Altair - the 'hottest' magnetically active star in X-rays

TL;DR

Altair, a hot A7 star with a shallow convection zone, exhibits a stable, low-activity corona with X-ray properties similar to the quiet Sun, studied through deep XMM-Newton observations revealing its coronal structure and plasma characteristics.

Contribution

This study provides detailed analysis of Altair's corona using X-ray observations, revealing its low activity level, plasma properties, and magnetic dynamo behavior in a star with a shallow convection zone.

Findings

Altair's corona is located mainly at low latitudes.

X-ray brightness varies by 30% likely due to rotational modulation.

No strong flares or hot plasma detected during observations.

Abstract

The A7 star Altair is one of the hottest magnetically active stars. Its proximity to the Sun allows a detailed investigation of a corona in X-rays for a star with a shallow convection zone. We used a deep XMM-Newton observation of Altair and analyzed X-ray light curves, spectra, and emission lines, investigated the temporal behavior and properties of the X-ray emitting plasma and studied the global coronal structure. We find that Altair's corona with an X-ray luminosity of L_X =1.4 x 10^27 erg/s and a very low activity level of log L_X/L_bol = -7.4, is located predominantly at low latitude regions. The X-ray emission is dominated by cool plasma (1-4 MK) at low density, and elemental abundances exhibit a solar-like FIP effect and Ne/O ratio. The X-ray brightness varies by 30 % over the observation, most likely due to rotational modulation and minor activity; in contrast, no strong flares or significant amounts of hot plasma were detected. The X-ray activity level of Altair is apparently close to the saturation level, which is reduced by roughly four orders of magnitude when compared to late-type stars. With its fast rotation, Altair provides an inefficient, but very stable dynamo that mainly operates in convective layers below its 'cooler' surface areas around the equator. This dynamo mechanism results in magnetic activity and leads to X-ray properties that are overall very similar to those of the quiescent Sun, despite very different underlying stars.