SIMBAD references

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, τ³ 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.