HEAVY, BUT NOT METAL: Could Metal-Free Stars be the Source of Massive Black Hole Mergers?

Title:  Can Population III Stars be Major Origins of both Merging Binary Black Holes and Extremely Metal-Poor Stars?

Authors: Ataru Tanikawa, Gen Chiaki, Tomoya Kinugawa, Yudai Suwa, and Nozomu Tominaga

First Author’s Institution: Department of Earth Science and Astronomy, Graduate School of Arts and Sciences, The University of Tokyo, Tokyo, Japan

Status: Accepted for the Publications of the Astronomical Society of Japan [open access]

Black holes have been literally making waves recently. Thanks to LIGO and recent gravitational wave observations, a large number of compact object mergers have been detected for the first time over the last few years. With these observations, scientists have been able to further develop and refine theories about the sources and evolution of these objects and their mergers. The authors of this paper are looking at the source of massive black hole mergers; specifically, the type of stars they originate from.

Blessed are the (Metal) Poor…

The main hypothesis this paper attempts to tackle is whether the oldest stars in the universe – first-generation, metal-free stars, or “Population III” stars – can be a major progenitor for black hole merger events. These stars are a tempting solution to the question of where BH mergers originate, as ‘Pop III’ stars are believed to be typically very massive (10 – 1000 solar masses), and thus are more likely to leave behind black holes than ‘Pop I’ or ‘Pop II’ stars. One of the major problems of this approach, however, is that Pop III star formation is very difficult to accurately determine, as no Pop III star has been actually observed to this day.

The authors elect to examine the population and distribution of Extremely Metal-Poor, or ‘EMP’ stars, as an approach to estimating Pop III star populations. EMP stars are stars with 1/1000th the metallicity of the sun or less, and the belief is that EMP stars should have formed in gas clouds that had been enriched by supernovae of Pop III stars, as these supernovae would have formed the metals detectable in EMP stars. Under this hypothesis, the authors analyze the BH merger scenario assuming that Pop III stars are the dominant source of ‘heavy’ (20-40 solar mass) BH binaries, and that the observed population of EMP stars today are dependent on Pop III stars as well.

Assuming that Pop III stars form in minihalos of about ~1 million solar masses at high redshifts of z > 5, the authors simulated a million Pop III binary stars and followed their evolution using the binary synthesis population code BSE, extending the model to fit EMP stars. Once a star went supernova, the authors calculated how much carbon and iron it would eject into the minihalo and whether the resultant minihalo would form an EMP star.

Terms and Conditions Apply

There are three conditions that are necessary to be fulfilled under this model for Pop III stars to actually be the major origins of massive black hole mergers, as well as the progenitors of EMP stars through their supernova ejecta.

The first condition is that the total mass of Pop III stars in each cubic gigaparsec of space does not exceed 1015 solar masses, an upper limit constrained by our current understanding of cosmic reionization.

The second condition is that the formation rate of 20-40 solar mass Black Hole binaries lie between 3 and 30 per cubic gigaparsec per year. While some argue that Pop III stars could account for all massive BH mergers (greater than 20 solar masses), the formation rate for Black Holes of 40 solar masses and greater depends highly on uncertainties of the selected model, and so the authors chose to cut off the mass range there, although they note that this constraint may be stricter than necessary.

The third condition is that the number ratio of “carbon-rich” EMPs to “carbon-normal” EMPs must lie between 0.01 and 1, but not exceed 1, so as to match EMP observations in reality. Carbon-normal EMPs have carbon-iron abundance ratios similar to the Sun’s, while carbon-rich EMPs have higher carbon abundances than the Sun. This is important, as each type of EMP is associated with a different kind of formation process. Carbon-normal EMPs form when a Pop III star undergoes a core-collapse supernova, while a carbon-rich EMP forms when a “failed supernova” occurs instead; with only the carbon-rich outer layers being ejected into the surrounding medium, and a significant amount of matter falling back onto the newly-formed black hole.

Results and Discussion

Figure 1 shows qualitative results of the simulation as the authors varied two free parameters in the initial mass function of their simulation: the minimum initial mass of the primary star (mmin) and the maximum mass of Pop III stars in each halo (Mmax). The figure aims to display which values of these parameters successfully satisfied all three conditions detailed above.

Figure 1: A qualitative diagram showing combinations of mmin and Mmax that satisfy the three necessary conditions. The areas of open circles with dots satisfy all three conditions, while those without dots satisfy them only when the second condition is relaxed such that the binary formation rate lies between 1 and 100 per cubic gigaparsec per year instead. The areas with crosses do not satisfy all conditions, and the color shading indicates which condition was not satisfied: red has too small binary formation rates, blue has too few carbon-rich EMPs, green has too many carbon-rich EMPs, and gray has too large total mass of Pop III stars per cubic gigaparsec. Adapted from Figure 1 in the paper, by the bite’s author, with the inclusion of a legend/key.

The results show that there does indeed exist a region where all three conditions are satisfied, meaning the hypothesis that Pop III stars are the dominant sources of black hole mergers is possible. In addition, the parameters that enclose this region give us some further constraints on the distribution of these progenitors.

The graph tells us that for this hypothesis to be satisfied, the formation rate of Pop III stars should be > 3×1014 solar masses per cubic gigaparsec, which constrains each minihalo to have a maximum mass of Pop III stars of 104 solar masses. This is larger than numerically predicted by other groups, but not large enough to violate the first condition.

In addition, the model only works if the minimum mass of Pop III stars lies between 15 and 27 solar masses, as falling outside these limits violates the third condition in either direction. This mass range overlaps with a range of 10 to 20 solar masses given by prior numerical simulations, and thus helps to further constrain the minimum mass of Pop III stars required for this hypothesis to work.

The authors end by reiterating the assumptions made in their model, and discuss some of its caveats and how it could be further expanded or refined, such as by considering alternative origins for black hole binaries, or the possibility that higher initial numbers of Pop III stars may ‘pollute’ minihalos more rapidly, creating Pop II/I stars and preventing more Pop III stars from forming.

Astrobite edited by Kayla Kornoelje

Featured image credit: NASA/JPL for the Red Giant, Shutterstock for the Metallica concert, Aldo Panfichi for the edit

About Aldo Panfichi

Hello! I'm currently a rising 2nd year Master's student in Physics at the Pontificia Universidad Católica del Peru in Lima, Peru, where I will be working on a thesis project related to asteroids. I previously got my BSc in Astronomy and Astrophysics at the University of Chicago. In my free time, I like spending time with my friends (and my dogs!), going on walks in the summer, and cozying up inside in the winter, playing games or reading science fiction.

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