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

Abstract

Aims: We aim at detecting and characterizing the main-sequence companion of the Cepheid AX Cir ($P_{\mathrm{orb}}\sim$ 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 $\rho =29.2\pm 0.2$ mas and projection angle $PA=167.6\pm 0.3^{\circ }$. We measured $H$-band flux ratios between the companion and the Cepheid of $0.90\pm 0.10$ % and $0.75\pm 0.17$ %, respectively at a pulsation phase for the Cepheid $\varphi =0.24$ and 0.48. The lower contrast at $\varphi =0.48$ is due to increased brightness of the Cepheid compared to the $\varphi =0.24$. This gives an average apparent magnitude $m{}_{\mathrm{H}}(\mathrm{comp})=9.06\pm 0.24$ mag. The limb-darkened angular diameter of the Cepheid at the two pulsation phases was measured to be ${\theta }_{\mathrm{LD}}=0.839\pm 0.023$ mas and ${\theta }_{\mathrm{LD}}=0.742\pm 0.020$ mas, respectively at $\varphi =0.24$ and 0.48. A lower limit on the total mass of the system was also derived based on our measured separation, we found $M_{\mathrm{T}}\ge 9.7\pm 0.6M_{\odot }$.

Accepted for publication in Astronomy and Astrophysics

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