Por que um quasistar não pode existir agora?

De minha pesquisa, descobri que os quasistares existiam teoricamente por causa de um núcleo de buraco negro cuja pressão de radiação neutralizava a gravidade dentro da estrela. No entanto, alguns sites afirmaram que os quasistares não existem agora porque existem metais contaminando hidrogênio e hélio.

Alguém poderia me explicar por que os metais (ou um pequeno deslocamento de hidrogênio e hélio) influenciam a pressão de radiação de um buraco negro, ou é apenas que não há onde essa massa (mais de mil massas solares) possa existir em qualquer lugar de um lugar agora ?

Procurei na Wikipedia (normalmente faço isso e depois pesquiso em sites diferentes, se encontrar algo interessante por lá) e aqui 1 .

Nuvens de gás com massas muito superiores a $10^3\,M_\odot$são abundantes em galáxias; a típica nuvem formadora de estrelas (as chamadas nuvens moleculares ) tem massas de$10^3\,M_\odot$ para $10^7\,M_\odot$. Quando os quasistares (estrelas hipotéticas alimentadas não pela fusão nuclear, mas pela acumulação em um buraco negro central) não podem existir hoje em dia, é porque todo o gás no Universo ficou poluído com metais.

Estrelas se formam a partir de nuvens de gás em colapso. Para que uma região de uma nuvem entre em colapso, ela deve ser suficientemente densa e suficientemente fria; se estiver muito diluído, não há gravidade suficiente e, se estiver muito quente, a energia dos átomos individuais neutraliza o colapso, fazendo os átomos escaparem.

Massa de jeans

Este critério é capturado na equação de instabilidade de Jeans . A relação pode ser expressa de várias maneiras; Uma maneira é dizer que a massa da nuvem - ou uma pequena região dela - deve exceder a "massa Jeans":

From this equation you see that the cooler the gas is, the smaller the threshold. In other words, the smaller stars you can form. If the gas is not able to cool, only the largest clumps will collapse, and hence such stars will be very massive.

Gas cooling

So, how does the gas cool? Hot gas means that the particles have large velocities. If the particles collide, they may excite each other, bringing an electron to a higher state at the expense of slowing down — i.e. cooling. When the electron de-excites, a photon is emitted, which may leave the system. Thus, the kinetic energy of the atoms is converted into electromagnetic energy which escapes.

However, an electron is only excited if the energy of the collision matches closely the energy needed for the excitation. If the collisional energy is too high, or too low, the atoms simply bounce off of each other, maintaining their total energy (although one may transfer some energy to the other).

The effect of metals

If the gas consists only of hydrogen and helium, there are only a few available energies for excitation. Hydrogen happens to be able to cool efficiently around $T\sim10^4\,\mathrm{K}$, while helium cools efficiently around $T\sim10^5\,\mathrm{K}$, but at other temperatures, the gas tends to stay at its given temperature.

However, as soon as there a some metals, the many electrons of these metals, with their many possible transitions, allow for atoms with many possible energies to be excited. Thus, before a gas cloud of $M\sim10^3\,M_\odot$ collapses to form a $10^3\,M_\odot$ star, it will fragment into smaller pieces, forming smaller stars.

If you build a very massive protostar, more than a thousand solar masses, then it is possible for the core of the protostar to collapse directly to a black hole whilst it is still surrounded by a massive envelope. The collapse will happen "inside out", so that the envelope collapses at a slower rate. However, there is a maximum rate at which black holes can grow, because the compressing material gets very hot and emits lots of radiation and the radiation pressure can stall the collapse (temporarily). This is a quasi star.

The key to a quasi star is it's large initial mass, which prevents the envelope being "blown away" by the initial release of energy during the black hole formation. Such massive protostars can only be built in the early universe from pristine material. If the material is polluted with heavier elements then it can cool more readily - the heavier atoms can form molecules and radiate away energy. This cooling allows a large cloud to fragment into much smaller pieces, so that in the present day universe, the collapse of such a large cloud would not lead to one massive protostar, but a cluster of smaller protostars.

The addition of metals (i.e., elements heavier than helium) to a stellar mixture makes it less transparent to radiation. Basically, hydrogen and helium have relatively simple and uncrowded spectra, but the "metals" add many new spectral lines and the mixture absorbs much more light and is more efficiently heated by it and picks up more momentum from it, also..

The gas in the early universe then had less metals, and was thus less affected by the radiation of a condensing new star. Consequently, the star could grow to higher mass before its radiation cut off the inflow of gasses allowing it to grow. (Today, the upper limit for star formation is around 100 Ssolar masses; with a mixture of H and He only, it appears to be as high as 250 solar masses.) See the Wikipedia article for a good explanation. These super-large stars are needed to form a quasistar, and can only form from unsullied H/He. So quasistars (if they exist at all) could only form very early in the evolution of the universe.