At what temperature does magnetic dynamo action cease? Or: How hot is the hottest cool star?

Cool stars on the main sequence generate X-rays from coronal activity, powered by a convective dynamo. With increasing temperature, the convective envelope becomes smaller and X-ray emission fainter, so some cool star must be the “hottest cool star” before emission ends. We present Chandra/HRC-I observations of four single stars with early A spectral types. Only the coolest star of this sample, \(\tau^3\) Eri (Teff = 8000 K), is detected with log(LX/Lbol)=-7.6, while the three hotter stars (Teff > 8100 K), namely \(\delta\) Leo, \(\beta\) Leo, and \(\iota\) Cen, remain undetected with upper limits log(LX/Lbol)<-8.4. The drop in X-ray emission thus occurs a in narrow range of effective temperatures around 8100 K and matches the end of activity in the C III and O VI transition region lines. Based on our observations,  \(\tau^3\) Eri might well be the hottest star that still shows cool-star like coronal activity.