The evolutionary status of Sher25 - implications for blue supergiants and the progenitor of SN1987A

Abstract

The blue supergiant Sher25 in the massive Galactic cluster NGC3603 is surrounded by a striking emission line nebula. The nebula contains an equatorial ring and probable bi-polar outflows, and is similar in morphology, mass and kinematics to the structure visible around SN1987A. It has been suggested that both nebulae were ejected while Sher25 and the progenitor of SN1987A were in previous red supergiant phases. We present optical high-resolution spectra of Sher25 and a model photosphere and unified stellar wind analysis which determines atmospheric parameters, mass-loss rate and photospheric abundances. We compare CNO abundances to other Galactic B-type supergiants and find that Sher25 does not appear extreme or abnormal in terms of its photospheric nitrogen and helium abundances. The C/N and N/O ratios are compared to surface abundances predicted by stellar evolutionary calculations and are incompatible with the star having a previous red-supergiant phase. The nebula is likely to have been ejected while the star was a blue supergiant. The results are compatible with some degree of rotationally induced mixing having occurred while the star was on or near the main-sequence. Our analsysis suggests the star has a relatively normal stellar wind and mass-loss rate, and sits comfortably within the wind momentum-luminosity relationship. In light of the evidence regarding massive evolved early-type stars in the Galaxy we suggest there is no object which shows clear evidence of having had a previous red supergiant phase and hence of undergoing blue loops in the HR diagram. [ABRIDGED]