Context :

Aims : 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 .

Methods : 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-2.2m/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 .

Results : 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 _ { \textnormal { \scriptsize { eff, 2012 } } } \sim 6 650 K ; the star resembles a late F supergiant .
R71 ’ s visual lightcurve had a maximum in 2012 with m _ { \textnormal { \scriptsize { V, 2012 } } } \sim 8.7 mag .
Given the uncertainty in the extinction towards R71 , this corresponds to a bolometric luminosity of M _ { \textnormal { \scriptsize { bol, 2012 } } } \sim - 9.8 mag to -10.3 mag .
R71 ’ s 2011–2012 spectra do not show H I and Fe II 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 Fe I 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 13 AU from the star .

Conclusions : 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 \Delta M _ { \textnormal { \scriptsize { bol } } } = 0.4 – 1.3 mag 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 } _ { \textnormal { \scriptsize { R 71 , 2012 } } } \sim 5 \times 10 ^ { -4 } M _ { \odot } yr ^ { -1 } compared to \dot { M } _ { \textnormal { \scriptsize { quiescence } } } \sim 3 \times 10 ^ { -7 } M _ { \odot } yr ^ { -1 } .
The apparent radius increased by a factor of 5 to about 500 R _ { \odot } .
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 .