We revisit the astrophysical constraints on a generic light CP-even scalar particle $S$, mixing with the Standard Model (SM) Higgs boson, from observed luminosities of the Sun, red giants, white dwarfs and horizontal-branch stars. The production of $S$ in the stellar core is dominated by the electron-nuclei bremsstrahlung process $e + N to e + N + S$. With the $S$ decay and reabsorption processes taken into consideration, we find that the stellar luminosity limits exclude a broad range of parameter space in the $S$ mass-mixing plane, with the scalar mass up to 350 keV and the mixing angle ranging from $7.0times10^{-18}$ to $3.4times10^{-3}$. We also apply the stellar limits to a real-singlet scalar extension of the SM, where we can relate the mixing angle to the parameters in the scalar potential. In both the generic scalar case and the real-singlet extension, we show that the stellar limits preclude the scalar interpretation of the recently observed XENON1T excess in terms of the $S$ particles emitted from the Sun.
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