properties.
What's Next
The work raises several open questions. Direct numerical simulations testing the stability predictions would strengthen confidence in the analytical results. Observers searching for failed supernova candidates should prioritize red supergiants and examine whether the predicted luminosity differences match real data. Additionally, the interplay between neutrino physics, shock dynamics, and fallback accretion deserves further investigation.
Starithm continuously monitors alerts for transient events, including the rare and faint signals that may herald failed supernovae.