2025ApJ...988L...9C

Polarization measurements provide strong constraints on magnetic fields in star-forming systems. While magnetic field estimates of a few kG have been obtained near the surfaces of low-mass protostars, there are no analogous measurements in the immediate vicinity of the surface of massive protostars. We report the measurement of radio continuum circular polarization (CP) toward a massive protostar, IRAS 18162-2048, for the first time, using Karl G. Jansky Very Large Array observations. The fractional CP varies between 3% and 5% across the observed frequency range of 4-6 GHz. We consider multiple hypotheses for the production of CP and propose (i) gyrosynchrotron emission and (ii) Faraday conversion due to turbulence in the magnetic medium-both driven by mildly relativistic electrons as plausible mechanisms. We estimate, for the first time, a magnetic field B ≳ 20-35 G close to the massive protostar. The Lorentz factor of the low-energy electrons is estimated to be in the range γ_min ∼ 5-7 for gyrosynchrotron emission and 80-100 for Faraday conversion from our observations. The magnetic field estimate can provide important constraints to the formation models of massive stars.