2013A&A...555A.116M

The luminous blue variable (LBV) R71 is currently undergoing an eruption, which differs photometrically and spectroscopically from its last outburst in the 1970s. Valuable information on the physics of LBV eruptions can be gained by analyzing the spectral evolution during this eruption and by comparing R71's present appearance to its previous outburst and its quiescent state. An ongoing monitoring program with VLT/X-shooter will secure key spectral data ranging from visual to near-infrared wavelengths. Here we present the first spectra obtained in 2012 and compare them to archival VLT/UVES and MPG/ESO-2.2 m/FEROS spectra from 2002 to 2011. The discussed data include pre-eruption spectra in 2002 and 2005, a spectrum of the transitionary phase between quiescent and eruptive state in 2007, and spectra of the eruption in 2011-2012. Information on R71's 1970s outburst is taken from the literature. The 2011-2012 spectra are dominated by strong neutral and singly ionized metal absorption lines likely formed in a large ``pseudo-photosphere''. We find an unusually low apparent temperature of R71 of only T_eff,2012∼6650K; the star resembles a late F supergiant. R71's visual lightcurve had a maximum in 2012 with m_V,2012∼8.7mag. Given the uncertainty in the extinction toward R71, this corresponds to a bolometric luminosity of M_bol,2012~-9.8mag to -10.3mag. R71's 2011-2012 spectra do not show HI and FeII P Cyg profiles, which were present during its last outburst in the 1970s and which are normally observed during LBV outbursts. Low-excitation forbidden emission lines and FeI P Cyg-like profiles from a slowly expanding nebula became apparent in late 2012. These lines originate likely in the rarefied region above the pseudo-photosphere up to 13AU from the star. The rise in R71's visual magnitude and the low apparent temperature of its pseudo-photosphere during the current eruption are unprecedented for this star. R71 most likely increased its bolometric luminosity by ΔM_bol=0.4-1.3mag compared to its quiescent state. The very low temperature of its pseudo-photosphere implies a very high-mass loss rate on the order of {dot}(M)_R71,2012∼5x10^–4M_☉/yr compared to {dot}(M)_quiescence∼3x10^–7M_☉/yr. The apparent radius increased by a factor of 5 to about 500R_☉. No fast-moving material indicative of an explosion is observed. The changes in R71's photometry and spectrum are thus likely consequences of a tremendously increased wind density, which led to the formation of a pseudo-photosphere.