Radio Emission from Supernovae in the Very Early Phase: Implications for the Dynamical Mass Loss of Massive Stars

Recent high-cadence transient surveys and rapid follow-up observations indicate that some massive stars may dynamically lose their own mass within decades before supernovae (SNe). Such a mass-loss forms "confined" circumstellar medium (CSM); a high-density material distributed only within a small radius ( 1015 cm with the mass-loss rate of 0.01 ∼ 10−4 M yr−1). While the SN shock should trigger particle acceleration and magnetic field amplification in the "confined" CSM, synchrotron emission may be masked in centimeter wavelengths due to free-free absorption; the millimeter range can, however, be a potential new window. We investigate the time evolution of synchrotron radiation from the system of a red supergiant surrounded by the "confined" CSM, relevant to typical Type II-P SNe. We show that synchrotron millimeter emission is generally detectable, and that the signal can be used as a sensitive tracer of the nature of the "confined" CSM; it traces different CSM density parameter space than in the optical. Furthermore, our simulations show that the "confined" CSM efficiently produces secondary electrons and positrons through proton inelastic collisions, which can become main contributors to the synchrotron emission in several ten days since the SN. We predict that the synchrotron emission is detectable by ALMA, and suggest that it will provide a robust evidence of the existence of the "confined" CSM.