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

TL;DR

This study investigates the origin and variability of near-infrared HI emission lines in the Herbig star VV Ser, revealing that an extended wind, possibly bipolar, dominates the line emission and that inflow and outflow processes coexist and vary over time.

Contribution

It provides multi-epoch spectroscopic and interferometric observations combined with radiative transfer models to identify the wind as the main contributor to HI line emission in VV Ser.

Findings

HI lines show significant profile variability, especially in the redshifted part.

Brγ emission region is smaller than the continuum emitting region.

An extended wind, likely bipolar, best explains the observed line profiles and interferometric data.

Abstract

The origin of the near-infrared (NIR) HI emission lines in young stellar objects are not yet understood. To probe it, we present multi-epoch LBT-LUCIFER spectroscopic observations of the Pa{\delta}, Pa{\beta}, and Br{\gamma} lines observed in the Herbig star VVSer, along with VLTI-AMBER Br{\gamma} spectro-interferometric observations at medium resolution. Our spectroscopic observations show line profile variability in all the HI lines. The strongest variability is observed in the redshifted part of the line profiles. The Br{\gamma} spectro-interferometric observations indicate that the Br{\gamma} line emitting region is smaller than the continuum emitting region. To interpret our results, we employed radiative transfer models with three different flow configurations: magnetospheric accretion, a magneto-centrifugally driven disc wind, and a schematic bipolar outflow. Our models suggest that the HI line emission in VVSer is dominated by the contribution of an extended wind, perhaps a bipolar outflow. Although the exact physical process for producing such outflow is not known, this model is capable of reproducing the averaged single-peaked line profiles of the HI lines. Additionally, the observed visibilities, differential and closure phases are best reproduced when a wind is considered. Nevertheless, the complex line profiles and variability could be explained by changes in the relative contribution of the magnetosphere and/or winds to the line emission. This might indicate that the NIR HI lines are formed in a complex inner disc region where inflow and outflow components might coexist. Furthermore, the contribution of each of these mechanisms to the line appears time variable, suggesting a non-steady accretion/ejection flow.