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

A. Gallenne, A. Mérand+7 more|arXiv (Cornell University)|Dec 6, 2013

Stellar, planetary, and galactic studies31 references21 citations

TL;DR

This study uses VLTI/PIONIER long-baseline interferometry to spatially resolve the close companion of the Cepheid AX Circini, measuring its projected separation (29.2 ± 0.2 mas) and H-band flux ratio (0.83 ± 0.14%), yielding a lower limit on the system's total mass of 9.7 ± 0.6 M☉. The results provide critical constraints for dynamical mass estimation and distance calibration in Cepheid binaries.

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 $ρ= 29.2 \pm 0.2$ 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 $ϕ= 0.24$ and 0.48. The lower contrast at $ϕ= 0.48$ is due to increased brightness of the Cepheid compared to the $ϕ= 0.24$. This gives an average apparent magnitude $m\mathrm{_H (comp)} = 9.06 \pm 0.24$ mag. The limb-darkened angular diameter of the Cepheid at the two pulsation phases was measured to be $θ_\mathrm{LD} = 0.839 \pm 0.023$ mas and $θ_\mathrm{LD} = 0.742 \pm 0.020$ mas, respectively at $ϕ= 0.24$ and 0.48. A lower limit on the total mass of the system was also derived based on our measured separation, we found $M_\mathrm{T} \geq 9.7 \pm 0.6 M_\odot$.

Motivation & Objective

• To detect and characterize the close main-sequence companion of the Cepheid AX Cir, which has a known orbital period of ~18 years.
• To measure the astrometric position and flux ratio of the companion using high-angular-resolution interferometry.
• To derive a lower limit on the total mass of the binary system using the measured projected separation and orbital period.
• To contribute to resolving the Cepheid mass discrepancy by enabling dynamical mass measurements in binary systems.
• To support geometric distance calibration via orbital parallax and improve the Leavitt Law calibration using a resolved binary system.

Proposed method

• Employed the PIONIER beam combiner at the Very Large Telescope Interferometer (VLTI) to obtain high-spatial-resolution visibility and closure phase measurements.
• Modeled the system as a uniform disk (Cepheid) plus a point source (companion), fitting the interferometric data to derive angular separation and flux ratio.
• Used limb-darkened disk modeling to measure the Cepheid's angular diameter at two pulsation phases (φ = 0.24 and φ = 0.48).
• Calibrated flux ratios using 2MASS H-band magnitudes and corrected for interstellar extinction using E(B−V) = 0.262 and RV = 3.23.
• Applied Kepler’s third law to derive a lower limit on the total system mass from the projected separation and orbital period.
• Combined the measured separation with the K-band period–luminosity distance (d = 500 ± 10 pc) to estimate absolute magnitudes and mass constraints.

Experimental results

Research questions

• RQ1 Can the close companion of AX Circini be spatially resolved using long-baseline interferometry?
• RQ2 What is the H-band flux ratio between the Cepheid and its companion at different pulsation phases?
• RQ3 What is the projected angular separation and position angle of the companion relative to the Cepheid?
• RQ4 What lower limit can be placed on the total mass of the binary system using interferometric measurements?
• RQ5 How do these measurements contribute to resolving the Cepheid mass discrepancy and improving distance calibration?

Key findings

• The companion of AX Circini was spatially resolved at a projected separation of 29.2 ± 0.2 mas and position angle of 167.8 ± 0.3°.
• The H-band flux ratio between the companion and the Cepheid was measured as 0.83 ± 0.14%, with a mean apparent magnitude of mH(comp) = 9.06 ± 0.24 mag.
• The limb-darkened angular diameter of the Cepheid decreased from 0.839 ± 0.023 mas at φ = 0.24 to 0.742 ± 0.020 mas at φ = 0.48, consistent with pulsational variability.
• A lower limit on the total system mass was derived as 9.7 ± 0.6 M☉, based on the projected separation and orbital period.
• The derived mass limit is consistent with the pulsation mass of ~5.1 M☉ for the Cepheid and the spectral type B6V estimate of 5 M☉ for the companion.
• The study demonstrates the feasibility of using interferometry to resolve and characterize close Cepheid companions, enabling future full orbital solution and dynamical mass determination.