Searching for visual companions of close Cepheids

VLT/NACO lucky imaging of Y Oph, FF Aql, X Sgr, W Sgr, and η Aql^⋆

¹ Universidad de ConcepciónDepartamento de Astronomía, Casilla 160-C, Concepción, Chile
e-mail: agallenne@astro-udec.cl
² LESIA, Observatoire de Paris, CNRS UMR 8109, UPMC, Université Paris Diderot, 5 place Jules Janssen, 92195 Meudon, France
³ European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Santiago 19, Chile
⁴ Smithsonian Astrophysical Observatory, MS 4, 60 Garden Street, Cambridge MA 02138, USA
⁵ Warsaw University Observatory, Al. Ujazdowskie 4, 00-478 Warsaw, Poland

Received: 24 March 2014
Accepted: 13 May 2014

Abstract

Aims. High-resolution imaging in several photometric bands can provide color and astrometric information of the wide-orbit component of Cepheid stars. Such measurements are needed to understand the age and evolution of pulsating stars. In addition, binary Cepheids have the potential to provide direct and model-independent distances and masses.

Methods. We used the NAOS-CONICA adaptive optics instrument (NACO) in the near-infrared to perform a deep search for wide components around the classical Cepheids, Y Oph, FF Aql, X Sgr, W Sgr, and η Aql, within a field of view of 1.7″ × 1.7″ (3.4″ × 3.4″ for η Aql).

Results. We were able to reach contrast ΔH = 5–8 mag and ΔK_s = 4–7 mag in the radius range r > 0.2″, which enabled us to constrain the presence of wide companions. For Y Oph, FF Aql, X Sgr, W Sgr, and η Aql at r> 0.2″, we ruled out the presence of companions with a spectral type that is earlier than a B7V, A9V, A9V, A1V, and G5V star, respectively. For 0.1″ < r < 0.2″, no companions earlier than O9V, B3V, B4V, B2V, and B2V star, respectively, are detected. A component is detected close to η Aql at projected separation ρ = 654.7 ± 0.9 mas and a position angle PA = 92.8 ± 0.1°. We estimated its dereddened apparent magnitude to be m_H⁰ = 9.34 ± 0.04 and derived a spectral type that ranges between an F1V and an F6V star. Additional photometric and astrometric measurements are necessary to better constrain this star and check its physical association to the η Aql system.