[Paper Review] The 2008 outburst in the young stellar system ZCMa: I. Evidence of an enhanced bipolar wind on the AU-scale

M. Benisty, F. Malbet+16 more|arXiv (Cornell University)|Jul 5, 2010

Astrophysics and Star Formation Studies38 references21 citations

TL;DR

Using VLTI/AMBER interferometry during Z CMa's 2008 outburst, the study detects a bipolar wind in the AU-scale inner region of the Herbig Be star, with spectro-astrometric and visibility data ruling out Keplerian disk or infall models. The disappearance of the Br γ line post-outburst indicates the outburst was driven by enhanced mass ejection, not dust obscuration, suggesting a link between high accretion and strong outflows in massive young stars.

ABSTRACT

Accretion is a fundamental process in star formation. Although the time evolution of accretion remains a matter of debate, observations and modelling studies suggest that episodic outbursts of strong accretion may dominate the formation of the protostar. Observing young stellar objects during these elevated accretion states is crucial to understanding the origin of unsteady accretion. ZCMa is a pre-main-sequence binary system composed of an embedded Herbig Be star, undergoing photometric outbursts, and a FU Orionis star. The Herbig Be component recently underwent its largest optical photometric outburst detected so far. We aim to constrain the origin of this outburst by studying the emission region of the HI Brackett gamma line, a powerful tracer of accretion/ejection processes on the AU-scale in young stars. Using the AMBER/VLTI instrument at spectral resolutions of 1500 and 12 000, we performed spatially and spectrally resolved interferometric observations of the hot gas emitting across the Brackett gamma emission line, during and after the outburst. From the visibilities and differential phases, we derive characteristic sizes for the Brackett gamma emission and spectro-astrometric measurements across the line, with respect to the continuum. We find that the line profile, the astrometric signal, and the visibilities are inconsistent with the signature of either a Keplerian disk or infall of matter. They are, instead, evidence of a bipolar wind, maybe partly seen through a disk hole inside the dust sublimation radius. The disappearance of the Brackett gamma emission line after the outburst suggests that the outburst is related to a period of strong mass loss rather than a change of the extinction along the line of sight. Based on these conclusions, we speculate that the origin of the outburst is an event of enhanced mass accretion, similar to those occuring in EX Ors and FU Ors.

Motivation & Objective

• To investigate the origin of the 2008 photometric outburst in the Z CMa binary system, which is the largest such event observed in 90 years.
• To constrain the physical conditions and geometry of the hot gas emitting in the HI Br γ line, a key tracer of accretion and ejection processes on AU-scales.
• To determine whether the outburst was caused by changes in extinction or by intrinsic changes in the accretion-ejection activity.
• To test whether the innermost environment of the Herbig Be star hosts a disk-wind or stellar wind structure, using high-spectral and spatial resolution interferometry.
• To assess the role of mass accretion and ejection in high-mass young stellar objects by analyzing the response of the Br γ line during extreme accretion states.

Proposed method

• Performed spatially and spectrally resolved interferometric observations using the AMBER instrument at the Very Large Telescope Interferometer (VLTI) at spectral resolutions of 1500 and 12,000.
• Measured visibilities and differential phases across the Br γ emission line to derive characteristic sizes and spatial offsets of the emitting region.
• Applied spectro-astrometry to detect small displacements between redshifted and blueshifted line components, indicating asymmetric emission structures.
• Compared the spatial and spectral characteristics of the Br γ line emission with those of the continuum to isolate the location and kinematics of the emitting gas.
• Used the observed astrometric signal and visibility modulations to rule out Keplerian disk and infall models, favoring a bipolar wind geometry.
• Combined results with existing data on Balmer line absorption and CO overtone emission to infer the mass accretion rate and wind properties.

Experimental results

Research questions

• RQ1 What is the origin of the 2008 photometric outburst in Z CMa—dust obscuration or enhanced accretion/ejection?
• RQ2 What is the geometry and kinematics of the hot gas emitting in the Br γ line during the outburst?
• RQ3 Is the emitting region consistent with a Keplerian disk, infalling matter, or a bipolar wind?
• RQ4 How does the disappearance of the Br γ line after the outburst inform the physical mechanism behind the event?
• RQ5 Can the observed interferometric signatures be explained by a disk-wind or stellar wind model, and what does this imply for accretion-ejection coupling in massive young stars?

Key findings

• The Br γ line profile, astrometric signal, and visibility data are inconsistent with a Keplerian disk or infall, instead indicating a bipolar wind structure.
• The spectro-astrometric displacement between redshifted and blueshifted components confirms asymmetric emission, supporting a bipolar geometry with emission seen through a disk hole.
• The characteristic size of the Br γ emission region is consistent with the dust sublimation radius, placing the emission inside the innermost disk cavity.
• The Br γ emission line vanishes after the outburst, indicating that the line emission was transient and linked to the outburst phase, not a permanent feature.
• The outburst is not due to changes in line-of-sight extinction, as the disappearance of the line is not accompanied by a change in the continuum flux.
• The results suggest that the outburst was triggered by a sudden increase in mass accretion, leading to a strong, transient bipolar wind, analogous to EXors and FUors in lower-mass stars.