Monthly Highlights

Dark matter in the first stars of the universe: evolution and rotation

Figure 1. left, Hertzsprung-Russell Diagram for a 20 M_☉ with different density of WIMPs in its surrounding. The stars are the position on the ZAMS of the same 20 M_☉ star in the paper of Taoso et al. 2008 where the WIMPs subroutines come from. The difference comes from the improvement in the stellar evolutionary code GENEC (from the Observatoire de Genève). right, The time of the end of the H-burning phase as a function of the WIMPs density.
Figure 2. Element abundances during the MS, inside a 20 M_☉ star with a WIMPs density of 1 x 10¹⁰ GeV/cm³, as a function of the radius of the star and as a function of the time, without rotation left and with an initial rotation of 40% the critical velocity right. Adding the rotation increase even more the lifetime of the star.

Anaïs Pauchet

We know that huge halos of Dark Matter (DM) that we call Dark Matter Halos (DMHs) induced the formation of galaxies by creating an important gravity potential. It is likely that in the center of those potentials, what will become the center of a primordial galaxy, the first stars of the universe were formed. Those stars are called Pop III stars and were very massive.
We don’t know what DM is composed of, nevertheless we suppose it has a mass, thus it interacts through gravitation with baryonic matter. It could also interact with baryonic matter through other effects but only for an infinitesimal proportion (with a really small cross section), then, which can only take place when DM and baryonic matter are in really high density simultaneously, i.e. in the center of DMHs. Hence, Except for gravity, the presence of DM could also have an impact on the structure and composition of Pop III stars. During my master’s thesis in the Observatoire de Genève with Pr. Meynet, I studied the effect of one DM candidate: the Weakly Interacting Massive Particle (WIMP), on the formation and evolution of Pop III stars.

WIMPs can interact in different ways with baryonic matter (only hydrogen and helium in the early universe) inside the star. (1) They can be scattered from one atom to another and lose momentum to finally be captured by one of them. This only takes place when the probability of collision is high enough to occur several times when the WIMP is passing the star i.e. in the densest part of the star, the center. (2) As WIMPs are their own antiparticle they can annihilate themselves and give the resulting energy to the atom that captured them. As capture occurs in the center of the star, the annihilation too, it then brings a new source of energy in the center of the star.
Other mechanisms exist but they are negligible for high mass stars so I won’t present them here. On the Figure 1, left, we can see the Hertzsprung–Russell diagram (HRD) of a 20 M_⊙ star surrounded by different densities of WIMPs. We can see that the more we increase the WIMPs density the more the position of the Zero Age Main Sequence (ZAMS) is shifted toward lower temperatures. This is due to the fact that the new source of energy in the center of the star makes the star inflate, which will decrease its surface temperature. Another effect is that the energy coming from the WIMP annihilation will overcome the energy produced by nuclear fusion and the star will be sustained longer without burning hydrogen. This results in increasing the lifetime of the star drastically. For example, a 20 M_⊙ star with no WIMP stays in the main sequence (MS) during ~10 Myr while we can see in Figure 1, right, that the same star with 2 x 10¹⁰ GeV/cm³ WIMPs density stays ~ 700 Myr.

During my Master’s thesis I also studied the effect of rotation in addition to the WIMP on Pop III star evolution. We found that, as the lifetime of the star was increased, the energy had more time to be transported toward the surface, thus, after the ZAMS the surface temperature and luminosity tend to be higher. The elements (H and He) had also more time to be mixed and the composition of the star is more homogeneous (see Figure 2). If the rotation speed and/or WIMP density were high enough we could reach a star only composed of He at the end of the MS (it could be a way to detect them). Unfortunately, as we increase the WIMP density and/or rotation speed, the rotation speed reaches the critical velocity (where the star is not stable anymore) more rapidly.