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, the convective envelope becomes smaller and X-ray emission fainter. We present Chandra/HRC-I observations of four single stars with early A spectral types. Only the coolest star of this sample, τ ³ Eri (T _eff ≈ 8, 000 K), is detected with $\mathrm{log}({L}_{X}/{L}_{\mathrm{bol}})=-7.6$ while the three hotter stars (T _eff ≥ 8, 000 K), namely δ Leo, β Leo, and ι Cen, remain undetected with upper limits $\mathrm{log}({L}_{X}/{L}_{\mathrm{bol}})\lt -8.4$ . The drop in X-ray emission thus occurs in a narrow range of effective temperatures around ~8100 K and matches the drop of activity in the C III and O VI transition region lines.