Dark Stars: A Review (1501.02394v2)

Abstract: Dark Stars are stellar objects made (almost entirely) of hydrogen and helium, but powered by the heat from Dark Matter annihilation, rather than by fusion. They are in hydrostatic and thermal equilibrium, but with an unusual power source. Weakly Interacting Massive Particles (WIMPs), among the best candidates for dark matter, can be their own antimatter and can annihilate inside the star, thereby providing a heat source. Although dark matter constitutes only $\lesssim 0.1\%$ of the stellar mass, this amount is sufficient to power the star for millions to billions of years. Thus, the first phase of stellar evolution in the history of the Universe may have been dark stars. We review how dark stars come into existence, how they grow as long as dark matter fuel persists, and their stellar structure and evolution. The studies were done in two different ways, first assuming polytropic interiors and more recently using the MESA stellar evolution code; the basic results are the same. Dark stars are giant, puffy ($\sim$ 10 AU) and cool (surface temperatures $\sim$10,000 K) objects. We follow the evolution of dark stars from their inception at $\sim 1 M_\odot$ as they accrete mass from their surroundings to become supermassive stars, some even reaching masses $> 10^6 M_\odot$ and luminosities $>10^{10} L_\odot$, making them detectable with the upcoming James Webb Space Telescope. Once the dark matter runs out and the dark star dies, it may collapse to a black hole; thus dark stars may provide seeds for the supermassive black holes observed throughout the Universe and at early times. Other sites for dark star formation may exist in the Universe today in regions of high dark matter density such as the centers of galaxies. The current review briefly discusses dark stars existing today, but focuses on the early generation of dark stars.

• The paper outlines how dark stars form when high dark matter densities, trapped annihilation products, and dominant DM heating overcome traditional fusion processes.
• The paper employs both polytropic models and the MESA code to reveal dark stars’ large sizes (about 10 AU), cooler surfaces (~10,000 K), and potential to grow into supermassive objects.
• The paper discusses observational prospects with JWST and posits that the collapse of dark stars might seed the formation of early supermassive black holes.

Overview of "Dark Stars: A Review"

The paper "Dark Stars: A Review" by Katherine Freese et al. provides a comprehensive analysis of the concept of Dark Stars (DSs), celestial objects hypothesized to be powered by dark matter (DM) annihilation instead of traditional nuclear fusion. The authors explore several facets of these stars, focusing particularly on their theoretical formation, structure, evolution, and implications for understanding stellar generation in the early Universe and their potential contribution to the formation of massive black holes.

Dark Stars and Their Formation

The proposed model posits that Dark Stars are formed predominantly from hydrogen and helium while being sustained by the thermal effects of DM annihilation. Weakly Interacting Massive Particles (WIMPs), hypothesized as prime candidates for DM, annihilate within these stars, releasing sufficient energy to sustain them for prolonged periods ranging from millions to billions of years, despite accounting for merely 0.1% of the stellar mass. The formation of DSs is suggested to have occurred during the earliest phases of stellar evolution in the Universe, coinciding with the high DM densities prevalent at that time.

The authors articulate three critical conditions for the formation of DSs: the necessity of high DM densities, trapping of annihilation products within the stellar structure, and the dominance of DM annihilation heating over other energy processes such as fusion. These criteria are crucial to instigating the unique properties of DSs, distinguishing them from traditional branches of stellar evolution.

Stellar Structure and Evolution

The review explores methodologies adopted for evaluating DS structures, notably through polytropic models and the more advanced MESA stellar evolution code. Both frameworks yield consistent fundamental insights, depicting DSs as extensive, with radii approximately 10 astronomical units and relatively cooler surface temperatures around 10,000 K. The growth trajectory of DSs, fueled by ambient DM, allows them to amass into supermassive entities potentially exceeding 10^6 solar masses, with luminosity surpassing 10^9 solar luminosities. Furthermore, the investigation into equilibrium dynamics reveals insights into energy contributions from gravitational contraction and nuclear fusion, and the intriguing potential of DM capture through elastic scattering.

Numerical Results and Future Implications

The paper presents detailed numerical results regarding the evolution and observational characteristics of DSs. These findings are pivotal in evaluating their potential detectability through the James Webb Space Telescope (JWST). Given the speculated masses and luminosities of supermassive DSs, JWST might observe these entities under specific conditions. Moreover, the theorized pulsations of DSs offer an additional mechanism for their identification, offering unique variability signatures distinguishable from galaxies.

Furthermore, the review considers the role of DSs in explaining the existence of supermassive black holes across the Universe. The eventual cessation of DM fuel could lead DSs to collapse into black holes, providing seeds for the observed supermassive black holes and helping reconcile their presence at relatively young cosmic ages.

Observational Prospects and Contemporary Existence

While the focus is largely on primordial DSs, the review acknowledges the possibility of analogous stars existing in high DM density regions today, such as galaxy centers. These modern DSs may offer observable insights into DM properties and the dynamics of stellar evolution under non-traditional energy sources.

Conclusion

"Dark Stars: A Review" contributes a crucial theoretical examination of DSs, positioning them as a significant chapter in the cosmic narrative. Their paper not only broadens understanding of stellar formations in the cosmological timeline but also offers avenues for advancing DM detection methodologies. As the astronomical community approaches new observational milestones, DSs present an exciting frontier intertwined with the quest to understand the Universe's earliest and most enigmatic phenomena.