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 { O / H } = 8.61 \pm 0.13 dex ) is similar to that in the background nebula ( 8.56 \pm 0.07 ) , 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 8.91 \pm 0.15 , compared to the background value of 7.47 \pm 0.18 .
A new analysis of the stellar spectrum with the 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 50M _ { \odot } , 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 .