The evolutionary status of Sher 25 – implications for blue supergiants and the progenitor of SN 1987A

Abstract

The blue supergiant Sher 25 in the massive Galactic cluster NGC 3603 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 kine- matics to the shell now visible around SN 1987A. It has been suggested that both nebulae were ejected while Sher 25 and the progenitor of SN 1987A were in previous red supergiant phases. In the case of Sher 25 this is based on the qualitative strengths of nebular (Nii) emission which is indicative of nitrogen enriched gas. This gas may have been dredged up to the stellar surface by convective mixing during a previous red supergiant phase. We present optical high-resolution spectra of Sher 25 and a model photosphere and unified stellar wind analysis which determines the atmospheric parameters, mass-loss rate and photospheric abundances for C, N, O, Mg, and Si. We compare these results, in particular CNO, to other Galactic B-type supergiants and find that Sher 25 does not appear extreme or abnormal in terms of its photospheric nitrogen abundance. The C/ Na nd N/O ratios are compared to surface abundances predicted by stellar evolutionary calculations which assume the star has gone through a red supergiant phase and convective dredge-up. In particular we find that the N/O abundance is incompatible with the star having a previous red supergiant phase, and that the nebulae 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. This is similar to what has recently been found for nebulae surrounding LBVs. In addition our wind analysis suggests the star currently has a relatively normal mass-loss rate in comparison with other Galactic B-type supergiants 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 any evidence of having had a previous red supergiant phase and hence of undergoing blue loops in the HR diagram.