Dark stars powered by self-interacting dark matter

Open Access

Dark stars powered by self-interacting dark matter

Youjia Wu, Sebastian Baum, Katherine Freese, Luca Visinelli, and Hai-Bo Yu

Phys. Rev. D 106, 043028 – Published 29 August 2022

Abstract

Dark matter annihilation might power the first luminous stars in the Universe. These types of stars, known as dark stars, could form in $(10^{6} - 1 0^{8}) M_{⊙}$ protohalos at redshifts $z \sim 20$ , and they could be much more luminous and larger in size than ordinary stars powered by nuclear fusion. We investigate the formation of dark stars in the self-interacting dark matter (SIDM) scenario. We present a concrete particle physics model of SIDM that can simultaneously give rise to the observed dark matter density, satisfy constraints from astrophysical and terrestrial searches, and address the various small-scale problems of collisionless dark matter via the self-interactions. In this model, the power from dark matter annihilation is deposited in the baryonic gas in environments where dark stars could form. We further study the evolution of SIDM density profiles in the protohalos at $z \sim 20$ . As the baryon cloud collapses due to the various cooling processes, the deepening gravitational potential can speed up gravothermal evolution of the SIDM halo, yielding sufficiently high dark matter densities for dark stars to form. We find that SIDM-powered dark stars can have similar properties, such as their luminosity and size, as dark stars predicted in collisionless dark matter models.

Received 30 May 2022
Accepted 11 August 2022

DOI:https://doi.org/10.1103/PhysRevD.106.043028

Published by the American Physical Society under the terms of the Creative Commons Attribution 4.0 International license. Further distribution of this work must maintain attribution to the author(s) and the published article’s title, journal citation, and DOI. Funded by SCOAP³.

Published by the American Physical Society

Physics Subject Headings (PhySH)

Formation & evolution of stars & galaxies Particle dark matter

Particles & FieldsGravitation, Cosmology & Astrophysics

Authors & Affiliations

Youjia Wu ^1,*, Sebastian Baum ^2,†, Katherine Freese^3,4,5,‡, Luca Visinelli ^6,7,§, and Hai-Bo Yu ^8,∥

¹Leinweber Center for Theoretical Physics, Department of Physics, University of Michigan, Ann Arbor, Michigan 48109, USA
²Stanford Institute for Theoretical Physics, Stanford University, Stanford, California 94305, USA
³Department of Physics, University of Texas, Austin, Texas 78712, USA
⁴The Oskar Klein Centre for Cosmoparticle Physics, Department of Physics, Stockholm University, AlbaNova, Roslagstullsbacken 21, 10691 Stockholm, Sweden
⁵Nordita, KTH Royal Institute of Technology and Stockholm University, Roslagstullsbacken 23, 10691 Stockholm, Sweden
⁶Tsung-Dao Lee Institute (TDLI), 520 Shengrong Road, 201210 Shanghai, People’s Republic of China
⁷School of Physics and Astronomy, Shanghai Jiao Tong University, 800 Dongchuan Road, 200240 Shanghai, People’s Republic of China
⁸Department of Physics and Astronomy, University of California, Riverside, California 92521, USA

^*youjiawu@umich.edu
^†sbaum@stanford.edu
^‡ktfreese@utexas.edu
^§luca.visinelli@sjtu.edu.cn
^∥haiboyu@ucr.edu

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Issue

Vol. 106, Iss. 4 — 15 August 2022

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Images

Figure 1
Velocity dependence of the annihilation cross section $(σ v)_{eff}$ , normalized to $(σ v)_{eff}^{FO} = 6 \times 10^{- 26} {cm}^{3} / s$ , for different choices of $m_{χ}$ and $m_{ϕ}$ . The shaded areas show characteristic velocities during the recombination epoch relevant for CMB constraints, in dark stars, and at freeze-out relevant for setting the relic density.
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Figure 2
Different constraints on our model in the $m_{ϕ} - m_{χ}$ plane as summarized in the final paragraph of Sec. 2. In the gray-shaded band dark matter self-interactions at dwarf galaxy scales could alleviate the small-scale problems of collisionless dark matter. The blue hatched area is disfavored, in this region of parameter space dark matter self-interactions at galaxy-cluster scales lead to conflicts with observations. In the orange-shaded regions (the different orange shades are for different values of the coupling to standard model fermions as indicated in the legend), the decay length of the $ϕ$ ’s from $\bar{χ} χ \to 2 ϕ$ annihilation are shorter than 1 AU such that dark matter annihilation could power a dark star. The red star marks the benchmark point we will use in the remainder of this work.
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Figure 3
Gravothermal evolution of an SIDM halo in the presence of a static gravitational potential source by baryonic gas (measured in units of the characteristic time $t_{0} = 1 / \sqrt{4 π G ρ_{s}} \approx 2.6 Myr$ ). The different lines show the dark matter density profiles at $t / t_{0} = 0$ (solid black), 2 (solid orange) and 8 (solid blue); based on SIDM simulations in Ref. [43], where the gas (dashed red) and initial dark matter (solid black) density profiles are from hydrodynamical cosmological simulations of protogalaxies [3]. For comparison, the dash-dotted black line shows the density profile for a WIMP halo with the same initial dark matter profile undergoing adiabatic contraction by the same gas profile.
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Figure 4
The profiles of gas (red) and dark matter (blue) densities (top), temperature (middle) and pressure (lower) of $10 M_{⊙}$ SIDM-powered (solid lines) and WIMP-powered (dashed lines) dark stars. Both cases are for a dark matter particle mass of $m_{χ} = 100 GeV$ . For the SIDM-powered dark star, we set the SIDM-to-baryon mass ratio in the dark star to $f_{DM} = 7.4 \times 10^{- 4}$ such that both dark stars have the same total luminosity, $L_{eff} = 1.5 \times 10^{5} L_{⊙}$ . The vertical dotted black line indicates the photosphere radius of the SIDM-powered dark star, $R_{⋆} = 3.2 AU$ , and the photosphere temperature of the SIDM dark star is $T_{eff} = 4300 K$ . For the WIMP dark star, we find $R_{⋆} = 2.9 AU$ and $T_{eff} = 4600 K$ .
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