Stationary bound-state massive scalar field configurations supported by spherically symmetric compact reflecting stars

Abstract: It has recently been demonstrated that asymptotically flat neutral reflecting stars are characterized by an intriguing no-hair property. In particular, it has been proved that these {\it horizonless} compact objects cannot support spatially regular {\it static} matter configurations made of scalar (spin-0) fields, vector (spin-1) fields, and tensor (spin-2) fields. In the present paper we shall explicitly prove that spherically symmetric compact reflecting stars can support {\it stationary} (rather than static) bound-state massive scalar fields in their exterior spacetime regions. To this end, we solve analytically the Klein-Gordon wave equation for a linearized scalar field of mass $\mu$ and proper frequency $\omega$ in the curved background of a spherically symmetric compact reflecting star of mass $M$ and radius $R_{\text{s}}$. It is proved that the regime of existence of these stationary composed star-field configurations is characterized by the simple inequalities $1-2M/R_{\text{s}}<(\omega/\mu)^2<1$. Interestingly, in the regime $M/R_{\text{s}}\ll1$ of weakly self-gravitating stars we derive a remarkably compact {\it analytical} formula for the discrete spectrum ${\omega(M,R_{\text{s}},\mu)}^{n=\infty}_{n=1}$ of resonant oscillation frequencies which characterize the stationary composed compact-reflecting-star-linearized-massive-scalar-field configurations. Finally, we verify the accuracy of the analytically derived resonance formula of the composed star-field configurations with direct numerical computations.