PHOTOIONIZATION MODELING BASED ON HST IMAGES OF MAGELLANIC CLOUD PLANETARY-NEBULAE .1. SMC-N-2 AND SMC-N-5

We construct fully self-consistent, detailed photoionization models fo r two planetary nebulae (PNe) in the Small Magellanic Cloud (SMC), nam ely SMC N 2 and SMC N 5, to fit optical and UV spectrophotometric obse rvations as well as HST Faint Object Camera (FOC) narrow-band images t aken in the light of H beta. The derived density structure shows that both PNe have a central cavity surrounded by a shell of decreasing den sity described by a parabolic curve. For both nebulae, our models fail to reproduce the HST images taken in the light of the [O III] lambda 5007 line, in the sense that the observed [O III] lambda 5007 surface brightness decreases more slowly outside the peak emission than predic ted. An effective temperature of T-eff = 111 500 K, a stellar surface gravity of log g = 5.45 and a luminosity of L() = 8430 L. are derived for the central star of SMC N2; similarly T-eff = 137 500 K, log g = 6.0 and L() = 5850 L. are derived for SMC N 5. SMC N 2 is optically t hin and has a total nebular mass (H plus He) of 0.180 M., while SMC N 5 is optically thick and has an ionized gas mass of 0.194 M.. Using th e H-burning SMC metal abundance (Z = 0.004) evolutionary tracks calcul ated by Vassiliadis & Wood, core masses of 0.674 M. and 0.649 M. are d erived for SMC N 2 and SMC N 5, respectively. Similarly, from the He-b urning evolutionary tracks of Vassiliadis & Wood for progenitor stars of mean LMC heavy-element abundance (Z = 0.008), we find M(c) = 0.695 and 0.675 M. for SMC N 2 and SMC N 5, respectively. We find that H bet a images are needed if one is to derive accurate stellar luminosities directly from photoionization modelling. However, in the absence of an H beta image, photoionization models based on [O III] images (and neb ular line intensities) yield accurate values of T-eff and log g, which in turn allow reliable stellar masses and luminosities to be derived from a comparison with theoretical evolutionary tracks. We show that t he correct nebular ionized mass can be deduced from the nebular H beta flux, provided the mean nebular density given by the C III] lambda 19 09/lambda 1907 ratio is also known.