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 t Cet,. 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 Ca 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. t Cet,. 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 t Cet and 1.6 for. 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 Atacama Large Millimeter/ Submillimeter Array observations, will enable the construction of temperature profiles of their chromospheres and lower transition regions.