First Detection of Thermal Radio Emission from Solar-Type Stars with the   Karl G. Jansky Very Large Array
  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. 
Jackie Villadsen, Gregg Hallinan, Stephen Bourke, Manuel G\"udel,   Michael Rupen
astro-ph.SR