Spectroscopy and linear spectropolarimetry of the early Herbig Be stars PDS 27 and PDS 37

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 recognized candidate Herbig Be stars, PDS 27 and PDS 37. Through analysis of spectral lines and photometry, we find that these two objects are hot, 17 500 ± 3500 K, have large radii, 17.0 ± 4.0 and 25.8 ± 5.0 R⊙, and are very massive, 15.3 (+5.4, -4.4) and 21.1 (+11.0, -5.3) M⊙ 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 M⊙ yr-1. A change in linear polarization across the absorption component of Hα 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 disc. Strong variability is seen in both objects in many lines as further indication of an active circumstellar environment.