Black holes with regular scalar hair in Brans-Dicke gravity via the Herglotz variational principle

Abstract

Brans-Dicke theory is reformulated within the Herglotz variational principle (HVP), and an exact black hole solution with scalar hair is obtained for $ω_{0}=0$ and vanishing potential $V(ϕ)=0$. The scalar profile is strictly positive and the resulting stealth Schwarzschild solution arises without fixing the otherwise arbitrary Herglotz function $η(r)$. Motivated by the weak-field limit, the explicit choice $η(r)=η_{0}(r-2M)^k/r^{k+2}$, with $η_0$ a constant of dimension length, produces a scalar field configuration remaining regular at the black hole horizon. Consequently, the HVP provides a new mechanism for evading standard no-hair theorems in scalar-tensor theories.