Disentangling the two sub-populations of early Herbig Be stars using VLT/X-Shooter spectra

TL;DR

This study classifies early Herbig Be stars into intense and weak sub-populations based on spectral features, revealing differences in their circumstellar environments and inner disk evolution.

Contribution

It introduces a new classification scheme for early Herbig Be stars using spectral emission features and infrared excess, enhancing understanding of their circumstellar evolution.

Findings

Intense HBe stars have active inner disks and circumstellar environments.

Weak HBe stars show cleared inner disks and more evolved circumstellar regions.

Spectroscopic analysis distinguishes sub-populations based on emission line profiles.

Abstract

Early Herbig Be (HBe) stars are massive, young stars accreting through the Boundary Layer mechanism. However, given the rapid ($<$ 2 Myr) evolution of early Herbig stars to the main-sequence phase, studying the evolution of the circumstellar medium around these stars can be a cumbersome exercise. In this work, we study the sample of early (B0-B5) HBe stars using the correlation between H$\alpha$ emission strength and near--infrared excess, complemented by the analysis of various emission features in the X-Shooter spectra. We segregate the sample of 37 early HBe stars based on the median values of H$\alpha$ equivalent width (EW) and near--infrared index (n(J$-$H)) distributions. The stars with |H$\alpha$ EW| $>$ 50 {\AA} and n(J$-$H) $>$ -2 are classified as intense HBe stars and stars with |H$\alpha$ EW| $<$ 50 {\AA} and n(J$-$H) $<$ -2 as weak HBe stars. Using the VLT/X--Shooter spectra of five intense and eight weak HBe stars, we visually checked for the differences in intensity and profiles of various H{\sc I} and metallic emission lines commonly observed in Herbig stars. We propose that the intense HBe stars possess an inner disk close to the star (as apparent from the high near-infrared excess) and an active circumstellar environment (as seen from high H$\alpha$ EW value and presence of emission lines belonging to Fe{\sc II}, Ca{\sc II}, O{\sc I} and [O{\sc I}]). However, for weak HBe stars, the inner disk has cleared, and the circumstellar environment appears more evolved than for intense HBe stars. Furthermore, we compiled a sample of $\sim$58,000 emission-line stars published in \textit{Gaia DR3} to identify more intense HBe candidates. Further spectroscopic studies of these candidates will help us to understand the evolution of the inner ($\sim$a few au) disk in early HBe stars.