The Unusual 2001 Periastron Passage in the ``Clockwork'' Colliding-Wind Binary WR 140

International audienceWe follow, using both optical spectroscopy and photometry, the ``textbook'' colliding-wind WR+O binary WR 140 through and between the periastron passages of 1993 and 2001. An extensive collection of high-quality spectra allows us to derive precise orbital elements for both components simultaneously. We confirm the extremely high eccentricity of the system, e=0.881+/-0.005, find an excellent match of the newly derived period to the previous estimates, P=2899.0+/-1.3 days, and improve the accuracy of the time of periastron passage, T0=HJD2,446,147.4+/-3.7. Around periastron, at orbital phases φ~0.995-1.015, additional emission components appear on the tops of the broad Wolf-Rayet emission lines of relatively low ionization potential. The phase-dependent behavior of these excess line emissions points to their origin in the wind-wind collision zone, which allows us to place some limits on the orbital inclination of the system, i=50deg+/-15deg, and half-opening angle of the bow shock cone, θ=40deg+/-15deg. The relatively sudden appearance and disappearance of the extra emission components probably signify a rapid switch from an adiabatically to a radiatively dominated regime and back again. Multiyear UBV photometry provides one more surprise: in 2001 at φ=0.02-0.06, the system went through a series of rapid, eclipse-like events. Assuming these events to be related to an episode of enhanced dust formation at periastron, we estimate the characteristic size of the dust grains to be a~0.07 μm