Classifying the secondary component of the binary star W Aquilae (Research Note)

Abstract

Aims. The object WAql is an asymptotic giant branch (AGB) star with a faint companion. By determining more carefully the properties of the companion, we hope to better constrain the properties of the AGB star. Methods. We present new spectral observations of the binary star WAql at minimum and maximum brightness and new photometric observations of WAql at minimum brightness. Results. The composite spectrum near minimum light is predominantly from the companion at wavelengths λ < 6000 A˚. This spectrum can be classified as F8 to G0, and the brightness of the companion is that of a dwarf star. Therefore, it can be concluded that the companion is a main sequence star. From this, we are able to constrain the mass of the AGB component to 1.04-3 M⊙ and the mass of the WAql system to 2.1-4.1 M⊙. Our photometric results are broadly consistent with this classification and suggest that the main sequence component suffers from approximately 2 mag of extinction in the V band primarily due to the dust surrounding the AGB component.