Abstract
Active pre-main-sequence binaries with separations of around 10 stellar radii present a wealth of phenomena unobserved in common systems. The study of these objects is extended from classical T Tauri stars to the Herbig Ae star HD 104237. The primary has a mass 2.2 +/- 0.2 M-circle dot and secondary 1.4 +/- 0.3 M-circle dot. Spectrointerferometry with the VLTI/AMBER in the K-band continuum and the Br gamma line is presented. It is found that the K-band continuum squared visibilities are compatible with a circumbinary disc with a radius of similar to 0.5 AU. However, a significant fraction (similar to 50 per cent) of the flux is unresolved and not fully accounted by the stellar photospheres. The stars probably do not hold circumstellar discs, in addition to the circumbinary disc, due to the combined effects of inner magnetospheric truncation and outer tidal truncation. This unresolved flux likely arises in compact structures inside the tidally disrupted circumbinary disc. Most (greater than or similar to 90 per cent) of the Br gamma line emission is unresolved. The line-to-continuum spectroastrometry shifts in time, along the direction of the Ly alpha jet known to be driven by the system. The shift is anticorrelated with the Br gamma equivalent width. It is shown that the unresolved Br gamma emission cannot originate in the jet but instead is compatible with stellar emission from the orbiting binary components. The increase in the absolute value of the equivalent width of the line takes place at periastron passage; it could arise in an accretion burst, a flare or in the increase in effective size of the emission region by the interaction of the magnetospheres. The binary longitude of the ascending node is found to be Omega = (235 +/- 3)degrees and the orbit retrograde. The origin of the jet is revisited. The tidal disruption of the circumstellar discs creates difficulties to ejection models that rely on stellar magnetosphere and disc coupling. A scenario of a stellar wind collimated by a circumbinary disc wind is suggested.