The highly magnetic Wolf-Rayet binary HD 45166 resolved with VLTI/GRAVITY

Abstract

HD 45166 was recently reported to be a long-period binary comprising a B7V star and a highly magnetic ($⟨B⟩=43.0\pm 0.5$kG) hot Wolf-Rayet-like component, dubbed as a quasi Wolf-Rayet (qWR) star in literature. While originally proposed to be a short-period binary, long-term spectroscopic monitoring suggested a 22.5 yr orbital period. With a derived dynamical mass of $2.03\pm 0.44M_{\odot }$, the qWR component is the most strongly magnetized non-degenerate object ever detected and a potential magnetar progenitor. However, the long period renders the spectroscopic orbital solution and dynamical mass estimates uncertain, casting doubts on whether the qWR component is massive enough to undergo core-collapse. Here, we spatially resolve the HD 45166 binary using newly acquired interferometric data obtained with the GRAVITY instrument of the Very Large Telescope Interferometer. Due to the calibrator star being a binary as well, we implement a new approach for visibility calibration and test it thoroughly using archival GRAVITY data. The newly calibrated HD 45166 data reveal the unmistakable presence of a companion to the qWR component with an angular separation of $10.9\pm 0.1$ mas (which translates to a projected physical separation of $10.8\pm 0.4$ au), consistent with the long-period orbit. We obtain a model-independent qWR mass $M_{\mathrm{qWR}}=1.96_{-0.54}^{+0.74}{M}_{\odot }$ using interferometric and spectroscopic data together. This observation robustly confirms that HD 45166 is truly a long-period binary, and provides an anchor point for accurate mass determination of the qWR component with further observations.

Accepted in A&A

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