The circumstellar environment of HD50138 revealed by VLTI/AMBER at high angular resolution

M. Koutoulaki, R. Garcia Lopez, A. Natta, A. Caratti o Garatti, D. Coffey, J. Sanchez-Bermudez, T. P. Ray

Published 2018-05-03Version 1

HD50138 is a Herbig B[e] star with a circumstellar disc detected at IR and mm wavelength. Its brightness makes it a good candidate for NIR interferometry observations. We aim to resolve, spatially and spectrally, the continuum and hydrogen emission lines in the 2.12-2.47 micron region, to shed light on the immediate circumstellar environment of the star. VLTI/AMBER K-band observations provide spectra, visibilities, differential phases, and closure phases along three long baselines for the continuum, and HI emission in Br{\gamma} and five high-n Pfund lines. By computing the pure-line visibilities, we derive the angular size of the different line-emitting regions. A simple LTE model was created to constrain the physical conditions of HI emitting region. The continuum region cannot be reproduced by a geometrical 2D elongated Gaussian fitting model. We estimate the size of the region to be 1 au. We find the Br{\gamma} and Pfund lines come from a more compact region of size 0.4 au. The Br{\gamma} line exhibits an S-shaped differential phase, indicative of rotation. The continuum and Br{\gamma} line closure phase show offsets of $\sim$-25$\pm$5 $^o$ and 20$\pm$10$^o$, respectively. This is evidence of an asymmetry in their origin, but with opposing directions. We find that we cannot converge on constraints for the HI physical parameters without a more detailed model. Our analysis reveals that HD50138 hosts a complex circumstellar environment. Its continuum emission cannot be reproduced by a simple disc brightness distribution. Similarly, several components must be evoked to reproduce the interferometric observables within the Br{\gamma}, line. Combining the spectroscopic and interferometric data of the Br{\gamma} and Pfund lines favours an origin in a wind region with a large opening angle. Finally, our results point to an evolved source.