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Detection of thermal radio emission from a single coronal giant

Abstract

We report the detection of thermal continuum radio emission from the K0 III coronal giant Pollux (β\beta Gem) with the Karl G. Jansky Very Large Array (VLA). The star was detected at 21 and 9 GHz with flux density values of 150±21150\pm21 and 43±8μ43\pm8\,\muJy, respectively. We also place a 3σrms3\sigma_{\mathrm{rms}} upper limit of 23μ23\,\muJy for the flux density at 3 GHz. We find the stellar disk-averaged brightness temperatures to be approximately 9500, 15000, and <71000<71000\,K, at 21, 9, and 3 GHz, respectively, which are consistent with the values of the quiet Sun. The emission is most likely dominated by optically thick thermal emission from an upper chromosphere at 21 and 9 GHz. We discuss other possible additional sources of emission at all frequencies and show that there may also be a small contribution from gyroresonance emission above active regions, coronal free-free emission and free-free emission from an optically thin stellar wind, particularly at the lower frequencies. We constrain the maximum mass-loss rate from Pollux to be less than 3.7×1011M3.7\times 10^{-11}\,M_{\odot} yr1^{-1} (assuming a wind terminal velocity of 215 km s1^{-1}), which is about an order of magnitude smaller than previous constraints for coronal giants and is in agreement with existing predictions for the mass-loss rate of Pollux. These are the first detections of thermal radio emission from a single (i.e., non-binary) coronal giant and demonstrate that low activity coronal giants like Pollux have atmospheres at radio frequencies akin to the quiet Sun

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