Galactic planetary nebulae with emphasis on those that are members of open star clusters

Abstract

The study of Planetary Nebulae (PNe) provides vital clues for understanding late stage stellar evolution and stellar mass loss. Despite their significance, various issues concerning these objects remain unsolved. Interstellar extinction by dust hampers the detection of many Galactic PNe at optical wavelengths while their largely unknown distances and progenitor masses are two key parameters that hamper deeper understanding. More reliable data is urgently required. Identification of dust obscured PNe can be partially solved with observations at longer wavelengths where extinction effects are less serious and by multi-wavelength techniques proposed for their detection. Multi-wavelength techniques are a proven effective method for identification of Galactic PN candidates. Here, the efficacy of these methods is evaluated for identification of new Galactic PN candidates. From 70 new PN candidates uncovered in this study only 11 have optical counterparts allowing spectroscopic follow-up. From the eight observed so far, seven are confirmed as PNe. One subset of PNe that avoid most of the above issues are PNe in Galactic star clusters. Such PNe have distances and progenitor masses determined independently from cluster color-magnitude diagrams. Observational data of Galactic cluster PNe and their central stars (CSPN) allows determination of their final core masses that can feed into the Initial-To-Final Mass (IFMR) relation which is crucial for understanding the chemical enrichment of the interstellar medium. Evidence of a PN-cluster association requires multiparameter agreement of reddening, distance, angular proximity, PN physical size and most importantly radial velocity. Here, we study the first confirmed PN-open cluster pair (PHR1315-6555 – AL 1) identifying its CSPN and measuring its physical properties. This study allowed the determination of AL 1’s physical parameters with precision and the identification of the core of PHR 1315-6555 that is a ̴ 0.6 Mʘ, hot (T= ̴ 110 kK) white dwarf with a progenitor mass of ̴ 2.5 Mʘ. The presence of an unresolved binary companion is unlikely. We also demonstrate in detail the physical association of PN BMP J1613-5406 with Galactic open cluster NGC 6067, the second confirmed PN-OC pair. The excellent agreement of all required parameters (e.g. radial velocity) strongly suggests association. Its progenitor mass is ̴ 5.6 Mʘ. The CSPN mass is estimated to be ̴ 0.94 Mʘ and a candidate CSPN of V ̴ 20.2 is found to be close to the apparent nebular centre. Finally, we provide preliminary details on several additional cases of possible PN-cluster associations we have uncovered. This is based on the close angular proximity of PN, cluster morphology and other preliminary indicators, but more data are required to confirm these links. This work suggests many Galactic PNe remain to be uncovered in areas of high extinction and that cluster-PN pairs offer a great opportunity for study. Our new PN-OC associations contribute to the improvement of the IFMR and our multi-wavelength study suggests that a new era begins for the identification of Galactic PNe hidden by dust.