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 V605 Aql that unveil for the first time the spatio-kinematic distribution of the molecular material associated to a born-again star. Both the continuum and molecular line emission exhibit a clumpy ring-like structure with a total extent of $\approx$1${}^{\mathrm{\prime }\mathrm{\prime }}$ 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${}_{\mathrm{e}\mathrm{x}\mathrm{p}}$$\sim$90 km s${}^{-1}$ and an inclination $i$$\approx$60${}^{\circ }$ with respect to the line-of-sight. The observations also reveal a compact high-velocity component, v${}_{\mathrm{e}\mathrm{x}\mathrm{p}}$$\sim$280 km s${}^{-1}$, that is aligned perpendicularly to the expanding disk. This component is interpreted as a bipolar outflow with a kinematical age $\tau$$\lesssim$20 yr, which could either be material that is currently being ejected from V605 Aql, or it is being dragged from the inner parts of the disk by a stellar wind. The dust mass of the disk is in the range $M_{\rm dust}\sim$0.2-8$\times$10$^{-3}$ M$_{\odot}$, depending on the dust absorption coefficient. The mass of the CO is $M_{\rm CO}\approx$1.1$\times10^{-5}$ $M_{\odot }$, which is more than three orders of magnitude larger than the mass of the other detected molecules. We estimate a ${}^{12}$C/${}^{13}$C ratio of 5.6$\pm$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.