Impact of a binary companion in AGB outflows on CO spectral lines

Context. In the late stage of their evolution, low- to intermediate-mass stars pass through the asymptotic giant branch (AGB) phase. AGB stars are characterised by strong mass loss through dust driven winds. High angular resolution interferometric observations reveal that these winds harbour strong deviations from spherical symmetry, such as spirals and arcs, believed to be caused by hidden (sub-)stellar companions. Such observations are scarce, and much more often observed features from these systems are molecular spectral lines, where the presence of a companion is less clear. Aims. Our aim is to study the impact of a binary companion on low-/ CO spectral lines of AGB star outflows. By varying the orbital separation and wind velocity, we aim to find line shapes characteristic of more complex binary-induced morphologies. This would allow us to quantify whether a companion can be identified from spectral lines, or if it remains hidden in the line profiles. Methods. We generated a new grid of nine 3D models of a mass-losing AGB star using the smoothed particle hydrodynamics (SPH) code P<ns0:sc>HANTOM</ns0:sc>, with three values for both the outflow velocity and orbital separation. We created a novel method for calculating the CO photodissociation in asymmetric outflows in order to determine the size of the emitting envelope. Utilising the 3D non-local thermodynamic equilibrium (NLTE) radiative transfer code M<ns0:sc>AGRITTE</ns0:sc>, we created synthetic spectral lines for the low rotational transitions of CO at different inclinations and position angles. Results. Our hydrodynamical simulations show a variety of morphologies, always with a pronounced spiral structure arising in the orbital plane, but with varying shapes in the meridional plane, and different degrees of global flattening. CO photodissociation closely follows the global morphology, thereby creating a non-spherical emitting region. We find that the low-/ CO line profiles can deviate strongly from the parabolic or flat-topped profiles expected from non-resolved spherically symmetric outflows. A variety of line shapes emerge, with two peaks near the terminal velocity, and a central bump near the central velocity being the most pronounced. The line shapes strongly depend on the underlying morphology and inclination. In specific cases the spectral lines can appear parabolic, hiding the presence of a binary companion. Conclusions. We find that a binary companion can have a pronounced impact on the resulting CO spectral lines, and thus the molecular line profiles can serve as a binary diagnostic. However, the influence of the companion on the line can also go easily unnoticed as the characteristic features can be concealed by the beam profile and the noise of the observations. Therefore, it is easy to misclassify systems as single stars. Neglecting the impact of a binary companion when modelling the CO spectral lines can thus cause systematic errors on the derived mass-loss rates.