  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. We use X-ray spectroscopy data from
XMM-Newton. 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 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. AB Aurigae reveals a
soft X-ray spectrum, most plasma being concentrated at 1-6 MK. The He-like
triplet reveals no signatures of increased densities and there are no clear
indications for strong abundance anomalies. The light curve displays modulated
variability, with a period of ~ 42 hr. It is unlikely that a nearby, undetected
lower-mass companion is the source of the X-rays. Accretion shocks close to the
star should 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 provide a very promising alternative. The X-ray
period is indeed close to periods previously measured in optical lines from the
wind.
