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.Comment: 9 pages, 2 figures. Accepted for publication in Ap
