Multiplicity of Galactic Cepheids from long-baseline interferometry. II. The Companion of AX Circini revealed with VLTI/PIONIER

Aims: We aim at detecting and characterizing the main-sequence companion of the Cepheid AX Cir (P_orb ~ 18 yrs). The long-term objective is to estimate the mass of both components and the distance to the system.
Methods: We used the PIONIER combiner at the VLT Interferometer to obtain the first interferometric measurements of the short-period Cepheid AX Cir and its orbiting component.
Results: The companion is resolved by PIONIER at a projected separation ρ = 29.2 ± 0.2 mas and projection angle PA = 167.6 ± 0.3°. We measured H-band flux ratios between the companion and the Cepheid of 0.90 ± 0.10% and 0.75 ± 0.17%, at pulsation phases for the Cepheid of φ = 0.24 and 0.48, respectively. The lower contrast at φ = 0.48 is due to the increased brightness of the Cepheid compared to φ = 0.24. This gives an average apparent magnitude m_H(comp) = 9.06 ± 0.24 mag. The limb-darkened angular diameter of the Cepheid at the two pulsation phases was measured to be θ_LD = 0.839 ± 0.023 mas and θ_LD = 0.742 ± 0.020 mas, at φ = 0.24 and 0.48, respectively. A lower limit on the total mass of the system was also derived based on our measured separation, and we found M_T ≥ 9.7 ± 0.6 M_⊙.

Based on observations made with ESO telescopes at Paranal observatory under program ID 090.D-0010.Table 2 is available in electronic form at http://www.aanda.org