Low-ionization structures in planetary nebulae – I. Physical, kinematic and excitation properties

Abstract

Though the small-scale, low-ionization knots, filaments and jets (LISs) of planetary nebulae (PNe) are known for ∼30 yr, some of their observational properties are not well established. In consequence our ability to include them in the wider context of the formation and evolution of PNe is directly affected. Why most structures have lower densities than the PN shells hosting them? Is their intense emission in low-ionization lines the key to their main excitation mechanism? Therefore, if considered altogether, can LISs line ratios, chemical abundances and kinematics enlighten the interplay between the different excitation and formation processes? Here we present a spectroscopic analysis of five PNe that possess LISs confirming that all nebular components have comparable electron temperatures, whereas the electron density is systematically lower in LISs than in the surrounding nebula. Chemical abundances of LISs versus other PN components do not show significant differences as well. By using diagnostic diagrams from shock models, we demonstrate that LISs' main excitation is due to shocks, whereas the other components are mainly photo-ionized. We also propose new diagnostic diagrams involving a few emission lines ([N ii], [O iii], [S ii]) and log(f shocks /f ), where f shocks and f are the ionization photon fluxes due to the shocks and the central star ionizing continuum, respectively. A robust relation differentiating the structures is found, with the shock-excited clearly having log(f shocks /f )>-1; while the photoionized show log(f shocks /f )<-2. A transition zone, with -2< log(f shocks /f )<-1 where both mechanisms are equally important, is also defined.

Characterization of the planetary nebula Tc 1 based on VLT X-shooter observations

“…These asymmetries may be associated with small-scale structures, bright in [N ii] or [S ii emission lines (e.g. LISs, Gonçalves et al 2001;Akras & Gonçalves 2016). The presence of small-scale structures, e.g., clumps or bipolar features, with different degree of ionization has also been proposed for Tc 1 by Williams et al (2008).…”

H2 emission in the low-ionization structures of the planetary nebulae NGC 7009 and NGC 6543

“…One of the most intriguing characteristic of LISs is the electronic density (determined from the common diagnostic line ratios), which is systematically lower than or at most equal to the electronic density of the surrounding nebular gas (e.g. Balick et al 1993;Hajian et al 1997;Gonçalves et al 2003Gonçalves et al , 2009Monteiro et al 2013;Akras & Gonçalves 2016;Ali & Dopita 2017). This finding contradicts the formation models of knots in which they are considered more dense than the surrounding nebular gas (e.g.…”

“…Low photoionization rate models gener- ate spectroscopic characteristics similar to shock-excited regions. A crucial parameter to distinguish the mechanisms is the distance between the LISs and the source of ionizing photons, which determines the photoionization rate at the position of the structures Akras & Gonçalves 2016). Based on Raga's simulations and spectroscopic data from a sample of PNe with LISs, Gonçalves et al (2009) argued that the spectra of high-velocity knots can be explained by shocks.…”

“…Based on Raga's simulations and spectroscopic data from a sample of PNe with LISs, Gonçalves et al (2009) argued that the spectra of high-velocity knots can be explained by shocks. A few years later, Akras & Gonçalves (2016) shown that the enhancement of low-ionization lines in knots relative to the surrounding medium is the result of a combination of the ultraviolet (UV) radiation from the central star and shock interactions. However, it is not easy to distinguish the contribution from each mechanisms.…”

Deciphering the bipolar planetary nebula Abell 14 with 3D ionization and morphological studies

“…Recently, Ali et al (2015) showed that a reverse shock of only 25 km s −1 in the PN G342.0-01.7, due to its motion through the ISM, can successfully explain the strong neutral emission lines. The low photo-ionization rate of their central stars, (very low L) and large nebular size, would make the contribution of shocks easily perceptible even at small velocities (Akras & Gonçalves 2016), or produce emission line spectra that resemble those of shock-excited regions (Raga et al 2008). …”