Altair – the “hottest” magnetically active star in X-rays

Universität Hamburg, Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany e-mail: jrobrade@hs.uni-hamburg.de

Received: 14 November 2008
Accepted: 22 February 2009

Abstract

Context. The onset of stellar magnetic activity is related to the operation of dynamo processes that require the development of an outer convective layer. This transition of stellar interior structure is expected to occur in late A-type stars.

Aims. 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.

Methods. We used a deep XMM-Newton observation of Altair and analyzed X-ray light curves, spectra, and emission lines. We investigated the temporal behavior and properties of the X-ray emitting plasma and studied the global coronal structure of Altair.

Results. Altair's corona with an X-ray luminosity of  erg/s and an activity level of is located predominantly at low latitude regions and exhibits X-ray properties that are overall very similar to those of other weakly active stars. 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.

Conclusions. 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 similar to those of the quiescent Sun, despite very different underlying stars.