Dark Matter and First Generation Stars
Dark matter has long remained one of the most outstanding puzzles in our universe. Its existence was first indirectly inferred by Zwicky in 1937 through the observation of the galaxy cluster and later confirmed by more evidence like the galaxy rotation curve and gravitational lensing.
As it outnumbered visible matter by several folds, people hypothesized various particle candidates, for instance, WIMP, Axions, and WIMPzilla, to explain the nature of dark matter. Currently, physicists around the globe are working on many dark matter detection experiments like XENON, LUX, and Panda-X to test on different dark matter candidates.
The first stars formed in the early stage of the universe mark the end of the dark ages. Numerical simulations indicate that they formed in the center of dark matter halos(our galaxy also exists in a dark matter halo) consisting of drifting dark matter particles. At the time when the first stars formed, the matter in the universe mostly consisted of hydrogen and helium.
The gas cloud aggregates and collapses, leading to the heating of the gas cloud and forms metal-poor protostar. This object continues to accrete material and become hotter and hotter until, eventually, hydrogen-burning has begun. A zero-age main sequence (ZAMS) zero metallicity star (a Pop. III star) is formed.
The aim is to find the effect of dark matter on the evolution of those first-generation stars. Dark matter particles drifting from far away in the dark matter halo, are accelerated through the gravitational attraction of the star. They enter the surface of the star, slowed down by collisions with the nucleons inside the star. Eventually, when the speeds of dark matter particles are lower than the escape velocity of the star, they can no longer escape from the star and get captured.
After capture, the first stars are heated up by the dark matter annihilation(similar to matter and anti-matter), and this new heat source can prolong the lifetime of Pop.III stars. The competition between capture and annihilation can model the number of dark matter particles in the star. At the equilibrium where the rate of change of the numbers of the dark matter particles in the star equals to zero, dark matter in the core of the star provides a stabilized power source. It could be much more powerful than fusion. Without this, Pop.III stars powered by fusion with masses typically up to ~1000 mass of the sun. This effects will allow those stars to grow much heavier. In other slightly different scenarios, those stars powered by the dark matter are called Dark Stars, and some of them can grow to be supermassive dark stars (SMDSs).
The model of dark stars was first proposed by Katherine Freese and her group from the University of Michigan in 2008. Later, they demonstrated that these distant but luminous objects would be detectable with the James Webb Space Telescope(JWST). This could also confirm the existence of a new phase of stellar evolution powered by dark matter. Meanwhile, with upcoming observations of first stars with JWST, the possibility to constrain properties of dark matter raises.
Written by Saiyang Zhang & Edited by Alexander Fleiss
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