Aims. The evolutionary status of the blue supergiant Sher 25 and its
membership to the massive cluster NGC 3603 are investigated. Methods. A hybrid
non-LTE (local thermodynamic equilibrium) spectrum synthesis approach is
employed to analyse a high-resolution optical spectrum of Sher 25 and five
similar early B-type comparison stars in order to derive atmospheric parameters
and elemental abundances. Fundamental stellar parameters are determined by
considering stellar evolution tracks, Gaia Data Release 3 (DR3) data and
complementary distance information. Interstellar reddening and the reddening
law along the sight line towards Sher 25 are constrained employing UV
photometry for the first time in addition to optical and infrared data. The
distance to NGC 3603 is reevaluated based on Gaia DR3 data of the innermost
cluster O-stars. Results. The spectroscopic distance derived from the
quantitative analysis implies that Sher 25 lies in the foreground of NGC 3603,
which is found to have a distance of dNGC3603 = 6250±150 pc. A
cluster membership is also excluded as the hourglass nebula is unaffected by
the vigorous stellar winds of the cluster stars and from the different
excitation signatures of the hourglass nebula and the nebula around NGC 3603.
Sher 25 turns out to have a luminosity of log L/L⊙ = 5.48±0.14,
equivalent to that of a ∼27 M⊙ supergiant in a single-star
scenario, which is about half of the mass assumed so far, bringing it much
closer in its characteristics to Sk-69{\deg}202, the progenitor of SN 1987A.
Sher 25 is significantly older than NGC 3603. Further arguments for a binary
(merger) evolutionary scenario of Sher 25 are discussed.Comment: 27 pages, 22 figures, Accepted for publication in Astronomy &
Astrophysics, Data: https://doi.org/10.5281/zenodo.823015