Spectroscopic survey of emission-line stars. I. B[e] stars

TL;DR

This study surveys emission-line B[e] stars, focusing on forbidden lines like [O I] and [Ca II], revealing their association with dense, high-temperature circumstellar environments and their potential as indicators of stellar mass and disc density.

Contribution

It provides new observational evidence linking forbidden line emission to dense circumstellar discs in massive B[e] stars, highlighting the conditions needed for these lines to appear.

Findings

[Ca II] lines are present in some B[e] supergiants.

Kinematic profiles suggest Keplerian discs in certain stars.

Forbidden lines are absent in lower-mass B[e] stars.

Abstract

Emission-line stars are typically surrounded by dense circumstellar material, often in form of rings or disc-like structures. Line emission from forbidden transitions trace a diversity of density and temperature regimes. Of particular interest are the forbidden lines of [O I] {\lambda}{\lambda}6300, 6364 and [Ca II] {\lambda}{\lambda}7291, 7324. They arise in complementary, high-density environments, such as the inner-disc regions around B[e] supergiants. To study physical conditions traced by these lines and to investigate how common they are, we initiated a survey of emission-line stars. Here, we focus on a sample of nine B[e] stars in different evolutionary phases. Emission of the [O I] lines is one of the characteristics of B[e] stars. We find that four of the objects display [Ca II] line emission: for the B[e] supergiants V1478 Cyg and 3 Pup the kinematics obtained from the [O I] and [Ca II] line profiles agrees with a Keplerian rotating disc scenario; the forbidden lines of the compact planetary nebula OY Gem display no kinematical broadening beyond spectral resolution; the LBV candidate V1429 Aql shows no [O I] lines, but the profile of its [Ca II] lines suggests that the emission originates in its hot, ionized circumbinary disc. As none of the B[e] stars of lower mass displays [Ca II] line emission, we conclude that these lines are more likely observable in massive stars with dense discs, supporting and strengthening the suggestion that their appearance requires high-density environments.