New Insights into Low-Ionisation Structures in Planetary Nebulae from [Fe II] Emission Line Survey

A recent imaging survey focused on the [Fe II] 1.644 μm emission line in planetary nebulae (PNe) has yielded significant findings regarding low-ionisation structures (LISs). Conducted by researchers Stavros Akras, Isabel Aleman, Denise R. Gonçalves, Gerardo Ramos-Larios, and Konstantinos Bouvis, the survey aimed to enhance the understanding of LISs, which are often observed in PNe but remain poorly understood.

The study reports the first detection of the [Fe II] 1.644 μm line directly associated with LISs in four out of five observed PNe. The researchers noted that the [Fe II] emission serves as a common tracer of shocks, which are critical for understanding the dynamics within these nebulae. The flux of the [Fe II] line varied between 1 to 40 x 10^-15 ergs cm^-2 s^-1, while the surface brightness ranged from 2 to 90 x 10^-5 erg cm^-2 s^-1 sr^-1.

Additionally, the study calculated the ratio of [Fe II] 1.644 μm to the Brγ line, denoted as R(Fe), which varied significantly among the observed PNe. For instance, in NGC 6543, the R(Fe) ratio was less than 0.15, indicating a strong influence from the ultraviolet radiation of the central star. In contrast, higher ratios in NGC 6571 and IC 4634 suggested shock contributions, highlighting the complex interplay of mechanisms at work in these celestial structures.

This research contributes to the broader understanding of planetary nebulae and their associated structures, potentially influencing future studies on stellar evolution and the chemical enrichment of the interstellar medium. The findings are documented in the paper titled "[Fe II] 1.644 μm imaging survey of planetary nebulae with low-ionisation structures," available on arXiv with the identifier arXiv:2409.00377.