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

Abstract

A prevailing open problem in planetary nebulae 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 (1967) 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, 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 abundance discrepancy factors and a binary nature of photoionizing stars. In this paper we show that large amplitude temperature fluctuations are expected to form in planetary nebulae photoionized by short-period binary stars. Resonant temperature fluctuations 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 temperatures 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.Further, 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 resonant temperature fluctuations in nebulae.

Physical Conditions and Chemical Abundances of the Variable Planetary Nebula IC 4997

“…Alternatively, large abundance discrepancies in PNe could be linked to binary central stars (Corradi et al 2015;Jones et al 2016;Bautista & Ahmed 2018;Wesson et al 2018;Jacoby et al 2020;García-Rojas et al 2022). In particular, Corradi et al (2015) found ADFs higher than 50 in the PNe Abell 46 and Ou 5 surrounding post common-envelope binary stars.…”

Deconstructing the Characteristics of the Ionized Gas Component in Planetary Nebulae From Their Internal Motions

“…Nevertheless, PNe with large ≳ 10 ADFs imply local temperature fluctuations whose origin is difficult to envisage. Bautista and Ahmed (2018) have modeled large temperature fluctuations that seem to propagate efficiently in the envelopes of PNe with short periods binary cores, as those expected from post common envelope systems where the period is shorter than the thermalization time in the plasma. Given the apparent close relationship between PNe with close binary cores (see §2.3) and large ADFs (Wesson et al, 2018) this is a very promising path to keep investigating.…”

NGC 6153: Reality is Complicated*

“…The work of Bautista & Ahmed (2018), who modeled a nebula whose central star is periodically eclipsed, may offer an alternative to the two plasma components we advocate here. The particular nebula they model is not a good approximation to NGC 6153, but, if the amplitude of their resonant temperature fluctuations varies sufficiently rapidly with depth, it might mimic the behavior of two plasma components within a single plasma and binary stars appear to be very common among planetary nebulae with large abundance discrepancies (e.g., Wesson et al 2018).…”