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

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.