What Are The Effects Of Dark Matter On Stars Present In Globular Clusters? (Cosmology)

If you would ever study rotational curves of spiral galaxies you will realize that 80% mass of galaxies is in the form of dark matter (DM). By studying DM density profile like NFW you will came to know that, the DM is distributed non-uniformly throughout the galaxies. It means, in central regions of galaxies, DM density is the highest and when you move towards the edges of galaxies, the DM density reduces. From this, you can say that all the objects inside galaxies like stars, stellar clusters etc. are immersed in the DM halo. Thus, you can say that DM will affect everything inside galaxies.

Stars which are present inside galaxies can absorb DM particle from their surrounding and this can affect their stellar properties in two ways: First, DM particles can transfer their energy between different layers of stars. This can affect chemical composition, temperature, pressure and many more properties of stars. Secondly, DM particles can act a new source of energy inside stars if they annihilate. This can also alter luminosity and temperature of stars. So, if you consider the two stars having same masses and initial chemical compositions but different DM densities, you will find that both the stars are following different evolutionary path in the Hertzsprung-Russell diagram (H-R) diagram.

According to classical view of globular clusters (GC’s), stars inside GC’s have similar chemical compositions as they evolved from the same gaint molecular cloud. But recent photometric and spectroscopic studies of globular clusters reveal the presence of more-than-one or multiple stellar populations (MSPP) inside globular clusters. This finding challenges our classical view of globular clusters. Now, Hassani and Mousavi investigated the possibility of solving MSPP in globular cluster using DM assumptions.

The results of their simulations, showed that stars in different locations of globular clusters (corresponding to different dark matter densities) follow different evolutionary paths (e.g. on Hertzsprung-Russell diagram). It means, stars in high dark matter density environments like the central region of globular clusters, are more affected by the presence of dark matter than the stars present in outer regions of same GC’s.

Depending on their mass, stars convert Hydrogen to Helium through PP or CNO energy production cycles. Thus, it has been shown that, the presence of dark matter can alter these energy production cycles inside stars and can affect their chemical compositions.

But, these two cycles are also strong function of temperature. Thus, if the DM annihilate inside stars, it can alter the core temperature too, which causes stars to consume Hydrogen atoms at different rates in comparison to the models without DM. Thus, dark matter presence can affect the elemental abundances of star too. Overall, they found that, if the presence of DM can alter the temperature of stars, then it can alter their age, luminosity, and many other physical parameters of stars too.

Finally, they concluded that, if the presence of DM alters the luminosity, temperature, chemical composition, age, etc. of stars, then its presence can be considered as a possible solution to the multiple stellar populations problem in GCs.

All images credit (except featured): Hassani and Mousavi

Reference: Ebrahim Hassani, Seyyed Milad Ghaffarpour Mousavi, “Dark Matter Effects on Stellar Populations in Globular Clusters”, Arxiv, pp. 1-11, 2021.

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