On the nature of the galactic early-B hypergiants

TL;DR

This study investigates four early-B hypergiants to understand their physical properties and evolutionary status, revealing their stability and potential links to luminous blue variables, while highlighting anomalies in one star challenging current models.

Contribution

It provides a detailed quantitative analysis of four early-B hypergiants, proposing their evolutionary connections to other massive star types and identifying inconsistencies with existing paradigms.

Findings

Zeta Sco, HD190603, and BP Cru are intermediate between B supergiants and LBVs.

Cyg OB2 #12's properties challenge current evolutionary models.

Most hypergiants studied are remarkably stable over decades.

Abstract

Despite their importance to a number of astrophysical fields, the lifecycles of very massive stars are still poorly defined. In order to address this shortcoming, we present a detailed quantitative study of the physical properties of four early-B hypergiants (BHGs); Cyg OB2 #12, zeta Sco, HD190603 and BP Cru. These are combined with an analysis of their long-term spectroscopic and photometric behaviour in order to determine their evolutionary status. The long-term datasets revealed that they are remarkably stable over long periods (>40yr), with the possible exception of zeta Sco prior to the 20th century, in contrast to the typical excursions that characterise luminous blue variables (LBVs). Zeta Sco, HD190603 and BP Cru possess physical properties intermediate between B supergiants and LBVs; we therefore suggest that BHGs are the immediate descendants and progenitors (respectively) of such stars (for initial masses in the range ~30-60Msun). In contrast, while the wind properties of Cyg OB2 #12 are consistent with this hypothesis, the combination of extreme luminosity and spectroscopic mass (~110Msun) and comparatively low temperature means it cannot be accommodated in such a scheme. Likewise, despite its co-location with several LBVs above the Humphreys-Davidson (HD) limit, the lack of long term variability and its unevolved chemistry apparently excludes such an identification. Since such massive stars are not expected to evolve to such cool temperatures, the properties of Cyg OB2 #12 are difficult to understand under current evolutionary paradigms. [ABRIDGED]