Low-ionization structures in planetary nebulae. IV. The molecular hydrogen counterpart

Low-ionization structures (LISs), found in all morphological types of planetary nebulae (PNe), are small-scale features that are prominent in emission from low-ionization species such as N ii S ii O ii and O i. Observational and theoretical efforts aim to better understand their origin and nature. The recent detection of molecular hydrogen (H₂) emission associated with LISs in a few PNe has added a new piece to the puzzle of understanding these nebular structures. Although observational studies indicate that LISs are characterized by lower electron densities than their host PNe, model predictions suggest that these structures have higher total densities. The detection of H₂ emission from LISs in more PNe could help reconcile the observations with model predictions. We conducted observations of five PNe with already known LISs using the Near InfraRed Imager and Spectrometer (NIRI) mounted on the 8 m Gemini North telescope. A narrowband filter, centered on the H₂ 1-0 2. 122 μm emission line, was used along with a continuum filter to ensure continuum subtraction. We present a deep, high-angular-resolution near-IR narrowband H₂ 1-0 S (1) imaging survey of five Galactic PNe with LISs. We nearly double the sample of LISs detected in the H₂ 1-0 2. 122 μm emission line, as well as the number of host PNe. These findings allow us to demonstrate that the systematically lower electron density in LISs -- relative to the rims and shells of their host nebulae -- is linked to the presence of H₂ molecular gas. Furthermore, we provide the first estimate of the excited H₂ molecular mass in LISs, which is between 200 and 5000 times lower than the corresponding ionized gas mass.