We present the first detections of thermal radio emission from the atmospheres of solar-type stars \tau Cet , \eta Cas A , and 40 Eri A .
These stars all resemble the Sun in age and level of magnetic activity , as indicated by X-ray luminosity and chromospheric emission in calcium-II H and K lines .
We observed these stars with the Karl G. Jansky Very Large Array with sensitivities of a few \mu Jy at combinations of 10.0 , 15.0 , and 34.5 GHz . \tau Cet , \eta Cas A , and 40 Eri A are all detected at 34.5 GHz with signal-to-noise ratios of 6.5 , 5.2 , and 4.5 , respectively .
15.0-GHz upper limits imply a rising spectral index greater than 1.0 for \tau Cet and 1.6 for \eta Cas A , at the 95 % confidence level .
The measured 34.5-GHz flux densities correspond to stellar disk-averaged brightness temperatures of roughly 10,000 K , similar to the solar brightness temperature at the same frequency .
We explain this emission as optically-thick thermal free-free emission from the chromosphere , with possible contributions from coronal gyroresonance emission above active regions and coronal free-free emission .
These and similar quality data on other nearby solar-type stars , when combined with ALMA observations , will enable the construction of temperature profiles of their chromospheres and lower transition regions .