On `On the Red Supergiant Problem': a rebuttal, and a consensus on the upper mass cutoff for II-P progenitors

TL;DR

This paper defends a higher upper mass cutoff for red supergiant progenitors of type II-P supernovae, clarifying previous statistical misinterpretations and emphasizing the need for long-term surveys to detect direct black hole formation.

Contribution

It corrects a statistical misinterpretation in recent work, reaffirming a higher upper mass cutoff around 19 solar masses for RSG progenitors, and discusses the observational challenges in detecting direct black hole formation.

Findings

The upper mass cutoff is around 19 M_sun, not 15.7 M_sun.

The statistical significance of the RSG Problem is less than 2σ.

Detecting RSGs collapsing directly into black holes requires decades of observation.

Abstract

The `Red Supergiant Problem' describes the claim that the brightest Red Supergiant (RSG) progenitors to type II-P supernovae are significantly fainter than RSGs in the field. This mismatch has been interpreted by several authors as being a manifestation of the mass threshold for the production of black holes (BHs), such that stars with initial masses above a cutoff of $M_{\rm hi}=17$M$_\odot$ and below 25$M_\odot$ will die as RSGs, but with no visible SN explosion as the BH is formed. However, we have previously cautioned that this cutoff is more likely to be higher and has large uncertainties ($M_{\rm hi}=19^{+4}_{-2}M_{\odot}$), meaning that the statistical significance of the RSG Problem is less than $2\sigma$. Recently, Kochanek (2020) has claimed that our work is statistically flawed, and with their analysis has argued that the upper mass cutoff is as low as $M_{\rm hi} = 15.7 \pm 0.8M_\odot$, giving the RSG Problem a significance of $>10\sigma$. In this letter, we show that Kochanek's low cutoff is caused by a statistical misinterpretation, and the associated fit to the progenitor mass spectrum can be ruled out at the 99.6\% confidence level. Once this problem is remedied, Kochanek's best fit becomes $M_{\rm hi} =19^{+4}_{-2}M_{\odot}$, in excellent agreement with our work. Finally, we argue that, in the search for a RSG `vanishing' as it collapses directly to a BH, any such survey would have to operate for decades before the absence of any such detection became statistically significant.