A few hundred million years after the Big Bang, the universe was a soup of ionized H (Hydrogen), He (Helium) and D (Deuterium) with trace amounts of Li (Lithium), B (Boron), Be (Beryllium) and H2 (molecular Hydrogen). With little amount of metal and H2, the conditions were far from ideal for star formation.

For star formation to begin, a sufficient amount of cold dense gas has to accumulate. But in the early universe, the primordial gas could not efficiently cool radiatively because atoms had excitation energies that were too high, and molecules, which had accessible rotational energies, were very rare and only trace amounts of molecular hydrogen (H2) were formed.

Under proper conditions, the formation of molecular hydrogen  allows the gas to cool. The cooling of the gas lead to thermal instability and a local density enhancement, which resulted in the fragmentation of the gas cloud. In the primordial gas clouds, the thermal instability took place at higher temperatures compared to leading to the formation of massive stars with mass of about 100 solar masses.

 

Primordial gas clouds undergo runaway collapse and forms a protostar when sufficient mass is accumulated at the centre. Then, the protostar starts to grow by accreting the surrounding gas to become a massive star. The ultimate mass of the star is determined by the mass of the cloud out of which it forms and by a number of feedback processes that occur during the evolution of the protostar.

The protostellar feedback processes affect the formation of primordial stars quite differently from the contemporary stars. First, primordial gas does not contain dust grains. As a result, radiative forces on the gas are much weaker. Second, the amplitudes of magnetic fields generated in the early Universe are so small that they never become dynamically significant in primordial star-forming gas. Magnetic fields have at least two important effects in contemporary star formation: they reduce the angular momentum of the gas out of which stars form, and they drive powerful outflows that disperse a significant fraction of the parent cloud. Third, primordial stars are much hotter than contemporary stars of the same mass, resulting in significantly greater ionizing luminosities.

The feedback processes affect the formation of individual stars also influence primordial star formation on large scales. The enormous fluxes of ionizing radiation and H2-dissociating Lyman–Werner radiation emitted by massive population III stars dramatically influence their surroundings, heating and ionizing the gas within a few kiloparsecs of the progenitor and destroying the H2 within a somewhat larger region. The effect of radiation from the first stars on their local surroundings has important implications for the numbers and types of population III stars that form. The photoheating of gas in the minihaloes hosting population III stars drives strong outflows, lowering the density of the gas in the minihaloes and delaying subsequent star formation by up to 100 Myr.

Abhinab Paul

(A Minerva M Writer)

Read more articles at www.minerva-m.com

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