Generalized Predictions for the Electromagnetic Signatures of Mirror Stars

Abstract

Mirror Stars are a generic prediction of dissipative dark matter models, including minimal atomic dark matter and twin baryons in the Mirror Twin Higgs. Mirror Stars capture regular atoms from the interstellar medium through highly suppressed kinetic mixing interactions between the regular and the dark photon. This results in the accumulation of a "nugget", which draws heat from the mirror star core and emits distinctive X-ray and optical signals. In this work, we solve the stellar structure equations of optically thick nuggets across a wide range of the effective mirror star parameter space, and characterize their emission spectra using stellar atmosphere models. This complements an earlier analysis of lower-mass optically thin nuggets. We find that optically thick mirror star nuggets occupy distinct regions of the (stellar surface temperature, luminosity, surface gravity) space, and can be distinguished from regular stars in both HR diagrams and temperature-surface-gravity diagrams using astrometric and spectroscopic stellar catalogues. Our detailed predictions, which are publicly available, now give for the first time a general picture of mirror star signals in the optical and IR to enable realistic mirror star searches using existing catalogues and new telescope observations.

Figures (6)

• Figure 1: Profiles of $T_{\rm{nugget}}(r)$ and $\rho_{\rm{nugget}}(r)$ for two nuggets, one dominantly convective (top) and one dominantly radiative (bottom). Convective regions where $\nabla_{\rm{rad}}>\nabla_{\rm{ad}}$, see Eqn. \ref{['e.PradGrad']}, are indicated in red. The dashed blue vertical line is the photospheric radius, where Eqn. \ref{['e.photoCondition']} holds. $\xi, \rho_\mathrm{core}, M_\mathrm{nugget}$ as well as the nugget luminosity $\mathcal{L}_{\rm{photo}}$ are listed at the top of each profile.
• Figure 2: Nugget properties in the in $(\xi,M_{\rm{nugget}})$ plane, for 7 representative values of the mirror star core density $\rho_\mathrm{core}$ relative to solar central density (different rows). Left to right columns correspond to nugget central density, central temperature, photosphere temperature, photosphere radius, luminosity relative to the Sun, and surface gravity. Optically thick nuggets are circular markers; optically thin nuggets from Armstrong:2023cis are star markers. Hollow markers correspond to interpolated solutions in either regime. Gray contours show $\log_{10}(\tau_\mathrm{MS,eff}/\mathrm{years})$, see Eqn. \ref{['e.tauMSeff']}, indicating the required capture time scale of a solar mass mirror star to accumulate the nugget in the geometric capture regime, which is valid above the gray dashed contour. Note that this must be rescaled to the expected mirror star mass for the given central density. We do not include nuggets with a mass above $10^{32}~\mathrm{g}$, which is approximately the minimum mass for the nugget to become a hydrogen-burning red-dwarf even in the absence of the external mirror star gravitational potential, and we also exclude nuggets that make up more than 10% of the mirror star mass within the nugget radius.
• Figure 3: Disk-integrated spectrum of the convective (top) and radiative (bottom) nuggets corresponding to the profiles in Figure \ref{['f.profile1']}.
• Figure 4: Temperature-surface-gravity diagram for low-luminosity ($10^{-9} < L/L_\odot < 10^{-3}$) (left) and high-luminosity ($10^{-3} < L/L_\odot < 1$) (right) mirror star nuggets. Our optically thick nugget solutions are shown as circular markers with a color indicating the mirror star core density. Colored rectangles indicate approximate regions populated by different types of Standard Model stars. The solid contours demarcate an approximate signal region for mirror stars in each luminosity range. Dashed contours indicate nugget luminosity $\log_{10} [(L/L_\odot )(\rho_{\mathrm{core}, \odot}/\rho_\mathrm{core})]$.
• Figure 5: Hertzsprung-Russell diagrams for mirror stars with optically thick nuggets, for different central mirror star densities $\rho_\mathrm{core} / \rho_{\mathrm{core},\odot}$ (different rows). Each panel shows absolute magnitude $M_G$ versus Gaia color index $(G_{\rm BP} - G_{\rm RP})$. Background stars from Gaia are shown in grayscale number density bins, while the mirror star tracks are plotted as colored scatter points. The color of mirror star markers in each scatter point indicates the surface gravity $\log_{10}(g/\mathrm{cm}~\mathrm{s}^{-2})$, nugget mass, and heating rate (left to right columns).