A Census of B[e] Supergiants

TL;DR

This paper reviews the characteristics, classification criteria, and population census of B[e] supergiants, a key transitional phase in massive star evolution, highlighting their uncertain origins and importance for understanding stellar evolution.

Contribution

It provides clear classification criteria for B[e] supergiants and critically assesses their populations across different galaxies, improving identification accuracy.

Findings

Established unambiguous classification criteria for B[e] supergiants.

Critically inspected B[e] supergiant populations in various galaxies.

Enhanced understanding of B[e] supergiants' role in stellar evolution.

Abstract

Stellar evolution theory is most uncertain for massive stars. For reliable predictions of the evolution of massive stars and their final fate, solid constraints on the physical parameters, and their changes along the evolution and in different environments, are required. Massive stars evolve through a variety of short transition phases, in which they can experience large mass-loss either in the form of dense winds or via sudden eruptions. The B[e] supergiants comprise one such group of massive transition objects. They are characterized by dense, dusty disks of yet unknown origin. In the Milky Way, identification and classification of B[e] supergiants is usually hampered by their uncertain distances hence luminosities, and by the confusion of low-luminosity candidates with massive pre-main sequence objects. The extragalactic objects are often mistaken as quiescent or candidate luminous blue variables, with whom B[e] supergiants share a number of spectroscopic characteristics. In this review, proper criteria are provided, based on which B[e] supergiants can be unambiguously classified and separated from other high luminosity post-main sequence stars and pre-main sequence stars. Using these criteria, the B[e] supergiant samples in diverse galaxies are critically inspected, to achieve a reliable census of the current population.