The first high-resolution X-ray spectrum of a Herbig star: AB Aurigae

Context. The X-ray emission from Herbig Ae/Be stars remains to be explained. In later-type T Tauri stars, X-rays are thought to be produced by magnetically trapped coronal plasma, although accretion-shock induced X-rays have also been suggested. In earlier-type (OB) stars, shocks in unstable winds are thought to produce X-rays. Aims. We present the first high-resolution X-ray spectrum of a prototypical Herbig star AB Aurigae), measure and interpret various spectral features, and compare our results with model predictions. Methods. We use X-ray spectroscopy data from the XMM-Newton Reflection Grating Spectrometers and the EPIC instruments. The spectra are interpreted using thermal, optically thin emission models with variable element abundances and a photoelectric absorption component. We interpret line flux ratios in the He-like triplet of O vii as a function of electron density and the UV radiation field. We use the nearby co-eval classical T Tauri star SU Aur as a comparison. Results. AB Aurigae reveals a soft X-ray spectrum, most plasma being concentrated at 1–6 MK. The He-like triplet reveals no signs of increased densities as reported for some accreting T Tau stars in the literature. There are also no clear indications of strong abundance anomalies in the emitting plasma. The light curve displays modulated variability, with a period of ≈42 h. Conclusions. It is unlikely that a nearby, undetected lower-mass companion is the source of the X-rays. Accretion shocks close to the star would be expected to be irradiated by the photosphere, leading to alteration in the He-like triplet fluxes of O vii, which we do not measure. Also, no indications for high densities are found, although the mass accretion rate is presently unknown. Emission from wind shocks is unlikely, given the weak radiation pressure. A possible explanation would be a solar-like magnetic corona. Magnetically confined winds are a very promising alternative. The X-ray period is indeed close to periods previously measured in optical lines from the wind.