Spectroscopy and Linear Spectropolarimetry of the early Herbig Be Stars PDS 27 and PDS 37

TL;DR

This study presents spectropolarimetric and spectroscopic observations of two candidate Herbig Be stars, revealing their physical properties, youth, active circumstellar environments, and evidence for inner accretion disks, contributing to understanding massive star formation.

Contribution

First detailed spectropolarimetric and spectroscopic analysis of PDS 27 and PDS 37, characterizing their physical properties and circumstellar structures, advancing knowledge of early Herbig Be stars.

Findings

Both stars are very young, massive, and hot.

Detection of linear polarisation change indicates inner accretion disks.

Evidence of active circumstellar environments and variability.

Abstract

The number of well-studied early-type pre-main-sequence objects is very limited, hampering the study of massive star formation from an observational point of view. Here, we present the results of VLT/FORS2 spectropolarimetric and VLT/X-shooter spectroscopic observations of two recently recognised candidate Herbig Be stars, PDS 27 and PDS 37. Through analysis of spectral lines and photometry, we find that these two objects are hot, 17500 $\pm$ 3500~K, have large radii, 17.0 $\pm$ 4.0 and 25.8 $\pm$ 5.0~${\rm R}_\odot$, and are very massive, 15.3 (+5.4, -4.4) and 21.1 (+11.0, -5.3)~M$_{\odot}$ for PDS 27 and PDS 37, respectively. This suggests that these two objects are very young in their evolution and may become O-type stars. Their youth is supported by their high accretion rates of the order of $10^{-3}$--$10^{-4.5}~{\rm M}_\odot$/yr. A change in linear polarisation across the absorption component of H$\alpha$ is detected in both objects. This change indicates that the circumstellar environment close to the star, at scales of several stellar radii, has a flattened structure, which we identify as an inner accretion disk. Strong variability is seen in both objects in many lines as further indication of an active circumstellar environment.