X-ray emission from the remarkable A-type star HR 8799

TL;DR

This study reports the detection of weak, stable X-ray emission from the A-type star HR 8799, challenging the expectation that mid A-type stars are X-ray dark, and provides insights into its coronal activity.

Contribution

First clear detection of X-ray emission from HR 8799, demonstrating that some A-type stars can exhibit coronal activity contrary to previous assumptions.

Findings

HR 8799 emits X-rays with Lx ≈ 1.3 x 10^28 erg/s

X-ray emission is consistent with late A/early F-type stars

Star shows long-term stable X-ray emission

Abstract

We present a Chandra observation of the exceptional planet bearing A5V star HR 8799, more precisely classified as a kA5hF0mA5 star and search for intrinsic X-ray emission. We clearly detect HR 8799 at soft X-ray energies with the ACIS-S detector in a 10 ks exposure; minor X-ray brightness variability is present during the observation. The coronal plasma is described well by a model with a temperature of around 3 MK and an X-ray luminosity of about Lx = 1.3 x 10^28 erg/s in the 0.2-2.0 keV band, corresponding to an activity level of log Lx/Lbol ~ -6.2. Altogether, these findings point to a rather weakly active and given a RASS detection, long-term stable X-ray emitting star. The X-ray emission from HR 8799 resembles those of a late A/early F-type stars, in agreement with its classification from hydrogen lines and effective temperature determination and thus resolving the apparent discrepancy with the standard picture of magnetic activity that predicts mid A-type stars to be virtually X-ray dark.