Hydrogen lines as a diagnostic tool for studying multicomponent emitting regions in hot young stars: magnetosphere, X-wind, and disk wind

Abstract

Infrared interferometric observations with high spatial and spectral resolution and their quantitative modeling provide us with a unique opportunity to improve our understanding of the circumstellar environment of young stars and the accretion-ejection process. The goal of this paper is to investigate various models of the emitting regions in young Herbig Ae/Be stars that consist of (i) a compact rotating magnetosphere; (ii) an X-wind; and (iii) a disk wind. These models can be used, for example, to quantitatively interpret line profile measurements and infrared interferometric observations with the AMBER instrument of the Very Large Telescope Interferometer (VLTI) in the high spectral resolution mode (R = 12 000). VLTI/AMBER observations allow us to resolve the disk wind region and study the flux contribution of the unresolved magnetosphere and X-wind region to the total line flux. Analyzing the results of our non-LTE calculations, we conclude that the mechanisms of the different broadening of emission lines of different series include (1) the kinematic expansion due to the motion of the outflowing, accelerating gas in the magneto-centrifugal disk wind; (2) the Stark effect; and (3) the rotation of the magnetosphere. We also investigated extinction effects that can influence the shape of the line profiles. We considered the obscuration of the outer disk wind by an opaque dust and gas disk, the obscuration of the disk wind by a flared disk or by dust in the disk wind itself, and absorption of the star and disk continuum radiation in the disk wind along the line of sight. We show that due to extinction effects, the line profiles can change from double-peaked to single-peaked and P Cygni profiles. We studied the contribution of the different components of the stellar environment to different hydrogen emission lines and investigated how this contribution is dependent on the model parameters. The results of this study can be used for the detailed modeling of the emitting regions of individual young stars.

Probing the accretion-ejection connection with VLTI/AMBER

“…However, these model parameters mainly modify the Brγ line profile (see e.g. Tambovtseva et al 2014), and have little effect on the line visibilities and differential phases which mostly depend on the ω 1 and ω N parameters. Therefore, a good match between our interferometric observables (line profile, visibilities, and differential and closure phases) and the modelled results can only be achieved when combining lower values of the inner and outer disc wind footpoints, that is, when a compact disc wind is considered.…”

“…Tambovtseva et al 2014, for examples of hybrid models). Because of the small magnetic fields measured in Herbig AeBe stars, the distance at which the circumstellar disc of Herbig AeBe stars is truncated by the stellar magnetic field (the truncation radius) is expected to be much smaller than in CTTSs and it is located within the corotation radius (R cor ; Shu et al 1994;Muzerolle et al 2004).…”

“…As a consequence, any additional emission in our model located within the corotation radius (∼0.02 AU) would increase the modelled Brγ visibilities of the disc wind intensity distribution. To illustrate this effect, we employed a hybrid model (disc wind plus magnetosphere) as described in Tambovtseva et al (2014), and add the contribution of a compact magnetosphere accounting for 40% of the total Brγ flux to our best model MW6 (a brief description of the magnestosphere parameters and their values can be found in Appendix D). To account for the extra emission from the magnetosphere and obtain a good fit to the line profile, the Brγ emitting region in model MW6 was reduced by increasing the size of the low-temperature region at the wind base where the gas does not produce Brγ emission (see Sect.…”

“…Following our previous work on the Herbig Be star MWC 297, we developed a disc wind model to explain the observed Brγ line profile, line visibilities, differential phases, and closure phase (Weigelt et al 2011;Grinin & Tambovtseva 2011;Tambovtseva et al 2014). A detailed description of the disc wind model, the complete algorithm for the model computation, and a detailed description on how the disc wind model solution depends on the kinematic and physical parameters can be found in these papers.…”

“…These parameters, along with our interferometric observations further constrain our disc wind model solution. For instance, the measured visibilities constrain the size of the disc wind launching region, and the adoptedṀ w and measured line profile constrain the wind dynamics (see Tambovtseva et al 2014, for a complete discussion).…”