We perform a comprehensive analysis of the planetary nebula ( PN ) NGC 6781 to investigate the physical conditions of each of its ionized , atomic , and molecular gas and dust components and the object ’ s evolution , based on panchromatic observational data ranging from UV to radio .
Empirical nebular elemental abundances , compared with theoretical predictions via nucleosynthesis models of asymptotic giant branch ( AGB ) stars , indicate that the progenitor is a solar-metallicity , 2.25 - 3.0 M _ { \sun } initial-mass star .
We derive the best-fit distance of 0.46 kpc by fitting the stellar luminosity ( as a function of the distance and effective temperature of the central star ) with the adopted post-AGB evolutionary tracks .
Our excitation energy diagram analysis indicate high excitation temperatures in the photodissociation region ( PDR ) beyond the ionized part of the nebula , suggesting extra heating by shock interactions between the slow AGB wind and the fast PN wind .
Through iterative fitting using the Cloudy code with empirically-derived constraints , we find the best-fit dusty photoionization model of the object that would inclusively reproduce all of the adopted panchromatic observational data .
The estimated total gas mass ( 0.41 M _ { \sun } ) corresponds to the mass ejected during the last AGB thermal pulse event predicted for a 2.5 M _ { \sun } initial-mass star .
A significant fraction of the total mass ( about 70 % ) is found to exist in the PDR , demonstrating the critical importance of the PDR in PNe that are generally recognized as the hallmark of ionized/H ^ { + } regions .