Resonant Temperature Fluctuations in Nebulae Ionized by Short-period Binary Stars

Abstract

Abstract A prevailing open problem in planetary nebulae (PNe) research, and photoionized gaseous nebulae research at large, is the systematic discrepancies in electron temperatures and ionic abundances as derived from recombination and collisionally excited lines. Peimbert proposed the presence of “temperature fluctuations” in these nebulae, but the apparent amplitude of such fluctuations, as deduced from spectral diagnostics and/or abundance discrepancy factors (ADFs), remain unexplained by standard photoionization modeling. While this and other alternative models to explain the temperature and abundance discrepancies remain inconclusive, recent observations seem to point at a connection between nebular ADFs and a binary nature of photoionizing stars. In this paper, we show that large amplitude temperature fluctuations are expected to form in PNe photoionized by short-period binary stars. Resonant temperature fluctuations (RTFs) are first formed along the orbital disk around the binary stars, as the periodically varying ionizing radiation field induces periodic oscillations in the heating-minus-cooling function. Then, the temperature fluctuations propagate vertically to the disk as thermal waves that later steepen into radiative shocks. The binary period of the ionizing stars is determinant in the formation and propagation of temperature fluctuations, as well as in associated density fluctuations. Fluctuations propagate efficiently only in systems with binary periods significantly shorter than the gas thermalization time, of the order of 10 days. Furthermore, we propose temperature diagnostic line ratios that combine [O iii] collisionally excited lines and O ii recombination lines to determine the equilibrium temperature and the magnitude of RTFs in nebulae.