Proof of shock-excited H2 in low-ionization structure of PNe

Abstract

We report the detection of near-IR H2 line emission from the low-ionization structures (LISs) in planetary nebulae. The deepest, high-angular resolution H2 1-0 S(1) at 2.122 μm, and H2 2-1 S(1) at 2.248 μm 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, with the H2 emission emanating from the walls of the outflows and a pair of knots at the tips of these outflows. The H2 1-0 S(1)/2-1 S(1) line ratio is ∼⃒7-8 which indicates shock interaction due to both the lateral expansion of the gas and the high-velocity knots. The strongest line, H2 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 H2 v = 2-1 S(1) line. These knots have H2 v = 1-0 S(1)/v = 2-1 S(1) values between 3 and 5. These data confirm the presence of molecular gas in both highly (K 4-47) and slowly moving LISs (NGC 7662). The H2 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 H2 v = 1-0 S(1)/H2 v = 2-1 S(1) and [N II]/Hα line ratios.