Eclipse of the B3V companion and flaring of emission lines in V838 Monocerotis

After four years during which only the spectacular light echo was showing continuous and rapid evolution while the central star was nearly constant, in autumn 2006 V838 Mon began a sequence of events which profoundly altered its spectroscopic and photometric behavior: (a) an eclipse of the B3V companion, characterized by the disappearance and reappearance of the B3V companion from optical spectra, and an eclipse-like lightcurve of ~70 day duration and Δ B ˜ 1.15 mag, Δ V ˜ 0.55 mag, Δ R[SUB]C[/SUB] ˜ 0.10 mag maximum depth; (b) a large increase in intensity of the [FeII] and FeII emission lines, and the appearance in emission for the first time since the 2002 outburst of Hα and higher Balmer series lines. While the [FeII] and FeII lines maintained a very sharp and unresolved profile, the Hα developed into a wide and structured profile, characterized by a sharp central reversal at the same velocity as one of the CO radio emission components. The disappearance of the B3V companion is equally well explained by a grazing eclipse from the outbursting L-type supergiant or by an eclipse from a dust cloud characterized by E[SUB]B-V[/SUB]=0.55 and R[SUB]V[/SUB]=3.1. We believe the flaring of the emission lines occurred at a similar time as the B3V eclipse just by chance. Tables 2-4 are only available in electronic form at http://www.aanda.org Based on observations made with the Asiago 1.82 m of the INAF Astronomical Observatory of Padova, and with 4.2 m WHT and 2.5 m INT operated by the Isaac Newton Group of Telescopes, and with the 2.6 m NOT operated jointly by Denmark, Finland, Iceland, Norway, and Sweden, in the Spanish Observatorio del Roque de Los Muchachos of the Instituto de Astrofísica de Canarias