Spectroscopic monitoring of the luminous blue variable Westerlund1-243 from 2002 to 2009

摘要

Context. The massive post-main sequence star W243 in thegalactic starburst cluster Westerlund1 has undergone a spectraltransformation from aB2Iasupergiant devoid of emissionfeatures in 1981 to an A-type supergiant with a rich emission-linespectrum by 2002/03. Aims. We examine the continued evolution of W243 from 2002 to2009 to understand its evolutionary state, current physical propertiesand the origin of its peculiar emission line spectrum. Methods. We used VLT/UVES and VLT/FLAMES to obtain highresolution, high signal-to-noise spectra on six epochs in 2003/04(UVES) and ten epochs in 2008/09 (FLAMES). These spectra are usedalongside other lower-resolution VLT/FLAMES and NTT/EMMI spectra tofollow the evolution of W243 from 2002 to 2009. Non-LTE models are usedto determine the physical properties of W243. Results. W243 displays a complex, time-varying spectrum with emission lines of hydrogen, helium and Lyman-pumped metals, forbidden lines of nitrogen and iron, and a large numberof absorption lines from neutral and singly-ionized metals. Many linesare complex emission/absorption blends, with significant spectralevolution occurring on timescales of just a few days. The LBV has atemperature of 8500K (spectral typeA3Ia+),and displays signs of photospheric pulsations and weak episodic massloss. Nitrogen is highly overabundant, with carbon and oxygen depleted,indicative of surface CNO-processed material and considerable previousmass-loss, although current time-averaged mass-loss rates are low. Theemission-line spectrum forms at large radii, when material lost by theLBV in a previous mass-loss event is ionized by an unseen hotcompanion. Monitoring of the near-infrared spectrum suggests that thestar has not changed significantly since it finished evolving to thecool state, close to the Humphreys-Davidson limit, in early2003. Key words: stars: individual: W243 - stars: evolution - supergiants - stars: variables: general - stars: winds, outflows