The merger of the champions

The other day, an article was published [ 1 ] about the confluence of two black holes registered by LIGO and Virgo detectors on May 19, 2019. Unusual in this event - the record masses of companions: the one that is smaller, from 50 to 80 solar masses; The one that is larger is from 70 to about 110 mʘ; The result of the merger is a hole from 135 to 180 mʘ (the limits of 90% of the trust interval are indicated).

In the figure - registered mergers of the most severe black holes in the order of increasing masses. Black squares indicate a lot of merging objects, red - a lot of fusion result. Please note that the scale is logarithmic and all participants in this event are approximately twice as heavy as previous record holders. But the intrigue here is not at all in record masses, but in the fact that black holes were merged, which are very difficult to attribute to the remnants of massive stars.

Many commentators write that these objects belong to the class of “black holes of intermediate masses”. This is not entirely accurate. Strictly speaking, the intermediate masses are that which is too difficult for stellar residues, but too easy for detected central black holes of galaxies. In principle, in the early Universe there were stars weighing, say, in 300 masses of the sun and collapsed entirely into black holes almost the same mass. Now there are no such stars: there are too many heavy elements in the universe that reduce thermal conductivity of stars - such giants simply cannot form. Both merged black holes are too heavy for modern stars, but it would seem that the relics of the early universe. The intrigue is just that they (in any case, a heavier companion) fall into the so -called “Mass Gap) - these masses cannot form with the collapse of ancient stars. Smaller and more - they can, but such - no.

The reason is not so simple and can cause distrust among a non -specialist. The existence of such a gap follows from the numerical modeling of combustion and explosions of stars, these calculations are brought to perfection and repeated by many independent authors. His nature is as follows.

The evolution of large stars, devoid of heavy elements, is relatively simple. In the center of the star, as the hydrogen burns out, an inert helium core grows. The pressure of hot helium resists gravity, but only to a certain limit. At a very high temperature, electron-positive pairs begin to be born, part of the energy is spent on them, and, most importantly, their state equation is softer due to relativistic effects-below the pressure with the same energy density. This effect is called Pair Instabide, but since in Russian the direct translation of “pair instability” sounds terribly and confuses, I propose the translation of “positive instability”. So, with the development of positron instability, the core begins to compress, a triple helium reaction turns on and, instead of scollaps into a black hole, the star is scattered without a trace. If the core is very heavy, about 200 m, the positron instability does not save from the collapse - the star still turns into a black hole, and the whole. It turns out that black holes are heavier than 50–65 mʘ and more easily about 150 mʘ cannot form. And then at least one of the merged black holes confidently falls into this range. We repeat: this is the result of calculations, but the calculations of well -worked out and independently reproduced by various authors. Specialists are confidently relying on them.

The article analyzes different versions.

The first-is there any unaccounted factors that have shifted this gap in the calculations of the positron instability of some unaccounted factors? In principle, there is, but making the lower border of the gap above 65 mʘ is very difficult. Another option, in my opinion, is much more natural: a hierarchical scenario is one of the merged holes or both were already the result of the merger of lighter predecessors. But for this we need special places where there are many black holes and they easily find a pair of themselves, where there are mechanisms of quick (shorter time of the existence of the Universe) the loss of the orbital moment of the couple.

This is, firstly, ball clusters. A typical cluster contains hundreds of thousands, large - millions of stars. They are remarkable in that all heavy objects, including black holes, because of multiple gravitational interactions with the stars are “drowning” into the center of cluster, where they easily “mate” with another heavy object. Again, due to multiple interactions, the couple manages to lose the corner moment during the cosmological time and merge. If the new black hole after the merger remains in the cluster, then the story may be repeated. The problem is that the radiation of gravitational waves during the fusion of black holes asymmetrically, how much - depends on the orientation of the axes of rotation of the merging objects. As a result, a return arises, and the resulting black hole, acquiring a speed of more than a thousand kilometers per second, leaves the cluster. It means that in order for the hierarchical scenario to work, the cluster should be very large. Huge clusters, similar to ball, only a couple of orders of magnitude heavier, exist in the centers of galaxies. In the same place, in the centers of galaxies, but not of any, but containing ultra -massive black holes with an accretion disc (active galactic nuclei), a more exotic version of the scenario can be realized.

In the centers of the galaxies, the same mechanism works as in ball clusters: heavy black holes are collected in the center (segregation by masses). Tens of thousands of black holes may be in the Central Parsek. Each of them, finding himself in a cramped orbit around the central black hole, interacts with its gas accrementary disk and is aligned with it. Thus, all holes find themselves in the same plane in a viscous environment, where they easily combine and merge. And already no return can knock out black holes from such a deep hole in gravitational potential.

In addition to the above scenarios, the authors of the article are considering the confluence of not black holes, but of giant stars with huge hydrogen membranes. In this embodiment, positive instability does not seem to arise, and after the merger a black hole is formed, for which the gap of the masses is not a decree.

One way or another, gravitational-wave astronomy is becoming more informative and excitingly interesting. Either we still find out when a large statistics of events will be accumulated and comprehended.

Boris Stern