Probing the wind launching regions of the Herbig Be star HD 58647 with high spectral resolution interferometry

TL;DR

This study uses high-resolution interferometry to investigate the wind launching region of the Herbig Be star HD 58647, revealing a small magnetosphere and disc wind that explain the observed Br-gamma emission and angular momentum loss.

Contribution

It introduces a radiative transfer model combining a small magnetosphere and disc wind to explain Br-gamma emission in HD 58647, highlighting the wind's role in angular momentum loss.

Findings

Br-gamma emission region is smaller than the dust sublimation radius.

Disc wind accounts for about 60% of angular momentum loss.

Small magnetosphere has minimal contribution to Br-gamma emission.

Abstract

We present a study of the wind launching region of the Herbig Be star HD 58647 using high angular (lambda/2B=0.003") and high spectral (R=12000) resolution interferometric VLTI-AMBER observations of the near-infrared hydrogen emission line, Br-gamma. The star displays double peaks in both Br-gamma line profile and wavelength-dependent visibilities. The wavelength-dependent differential phases show S-shaped variations around the line centre. The visibility level increases in the line (by ~0.1) at the longest projected baseline (88 m), indicating that the size of the line emission region is smaller than the size of the K-band continuum-emitting region, which is expected to arise near the dust sublimation radius of the accretion disc. The data have been analysed using radiative transfer models to probe the geometry, size and physical properties of the wind that is emitting Br-gamma. We find that a model with a small magnetosphere and a disc wind with its inner radius located just outside of the magnetosphere can well reproduce the observed Br-gamma profile, wavelength-dependent visibilities, differential and closure phases, simultaneously. The mass-accretion and mass-loss rates adopted for the model are Mdot_a = 3.5 x 10^{-7} Msun/yr and Mdot_dw = 4.5 x 10^{-8} Msun/yr, respectively (Mdot_dw/Mdot_a =0.13). Consequently, about 60 per cent of the angular momentum loss rate required for a steady accretion with the measured accretion rate is provide by the disc wind. The small magnetosphere in HD 58647 does not contribute to the Br-gamma line emission significantly.