Coronal and chromospheric emission in A-type stars

Abstract

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, $\ddot{I}{„}^3$ Eri ($T_{\mathrm{eff}}\approx 8,000$ K), is detected with $\log (L_X/L_{\mathrm{bol}})=-7.6$ while the three hotter stars ($T_{\mathrm{eff}}\ge 8,000$ K), namely $\hat{I}´$ Leo, ${\hat{I}}^2$ Leo, and ${\hat{I}}^1$ Cen, remain undetected with upper limits $\log (L_X/L_{\mathrm{bol}})<-8.4$. The drop in X-ray emission thus occurs in a narrow range of effective temperatures around $\sim 8100$ K and matches the drop of activity in the C III and O VI transition region lines.

11 pages, 3 figures, accepted by AJ