Millicharged Particle Production During Late-Stage Stellar Evolution

Abstract

Stars are natural sources of feebly interacting particles, including putative particles with mass $m_χ$ and electric charge $qe$. The emission of such millicharged particles (MCPs) causes an energy loss which can alter stellar evolution. While MCP production rates have been computed for different plasma parameters, they have yet to be derived for the conditions relevant to late stages of stellar evolution, in which the temperature can reach values $T\simeq 10-100\,\rm keV$ while the plasma frequency is $ω_{\rm pl}\ll T$. In this paper, we compute the MCP energy-loss rates relevant for pre-supernova objects, finding three different regimes in which the dominant processes are respectively plasmon decay ($m_χ< ω_{\rm pl}/2$), Compton-like scattering ($m_χ> ω_{\rm pl}/2$, $T\lesssim 0.5\,\rm MeV$), and electron-positron annihilation. We obtain semi-analytical fits for the energy-loss rates suitable for implementation in stellar evolution codes.

Figures (7)

• Figure 1: Left panel: Evolutionary track of a $20\,M_\odot$ stellar model in the $T_C\,–\,\rho_C$ plane. Colored markers indicate key evolutionary stages. Right panel: Evolution of the ratio $T/\omega_{\rm pl}$ for the same $20\,M_\odot$ model.
• Figure 2: Processes for millicharged particles pair production.
• Figure 3: Left panel: MCP emissivity via Compton as a function of the temperature $T$ for different values of MCP masses at $\rho Y_e = 10^{5}\,\rm g \,cm^{-3}$. The dashed lines represent the non-relativistic limit given by Eq. \ref{['eq:Compt_NR']} while the ultra-relativistic limit given by Eq. \ref{['eq:Compt_UR']} is shown by the dotted lines. The solid lines show the emissivity given by Eq. \ref{['eq:Compt_int']}, obtained by interpolating the two limits. Right panel: Relative error in the fitting functions $\Lambda$ for the non-relativistic limit in Eq. \ref{['eq:NRfunc']} (blue curve) and $\Phi$ for the relativistic limit given by Eq. \ref{['eq:URfunc']} (red curve).
• Figure 4: Left panel: MCP emissivity via pair production (see Eq. \ref{['eq:em_pair']}) as a function of the temperature $T$ for different values of MCP masses. Right panel: Relative error in the fit of the function $F$ defined in Eq. \ref{['eq:F_pair']}.
• Figure 5: Emissivities as a function of temperature and electron density for pair annihilation (A, top panels), Compton scattering (C, central panels), and plasmon decay (D, lower panels). Results are shown for $m_\chi = 1\,{\rm keV}$ (left panels) and $m_\chi = 100\,{\rm keV}$ (right panels). The black region in the upper panels corresponds to emissivities smaller than $10^{16}\,{\rm erg\,cm^{-3}\,s^{-1}}$. The hatched region in the central panels marks the conditions under which the plasma frequency cannot be neglected, since $\omega_{\rm pl}>T$. The white regions in the lower panels indicate where the D-process emissivity vanishes because $m_\chi > \omega_{\rm pl}/2$.