摘要：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 (Teff ≈ 8, 000 K), is detected with while the three hotter stars (Teff ≥ 8, 000 K), namely δ Leo, β Leo, and ι Cen, remain undetected with upper limits . 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.
