The maximum mass of dark matter existing in compact stars based on the self-interacting fermionic model

By assuming that only gravitation acts between dark matter (DM) and normal matter (NM), we studied DM admixed neutron stars (DANSs) using the two-fluid TOV equations. The NM and DM of compact stars are simulated by the relativistic mean field (RMF) theory and non-self-annihilating self-interacting fermionic model, respectively. The effects of the particle mass of fermionic DM $m_f$ and the interaction strength parameter $y$ on the properties of DANSs are investigated in detail. $m_f$ and $y$ are considered as the free parameters due to the lack of information about the particle nature of DM so far. For a DANS, we suggest a simple universal relationship $M_D^{\max}=(0.267 y +0.627-3.21\frac{M_N}{\M_{\odot}})( \frac{1\GeV}{{m_f}})^2 \M_{\odot}$ for $y>100$, where $M_D^{\max}$ is the maximum mass of DM existing in DANSs and $M_N$ is the mass of the neutron star without DM. For free fermion DM model ($y$=0), the relationship becomes $ M_D^{\max}=(0.627-0.027\frac{M_N^2}{\M_{\odot}^2}) ( \frac{1\GeV}{{m_f}})^2 \M_{\odot}$. The radius of DM $R_D$ shows a linear relationship with $M_D^{\max}$ in DANSs, namely $R_D=(7.02 \frac{M_D^{\max}}{ \M_{\odot}}+1.36)$~km. These conclusions are independent of the different NM EOSs from RMF theory. Such a kind of universal relationship connecting the nature of DM particle and mass of stars might shed light on the constraining the nature of the DM by indirect method.
Comment: 13 pages, 7 figures