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

Context. Low-ionization structures (LISs), found in all morphological types of planetary nebulae (PNe), are small-scale features prominent in emission from low-ionization species such as [N ii], [S ii], [O ii] and [O i]. Observational and theoretical efforts have aimed to better understand their origin and nature. Recently, the detection of molecular hydrogen (H$_2$) emission associated with LISs in a few PNe has added a new piece to the puzzle of understanding these nebular structures. Aims. Although observational studies indicate that LISs are characterized by lower electron densities than their host PNe, model predictions suggest higher total densities in these structures. The detection of H$_2$ emission from LISs in more PNe could help reconcile the observations with model predictions. Methods. Observations of five PNe with already known LISs were conducted using the Near InfraRed Imager and Spectrometer (NIRI) mounted on the 8 m Gemini North telescope. A narrow band filter, centered on H$_2$ 1-0 2.122 ${\mu}$m emission line, was used along with a continuum filter, to ensure continuum subtraction. Results. We present a deep, high-angular resolution near-IR narrowband H$_2$ 1-0 S(1) imaging survey of five Galactic PNe with LISs. We nearly double the sample of LISs detected in the H$_2$ 1-0 2.122 ${\mu}$m emission line as well as the number of host PNe. These findings allows us to prove 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. Additionally, we provide the first estimation of the excited H$_2$ molecular mass in LISs, which is found between 200 and 5000 times lower than the corresponding ionized gas mass.