Missing Black Holes Unveil The Supernova Explosion Mechanism

Abstract: It is firmly established that the stellar mass distribution is smooth, covering the range 0.1-100 Msun. It is to be expected that the masses of the ensuing compact remnants correlate with the masses of their progenitor stars, and thus it is generally thought that the remnant masses should be smoothly distributed from the lightest white dwarfs to the heaviest black holes. However, this intuitive prediction is not borne out by observed data. In the rapidly growing population of remnants with observationally determined masses, a striking mass gap has emerged at the boundary between neutron stars and black holes. The heaviest neutron stars reach a maximum of two solar masses, while the lightest black holes are at least five solar masses. Over a decade after the discovery, the gap has become a significant challenge to our understanding of compact object formation. We offer new insights into the physical processes that bifurcate the formation of remnants into lower mass neutron stars and heavier black holes. Combining the results of stellar modeling with hydrodynamic simulations of supernovae, we both explain the existence of the gap, and also put stringent constraints on the inner workings of the supernova explosion mechanism. In particular, we show that core-collapse supernovae are launched within 100-200 milliseconds of the initial stellar collapse, implying that the explosions are driven by instabilities with a rapid (10-20 ms) growth time. Alternatively, if future observations fill in the gap, this will be an indication that these instabilities develop over a longer (>200 milliseconds) timescale.

Analyzing the Mass Gap in Compact Stellar Remnants and Supernova Explosions

The paper "Missing Black Holes Unveil The Supernova Explosion Mechanism" by Belczynski et al. explores the enigmatic mass gap in the observed population of compact stellar remnants, specifically between neutron stars (NS) and black holes (BH). This mass gap, ranging between approximately 2 to 5 solar masses, presents a significant challenge to conventional models of stellar evolution and supernova (SN) explosions, which traditionally predict a continuous distribution of remnant masses. The authors offer a hypothesis that could reconcile these observations with theoretical models, providing crucial insights into the core-collapse supernova process and the formation of compact objects.

Stellar Mass Distribution and the Mass Gap Phenomenon

The observed mass gap poses an intriguing problem, as conventional evolutionary theories suggest a smooth transition in remnant mass distribution matching the smooth distribution of progenitor stellar masses. In contrast, the empirical data indicates that the heaviest neutron stars do not exceed about two solar masses, and the lightest black holes are not less than around five solar masses. This discrepancy is pivotal for understanding the equations of state of dense nuclear matter and the mechanics of stellar explosions.

Proposed Models and Mechanisms

The authors combine stellar modeling with hydrodynamic simulations to address the supernova engine—a process powered by the collapse of massive stars that can lead to neutron star and black hole formation. The paper proposes two fundamental scenarios:

1. Rapid Instability Model: Characterized by a 10-20 ms growth time for instabilities, leading to successful core-collapse supernova explosions within 100 to 200 ms after initial stellar collapse. This model supports the mass gap observed in remnants, suggesting rapid explosions only form neutron stars up to about two solar masses and black holes above five solar masses.

2. Delayed Instability Model: Exhibits growth times greater than 200 ms. Should observational data begin to fill the mass gap with remnants within the 2-5 solar mass range, it would imply that supernovae can also result from slower-developing instabilities that lead to a continuous mass distribution.

Implications for Astrophysics

These findings have substantial implications for both the theoretical modeling of supernova mechanisms and practical observation of compact objects. The distinction between the rapid and delayed models influences predictions about the nature and frequency of particular remnant masses, which could guide observational strategies in both electromagnetic and gravitational-wave astronomy. Moreover, understanding the mechanism underlying core-collapse supernovae is crucial to broader astrophysical phenomena, including nucleosynthesis and the dynamics of supernova remnants.

Future Directions in Research

Confirmation or refutation of the proposed mass gap could significantly revise current models of stellar evolution and supernova mechanics. If future observations detect compact objects within the mass gap, it could necessitate a reassessment of the energy scales and timescales in supernova simulations. Investigating the complex physics of convection, neutrino interaction, and hydrodynamics under extreme conditions remains a crucial pursuit, as does the refinement of numerical models to more accurately replicate the physical processes at play in supernova explosions.

In sum, while the existence of a mass gap in the remnant population is a considerable challenge to current astrophysical understanding, it also presents a valuable opportunity to refine theoretical frameworks and enhance our comprehension of the final stages of stellar evolution.