Southern massive stars at high angular resolution: WR 25 is a massive hierarchical triple system

WR 25 is a massive colliding-wind binary in the Carina nebula comprising a WN6ha primary with an O5 companion in an eccentric 208-d orbit. Recent spectroscopic analysis estimates the total binary mass to approach 100,M, and a primary-to-secondary mass ratio of q= M 1/M 2 about2. The presence of additional spectroscopic signatures from a third, intruder star was also noted, making it a candidate hierarchical triple system. In this study, we present a VLTI/PIONIER interferometric observation of WR 25, spatially resolving all three components for the first time. For the inner WN6ha + O5 binary, we find an angular separation of 1.68 plus or minus0.02 milliarcseconds (mas). Leveraging the fortunate timing of the VLTI observation, which was obtained when the two components were passing the line of nodes, we determined the semi-major axis a=3.11 plus or minus0.20 au. Subsequently, the newly constrained total dynamical binary mass is 93 plus or minus18,M, with a primary mass M 1=62 plus or minus13, M and secondary mass M 2=31 plus or minus7, M. We detect the tertiary with an angular separation of 27.69 plus or minus0.02 mas from the primary, with a chance alignment probability lower than 10-4. Using newly obtained brightness ratios between all components, we revisit archival spectroscopic data of WR 25 to disentangle spectra for individual components and derive their stellar parameters. The tertiary, which has a spectral type O7, is coeval with the inner binary and has an evolutionary mass M 3=25.6+2.8-2.3,M. Based on simulations, we estimate the tertiary period to be in the range 19 - 82 yr. The newly confirmed triple nature of WR 25 makes it an important benchmark system to measure accurate dynamical masses of the inner binary and potentially the tertiary, to calibrate stellar evolution and atmosphere models, and to study its formation and stability as a hierarchical triple system.