The 2008 outburst in the young stellar system Z?CMa - I. Evidence of an enhanced bipolar wind on the AU-scale

摘要

Context. Accretion is a fundamental process in starformation. Although the time evolution of accretion remains a matter ofdebate, observations and modelling studies suggest that episodicoutbursts of strong accretion may dominate the formation of the centralprotostar. Observing young stellar objects during these elevatedaccretion states is crucial to understanding the origin of unsteadyaccretion. Aims. ZCMais a pre-main-sequence binary systemcomposed of an embedded Herbig Be star, undergoing photometricoutbursts, and a FUOrionis star. This system therefore provides aunique opportunity to study unsteady accretion processes. The Herbig Becomponent recently underwent its largest optical photometric outburstdetected so far. We aim to constrain the origin of this outburst bystudying the emission region of the HI Brline, a powerful tracer of accretion/ejection processes on the AU-scale in young stars. Methods. Using the AMBER/VLTI instrument at spectral resolutionsof 1500 and 12000, we performed spatially and spectrally resolvedinterferometric observations of the hot gas emitting across the Bremissionline, during and after the outburst. From the visibilities anddifferential phases, we derive characteristic sizes for the Bremission and spectro-astrometric measurements across the line, with respect to the continuum. Results. We find that the line profile, the astrometric signal,and the visibilities are inconsistent with the signature of either aKeplerian disk or infall of matter. They are, instead, evidence of abipolar wind, maybe partly seen through a disk hole inside the dustsublimation radius. The disappearance of the Bremissionline after the outburst suggests that the outburst is related to aperiod of strong mass loss rather than a change of the extinction alongthe line of sight. Conclusions. Apart from the photometric increase of the system,the main consequence of the outburst is to trigger a massive bipolaroutflow from the Herbig Becomponent. Based on these conclusions,we speculate that the origin of the outburst is an event of enhancedmass accretion, similar to those occuring in EXOrs andFUOrs. Key words: circumstellar matter - stars: variables: T Tauri -herbig Ae/Be - stars: winds - outflows - methods: obsevational - stars:individual: Z CMa - techniques: interferometric