Coronal and Chromospheric Emission in A-type Stars

Cool stars on the main sequence generate X-rays from coronal activity, powered by a convective dynamo. With increasing temperature, envelope becomes smaller and X-ray emission fainter. We present Chandra/HRC-I observations of four single with early A spectral types. Only coolest star this sample, $\tau^3$ Eri ($T_\mathrm{eff}\approx8\,,000$ K), is detected $\log(L_X/L_\mathrm{bol})=-7.6$ while three hotter ($T_\mathrm{eff}\geq8\,,000$ namely $\delta$ Leo, $\beta$ $\iota$ Cen, remain undetected upper limits $\log(L_X/L_\mathrm{bol})<-8.4$. The drop in thus occurs narrow range effective temperatures around $\sim 8100$ K matches activity C III O VI transition region lines.