2009A&A...507.1597R

The massive post-main sequence star W243 in the galactic starburst cluster Westerlund 1 has undergone a spectral transformation from a B2Ia supergiant devoid of emission features in 1981 to an A-type supergiant with a rich emission-line spectrum by 2002/03. We examine the continued evolution of W243 from 2002 to 2009 to understand its evolutionary state, current physical properties and the origin of its peculiar emission line spectrum. We used VLT/UVES and VLT/FLAMES to obtain high resolution, high signal-to-noise spectra on six epochs in 2003/04 (UVES) and ten epochs in 2008/09 (FLAMES). These spectra are used alongside other lower-resolution VLT/FLAMES and NTT/EMMI spectra to follow the evolution of W243 from 2002 to 2009. Non-LTE models are used to determine the physical properties of W243. W243 displays a complex, time-varying spectrum with emission lines of hydrogen, helium and Lyman-α pumped metals, forbidden lines of nitrogen and iron, and a large number of absorption lines from neutral and singly-ionized metals. Many lines are complex emission/absorption blends, with significant spectral evolution occurring on timescales of just a few days. The LBV has a temperature of ∼8500K (spectral type A3Ia⁺), and displays signs of photospheric pulsations and weak episodic mass loss. Nitrogen is highly overabundant, with carbon and oxygen depleted, indicative of surface CNO-processed material and considerable previous mass-loss, although current time-averaged mass-loss rates are low. The emission-line spectrum forms at large radii, when material lost by the LBV in a previous mass-loss event is ionized by an unseen hot companion. Monitoring of the near-infrared spectrum suggests that the star has not changed significantly since it finished evolving to the cool state, close to the Humphreys-Davidson limit, in early 2003.