A resolved, au-scale gas disk around the B[e] star HD 50138

TL;DR

This study uses high-resolution interferometry to map the gas kinematics around the B[e] star HD 50138, revealing a compact, elongated Keplerian disk with complex features that inform its evolutionary status.

Contribution

First high-resolution interferometric observations of HD 50138's Br-gamma line, providing detailed insights into its gas disk structure and kinematics.

Findings

Gas emission originates within 3 au, more compact than the continuum.

Blue- and red-shifted emissions come from opposite hemispheres of an elongated structure.

Most data fit a Keplerian disk model, but some features remain unexplained.

Abstract

HD 50138 is a B[e] star surrounded by a large amount of circumstellar gas and dust. Its spectrum shows characteristics which may indicate either a pre- or a post-main-sequence system. Mapping the kinematics of the gas in the inner few au of the system contributes to a better understanding of its physical nature. We present the first high spatial and spectral resolution interferometric observations of the Br-gamma line of HD~50138, obtained with VLTI/AMBER. The line emission originates from a region more compact (up to 3 au) than the continuum-emitting region. Blue- and red-shifted emission originates from the two different hemispheres of an elongated structure perpendicular to the polarization angle. The velocity of the emitting medium decreases radially. An overall offset along the NW direction between the line- and continuum-emitting regions is observed. We compare the data with a geometric model of a thin Keplerian disk and a spherical halo on top of a Gaussian continuum. Most of the data are well reproduced by this model, except for the variability, the global offset and the visibility at the systemic velocity. The evolutionary state of the system is discussed; most diagnostics are ambiguous and may point either to a post-main-sequence or a pre-main-sequence nature.