Letter to the Editor

Multiplicity of Galactic Cepheids from long-baseline interferometry

II. The Companion of AX Circini revealed with VLTI/PIONIER^⋆,⋆⋆

¹ Universidad de ConcepciónDepartamento de Astronomía Casilla 160-C Concepción Chile
e-mail: agallenne@astro-udec.cl
² European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Santiago 19, Chile
³ LESIA, Observatoire de Paris, CNRS UMR 8109, UPMC, Université Paris Diderot, 5 place Jules Janssen, 92195 Meudon, France
⁴ UJF-Grenoble 1/CNRS-INSU, Institut de Planétologie et d’Astrophysique de Grenoble (IPAG) UMR 5274, Grenoble, France
⁵ Astronomy Department, University of Michigan, 1034 Dennison Bldg, Ann Arbor, MI 48109-1090, USA
⁶ Warsaw University Observatory, Al. Ujazdowskie 4, 00-478 Warsaw, Poland

Received: 21 October 2013
Accepted: 6 December 2013

Abstract

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_⊙.