J. S. Clark, F. Najarro, I. Negueruela, B. W. Ritchie, M. A. Urbaneja, I. D. Howarth
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]
View original:
http://arxiv.org/abs/1202.3991
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