Proof of shock-excited H$_2$ in low-ionization structures of PNe

TL;DR

This study detects near-infrared H$_2$ emission from low-ionization structures in planetary nebulae, revealing shock excitation in some cases and photo-ionization in others, with implications for understanding nebular dynamics.

Contribution

It provides high-resolution imaging of H$_2$ emission in planetary nebulae, demonstrating shock excitation in bipolar outflows and photo-ionization in peripheral knots, a novel detailed analysis of molecular gas excitation mechanisms.

Findings

H$_2$ emission originates from bipolar outflows and knots.

H$_2$ 1-0 S(1)/2-1 S(1) ratio indicates shock interaction or photo-ionization.

Correlation between H$_2$ line ratios and [N II]/H$eta$ line ratios.

Abstract

We report the detection of near-IR H$_2$ lines emission from low-ionization structures (LISs) in planetary nebulae. The deepest, high-angular resolution H$_2$ 1-0 S(1) at 2.122 $\mu$m, and H$_2$ 2-1 S(1) at 2.248 $\mu$m images of K 4-47 and NGC 7662, obtained using NIRI@Gemini-North, are presented here. K 4-47 reveals a remarkable high-collimated bipolar structure. The H$_2$ emission emanates from the walls of the bipolar outflows and a pair of knots at the tips of these outflows. The H$_2$ 1-0 S(1)/2-1 S(1) line ratio is $\sim$7-10 which indicates shock interaction due to both the lateral expansion of the gas in the outflows and the high-velocity knots. The strongest line, H$_2$ v=1-0 S(1), is also detected in several LISs located at the periphery of the outer shell of the elliptical PN NGC 7662, whereas only four knots are detected in the H$_2$ v=2-1 S(1) line. These knots have H$_2$ v=1-0 S(1)/v=2-1 S(1) values between 2 and 3. These data confirm the presence of molecular gas in both highly (K 4-47) and slowly moving LISs (NGC 7662). The H$_2$ emission in K 4-47 is powered by shocks, whereas in NGC 7662 is due to photo-ionization by the central star. Moreover, a likely correlation is found between the H$_2$ v=1-0 S(1)/H$_2$ v=2-1 S(1) and [N II]/H$\alpha$ line ratios.