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Register a new userThe large-scale magnetic field of the eccentric pre-main-sequence binary system V1878 Ori
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Alexis Lavail
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We report time-resolved, high-resolution optical spectropolarimetric observations of the young double-lined spectroscopic binary V1878 Ori. Our observations were collected with the ESPaDOnS spectropolarimeter at the Canada-France-Hawaii Telescope through the BinaMIcS large programme. V1878 Ori A and B are partially convective intermediate mass weak-line T Tauri stars on an eccentric and asynchronous orbit. We also acquired X-ray observations at periastron and outside periastron. Using the least-squares deconvolution technique (LSD) to combine information from many spectral lines, we clearly detected circular polarization signals in both components throughout the orbit. We refined the orbital solution for the system and obtained disentangled spectra for the primary and secondary components. The disentangled spectra were then employed to determine atmospheric parameters of the two components using spectrum synthesis. Applying our Zeeman Doppler imaging code to composite Stokes $IV$ LSD profiles, we reconstructed brightness maps and the global magnetic field topologies of the two components. We find that V1878 Ori A and B have strikingly different global magnetic field topologies and mean field strengths. The global magnetic field of the primary is predominantly poloidal and non-axisymmetric (with a mean field strength of 180 G). while the secondary has a mostly toroidal and axisymmetric global field (mean strength of 310 G). These findings confirm that stars with very similar parameters can exhibit radically different global magnetic field characteristics. The analysis of the X-ray data shows no sign of enhanced activity at periastron, suggesting the lack of strong magnetospheric interaction at this epoch.
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The bulk of X-ray emission from pre-main-sequence (PMS) stars is coronal in origin. We demonstrate herein that stars on Henyey tracks in the Hertzsprung-Russell diagram have lower $log(L_X/L_ast)$, on average, than stars on Hayashi tracks. This effect is driven by the decay of $L_X$ once stars develop radiative cores. $L_X$ decays faster with age for intermediate mass PMS stars, the progenitors of main sequence A-type stars, compared to those of lower mass. As almost all main sequence A-type stars show no detectable X-ray emission, we may already be observing the loss of their coronae during their PMS evolution. Although there is no direct link between the size or mass of the radiative core and $L_X$, the longer stars have spent with partially convective interiors, the weaker their X-ray emission becomes. This conference paper is a synopsis of Gregory, Adams and Davies (2016).
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