Near‐infrared spectroscopic monitoring of WR 140 during the 2001 periastron passage

ABSTRACT We present new spectra of WR 140 (HD 193793) in the JHK bands, with some covering the 1.083‐μm He  i emission line at higher resolution, observed between 2000 October and 2003 May to cover its 2001 periastron passage and maximum colliding‐wind activity. The WC7 + O4–5 spectroscopic binary WR 140 is the prototype of colliding‐wind, episodic dust‐making Wolf–Rayet systems, which also show strong variations in radio and X‐ray emission. The JHK spectra showed changes in continuum and in the equivalent widths of the WC emission lines, consistent with the formation of dust, starting between 2001 January 3 and March 26 (orbital phases 0.989 and 0.017) and its subsequent fading and cooling. The 1.083‐μm He  i line has a P Cygni profile, which showed variations in both absorption and emission components as WR 140 went through periastron passage. The variation of the absorption component of the profile yielded tight constraints on the geometry of the wind‐collision region, giving θ= 50°± 8° for the opening semi‐angle of the interaction region ‘cone’, indicating a wind‐momentum ratio , about three times larger than previously believed. As the system approached periastron, the normally flat‐topped emission component of the 1.083‐μm line profile showed the appearance of a significant subpeak. The movement of the subpeak across the profile was seen to be consistent with its formation in wind material flowing along the contact discontinuity between the WC7 and O4–5 stellar winds and the changing orientation of the colliding‐wind region as the stars moved in their orbits. The flux carried in the subpeak was significant, exceeding the X‐ray fluxes measured at previous periastron passages. This additional source of radiative cooling of the shock‐heated gas probably causes it to depart from being adiabatic around periastron passage, thereby accounting for the departure of the X‐ray flux from its previously expected 1/ d dependence.