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A&A
Volume 689, September 2024
Article Number A70
Number of page(s) 13
Section Interstellar and circumstellar matter
DOI https://doi.org/10.1051/0004-6361/202449580
Published online 03 September 2024

© The Authors 2024

Licence Creative CommonsOpen Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

This article is published in open access under the Subscribe to Open model. Subscribe to A&A to support open access publication.

1 Introduction

Planetary nebulae (PNe) are the result of the complex interac tion between stellar winds (interacting stellar wind model-ISW, Kwok et al. 1978, and generalised interacting stellar winds model-GISW, Kwok 1982; Balick 1987), the ionisation and exci tation processes of the expelled gas illuminated by the intense far-ultraviolet (far-UV) radiation from the central stars and the supersonic shock waves that propagate outward into the AGB circumstellar medium (e.g. Mellema 2004; Perinotto et al. 2004; Schönberner et al. 2005b,a, and references therein).

The illumination of the circumstellar gas by highly energetic UV photons is the main heating mechanism in PNe, yet shocks can also act as an extra heating process. Electron temperature augmentation is corroborated by a noticeable enhancement of the [O III]/Hα line ratio and may indicate shock interaction (e.g. Guerrero et al. 2013). Collimated fast outflows that are frequently found in PNe (e.g. Akras & López 2012; Akras et al. 2015; Miranda et al. 2017; Sabin et al. 2017; Sowicka et al. 2017; Derlopa et al. 2019; Rechy-García et al. 2020) are typical exem plars of shock-heated structural components. Several studies have demonstrated that collimated outflows or jets are strongly related to the formation of aspherical PNe, toroidal and ring structures, and dense knots (e.g. Sahai & Trauger 1998; Akashi & Soker 2008; Sahai et al. 2011; Akashi & Soker 2017, 2018; Balick et al. 2018, 2020; Bermúdez-Bustamante et al. 2020; García-Segura et al. 2020; Soker 2020; Akashi & Soker 2021, among others). A recent statistical analysis of collimated out flows and jets in PNe has shown that the bulk of their velocity is around 60 km s−1 and the host nebulae are typically young (<3000 yr; Guerrero et al. 2020).

The enhancement of low-ionisation emission lines such as [N II] λλ6548,6584, [S II] λλ6716,6731, and [O I] λλ6300,6363 at distinct microstructures in PNe (e.g. knots, filaments; here after low-ionisation structures, LISs, e.g. Gonçalves et al. 2001; Mari et al. 2023a) has been attributed to the intense UV radiation field from the central star (e.g. Ali & Dopita 2017) or in con junction with a shock interaction (e.g. Dopita 1997; Phillips & Guzman 1998; Gonçalves et al. 2004, 2009; Akras & Gonçalves 2016; Mari et al. 2023b,a). In particular, the intensity of the [O I] λ6300 and [S II] λλ6716,6731 emission lines are stronger than the photoionisation process predicts and they are attributed to a shock-heated gas (Phillips & Guzman 1998). Although the [S II]/Hα line ratio has been widely used to identify supernova remnants (shock-dominated) and distinguish them from PNe and H II regions that are UV-dominated (e.g. Leonidaki et al. 2013; Sabin et al. 2013; Fesen et al. 2015; Kopsacheili et al. 2020; Akras et al. 2020b), this ratio is not straightforwardly applica ble to trace shocks in PNe (e.g. Mari et al. 2023a). Numerical simulations have demonstrated that models of ionised gas with low photo-ionisation rates and shock-heated gas can generate very similar spectroscopic characteristics Raga et al. (2008, see also Kopsacheili et al. 2020). This makes very hard to determine the contribution of shocks in PNe dominated by UV radiation through optical emission lines only.

In the infrared (IR) wavelength regime, H2 1−0 S(1)/2− 1 S(1)≥4 and H2 1−0 S(1)/Brγ ≥ 10 line ratios are also considered as shock indicators for molecular gas (e.g. Beckwith et al. 1980; Marquez-Lugo et al. 2015), while the lower values are associ ated with UV radiation (Aleman & Gruenwald 2011; Aleman et al. 2011). However, a high density medium (≥104−5 cm−3) illuminated by intense UV radiation field can imitate the line ratios of shock-heated gas, considering that the collisional de-excitation process becomes significant (Sternberg & Dalgarno 1989). Recently, Aleman (2020) claimed that the large H2 1−0 S(1)/2−1 S(1) and H2 1−0 S(1)/Brγ observed ratios from several PNe can be naturally explained by photoionisation pro cess if the slit configuration used in the observations is taken into account. Overall, the detection of the H2 1−0 S(1) emission line directly originated from the LISs in some PNe (Akras et al. 2017, 2020a) did not provide a clear answer regarding the domi nant mechanisms, as both UV radiation and collisions by shock waves are able to reproduce the observations.

Other common tracers of shocks in IR wavelengths are the [Fe II] emission lines centered at 1.257 and 1.644 µm. They have been detected in a wide variety of sources such as active galactic nuclei (AGNs), supernova remnants, Herbig-Haro objects (HH), and PNe. In particular, in PNe, the [Fe II] 1.644 µm line has been mainly detected with spectroscopic observations (Lumsden et al. 2001).

Shock waves very likely destroy the dust grains and liberate iron into the gas phase, resulting in stronger [Fe II] lines com pared to an unshocked photoionised gas (Graham et al. 1987). In particular, the R(Fe)=[Fe II] 1.644 µm/Brγ line ratio > 0.1 has been widely considered as an indicator of shock activity. Yet, the [Fe II] 1.644 µm line can also be found in photoionised gas such as the Orion H II region, where R(Fe) < 0.06 (Lowe et al. 1979), and PNe (Lumsden et al. 2001). The compara ble ionisation potentials of Fe+ (16.2 eV) and O0 (13.6 eV) implies co-spatially [Fe II] 1.644 µm and [O I] 6300 Å emission lines, both emanate from the same partially ionised region. The [Fe II]/Brγ-[O I]/Hα diagnostic diagram as well as the excita tion mechanisms of the iron lines by UV radiation and collisions (shocks) on Seyfert and starburst galaxies have been discussed by Mouri et al. (1990, 2000). All the aforementioned optical and IR tracers of shocks, along with other potential tracers, are discussed in Hollenbach & McKee (1989).

With respect to planetary nebulae, it has not been reported any detection of iron emission lines directly associated with the LISs, despite their enhanced [O I] 6300 Å emission due to the lack of spatially resolved images. In this paper, we present a pilot narrow-band [Fe II] 1.644 µm imaging survey of PNe with particular interest in their LISs. The paper is organised as fol lows. In Sect. 2, we make a brief review of the [Fe II] 1.644 µm emission line detections in PNe. The sample selection and the observations are described in Sects. 3 and 4, respectively. The contribution of the H I 12-4 emission line centered at 1.640 µm is also discussed. The results of the survey are discussed in Sect. 5 and we finish with our conclusions in Sect. 6.

2 [Fe II] 1.644 µm emission line in PNe

Spectroscopic observations in the H and K bands for a number of PNe were presented by Hora et al. (1999); Lumsden et al. (2001) and Likkel et al. (2006). In some of the spectra, the [Fe II] 1.644 µm line is blended with the H I 12-4 1.640 µm line due to the low spectral resolution of the spectrograph and its detection is questionable.

Nevertheless, the theoretical flux of the H I 12-4 line can be derived from the Brγ flux and then subtracted from the [Fe II]+H I 12-4 blended observed flux. Considering the Case B recombination theory, an electron density (ne) of 104 cm−3 and electron temperature (Te) of 104 K, the theoretical flux of H I 12 4 line is approximately 19% of the Brγ flux (H I 12-4/Brγ ~0.19, Hummer & Storey 1987).

For the majority of the PNe in the sample of Lumsden et al. (2001), we find that the blended [Fe II]+H 112-4 intensity is con sistent with the expected flux of the H I 12-4 line. Therefore, the [Fe II] 1.644 µm line is certainly detected via spectroscopy only in the following PNe: CRL 618, BD +30° 3639, NGC 7027, M 4-18, M 1-16, M 1-92, M 2-9, Vy 2-2, and M 1-78.

Besides the aforementioned spectroscopic data, [Fe II] narrow-band images have been obtained only for a handful of pre-PNe and PNe: Hubble 12, M 2-9, CRL 618, NGC 6302, and NGC 7027. Hubble 12 displays strong [Fe II] 1.644 µm emission from an inner hourglass structure with an intensity around 8×10−15 erg cm−2 s−1 arcsec−2, being engulfed by a more extended H2 envelope (Hora & Latter 1996; Welch et al. 1999; Clark et al. 2014). The contribution of the H I 12-4 emission line to the total emission was taken into consideration, and the [Fe II] emission was found to be consistent with J-shock models of ~ 100 km s−1. The core of the nebula exhibits R(Fe)<0.1 as it is expected for a photodominated gas, while the spectroscopic data obtained with an offset from the core result in higher ratios of 0.14 (Luhman & Rieke 1996) and up to 3 (Hora & Latter 1996), implying a shock-heated gas.

The intriguing PN M 2-9 has also been imaged in the [Fe II] 1.644 µm line and a collimated bipolar structure enclosed by an alike but larger H2 and ionised structure has been unveiled (Smith et al. 2005; Clyne et al. 2015; Balick et al. 2018). Interest ingly, the young PN K 4-47 displays a very similar H2 structure (Akras et al. 2017) but no [Fe II] images are available yet. According to the spectroscopic data of M 2-9, [Fe II] 1.644 µm emission is stronger in the northern knot and bipolar lobes in comparison to the core. The assumed mechanism responsible for the emission is shocks (Hora & Latter 1994).

The highly collimated pre-PN CRL 618 is the third nebula imaged in the [Fe II] 1.644 µm line, displaying emission at the edges of the outflows. Because of the highly expanding outflows with velocities of 300 km s−1, the [Fe II] 1.644 µm emission has also been attributed to shock-heated gas (Balick et al. 2013). Shocks have also been proposed as the principal mechanism responsible for the excitation of H2 emission along the outflows of CRL 618 (Cox et al. 2003; Kastner et al. 2001). Unfortunately, the low spatial resolution of the H2 image does not allow for any comparison with the higher spatial resolution [Fe II] image to be performed.

The ‘Butterfly nebula’ NGC 6302 (Kastner et al. 2022) and the ‘Jewel Bug nebula’ NGC 7027 (Moraga Baez et al. 2023) have recently mapped in the [Fe II] 1.644 µm emission line using the HST and the narrow-band filter F164N. The [Fe II] 1.644 µm line is clearly detected at the edges of the bipolar outflows (‘wedge’) in NGC 6302, displaying an S-shape structure with a peak intensity of ~3×10−14 erg cm−2 s−1 sr−1 (Kastner et al. 2022). The same authors have argued that the dominant exci tation mechanism is shocks between a fast stellar wind with a velocity of >100 km s−1 and the previously formed bipolar lobes. HST [Fe II] 1.644 µm narrow-band images of NGC 7027 have also been presented by Moraga Baez et al. (2023). The [Fe II] 1.644 µm emission is detected at the tips of a symmet ric outflow along the south-east and north-west direction. The Chandra observations of NGC 7027 have revealed the presence of a very hot bubble with temperature of up to 3.6 MK. The X-ray emission detected in this nebula is also extended along the same direction (Montez & Kastner 2018). A shock interaction between the collimated outflow and the nebular shell have been proposed to be responsible for this [Fe II] emission (Moraga Baez et al. 2023).

Overall, the [Fe II] 1.644 µm emission line is undoubtedly found only in sixteen PNe or pre-PNe, morphologically char acterised as bipolar with collimated outflows or jet structures. It is worth noticing that most of these PNe are younger than 3000–3500 yr, while the [Fe II] 1.644 µm line is more commonly detected in nebular components younger than 1000–1500 yr (see Table 1).

Table 1

Ages of planetary nebula where the [Fe II] 1.644 µm emission line was detected.

3 Sample selection

Five PNe with LISs were selected for this pilot narrow-band [Fe II] 1.644 µm imaging survey. All but one PN (NGC 6751) are described as elliptical with collimated outflows. NGC 6751 is a complex multi-shell PNe with an equatorial ring fragmented in knots and a pair of faint jet-like features (Clark et al. 2010).

Based on HST [O III]/Hα line ratio maps, Guerrero et al. (2013) classified four of them (NGC 6201, NGC 7009,

NGC 6543, and IC 4634) as types A/B. The common character istic of these types is the enhancement of [O III] emission line at the tips of collimated outflows (see Fig.1 in Guerrero et al. 2013). NGC 6751 was not included in the previous analysis, but we assigned a D-type1 classification because of its similarity with other PNe from the same type (e.g. NGC 2392 or NGC 6369).

Two of the PNe (NGC 7009 and NGC 6543) in our sample display H2 emission that emanate from their LISs (Fang et al. 2018; Akras et al. 2020a). However, the H2 1−0/H2 2−1 and H2 1−0/Brγ ratios do not allow for the origin of the H2 emission (UV-pumping or collisional excitation) to be traced. No infor mation about any molecular gas in the remaining three PNe is available yet.

Table 2

Observations log.

4 Observations

A near-IR (NIR) narrow-band imaging survey of PNe with LISs was carried out in 2017 (February 20, March 25, June 6 and 9, and July 2 and 3, Program ID: GN–2017A–Q–58, PI: S. Akras) in service mode, using the Near InfraRed Imager and Spectrometer on the Gemini-North Telescope at Mauna Kea in Hawaii.

The narrow-band filter G0215 was used to isolate the [Fe II] line centered at 1.644 µm, while the narrow-band H-continuum filter (G0214) at 1.570 µm was used to determine and finally remove any adjacent continuum emission. The contribution of the [Si I] line centered at λ1.645 µm to the total emis sion is expected to be negligible or even totally absent (e.g. Lumsden et al. 2001). On the other hand, the contribution of the H I 12-4 line (λ1.640 µm) to the total emission may be signifi cant, and it is necessary to subtract it. Hereafter, we refer to the resultant observed continuum-subtracted images of our survey as [Fe II]+H I 12−4 images, and the H I 12-4 subtracted one as [Fe II] 1.644 µm images.

The f/6 configuration was selected for this narrow-band sur vey because of the large angular size of the targets providing a field of view of 120×120 arcsec2 and a pixel size of 0.117 arc-sec. Several individual frames were obtained for each target with exposure times between 170 and 180 s. The observations were carried out under poor weather conditions, which introduces extra uncertainty on the absolute fluxes. The seeing during the observations varied between 0.6 and 1.0 arcsec. For some PNe, it was necessary to degrade images quality in order to prop erly subtracted the continuum emission. The observing log is summarised in Table 2.

The thermal emission, dark current and hot pixels on the detector were corrected using the darks and GCAL frames. In order to reduce the total time of the observations, the major axis of each nebula was oriented in the up-down direction on the detector and the dithering was carried out across the left-right direction (minor axis of the nebula). The images were flux-calibrated observing standard stars with the same configuration as the science data.

Before started the reduction process, the first frames from each sequence were excluded as recommended by Gemini staff. The PYTHON routines CLEARIR.PY and NIRLIN.PY were applied to all the frames for the correction of the vertical stripping and the non-linearity of the detector. The reduction of the data was then performed with the Gemini IRAF2 package for NIRI. The routines Nprepare, Nisky, Niflat, Nireduce, and Imcoadd were used for each imaging set accordingly.

thumbnail Fig. 1

Gemini NIRI images of NGC 7009. Panel a: total [Fe II]+H I line image. Panel b: continuum-subtracted [Fe II]+H I line image. Panels c and d: zoom-in of the continuum-subtracted [Fe II]+H I image to the LIS-W1 and LIS-E1 overlaid by the H2 1−0 emission (orange contours) and Brγ emission (dashed red contours). Panel a is on logarithmic scale and panels b-d on a linear scale.

5 The [Fe II] 1.644 µm narrow-band imagery

The first [Fe II]+H I 12-4 line images for a sample of five PNe with NIRI@Gemini are presented with the aim to detect the [Fe II] 1.644 µm emission line associated with their LISs and provide further insights into the formation and excitation mechanisms of these intriguing microstructures.

5.1 NGC 7009

The strong emission of low ionisation lines such as [N II], [O I], and [S II] detected at the tips of the jet-like structures in NGC 7009 has been intriguing scientists for years. The recent detection of H2 emission from the same LISs has raised new questions about their nature. The observed H2/Brγ and H2 1−0/2−1 line ratios can be attributed either to UV-pumping or collisional excitation mechanism (Akras et al. 2020a).

The [Fe II] 1.644 µm emission line was not detected in the K-band spectra of this nebula published by Hora et al. (1999). Probably, the slit did not cover the LISs, as stated by the authors. It should be noted that the H2 2.12 µm emission line had not been previously detected in the LISs of this nebula either (see Latter et al. 2000).

In Fig.1, we present the total and continuum-subtracted [Fe II]+H I 12-4 image of NGC 7009. Strong emission is detected in the main inner nebula and it is accounted for in the H I 12-4 1.640 µm line. Interestingly, a fainter emission is easily per ceptible at the eastern LIS but only marginally detected at the western LIS. In both cases, the emission is engulfed by the more extended Brγ emission (dashed red contours in panels c and d). This implies that part (if not all) of the [Fe II]+H I 12-4 emission may originate from the ionised hydrogen gas. It should be noted the offset of approximately 1 arcsec between the [Fe II]+H I 12-4 and H2 emission (black contours in panels c and d) found for both LISs, with the [Fe II]+H I 12-4 emission laying closer to the central star. The emission flux of the eastern LISs is determined 1.87×10−15 erg s−1 cm−2, with a signal-to-noise ratio (S/N) of 4 while for the western LIS only an upper limit is presented <5.11×10−15 erg s−1 cm−2.

To calculate how much of the observed [Fe II]+H I 12-4 flux corresponds to the H I 12-4 line, the Brγ image of NGC 7009 from Akras et al. (2020a) was used to construct the theoretical H I 12-4 image3 (H I 12-4/Brγ ~0.19) and subtracted it from the observed [Fe II]+H I 12-4 image.

The first [Fe II] 1.644 µm image of NGC 7009 is presented in Fig. 2, showing a residual emission in the eastern LIS and a barely visible emission in the western LIS. [Fe II] 1.644 µm emission is also unveiled in an inner LIS/knot (Gonçalves et al. 2003). The [Fe II] 1.644 µm fluxes are estimated as 1.1 and 12.7 (×10−15) erg s−1 cm−2 for the LIS-E and LIS-K, respectively. The intensity varies from 3.36 to 8.85 (×10−5) erg s−1 cm−2 sr−1 (Table 3).

Table 3

NGC 7009 [Fe II] 1.644 µm+HI 12-4 1.640 µm and [Fe II] 1.644 µm flux and intensity.

thumbnail Fig. 2

[Fe II] 1.644 µm image of NGC 7009 on a linear intensity scale. Regions that the line fluxes and intensities are measured from are indi cated with red rectangles.

5.2 NGC 6543

NGC 6543 is the second PN in our sample for which H2 emission is associated with LISs (Akras et al. 2020a). For this PN, the observed H2/Brγ and H2 1−0/2−1 S(1) line ratios indicate a non thermal origin (UV-pumping mechanism). The [Fe II] 1.644 µm emission line was not detected in the K-band spectroscopic data of the southern knot of NGC 6543 (Hora et al. 1999).

The Brγ (panel a) and continuum-subtracted [Fe II]+H I 12-4 (panel b) images of NGC 6543 are presented in Fig. 3. Both emission lines show very similar spatial distribution, clearly dis playing the ellipse E 105 (major axis at PA = 105 degrees), the edges of the ellipse E25 (major axis at PA = 25 degrees), and the [N II] caps (for the definition of these structures see, Reed et al. 1999). Panels c and d show a zoom-in to the regions of the NE and SW LISs, respectively (see caption in Fig. 3).

[Fe II]+H I 12-4 emission is found to be co-spatial with H2 emission, while Brγ is more extended and encloses both emis sion lines. Fluxes were calculated from the same regions where H2 emission was also measured (Akras et al. 2020a) and they are equal to 7.86×10−15 erg s−1 cm−2, with an S/N of 5 for the NE and an upper limit for the SW LIS <12.2 ×10−15 erg s−1 cm−2.

Similarly to NGC 7009, the theoretical H I 12-4 line image was constructed from the corresponding Brγ line image (Akras et al. 2020a) and subtracted from the total [Fe II]+H I 12-4 image. The resulting [Fe II] 1.644 µm image of NGC 6543 is displayed in Fig. 4 with significant emission emanate from the edges of the ellipse E25 and the [N II] caps structures. A weak emission is also detected in the ellipse E105 and the NE/SW LISs.

[Fe II] 1.644 µm line fluxes are measured for various regions in the nebula (see Fig. 4) ranging from 2.82×10−15 to 4.08×10−14 erg s−1 cm−2 and the intensity from 6.95×10−5 to 9.42×10−4 erg s−1 cm−2 sr−1, respectively (Table 4).

5.3 NGC 6210

NGC 6210, also known as the Turtle-Shape nebula, is morpho logically characterised by a bright inner part with several arcs, filaments, and much fainter collimated outflows (Bohigas et al. 2015; Rechy-García et al. 2020).

The total (right panel) and continuum-subtracted (left panel) [Fe II]+H I 12-4 images of NGC 6210 are shown in Fig. 5. Emis sion emanates mainly from the bright inner part of the nebula, without any association with the outflows. The [Fe II]+H I 12-4 line image of NGC 6210 is not flux calibrated because no suit able standard stars were observed. Besides the overall spatial distribution of the [Fe II]+H I 12-4 emission, any link between [Fe II] 1.644 µm emission and the LISs distributed in the inner part of the nebula cannot be confirmed or ruled out yet.

thumbnail Fig. 3

Gemini NIRI images of NGC 6543. Panel a: Brγ line image. Panel b: continuum-subtracted [Fe II]+H I line image. Panels c and d: zoom-in of the continuum-subtracted [Fe II]+H I image to the LIS-W1 and LIS-E1 overlaid by the H2 1−0 emission (black contours) and Brγ emission (dashed red contours). Panel a is on logarithmic scale and panels b-d on a linear scale.
thumbnail Fig. 4

[Fe II] 1.644 µm image of NGC 6543 on a linear intensity scale. Regions that the line fluxes and intensities are measured from are indi cated with red rectangles.

5.4 NGC 6751

NGC 6751 is a complex multi-shell planetary nebula (Clark et al. 2010). The most intriguing part is the inner shell or ring frag mented into knots and filaments, similar to NGC 2392 nebula (García-Díaz et al. 2012).

The total and continuum-subtracted [Fe II]+H I 12-4 images of NGC 6751 are presented in Fig. 6 (panels a and b). The knotty and filamentary structure of the nebula is easily discerned in both images, despite the lower spatial resolution compared to the HST [N II] image. Panel b displays the [Fe II]+H I 12-4 con tinuum subtracted image with the [N II] λ6584 contours (black) from the HST image overlaid, showing a very good spatial matching. Most of the knots are detected in the [Fe II]+H I 12-4 and [N II] emission lines. [Fe II]+H I 12-4 flux has been deter mined for several regions in NGC 6751 varying by an order of magnitude. The faintest and brightest structures have fluxes of 1.26×10−15 and 3.94×10−14 erg s−1 cm−2, respectively (see Table 5).

The absence of a Brγ line image prevents us from con structing a theoretical image of H I 12-4 line and, as a con sequence, the [Fe II] 1.644 µm line image of NGC 6751 as well. Nevertheless, we used the integrated dereddened Hβ flux (2.79×10−13 erg s−1 cm−2) measured at the C1 region with size of 0.5x10.8 arcsec2 (Chu et al. 1991) to obtain the expected flux of the H I 12-4 line adopting the Case B recombination theory, ne =104 cm−3 and Te=104 K (Brγ/Hβ ~0.0275, H I 12-4/Brγ ~0.19). Following this approximation, the Brγ and H I 12-4 fluxes were calculated for each aperture, as well as the expected flux of the [Fe II] 1.644 µm line (Table 5). The [Fe II] 1.644 µm flux varies from 2×10−15 to 3.79×10−14 erg s−1 cm−2 depending on the size and the position of each region. The regions consid ered for the [Fe II] 1.644 µm flux measurements are depicted in Fig. 7. These values must be used with caution due to the uncer tain estimation of the H I 12-4 fluxes. However, there is no doubt about the detection of the [Fe II] 1.644 µm line in NGC 6751 given that the C1 region is the brightest structure of the nebula (Chu et al. 1991) and the contribution of the H I 12-4 line is very likely overestimated. The high R(Fe) values strongly support the presence of shocks in NGC 6751.

Table 4

NGC 6543 [Fe II] 1.644 µm+H I 12-4 1.640 µm and [Fe II] 1.644 µm fluxes and intensities.

thumbnail Fig. 5

Gemini NIRI images of NGC 6210. Panel a: total [Fe II] 1.644 µm+H I 12-4 1.640 µm line image. Panel b: continuum-subtracted [Fe II] 1.644 µm+H I 12-4 1.640 µm line image overlaid by the HST [N II] 6584 Å emission (black contours). All panels are on logarithmic scale.

5.5 IC 4634

The last PN observed in this pilot imaging survey is IC 4634. It is described by an inner and outer S-shape morphology with several individual microstructures bright in low-ionisation lines. Hereafter, we follow the nomenclature given by Guerrero et al. (2008): (i) a pair of bow-shock features (A/B and A′/B′) forming the outer S-shape, (ii) a pair of bow shock-like features (C/C′), (iii) a pair of knotty arcs (D/D′) forming the inner S-shape, and (iv) the bright inner shell.

Figure 8 displays the total and continuum-subtracted [Fe II]+H I 12-4 images of IC 4634 (panels a and b). The S-shape structure of the inner and outer nebula is visible in both images, with a faint [Fe II]+H I 12-4 emission detected in the A/A′ (white squares). The C/C′ outflows indicated by red ellipses are barely detected as well. Panels c and d show a zoom-in view of the A/A′ features in [Fe II]+H I 12-4 continuum-subtracted image with the HST [N II] emission line contours overlaid. Both the [Fe II]+H I 12-4 and [N II] emission lines are co-spatial.

The [Fe II]+H I 12-4 fluxes are estimated for the A/A′ and D/D′ features (see Fig. 9). The D/D′ features are found to be at least one magnitude brighter (3.79 and 2.69 ×10−14 erg s−1 cm−2) compared to the A/A′ features (4.30 and 4.87×10−15 erg s−1 cm−2), with S/N values of 39/28 and 5/6 for each feature, respectively (Table 6). Figure 9 displays the regions (black rectangular shape) where fluxes were calculated.

The absence of Brγ imagery or K-band spectroscopy of IC 4634 forced us to use the Hβ image of the nebula obtained with the HST in order to get the theoretical flux of the Brγ and H I 12-4 1.640 µm lines. The Hβ fluxes from the A/A′ features as defined in Fig. 9 are 7.55 and 6.15×10−14 erg s−1 cm−2 or 1.43 and 1.33×10−13 erg s−1 cm−2, respectively, after applying a correction for the extinction (0.34, Guerrero et al. 2008). These fluxes are two orders of magnitude lower than those reported by Guerrero et al. (2008) while the Hβ fluxes from the D/D’ features are comparable. The theoretical H I 12-4 line fluxes for the A/A′ features are computed 7.52 and 6.92×10−16 erg s−1 cm−2, respectively. According to this analy sis, the flux of the [Fe II] 1.644 µm line in the A/A′ features is approximately 3.5 and 4.2 × 10−15 erg s−1 cm−2, respectively.

thumbnail Fig. 6

Gemini NIRI images of NGC 6751. Panel a: total [Fe II] 1.644 µm+H I 12-4 1.640 µm line image. Panel b: continuum-subtracted [Fe II] 1.644 µm+H I 12-4 1.640 µm line image overlaid by the HST [N II] 6584 Å emission (black contours). All panels are on a linear scale.
thumbnail Fig. 7

Continuum-subtracted [Fe II] 1.644 µm+H I 12-4 1.640 µm image of NGC 6751 on a linear scale. Regions, from which line fluxes and intensities measured, are indicated with black rectangles.

6 Discussion

The subgroup of fast moving LISs, known as FLIERs (see Balick et al. 1993, 1998), are the ideal targets to search for evidence of shocks in PNe. Their high expansion velocities (supersonic velocities >40–50 km s−1) and interactions with the surrounding nebular gas or interstellar medium (ISM) can be responsible for the enhancement of the emission from low-ionisation species such as singly ionised nitrogen, sulphur ([N II] λλ6548,6584, and [S II] λλ6716,6731) or neutral oxygen and nitrogen ([O I] λ6300, [N I] λ5200) relative to the emis sion from the surrounding nebular gas (e.g. Balick et al. 1993; Phillips & Guzman 1998; Dopita 1997; Gonçalves et al. 2003, 2004; Raga et al. 2008; Akras & Gonçalves 2016).

The outward motion of LISs should result in some additional signs such as bow-shocks, which are not frequently observed though. Nevertheless, indirect evidence of shocks is apparent as the augmentation of the electron temperature (Guerrero et al. 2013) or the presence of a strong [Fe II] 1.644 µm line and high R([Fe II]) ratio.

Deep, spatially resolved images of the [Fe II] 1.644 µm emis sion line in NGC 7009, NGC 6543, NGC 6751 and IC 4634 have verified the presence of singly ionised iron in LISs and demonstrated that it is cospatial with other low-ionisation emis sion lines (e.g. [N II] λ6584). The R(Fe) ratio has been computed for several LISs in the PNe of our list, and it ranges from 0.5 up to 7. The low values are attributed to UV pumping process and the higher values to shocks.

In particular, the eastern LIS of NGC 7009 has R(Fe)=0.25, four times higher than the value measured in the Orion H II region (Lowe et al. 1979). Despite the fact that we cannot defini tively argue for the presence of shocks, the low R(Fe) value entails only low velocity shock waves. Based on the [O III]/Hα line ratio maps from HST and MUSE, it has been found that the outer pair of LISs are characterised by an increased ratio at the outer edges (see, Guerrero et al. 2013; Walsh et al. 2018; Akras et al. 2022), which may imply a higher electron temper ature due to the shock interaction of LISs with the surrounding gas. It should, however, be noted that the photoelectric heating process from dust grains can also be important in high density structures and provide a reasonable explanation for the enhanced [O III]/Hα ratio (e.g. Dopita & Sutherland 2000; van Hoof et al. 2004).

The lowest ratio in our sample is measured for NGC 6543, 0.05<R(Fe)<0.15 and we argue that UV process is more likely responsible for the ionisation of the gas. The same mechanism has also been proposed to explain the low R(H2) and R(Brγ) ratios found in this nebula (Akras et al. 2020a).

On the other hand, significantly high R(Fe) ratios have been measured for NGC 6751 (2<R(Fe)<7) and IC 4634 (R(Fe)~1; A/A′ features), providing strong evidence that shocks are the dominant mechanism. It is worth noticing that the A/A′ features in IC 4634 have noticeable higher Te([O III]) by 1000–1500 K, compared to the rest of the components (Hajian et al. 1997), while Te([N II]) is the same in all nebular structures. This tem perature difference may be associated with the interaction of the A/A′ features with the ISM. The high velocities up to 70– 100 km s1 measured at the A/A′ (features 1 and 5 in Hajian et al. 1997) support this hypothesis. From a statistical point of view, Mari et al. (2023a) have also shown that Te([O III]) is systematically higher than Te([N II]) in LISs.

Overall, four out of five PNe (NGC 7009, NGC 6543, NGC 6571, and IC 4634) observed in this pilot [Fe II]+H I 12-4 imaging survey provide sufficient evidence to claim for the first detection of the [Fe II] 1.644 µm line in the LISs of these PNe. However, the absolute fluxes of the [Fe II] 1.644 µm line com puted in this work are characterised by high uncertainties due to the accumulative uncertainties in all the steps followed to correct the observed emission for the contribution of the H I 12-4 line (Hβ and Brγ fluxes, ne= 104 cm−3, and Te=104 K assumptions for case B, the different weather conditions between the obser vations, the different instruments, slit positions, and extracted windows).

Scrutinising the available NIR spectroscopic data from PNe by Hora et al. (1999); Lumsden et al. (2001) and correcting for the contribution of the H I 12-4 1.640 µm line, we find that the R(Fe) ratio ranges from 0.06 up to 4. Five out of 15 PNe or 33 percent have R(Fe)<0.1, which is indicative of photo-heated gas. Shocks can only be attributed to a handful of PNe or spe cific nebular structures with R(Fe)> 1 (e.g. the south-east region in Hb 12, the northern knot in M 2-9, the north-east region in NGC 2440, CRL 618, M 1-78, and BD +30 3639). It should be noted though that the aforementioned PNe exhibit R(H2)<4, which is usually attributed to the UV-pumping process. There are also two PNe that have R(H2)>8 and may be associated with shock, whereas their low R(Fe) is only twice large the Orion’s value (~0.13). Photoionisation and shock models produce a wide range of R(H2) with significant overlapping that impedes the dif ferentiation between two mechanisms (see Fig. 6 in Akras et al. 2020a, and references therein).

Our detection of the [Fe II] 1.644 µm line depends on the estimate and subtraction of the H I 12-4 1.640 µm. To corrob orate our detection, we searched the literature for the potential detection of other optical Fe lines. Several lines, from a singly ionised and up to six times ionised Fe (either collisionally excited or recombined) have been reported for the four PNe; for instance, [Fe II] λ5261, λ7154, λ8617, and/or [Fe III] λ4658, λ4701, λ4881, λ5270, λ54124, among others (NGC 7009; Fang et al. 2018), (NGC 6543; Perinotto et al. 1999; Hyung et al. 2000), (IC 4634; Hyung et al. 1999; Guerrero et al. 2008), and (NGC 6751; Chu et al. 1991).

We note that these identifications are not all directly asso ciated with the LISs, where the near-IR [Fe II] 1.644 µm line is detected, except for the line λ5270 from the A′ feature in IC 4634 and the line λ8617 from the north [N II] cap feature in NGC 6543. We have to mention that the [Fe II] λ8617 line has also been detected in the MUSE data of NGC 7009 and specifically in the eastern LIS. The theoretical [Fe II] 1.644 µm/8617 Å line ratio is ~9, 5 and 1 for Ne= 103, 104 and 10s cm−3, respectively (Koo et al. 2016). The [Fe II] λ8617 line flux from the MUSE data is computed ~1.5e−16 erg cm−2 s−1, which yields a ratio of ~7 (Bouvis et al. in prep.). This ratio implies an electron density close to 103 cm−3, which agrees with the observations.

The comparison of the [Fe II] 1.644 µm intensities with the predictions from the shock models of Hollenbach & McKee (1989) has pointed out to the following model, a high density (104 cm−3) shock model with velocities between 80 and 100 km s−1 can explain the average intensity of the [Fe II] 1.644 µm line (~2.10×10−4 erg s−1 cm−2 sr−1) of NGC 6751, as well as the average intensity of the [N II] caps in NGC 6543 (~3.60×10−4 erg s−1 cm−2 sr−1). Regarding the eastern LIS of NGC 7009, we conclude that two shock models (i) n~ 104 cm−3 and v ~ 50 km s−1, or (ii) n ~ 103 cm−3 and v ~80 km s−1 can both explain the observed intensity of the [Fe II] 1.644 µm line (~3.3×10−5 erg s−1 cm−2 sr−1). Despite the fact that Hollenbach & McKee (1989) did not provide the predic tions for a denser molecular gas, we presume that a slower shock of ~30 km s−1 is likely able to reproduce the observed intensities of the [Fe II] line in denser gas. The observed intensities of the H2 2.12µm line from the eastern LIS in NGC 7009 (Akras et al. 2020a) are higher than the two models predicted, but comparable with the prediction from a highly dense gas (~105 cm−3). High densities between 103 and 105 cm−3 have been reported for the shell and knots of NGC 7009 by Hyung et al. (2023). Regarding the Hβ emission from all the aforementioned structures, shock models with velocity <100 km s−1 and densities between 103 to 107 cm−3 can reproduce the observed intensity (see Table 1 in Hollenbach & McKee 1989). It is important to point out that slow shock models require denser gas. Based on the more recent shock models by Koo et al. (2016), the observed [Fe II] 1.644 µm line fluxes can be reproduced either by fast velocity shock models (90–100 km s−1) and low pre-shock density of 10 cm−3, or shock models with velocities lower than 80 km s−1 and higher pre-shock density of ~100 cm−3. All these models are characterised by R([Fe II]) > 1, which increases for higher velocities.

Thus, we suggest that the shock interaction of the LISs with the low density surrounding gas or ISM requires high velocities, which are not always observed. On the contrary, a slow shock wave has to interact with denser gas in order to have the same emission. In the first case, the interaction of LISs with the sur rounding nebular due to their outwards motion, the H2 2.12 µm line should lie at a larger distance behind the shock front than the [Fe II] 1.644 µm line (see Fig. 1 in Hollenbach & McKee 1989) or emanate from at least the same region behind the shock front (Novikov & Smith 2018, 2019). However, our observations show the exact opposite result, along with an offset between the two lines of ~1 arcsec.

An alternative scenario to explain the enhanced emission from the optical low-ionisation lines, [Fe II] 1.644 µm, and H2 2.12 µm is the photoevaporation process of clumps illumi nated by a strong UV radiation field. This process has been discussed by several authors (e.g. Sternberg & Dalgarno 1989; Burton et al. 1990; Störzer & Hollenbach 2000). For instance, Mellema et al. (1998) performed a detailed analysis of photo-evaporated clumps in PNe carrying out both an analytic and a numerical approach. This process offers an adequate explana tion for several of the LISs’ characteristics (see also Raga et al. 2005). According to the PDR models by Burton et al. (1990), a high dense clump (106–7 cm−3) illuminated by a strong UV radiation field (G0 = 104–5 )5 can reproduce the intensity of the H2 lines as well as the high line ratio because of collisional de-excitation, measured at the eastern LIS in NGC 7009 (Akras et al. 2020a). Photoevaporation process has also been proposed to explain the cometary shapes of knots in the Helix nebula (e.g. López-Martín et al. 2001).

More specifically, the photons from the central star (or UV source) interact with the inner face of the clump and start evapo rating the molecular gas. This evaporating flow moves backward and interacts with the stellar wind or the nebular gas. As a result, a reverse shock wave is formed moving in the opposite direction and propagates through the clump. The pressure that drives the shock wave depends on the ionising photon flux of the central star. According to the photoevaporation model of Mellema et al. (1998), the velocity of the reverse shock can be as high as 20– 30 km s−1 for a central source of 50000 K and 7000 L⊙. Such a slow velocity shock interacts with the highly dense molecu lar code of the LISs (i.e. in the second scenario) and this is likely to be the mechanism that leads to the observed emission spectrum. Moreover, the same shock wave can also lead to the destruction of the dust grains and the release of Fe to gas-phase which would explain our detections. Meanwhile, the high den sity of molecular core of the LISs provide the necessary shield from being fully dissociated. The hypothesis of the photoevap-oration can also account for the observed offset between the H2 and [Fe II] lines, with the latter being closer to the central star.

Two noteworthy outcomes of the photoevaporation process are (i) the atomic hydrogen emission (Brγ) is more extended than the [N II] emission (or other line) due to the ionisation of the evaporated flow (see Fig. 7 Mellema et al. 1998) and (ii) the spatial offset between the Brγ and H2 emission lines. Such a spatial offset between the two hydrogen lines has been observed in molecular clouds (Hartigan et al. 2015; Carlsten & Hartigan 2018) as well as in the outer pair of LISs in NGC 7009 (Akras et al. 2020a).

Table 5

NGC 6751 [Fe II] 1.644 µm+H I 12-4 1.640 µm and [Fe II] 1.644 µm fluxes and intensities.

thumbnail Fig. 8

Gemini NIRI images of IC 4634. Panel a: total [Fe II] 1.644 µm+H I 12-4 1.640 µm image. Panel b: continuum-subtracted [Fe II] 1.644 µm+H I 12-4 1.640 µm image. Panels c and d: zoom-in of the continuum-subtracted [Fe II] 1.644 µm+H I 12-4 1.640 µm image to the LIS-NW and LIS-SE overlaid by the HST [N II] 6584 Å emission (black contours). All panels are on a linear scale.
thumbnail Fig. 9

Continuum-subtracted [Fe II] 1.644 µm+H I 12-4 1.640 µm image of IC 4634 on a logarithmic scale. Black rectangular shapes indi cate the windows where the line fluxes and intensities are measured.
Table 6

IC 4634 [Fe II] 1.644 µm+H I 12-4 1.640 µm and [Fe II] 1.644 µm fluxes and intensities.

7 Conclusions

The [Fe II] narrow-band imaging survey of five PNe with LISs was carried with NIRI@Gemini and presented here. Three emission lines, [Fe II] 1.644 µm, H I 12-4 1.640 µm, and [Si I] 1.645 µm, were covered by the Gemini filter. [Si I] 1.645 µm is in general very faint in PNe and its contri bution is negligible, while H I 12-4 1.640 µm is relatively strong (0.5 percent of Hβ); thus, it was taken into account in order to obtain the [Fe II] 1.644 µm fluxes.

The [Fe II] 1.644 µm + H I 12-4 1.640 µm blended emission was detected in all five PNe. The contribution of the H I 12-4 1.640 µm line was determined from the available Brγ images or Hβ fluxes. After correcting for the contribution of the H I 12-4 1.640 µm line, the detection of [Fe II] 1.644 µm line was con firmed in four out of five PNe. In all cases, it is found to be directly associated with LISs.

Beside the detection of the [Fe II] 1.644 µm line, its direct association with shocks is not confirmed. Based on the [Fe II] 1.644 µ/Brγ line ratio, we argue that the [Fe II] 1.644 µm emission is ascribed to photoionisation process in NGC 6543 (<0.15) and to shocks in IC 4634 (~1) and NGC 6571 (between 2 and 7), attributed to the high ratios. Both mechanisms are seen in the case of the eastern LIS in NGC 7009 (< 0.25).

The observed displacement between the [Fe II], H2, and Brγ lines is an indicative of the photoevaporation process in the LISs, while it contradicts the predictions from shock interaction with the nebular gas or ISM. It is likely that a slow-moving shock in the LISs is responsible for the release of Fe in gas phase and the observed emission lines. The presence of such a shock wave ought to be validated with an observational confirmation.

Acknowledgements

We would like to thank the anonymous referee for the con structive comments that helped us to improve the quality of the paper. The authors would like to thank Dr. Martin Guerrero for providing us the flux cal ibrated HST Hβ image of IC 4634. The research project is implemented in the framework of H.F.R.I. call “ Basic research financing (Horizontal support of all Sciences)” under the National Recovery and Resilience Plan “Greece 2.0” funded by the European Union 0 NextGenerationEU (H.F.R.I. Project Number: 15665). S.A. and K.B. acknowledge support from H.F.R.I. IA acknowledges support from CAPES, Ministry of Education, Brazil, through a PNPD fellowship. DGR acknowledges support from the CNPq grants 428330/2018-5 and 313016/2020-8. G.R.-L. acknowledges support from CONACyT (grant 263373). Based on observations obtained at the international Gemini Observatory, a program of NSF’s OIR Lab [processed using the Gemini IRAF package and DRAGONS (Data Reduction for Astronomy from Gemini Observatory North and South)], which is managed by the Association of Universities for Research in Astronomy (AURA) under a cooperative agreement with the National Science Foundation. on behalf of the Gemini Observatory partnership: the National Science Foun dation (United States), National Research Council (Canada), Agencia Nacional de Investigación y Desarrollo (Chile), Ministerio de Ciencia, Tecnología e Innovación (Argentina), Ministério da Ciência, Tecnologia, Inovações e Comu-nicações (Brazil), and Korea Astronomy and Space Science Institute (Republic of Korea). The following software packages in Python were used: Matplotlib (Hunter 2007), NumPy (van der Walt et al. 2011), SciPy (Virtanen et al. 2020) and AstroPy Python (Astropy Collaboration et al. 2013, 2018).

Data availability

The data presented in this work are available at the CDS or upon request to the corresponding author. The observations are also available in the Gemini Science Archive.

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1

Guerrero et al. (2013) define the D-type PNe as those with a nearly flat [O III]/Hα ratio map and a complex structure that does not fit with the other types.

2

IRAF is distributed by the National optical Astronomy observatories, which are operated by the Association of Universities for Research in Astronomy, Inc., under cooperative agreement with the National Science Foundation.

3

Both [Fe II]+H I 12-4 and Brγ images were obtained with the same telescope (Gemini), the same instrument (NIRI) and same configuration. Hence, the subtraction of the images using the central star of the nebula for their alignment becomes straightforward with small error of 2–3 pixels.

4

Due to the presence of the He I λ5412, this detection is highly uncertain.

5

G0 gives the intensity of the UV radiation field in terms of the Habing field, and it is equal to 1.6×10−3 erg cm−2 s−1 (Habing 1968).

All Tables

Table 1

Ages of planetary nebula where the [Fe II] 1.644 µm emission line was detected.

In the text
Table 2

Observations log.

In the text
Table 3

NGC 7009 [Fe II] 1.644 µm+HI 12-4 1.640 µm and [Fe II] 1.644 µm flux and intensity.

In the text
Table 4

NGC 6543 [Fe II] 1.644 µm+H I 12-4 1.640 µm and [Fe II] 1.644 µm fluxes and intensities.

In the text
Table 5

NGC 6751 [Fe II] 1.644 µm+H I 12-4 1.640 µm and [Fe II] 1.644 µm fluxes and intensities.

In the text
Table 6

IC 4634 [Fe II] 1.644 µm+H I 12-4 1.640 µm and [Fe II] 1.644 µm fluxes and intensities.

In the text

All Figures

thumbnail Fig. 1

Gemini NIRI images of NGC 7009. Panel a: total [Fe II]+H I line image. Panel b: continuum-subtracted [Fe II]+H I line image. Panels c and d: zoom-in of the continuum-subtracted [Fe II]+H I image to the LIS-W1 and LIS-E1 overlaid by the H2 1−0 emission (orange contours) and Brγ emission (dashed red contours). Panel a is on logarithmic scale and panels b-d on a linear scale.
In the text
thumbnail Fig. 2

[Fe II] 1.644 µm image of NGC 7009 on a linear intensity scale. Regions that the line fluxes and intensities are measured from are indi cated with red rectangles.
In the text
thumbnail Fig. 3

Gemini NIRI images of NGC 6543. Panel a: Brγ line image. Panel b: continuum-subtracted [Fe II]+H I line image. Panels c and d: zoom-in of the continuum-subtracted [Fe II]+H I image to the LIS-W1 and LIS-E1 overlaid by the H2 1−0 emission (black contours) and Brγ emission (dashed red contours). Panel a is on logarithmic scale and panels b-d on a linear scale.
In the text
thumbnail Fig. 4

[Fe II] 1.644 µm image of NGC 6543 on a linear intensity scale. Regions that the line fluxes and intensities are measured from are indi cated with red rectangles.
In the text
thumbnail Fig. 5

Gemini NIRI images of NGC 6210. Panel a: total [Fe II] 1.644 µm+H I 12-4 1.640 µm line image. Panel b: continuum-subtracted [Fe II] 1.644 µm+H I 12-4 1.640 µm line image overlaid by the HST [N II] 6584 Å emission (black contours). All panels are on logarithmic scale.
In the text
thumbnail Fig. 6

Gemini NIRI images of NGC 6751. Panel a: total [Fe II] 1.644 µm+H I 12-4 1.640 µm line image. Panel b: continuum-subtracted [Fe II] 1.644 µm+H I 12-4 1.640 µm line image overlaid by the HST [N II] 6584 Å emission (black contours). All panels are on a linear scale.
In the text
thumbnail Fig. 7

Continuum-subtracted [Fe II] 1.644 µm+H I 12-4 1.640 µm image of NGC 6751 on a linear scale. Regions, from which line fluxes and intensities measured, are indicated with black rectangles.
In the text
thumbnail Fig. 8

Gemini NIRI images of IC 4634. Panel a: total [Fe II] 1.644 µm+H I 12-4 1.640 µm image. Panel b: continuum-subtracted [Fe II] 1.644 µm+H I 12-4 1.640 µm image. Panels c and d: zoom-in of the continuum-subtracted [Fe II] 1.644 µm+H I 12-4 1.640 µm image to the LIS-NW and LIS-SE overlaid by the HST [N II] 6584 Å emission (black contours). All panels are on a linear scale.
In the text
thumbnail Fig. 9

Continuum-subtracted [Fe II] 1.644 µm+H I 12-4 1.640 µm image of IC 4634 on a logarithmic scale. Black rectangular shapes indi cate the windows where the line fluxes and intensities are measured.
In the text

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