Coronal and chromospheric emission in A-type stars

TL;DR

This study investigates X-ray emissions in early A-type stars, finding that only the coolest among them emit detectable X-rays, indicating a sharp decline in coronal activity around 8100 K.

Contribution

First observational evidence of the temperature threshold for X-ray emission in A-type stars using Chandra data.

Findings

Only the coolest star detected in X-rays.

X-ray emission drops sharply around 8100 K.

Coronal activity decline matches transition region line activity.

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, $\tau^3$ Eri ($T_\mathrm{eff}\approx8\,,000$ K), is detected with $\log(L_X/L_\mathrm{bol})=-7.6$ while the three hotter stars ($T_\mathrm{eff}\geq8\,,000$ K), namely $\delta$ Leo, $\beta$ Leo, and $\iota$ 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.