Astrophysics papers — 2026-08-18

The research presented today covers two distinct but equally critical areas of astrophysics, spanning from the fundamental physics governing how massive stars die to detailed observations of binary systems that evolve over cosmic timescales.

First, we have a deep dive into the process of core-collapse supernovae or CCSNe. The study addresses a major unresolved question: determining whether a massive star will explode as a visible supernova or fail to explode, resulting in the formation of a black hole. This uncertainty directly impacts our understanding of how many compact remnants are born and what their mass distribution looks like. While earlier models suggested that only the most massive stars could fail to explode, recent simulations have shown that failures can occur in progenitors as low as thirteen solar masses. Observing these failed collapses suggests that between five and fifty percent of all such stellar deaths do not produce a visible explosion. These failures are thought to help explain two persistent astrophysical puzzles: the red supergiant problem, which is the lack of red supergiant progenitors in a certain mass range, and the CCSN rate problem, where the observed rate falls short of what we expect based on star formation.

The core mechanism driving these events involves neutrinos, which carry away about ninety-nine percent of a star's gravitational binding energy. However, current state-of-the-art simulations do not fully account for the fact that these neutrinos can change their flavor as they travel through the core. This paper introduces and tests this concept of neutrino flavor conversion, or FC. The researchers simulated one hundred ninety-five different progenitors, ranging from nine to one hundred twenty solar masses. They implemented a parametric scheme for FC that allows flavors to become equipartition while preserving electron lepton number and neutrino momentum within each energy bin. This was applied in regions where the matter density was below a certain threshold, specifically between the neutrinosphere and the stalled shock region.

The results are highly significant. The study demonstrates that flavor conversion has a profound impact on whether an explosion succeeds or fails. Overall, the findings suggest that FC increases the fraction of failed explosions, particularly for stars in the sixteen to thirty solar mass range. The data shows a strong correlation between how deep inside the proto-neutron star flavor equilibration is achieved and how much shock revival is suppressed. As we move deeper inside the flavor equilibration occurs, the suppression of shock revival becomes greater. Quantitatively, when there is no flavor conversion at a density of one-zero-to-the-power-nine grams per cubic centimeter, the failure rate stands at twenty five point six percent. However, when FC is applied at that same density level, the IMF weighted fraction of failed explosions rises substantially to forty seven point six percent. At a much higher density of one-zero-to-the-power-thirteen grams per cubic centimeter, without FC, the failure rate is ninety three point three percent; with FC, it jumps to eighty eight point three percent. The authors conclude that because FC reduces the net energy deposition behind the stalled shock, it systematically favors failed explosions. This effect is particularly pronounced in this sensitive mass range of sixteen to thirty solar masses. and they suggest that these changes could help alleviate both the red supergiant problem and the CCSN rate problem, asserting that neutrino flavor conversion is a key ingredient in modeling how black holes and neutron stars are born.

Moving from stellar death, we turn to observations of binary systems. The second paper details a study of two specific Long Period Transients or LPTs, which involve white dwarfs paired with M dwarf stars. The goal was to analyze their orbital parameters and constrain their physical nature within the context of WD plus M dwarf binaries. Using new phase-resolved spectroscopy from the Keck I/LRIS telescope, the researchers achieved high-precision measurements of radial velocities. The analysis revealed that both systems harbor unusually massive and cool white dwarfs, with inferred temperatures and masses that are quite high. These core materials are found to be nearly entirely crystallized.

Kinematic analysis using data from Gaia Release three shows these systems are kinematically hotter and less concentrated in the Galactic plane than other LPTs, placing them within the Galactic thick disk. The systems were found to be close to being face-on binaries, which is important because the production of coherent radio pulses may be strongly dependent on this inclination. Furthermore, observations of a hydrogen emission line confirmed that this light stems from the M dwarf due to its chromospheric activity in such a compact orbit.

The study then employed Modules for Experiments in Stellar Astrophysics or MESA models to simulate their evolution. These simulations predict that both systems will fill their Roche lobe within approximately one hundred thousand years, ultimately becoming cataclysmic variables, driven by the loss of angular momentum through gravitational wave radiation. Based on these observations, the researchers placed lower limits on the local space density of these WD plus M dwarf LPTs. By comparing this to the broader population, they estimate that there are between one hundred and two thousand such systems within a two kiloparsec radius. The paper concludes that this specific sub-population of LPTs favors low orbital inclinations and cool, massive white dwarfs that have undergone significant core crystallization.

In summary, the day's research has provided critical insights into the fundamental physics governing stellar collapse through neutrino flavor conversion in supernovae, while simultaneously offering a detailed look at the evolutionary paths and population constraints of specific binary systems involving white dwarfs and M dwarf companions.

Today's papers

The papers

Important terms

Core-Collapse Supernovae (CCSNe)
The explosive death of a massive star's core, where gravitational collapse occurs. The research investigates whether these explosions successfully happen or fail, forming black holes instead.
Neutrino Flavor Conversion (FC)
A process where neutrinos change their type or 'flavor' as they travel through dense stellar matter. This conversion significantly alters the energy deposition behind a stalled supernova shock.
Red Supergiant Problem
An astrophysical puzzle concerning the observed lack of red supergiant stars within specific mass ranges. Failed supernovae are proposed to help explain this missing population.
Long Period Transients (LPTs)
A type of binary system involving a white dwarf and an M dwarf star. The study uses these systems to constrain the local density and evolutionary fate of such binaries.
Core Crystallization
The process where the dense, solid core material within a white dwarf begins to crystallize. This is observed in the studied LPTs, indicating advanced stellar evolution.