Gas dynamics in the inner few AU around the Herbig B[e] star MWC297

Abstract

Aims. Circumstellar accretion disks and outflows play an important role in star formation. By studying the continuum and Brγ-emitting region of the Herbig B[e] star MWC297 with high-spectral and high-spatial resolution we aim to gain insight into the wind-launching mechanisms in young stars. Methods. We present near-infrared AMBER (R = 12 000) and CRIRES (R = 100 000) observations of the Herbig B[e] star MWC297 in the hydrogen Brγ-line. Using the VLTI unit telescopes, we obtained a uv-coverage suitable for aperture synthesis imaging. We interpret our velocity-resolved images as well as the derived two-dimensional photocenter displacement vectors, and fit kinematic models to our visibility and phase data in order to constrain the gas velocity field on sub-AU scales. Results. The measured continuum visibilities constrain the orientation of the near-infrared-emitting dust disk, where we determine that the disk major axis is oriented along a position angle of ∼99.6 ± 4.8 • . The near-infrared continuum emission is ∼3.6× more compact than the expected dust-sublimation radius, possibly indicating the presence of highly refractory dust grains or optically thick gas emission in the inner disk. Our velocity-resolved channel maps and moment maps reveal the motion of the Brγ-emitting gas in six velocity channels, marking the first time that kinematic effects in the sub-AU inner regions of a protoplanetary disk could be directly imaged. We find a rotation-dominated velocity field, where the blue-and red-shifted emissions are displaced along a position angle of 24 • ± 3 • and the approaching part of the disk is offset west of the star. The visibility drop in the line as well as the strong non-zero phase signals can be modeled reasonably well assuming a Keplerian velocity field, although this model is not able to explain the 3σ difference that we measure between the position angle of the line photocenters and the position angle of the dust disk. We find that the fit can be improved by adding an outflowing component to the velocity field, as inspired by a magneto-centrifugal disk-wind scenario.Conclusions. This study combines spectroscopy, spectroastrometry, and high-spectral dispersion interferometric, providing yet the tightest constraints on the distribution and kinematics of Brγ-emitting gas in the inner few AU around a young star. All observables can be modeled assuming a disk wind scenario. Our simulations show that adding a poloidal velocity component causes the perceived system axis to shift, offering a powerful new diagnostic for detecting non-Keplerian velocity components in other systems.

Compact gaseous accretion disk in Keplerian rotation around MWC 147

“…The asymmetric line profile of MWC 297 was well reproduced by a more complex disk-wind model whereas, for MWC 147, the strongly symmetrical observed Brγ-line suggests that the emission arises in a rotating circumstellar disk or a region of a disk wind near the disk surface where material has not been significantly accelerated. The differences between these two scenarios are subtle (as we discuss in Hone et al 2017) and it is likely only possible to distinguish between the two cases if there is a more significant out-of-plane velocity component present in the velocity field traced by the Brγ emission. Such out-of-plane velocities are found in disk wind scenarios in the outer regions where material is more accelerated but also more diffuse, leading to a much smaller contribution to the overall line emission.…”

“…Article number, page 5 of 10 A&A proofs: manuscript no. main To derive the wavelength-dependent photocentre shifts from our differential phase data we use the fitting method outlined in Hone et al (2017) that is based on the equation:…”

“…Kurosawa et al (2016) also fit their spectro-interferometric data with a disk wind model but noted that the expected emission from the base of the wind strongly resembled a Keplerian rotation velocity field, making it difficult to distinguish between the two line-emitting scenarios. In their modelling of Brγ emission from MWC 297, Hone et al (2017) demonstrated that it was possible to distinguish between a disk wind and Keplerian rotation, as it was shown that an out-of-plane velocity component shifts the perceived rotation axis 1 away from the disk axis. In a Keplerian rotation case, it would be expected that the axis of rotation is aligned with the axis of the continuum-emitting disk.…”

Why chromatic imaging matters

“…This method was subsequently used in a few papers to reconstruct images of supergiant stars [see e.g., 25, 27] and Eta Carinae [e.g., 33, 48]. More recently, [14] published the first chromatic reconstruction of the Brγ structure in a YSO with IRBis. [24] presented an extended method to tackle the image reconstruction challenges posed by the lacking of the closure phases and a proper calibration of the spectrally-dispersed visibilities.…”

“…After choosing the regularisation, MiRA needs the following inputs: 14 Environment variables considered by make config are: CC, CFLAGS, LDFLAGS, RANLIB and AR (i) The data, in the OIFITS standard format, either table versions 1 (Pauls et al, 2005) or 2 (Duvert et al, 2015). (ii) An optional estimate for the image -important when only power-spectra and bispectra are present.…”