Researchers Find the Origin and Maximum Mass of Massive Black Holes Observed by Gravitational Wave Detectors


Through simulations of a dying star, a team of theoretical physics researchers have found the evolutionary origin and the maximum mass of black holes which are discovered by the detection of gravitational waves as shown in Figure 1.

The exciting detection of gravitational waves with LIGO (laser interferometer gravitational-wave observatory) and VIRGO (Virgo interferometric gravitational-wave antenna) have shown the presence of merging black holes in close binary systems.

The masses of the observed black holes before merging have been measured and turned out to have a much larger than previously expected mass of about 10 times the mass of the Sun (solar mass). In one of such event, GW170729, the observed mass of a black hole before merging is actually as large as about 50 solar mass. But it is not clear which star can form such a massive black hole, or what the maximum of black holes which will be observed by the gravitational wave detectors is.

To answer this question, a research team at the Kavli Institute for the Physics and Mathematics of the Universe (Kavli IPMU) consisting of Project Researcher at the time Shing-Chi Leung (currently at the California Institute of Technology), Senior Scientist Ken’ichi Nomoto, and Visiting Senior Scientist Sergei Blinnikov (professor at the Institute for Theoretical and Experimental Physics in Mosow) have investigated the final stage of the evolution of very massive stars, in particular 80 to 130 solar mass stars in close binary systems. Their finding are shown in Illustrations (a – e) and Figures (1 – 4).

In close binary systems, initially 80 to 130 solar mass stars lose their hydrogen-rich envelope and become helium stars of 40 to 65 solar mass. When the initially 80 to 130 solar mass stars form oxygen-rich cores, the stars undergo dynamical pulsation (Illustrations a – b and Figure 2), because the temperature in the stellar interior becomes high enough for photons to be converted into electron-positron pairs. Such “pair-creation” makes the core unstable and accelerates contraction to collapse (Illustration b).

In the over-compressed star, oxygen burns explosively. This triggers a bounce of collapse and then rapid expansion of the star. A part of the stellar outer layer is ejected, while the inner part cools down and collapses again (Illustration c). The pulsation (collapse and expansion) repeats until oxygen is exhausted (Illustration d). This process is called “pulsational pair-instability”(PPI). The star forms an iron core and finally collapses into a black hole, which would trigger the supernova explosion (Illustration e), being called PPI-supernova (PPISN).

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