The blue supergiant Sher 25 and its intriguing hourglass nebula

M. A. Hendry (1,2), S. J. Smartt (1), E. D. Skillman (3), C. J. Evans (4), C. Trundle (1), D. J. Lennon (5,6), P. A. Crowther (7), I. Hunter (1)

(1) Queen's University Belfast
(2) IoA, University of Cambridge
(3) University of Minnesota
(4) UKATC, Edinburgh
(5) STScI, Baltimore
(6) IAC, Tenerife
(7) University of Sheffield

The blue supergiant Sher,25 is surrounded by an asymmetric,
hourglass-shaped circumstellar nebula. Its structure and dynamics have
been studied previously through high-resolution imaging and
spectroscopy, and it appears dynamically similar to the ring structure
around SN,1987A. Here we present long-slit spectroscopy of the
circumstellar nebula around Sher,25, and of the background nebula of
the host cluster NGC,3603. We perform a detailed nebular abundance
analysis to measure the gas-phase abundances of oxygen, nitrogen,
sulphur, neon and argon. The oxygen abundance in the circumstellar
nebula ($12 + log {rm O/H} = 8.61pm0.13$,dex) is similar to that
in the background nebula ($8.56pm0.07$), suggesting the composition
of the host cluster is around solar. However, we confirm that the
circumstellar nebula is very rich in nitrogen, with an abundance of
$8.91pm0.15$, compared to the background value of $7.47pm0.18$. A new
analysis of the stellar spectrum with the {sc fastwind} model
atmosphere code suggests that the photospheric nitrogen and oxygen abundances in
Sher,25 are consistent with the nebular results. While the nitrogen
abundances are high, when compared to stellar evolutionary models they
do not unambiguously confirm that the star has undergone convective
dredge-up during a previous red supergiant phase. We suggest that the
more likely scenario is that the nebula was ejected from the star
while it was in the blue supergiant phase. The star's initial mass was
around 50msun, which is rather too high for it to have had a
convective envelope stage as a red supergiant. Rotating stellar models
that lead to mixing of core-processed material to the stellar surface
during core H-burning can quantitatively match the stellar results
with the nebula abundances.

Reference: Accepted by MNRAS, arXiv:0803.4262
Status: Manuscript has been accepted