Planetary nebulae with Wolf-Rayet-type central stars -- IV. NGC 1501 and its mixing layer

TL;DR

This study models the mixing layers in planetary nebula NGC 1501 to understand the origin of X-ray-emitting gas and predicts similar phenomena in other nebulae, highlighting the role of stellar winds and ionized species.

Contribution

It demonstrates that hot central stars cannot produce certain emission lines via photoionization, emphasizing the importance of mixing layers and stellar wind absorption in hot bubble formation.

Findings

NGC 1501's mixing layer properties are characterized using C IV lines.

Most energetic photons are absorbed in dense stellar winds of [WR] stars.

UV observations support the presence of mixing layers and hot bubbles in several planetary nebulae.

Abstract

Theory predicts that the temperature of the X-ray-emitting gas ($\sim$10$^{6}$ K) detected from planetary nebulae (PNe) is a consequence of mixing or thermal conduction when in contact with the ionized outer rim ($\sim$10$^{4}$ K). Gas at intermediate temperatures ($\sim$10$^{5}$ K) can be used to study the physics of the production of X-ray-emitting gas, via C IV, N V and O VI ions. Here we model the stellar atmosphere of the CSPN of NGC 1501 to demonstrate that even this hot H-deficient [WO4]-type star cannot produce these emission lines by photoionization. We use the detection of the C IV lines to assess the physical properties of the mixing region in this PNe in comparison with its X-ray-emitting gas, rendering NGC 1501 only the second PNe with such characterization. We extend our predictions to the hottest [WO1] and cooler [WC5] spectral types and demonstrate that most energetic photons are absorbed in the dense winds of [WR] CSPN and highly ionized species can be used to study the physics behind the production of hot bubbles in PNe. We found that the UV observations of NGC 2452, NGC 6751 and NGC 6905 are consistent with the presence mixing layers and hot bubbles, providing excellent candidates for future X-ray observations.