New light on the dark ages of the Universe

During the dark ages, the Universe was full of neutral hydrogen and stars had not yet been formed. In this context, as soon as stars began to be formed they « re-ionized » the hydrogen, , and photons were as a consequence able to propagate in this medium, and so eventually reach us… To-day, we can see the weak glow of this first generation of stars.

The very first stars in the Universe were massive and evolved rapidly. They created gradually very small quantities of heavy elements (such as carbon, iron…) which they then disperesed into the interstellar medium via their final explosions. The only stars which can still bear witness to these dark ages are those of small mass- smaller than that of the Sun : they evolve very slowly, have a long life-tim and have survived till now. Their primordial matter does not is poor in heavy elements, since their formation has hardly begun in the Universe. Now, according to theory, small mass stars are are not easily formed in a metal poor medium, since metals are a necessary prerequisite to cool the primordial matter in order that it can dondense and form stars.

The three newly discovered stars date from the dark ages (13 billion years ago), are of small mass and contain one hundred thousand times less iron than the Sun, in contrast to theoretical predictions. This shows that that the mechanism responsible for the formation of the first generation of stars must necessarily be able to form small mass stars like the Sun, or even smaller (in a medium which is nevertheless poor in heavy elements).

The study of the relative abundances of the elements which make up these stars shows that they are relatively rich in carbon, with respect to heavier elements such as iron. One concludes that these stars are members of a special class of stars whose first member had already been identified in 1998 [1]. It could well be that a significant amount of carbon is essential for the formation of small mass stars, due to the high efficiency of carbon in all its forms as a cooling agent of the primordial clous during its contraction.

However, in a different study, the same group discovered the star SDSS J102915+172927 [2] a remarkable star since it has as much iron as the 3 stars which are the subject of the present work. To be more precise, this star is not overabundant in carbon with rerspect to iron.

Now, while carbon contributes significantly to the cooling process, it is classically insufficient to lead to the formation of stars less massive than 10 solar masses. To cross this frontier and enable the formation of stars such as have been observed here, an additional cooling mechanism, such as cooling by dust grains, is required. This is the only process which can work for the appropriated mass regime in the primordial cloud.

The study of the elements in these stars leads to a new scenario for the formtion of the first generation of stars. We must also explain the particularly high level of carbon observed in one of these stars. Stars do not form separately, but in groups within small halos of dark matter. The massive stars, at the end of their lives, eject the matter which has been formed, of which, however, a small fraction , in particlar the light elements such as carbon and oxygen, fall back onto the star. Certain supernovae, which do not explode too energetically, only eject their external layers, and in particular those which contain the light elements such as cabon and oxygen.. We can in this way understand the specific enrichment in carbon of the stars formed later from this ejected matter.

While this scenario throws new light on the formation of the first generations of stars, these observations present a new conundrum, since one expects to have observed lithium in the atmospheres of these stars, , since this element was produced at the same time as helium during the big bang. However, there is no trace of lithim has been seen in these stars. Yet another mystery which makes these stars of the earliest times that much more fascinating to study.

Source

• TOPoS : II. On the bimodality of carbon abundance in CEMP stars - Implications on the early chemical evolution of galaxies, P. Bonifacio et al., Astronomy and Astrophysics, juin 2015.