Probing the accretion-ejection connection with VLTI/AMBER

Abstract

Context. Accretion and ejection are tightly connected and represent the fundamental mechanisms regulating star formation. However, the exact physical processes involved are not yet fully understood. Aims. We present high angular and spectral resolution observations of the Brγ emitting region in the Herbig Ae star HD 163296 (MWC 275) in order to probe the origin of this line and constrain the physical processes taking place at sub-AU scales in the circumstellar region. Methods. By means of VLTI-AMBER observations at high spectral resolution (R ∼ 12 000), we studied interferometric visibilities, wavelength-differential phases, and closure phases across the Brγ line of HD 163296. To constrain the physical origin of the Brγ line in Herbig Ae stars, all the interferometric observables were compared with the predictions of a line radiative transfer disc wind model. Results. The measured visibilities clearly increase within the Brγ line, indicating that the Brγ emitting region is more compact than the continuum. By fitting a geometric Gaussian model to the continuum-corrected Brγ visibilities, we derived a compact radius of the Brγ emitting region of ∼0.07 ± 0.02 AU (Gaussian half width at half maximum; or a ring-fit radius of ∼0.08 ± 0.02 AU). To interpret the observations, we developed a magneto-centrifugally driven disc wind model. Our best disc wind model is able to reproduce, within the errors, all the interferometric observables and it predicts a launching region with an outer radius of ∼0.04 AU. However, the intensity distribution of the entire disc wind emitting region extends up to ∼0.16 AU. Conclusions. Our observations, along with a detailed modelling of the Brγ emitting region, suggest that most of the Brγ emission in HD 163296 originates from a disc wind with a launching region that is over five times more compact than previous estimates of the continuum dust rim radius.

Investigating the origin and spectroscopic variability of the near-infrared H i lines in the Herbig star VV Ser

“…Our modelling suggests, that in the case of VV Ser, the contribution of a fast rotating magnetosphere to the H I NIR emission line profiles is very small in comparison with the contribution of a disc wind or bipolar outflow. Similar results have been previously found in other Herbig AeBe stars by analysing their NIR line profiles alone or in combination with NIR spectrointerferometric techniques (Weigelt et al 2011;Tambovtseva et al 2014;Garcia Lopez et al 2015;Caratti o Garatti et al 2015). In contrast with these results, where a disc wind model could be used to model the Brγ line profile and the spectro-interferometric observables, in the case of VV Ser only a bipolar outflow can approximately reproduce the averaged single-peaked line profiles of the Paβ, Paδ, and Brγ lines observed with the LBT, as well as, the Brγ spectro-interferometric observables.…”

“…At the longer baseline, a faint increase of the visibilities within the Brγ line is detected. This would indicate that most of the Brγ emission is produced in a region smaller than the dust sublimation radius, as previously observed in other Herbig AeBe stars (e.g., Kraus et al 2008;Garcia Lopez et al 2015;Caratti o Garatti et al 2015).…”

Determining the mid-plane conditions of circumstellar discs using gas and dust modelling: a study of HD 163296

“…This treatment would probably not be suitable for the inner few au of the mid-plane, where viscous heating usually dominates. The mass accretion rate of HD 163296 is derived to be within the range (0.8-4.5) ×10 −7 M yr −1 (Garcia Lopez et al 2015) from Br-γ observations, so depending on its value, viscous heating could potentially be important in the very inner disc regions (∼few au). Given that we are interested in mid-plane regions further out, we do not take this effect into account.…”

“…Viscosity Using the masses of H2 within a radius of 90 au (Table 6), we calculate the viscosity parameter αvisc of the inner disc regions. The mass accretion rate of HD 163296 is measured to be within the range (0.8-4.5)×10 −7 M yr −1 (Garcia Lopez et al 2015). The viscosity parameter is given by αvisc = H r −2 τdyn τvisc .…”

On the Brγ line emission of the Herbig Ae/Be star MWC 120

“…The scientific merit of spectrally and spatially resolved Brγ line observations of Herbig Ae/Be stars and its comparison to disc-wind and magnetosphere models was shown in literature (e.g. Weigelt et al 2011;Caratti o Garatti et al 2015;Garcia Lopez et al 2015;Kurosawa et al 2016). In this work, we expand the sparse sample of available detailed interferometric studies on the Brγ line emission region in Herbig stars by presenting new observations of the Herbig Ae/Be star MWC 120.…”