First Images of the Molecular Gas around a Born-again Star Revealed by ALMA

Born-again stars allow probing stellar evolution in human timescales and provide the most promising path for the formation of hydrogen-deficient post-asymptotic giant branch objects, but their cold and molecular components remain poorly explored. Here we present ALMA observations of V 605 Aql that unveil for the first time the spatio-kinematic distribution of the molecular material associated with a born-again star. Both the continuum and molecular line emission exhibit a clumpy ring-like structure with a total extent of ≈1'' in diameter. The bulk of the molecular emission is interpreted as being produced in a radially expanding disk-like structure with an expansion velocity ${v}_{\exp }$ ∼ 90 km s−1 and an inclination i ≈ 60° with respect to the line of sight. The observations also reveal a compact high-velocity component, ${v}_{\exp }$ ∼ 280 km s−1, that is aligned perpendicularly to the expanding disk. This component is interpreted as a bipolar outflow with a kinematical age τ ≲ 20 yr, which could either be material that is currently being ejected from V 605 Aql, or is being dragged from the inner parts of the disk by a stellar wind. The dust mass of the disk is in the range Mdust ∼ 0.2–8 × 10−3 M⊙, depending on the dust absorption coefficient. The mass of the CO is MCO ≈ 1.1 × 10−5 M⊙, which is more than three orders of magnitude larger than the mass of the other detected molecules. We estimate a 12C/13C ratio of 5.6 ± 0.6, which is consistent with the single stellar evolution scenario in which the star experienced a very late thermal pulse instead of a nova-like event as previously suggested.