The Binary Central Star of the Planetary Nebula A35

Abstract

Using new Far Ultraviolet Spectroscopic Explorer (FUSE) observations in conjunction with Space Telescope Imaging Spectrograph (STIS) and International Ultraviolet Explorer archive data, we have modeled both components of the binary central star of the planetary nebula A35. The white dwarf (the ionizing star) was modeled using the non-LTE, plane-parallel code TLUSTY. We find its parameters to be Teff = 80 ? 3 kK, log g = 7.70 cm s-2, and [He/H] = -4 ? 1 and C, N, O, Si, and Fe to be underabundant by 2 orders of magnitude with respect to their solar values. This confirms its classification as a DAO white dwarf, and using the Hipparcos distance D = 163 pc, we derive a radius of RWD 1.65 ? 10-2 R? and a mass of M 0.5 M?. The modeling of the far-ultraviolet spectra also constrains the extinction value; EB-V = 0.04 ? 0.01. Furthermore, the FUSE and STIS data allow us to measure the molecular hydrogen (H2) and neutral hydrogen (H I) column densities along the sight line, the majority of which we believe is associated with the circumstellar material. The FUSE spectrum is best fitted with a two-component model for H2, consisting of a cool component (T = 200 K) with log N(H2,cool) = 19.6 cm-2 and a hot component (T 1250 K) with log N(H2,hot) = 17.4 cm-2. The H I column density is log N(H ) = 20.9 ? 0.1 cm-2. Assuming a typical gas/dust ratio for the interstellar medium, our value of EB-V implies that log N(H ) = 20.8 cm-2 of this is circumstellar. Our low extinction value and the measured column densities imply that there is essentially no dust in the nebula. Assuming that the neutral and molecular hydrogen is contained in a sphere of comparable dimensions to the ionized shell, we derive the combined mass of the circumstellar H I and H2 to be ~2.7 M?. Other geometries, such as a shell surrounding the ionized region, can be excluded. The mass of the ionized hydrogen is 1% that of the neutral material. From comparison with evolutionary calculations, we estimate the progenitor mass to be ~3.2 M?.