Detection of 2-4 GHz Continuum Emission from ɛ Eridani

Abstract

The nearby star ɛ Eridani has been a frequent target of radio surveys for stellar emission and extraterrestrial intelligence. Using deep 2-4 GHz observations with the Very Large Array, we have uncovered a 29 μJy compact, steady continuum radio source coincident with ɛ Eridani to within 0"06 (≲2σ; 0.2 au at the distance of the star). Combining our data with previous high-frequency continuum detections of ɛ Eridani, our observations reveal a spectral turnover at 6 GHz. We ascribe the 2-6 GHz emission to optically thick, thermal gyroresonance radiation from the stellar corona, with thermal free-free opacity likely becoming relevant at frequencies below 1 GHz. The steep spectral index (α ≃ 2) of the 2-6 GHz spectrum strongly disfavors its interpretation as stellar-wind-associated thermal bremsstrahlung (α ≃ 0.6). Attributing the entire observed 2-4 GHz flux density to thermal free-free wind emission, we thus derive a stringent upper limit of 3 × 10^-11 M_⊙ yr^-1 on the mass-loss rate from ɛ Eridani. Finally, we report the nondetection of flares in our data above a 5σ threshold of 95 μJy. Together with the optical nondetection of the most recent stellar maximum expected in 2019, our observations postulate a likely evolution of the internal dynamo of ɛ Eridani.