The secular evolution of planetary nebula IC 418 and its implications for carbon star formation

The rate of stellar evolution can rarely be measured in real time. The fastest evolution (excluding event-driven evolution), where stars may evolve measurably over decades, is during the post-AGB phase. In this paper we provide direct evidence for such a case. A secular, linear, factor of ∼2.5 increase is found in the strength of the [O iii] lines relative to Hβ over an 130 year period in the young, well-known, low excitation planetary nebula IC 418. The increase is caused by the rising temperature of the central star. We use photo-ionization models to derive a model dependent heating rate for the central star in the range 15–42 K yr⁻¹ . These derived heating rates are very sensitive to the stellar mass, and yield a central-star mass of ∼ 0.560–0.583 M_⊙. Initial-final mass relations based on the Miller-Bertolami models give a progenitor main-sequence mass of 1.25–1.55 M_⊙. IC 418 is a carbon rich planetary nebula and its central star, HD 35914, has evolved from an AGB carbon star. This result shows that carbon star formation at solar metallicity extends to these low masses. This is lower than commonly assumed and suggests that post-AGB evolution may be slower than recent post-AGB models predict.