The physical association of planetary nebula NGC 2818 with open cluster NGC 2818A

Abstract

Context. Planetary nebulae (PNe) that are physical members of open star clusters (OCs) provide us with highly valuable data for stellar evolution studies. Unfortunately, they are extremely rare, with only three such instances previously confirmed in our Galaxy. Aims. In this study, we confirm the physical association of PN NGC 2818 with OC NGC 2818A, an association long debated in literature studies. Hence, we add a fourth object to the sample of rare OC-PN pairs. The physical properties of the PN can then be linked to those of its progenitor star. Methods. Using GHOST at Gemini high-resolution nebular spectra, we measured the PN systemic radial velocity to compare it with that of the putative host cluster. This was well determined from Gaia. We estimated the physical parameters of both the OC NGC 2818A and the PN using our data, together with estimates from previous studies and theoretical cluster isochrones and evolutionary tracks to show they are compatible. Results. The highly precise, systemic radial velocity of the PN that was determined, is consistent to within the small errors with that of the cluster and its 1 km s^-1 associated velocity dispersion. This is a tight and primary requirement for cluster membership. We have determined other physical parameters of the PN and cluster, such as age and distance. These also match within the errors. Taken together, these results present compelling evidence for the physical association of the PN and cluster. The PN age was found to be around 11 kyr and the effective temperature of its central star was estimated as 130 kK. The initial and final masses of the progenitor star were determined to be 2.33 ± 0.10 M⊙ and 0.58 ± 0.10 M⊙, respectively. We plotted the resulting initial-to-final data point on the latest initial-to-final-mass relation and also include the data points from our other three OC-PN associations known. This new data point agrees with the published trends from previous studies and further delineates the 'kink' found at relatively low initial masses. Conclusions. We show that all four OC-PN associations, identified thus far, share a number of common properties. These rare cases merit detailed cluster-PN studies and work to further extend the sample.