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A&A
Volume 691, November 2024
Article Number A321
Number of page(s) 11
Section Interstellar and circumstellar matter
DOI https://doi.org/10.1051/0004-6361/202451719
Published online 22 November 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.

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1 Introduction

Planetary nebulae (PNe) consist of circumstellar matter that is photoionised by hot central stars that evolve towards the cooling white dwarf phase. This ionised gas is the origin of most of the radiation from PNe. The emission at optical and infrared wavelengths is dominated by forbidden and recombination emission lines, while in the radio domain, the main emitting mechanism is bremsstrahlung (or free-free radiation) from free electrons in the plasma. This thermal continuum free-free radiation is characterised by a spectral index (α, where Sv ∝ vα) ranging from +2 (optically thick regime at low frequencies) to −0.1 (optically thin regime at high frequencies; e.g. Aaquist & Kwok 1991). However, Suárez et al. (2015) showed that the young PN IRAS 15103 –5754 presents non-thermal radio continuum emission, with α ≃ −0.54, compatible with synchrotron radiation. Several other possible PNe also present some evidence of non-thermal emission (Cerrigone et al. 2011, 2017). The detection of non-thermal radio continuum emission in a PN could be an indication that it is in a nascent stage, since the free-free radiation in more evolved PNe would completely veil this non-thermal continuum emission.

IRAS 22568+6141 (PNG110.1+01.9, α(2000) =22h58m51.6s, δ(2000)= +61º57′43.5″; hereafter IRAS 22568) could be one such nascent PNe. The optical spectrum presented by García Lario et al. (1991, hereafter GL+91) shows emission lines that are compatible with a low-excitation PN, and it is characterised by relatively prominent [N II]] and absent [O III] emission lines. The radio continuum emission reported by Cerrigone et al. (2011, 2017) suggests a negative spectral index (α ≃ −0.35), which was confirmed by Gómez et al. (in prep.) with quasi-simultaneous observations at different frequencies. Figure 1 presents the Hubble Space Telescope (HST) image of IRAS 22568 in the F606W filter. The nebula is bipolar and extends over ≃8” along the main axis, which is oriented at a position angle PA ≃ −30º. The bipolar lobes are separated by a dark lane of width ≃0″.25, where no optical emission is observed, and which seems to obscure the central star. Radio continuum observations by Cerrigone et al. (2017) at 8.4 GHz resolved the two lobes and showed emission from a central core between the lobes.

As part of a project to identify, confirm, and characterise new nascent PNe with non-thermal radio continuum emission, we obtained intermediate- and high-resolution long-slit spectra of IRAS 22568 at different epochs. In this paper, we show that the source presents a peculiar variability on timescales of decades and a few years, as well as an internal kinematics that is not usually seen in PNe. The data presented here are crucial for understanding the nature of this possible nascent PN.

thumbnail Fig. 1

HST (F606W) image of IRAS 22568+6141 with the long-slit position used for our spectra represented in blue (width = 2″, PA −30°), and the spectra used by GL+91 represented in red (width =1″.5, PA −40°).

2 Observations and data reduction

2.1 Intermediate-resolution long-slit optical spectroscopy

We used the Calar Alto Faint Object Spectrograph (CAFOS) at the 2.2 m telescope of the Calar Alto Observatory (Spain) on 2021 August 2 (proposal ID: F21-2.2-0.17; PI: R.A. Cala), and 2023 November 17 (proposal ID: DDT23B.320; PI: R. A. Cala) to obtain intermediate-resolution long-slit spectra on IRAS 22568. In Fig. 1 we show with blue lines the long slit (width = 2″.0) oriented at a PA −30° along the major nebular axis. In both epochs, the sky was photometric during the observations, with a mean seeing of 2″.1, and in both epochs, we observed the standard star BD+28°4211 for the flux calibration. The spectra were recorded in a SITe CCD with 2048 × 2048 pixel2. The spatial scale in the detector is 0.53 arcsec pixel−1. In 2021 and 2023, we obtained four and three spectra using grism B-100 (3200-6200 Å) with 1800 s exposure time each, and we combined them into a single spectrum for each epoch. In each epoch, we also obtained one spectrum with grism R-100 (5800–9600Å) and 1800 s of exposure time. The spectra were calibrated following standard procedures in IRAF. We obtained a dispersion of ~2 Å pixel−1 after the wavelength calibration. PyNeb (version 1.1.19; Luridiana et al. 2015) was used to obtain the physical properties of the ionised gas, and Table A.1 lists the atomic constants we employed.

2.2 High-resolution long-slit optical spectroscopy

High-resolution long-slit spectroscopy of IRAS 22568 was conducted on 2023 August 5–7 using the Manchester Echelle Spectrometer (MES) at the 2.1 m telescope on the San Pedro Mártir Observatory (Mexico; proposal ID: FM-15; PI: R. Vázquez). The long slit (2” width) was oriented at a PA −30° (Fig. 1). The spectra were recorded in an E2V-4240 CCD with 1024 × 1024 pixel2 and 4 × 4 binning, resulting in a spatial scale of 0″.704 pixel−1. Spectra of the Hα, [N II]] λ6584, [S II]] λ6730, and [O III] λ5007 emission lines (hereafter [N II]], [S II]], and [O III]) were obtained with exposure times of 1800, 1800, 3600, and 3600 s, respectively. A Th-Ar lamp spectrum was obtained immediately after each source spectrum for the wavelength calibration. The spectra were reduced with IRAF. The average seeing during the observations was ~1″.8, and the spectral resolution is ~11 km s−1, as indicated by the full width at half maximum (FWHM) of the emission lines in the ThAr spectra.

3 Results

3.1 Spectral variability and physical properties of the ionised gas

Fig. 2 shows the intermediate-resolution spectra of IRAS 22568 integrated over its two bipolar lobes. Fig. 2 and Fig. 3 in GL+91 immediately reveal significant differences between the 1988, 2021, and 2023 spectra. Although our spectra resolve the two bipolar lobes of the object, we used the observed emission line fluxes in the integrated spectra for the analysis for a better comparison between the spectra at the three epochs. The observed fluxes in 1988 can be found in GL+91 (their Table 2, column 3), and those in 2021 and 2023 are listed in Table B.1.

We used the interstellar extinction law of Seaton (1979) to obtain the logarithmic extinction coefficient at Hβ, c(Hβ), and hereby, the intrinsic emission line intensities in the three epochs. The c(Hβ) was derived from the Hα and Hβ emission lines and a theoretical Hα/Hβ line intensity ratio of 2.85. We note that a similar c(Hβ) is obtained from the Paschen 9 (P 9) and Hβ emission lines, and a theoretical P 9/Hβ line intensity ratio of 18.4×10−3. The intrinsic line intensities, Hβ flux, and c(Hβ) at the three epochs are listed in Table 1. The differences between the intrinsic line intensities in 1988 obtained by us (listed in our Table 1, Col. 4) and those obtained by GL+91 (listed in their Table 2, Col. 4) are not significant and are due to the different extinction law employed.

The most astonishing result is the presence in 2021 of the [O III] / 4363,4959,5007 emission lines in the nebula with I([O III])/I(Hβ)≃1.7, which were absent in 1988 (GL+91). Based on the intensity of the weakest emission line detected in the spectrum from 1988 (I(P 10)/I(Hβ)≃0.016; Table 1) as an upper limit to the detection of the [O III] lines, they have emerged between 1988 and 2021 or, at least, their intensity has increased by more than two orders of magnitude in that time span. The subsequent variability of the [O III] λ4959,5007 emission lines is also impressive. Their intensity relative to Hβ has decreased by a factor of ≃3 between 2021 and 2023, while the [O III] λ363 line is not detected in the last epoch.

Other emission properties of the ionised gas have also substantially changed in IRAS 22568. Table 1 shows that the Hβ flux increased by a factor ≃6 between 1988 and 2021. This value should be taken with some caution because the long slits used in 1988 and 2021 do not cover exactly the same regions of the object (Fig. 1), and extinction (see below) and sensitivity of the observations were different. However, the strong increase in the Hβ flux between 1988 and 2021 should be considered real. Between 2021 and 2023, the Hβ flux decreased by a factor of ≃1.7, and we note that these two spectra were obtained under almost identical conditions (Sect. 2.1).

The value of c(Hβ) (Table 1) decreased by factors of ≃1.06 between 1988 and 2021, and by factors of ≃1.8 between 2021 and 2023. Given that the nebular reddening E(B–V) and the nebular extinction Av are related by c(Hβ)=1.45 × E(B–V), and c(Hβ)=0.47 × Av (using the Seaton 1979, extinction law), we obtain Av(1988) = 7.88±0.34 mag, Av(2021) = 6.48±0.10 mag, and Av(2023) = 3.53±0.10 mag. Therefore, the nebular extinction of IRAS 22568 decreased 1.40±0.35 mag between 1988 and 2021, and 2.95±0.10 mag between 2021 and 2023.

Other emission lines present a variable intensity relative to Hβ between 1988 and 2023. For the very weak emission lines (e.g., Hγ, Hδ, [Cl II]], and [Fe II]]) that were not present in the 1988 spectrum, their detection in 2021 and/or 2023 may be due to a combination of an increased nebular flux and better sensitivity in our spectra. The variability in the [S II]], [O I], [O II]], and He I emission lines can be considered real because these lines are relatively strong and were detected in all three epochs. Moreover, an inspection of the 2021 and 2023 line intensities reveals that IRAS 22568 has recombined in this time span. This is directly recognizable in (a) the decrease in the O2+, O+, and S2+ line intensities (relative to Hβ) in favour of those of O+, O0, and S+, respectively, (b) the decreasing intensity in the Ar2+ emission lines, (c) the increasing intensity in the Fe+ and Ni+ lines, and (d) the decrease in the Balmer emission intensities. The [N II]]26548,6583 emission lines increased slightly in their relative intensities between 2021 and 2023, probably due to recombination from N2+, which proceeds faster than recombination from N+ to N0. In contrast to the line intensity observed in the nebular [N II]] lines, the auroral [N II]] 25755 line intensity clearly decreased between 2021 and 2023, implying that the nebula also cooled.

To calculate the electron temperature (Te), we used the [N II]](6584+6548)/5755 and [O II](4959+5007)/4363 emission lines ratios, while the electron density (Ne) was obtained from the [S II]](6716/6731) line intensity ratio. These values were obtained making use of CROSSTEMDEN in PyNeb. The results are listed in Table 2. The values of Te([N II]]) are consistent with an increase in the excitation between 1988 and 2021, and with cooling between 2021 and 2023. In 2021, when the [O II]] λ4363 was detected, we obtained Te([O III]) ≃ 38400 K and Te([N II]]) ≃ 17 300 K. Te([O III]) is anomalously high and very much higher than Te([N II]]). These values can hardly be explained with photoionisation, but clearly indicate shockexcita-tion. This is discussed in detail in Sect. 4.1. In 2023, we obtained an upper limit for the [OIII]λ4363 line intensity (Table B.1) that prevented us from deriving Te([O III]), while the [OIII]lines were not detected in 1988.

As for the electron density (Table 2), the uncertainties in the line intensities in 1988 do not allow us to obtain an upper limit of Ne, suggesting that the nebula could have been in a high-density regime (see GL+91). On the other hand, the nominal values of Ne agree with each other within the errors. We note that the values of Ne are averaged values that were integrated along the line of sight. The knotty structure of IRAS 22568 suggests that Ne may present a relatively wide range of values throughout the nebula (see e.g. Lee et al. 2022; Hyung et al. 2023).

With the values obtained for Ne (Table 2), we can estimate the recombination timescales, assuming a recombination rate per ion Ri ≃ 105 yr/(Z2 Ne), where Z is the ionic charge (Osterbrock 1989). The recombination timescales of single- and double-ionised species are 8–12 and 2–3 yr, respectively, which are consistent with the fact that the [OIII]emission lines have almost vanished between 2021 and 2023, while the Balmer lines show a more moderate decrease in intensity between these two epochs.

thumbnail Fig. 2

Intermediate-resolution optical spectra of IRAS 22568+6141 obtained at the Calar Alto Observatory in 2021 (blue) and 2023 (red), normalised to the peak flux of Hβ. The indicated [O III] λ4959,5007 emission lines were not detected in 1988 (see the text).

3.2 Light curve

In order to further investigate the optical variability of IRAS 22568, we searched for available photometric broadband r-filter observations in the literature and public surveys. We found six photometric values for the r magnitude from 1953 to 2020, which are reported in Table 3. However, the characteristics of the filters are not the same in the six epochs. The Panoramic Survey Telescope and Rapid Response System (PanSTARRS) filter is similar to that of GL+91, and we therefore transformed the magnitudes of the Sloan Digital Sky Survey (SDSS) to the PanSTARRS system magnitude (see Chambers et al. 2019). The filter of the Zwicky Transient Facility (ZTF) is broader and its transmission is higher than the other filters, thus the magnitude from 2019.5 should be seen as a lower limit to the r magnitude measured in PanSTARRS. Finally, the Palomar Observatory Sky Survey first (POSS-I) and second (POSS-II) filters are narrower than the other filters, and their r magnitudes should be considered as upper limits to the r magnitude obtained in PanSTARRS.

Figure 3 shows the r light curve of IRAS 22568. Despite the limited temporal sampling and the lower and upper limits at some epochs, some trends emerge. The r magnitude seems to have decreased ≥0.54 mag between 1953 and 1990.5, suddenly rose ≥ 1.5 mag between 1990.5 and 1991.7, and it steadily decreased by ≥1 mag between 1991.7 and 2019.5. Our 2021 and 2023 spectra are consistent with this decrease (Table 1). Moreover, the flux density of the radio continuum emission also faded between 2005 and 2012 (Cerrigone et al. 2017).

Table 1

Intrinsic line intensities (I(Hβ)=100) in IRAS 22568.

Table 2

Electron temperature and electron density of the ionised gas in IRAS 22568 in the three epochs.

Table 3

Multi-epoch broadband r-filter photometry on IRAS 22568.

thumbnail Fig. 3

Light curve of IRAS 22568 in the r filter. The arrows represent lower or upper limits. See the text for details.

3.3 Internal kinematics

Fig.4 displays position-velocity (PV) maps of the Hα, [N II]], [S II]], and [OIII]emission lines derived from the high-resolution long-slit MES spectra obtained in 2023.68, which are close in time to the intermediate-resolution spectra from 2023.88. The MES spectra spatially resolve the north-west (NW) and southeast (SE) lobes of IRAS 22568. The emission lines arise in two relatively compact regions and are characterised by a large velocity width that reaches up to 450 and 380 km s−1 in Hα and [N II], respectively, as measured at the 3σ level above the background (white contours in Fig. 4). The [S II]] and [O III] emissions are much fainter than those of Hα and [N II]], although they seem to present a similar profile. Figure 5 presents the emission line profiles as obtained by integrating the emission of each lobe in a region of ≃2″ centred on the emission peaks. Despite the differences in S/N, the [S II]] and [OIII]emission lines also exhibit large velocity widths that seem to be comparable to those observed in Hα and [N II].

In Table 4, we list the spectral, spatial, and emission properties of the NW and SE lobes derived from the PV maps (Fig. 4). We measured the radial velocity of the emission peaks (Vpeak), the observed minimum (Vmin) and maximum (Vmax) radial velocities in the integrated spectra (see above) at the 10% level of the intensity peak (FW0.1; which avoided the insertion of noise in the [S II]] and [OIII]emission lines and allowed us to obtain a coherent value from the four emission lines), the angular separation between the emission peaks of the two lobes as obtained from Gaussian line fits to the spatial intensity distribution at the position of the intensity peaks, and the relative intensity of the emission peaks. Throughout this paper, radial velocities are quoted with respect to the local standard of rest (LSR).

The emission peaks are separated by ≃37 km s−1, and the NW lobe is redshifted with respect to the SE lobe (Table 4). The centroid radial velocity of the two emission peaks is very similar in the four emission lines, and the peak values are included in Table 4; their mean value is −93±1 km s−1, which we considered the systemic velocity of the optical nebula. It is similar to but somewhat higher than the value of −85±2 km s−1 obtained by Sánchez-Contreras et al. (2012) from the CO emission and also higher than the value of −80 km s−1 obtained by GL+91, perhaps because of the different spectral resolution. Secondary peaks are observed in the emission features, but they are more clearly detected in the SW than in the NW lobe.

The FW0.1 in Hα and [N II]] is about 200 km s−1 in the NW lobe and about 170 km s−1 in the SE lobe. In [S II]] and [O III], the values of FW0.1 are similar in both lobes and are ≃173 and ≃147 km s−1, respectively. It should be noted that the observed differences in FW0.1 are probably due to the different S/N ratio in the four emission lines.

The angular separation between the intensity peaks (Table 4) is different in each emission line. However, given the errors in the [S II]]and [OIII]emission lines and that the line profiles of Hα and [N II]] are very well defined, the only conclusion is that the angular separation between Hα and [N II]] is different, while the separation in [S II]] and [OIII]seems to be similar to that of Hα. Finally, the SE lobe is stronger than the NW lobe in Hα and [N II]], the opposite is observed in [O III], and both lobes present a similar intensity in [S II]] (Table 4).

thumbnail Fig. 4

Position-velocity maps of the Hα, [N II]], [S II]], and [OIII]emission lines obtained at a position angle PA–30 in 2023.68. The increment step of the contours is 3σ ×2n, starting from n = 0 (white). The SE lobe and NW lobe are indicated.
thumbnail Fig. 5

Normalised line profiles of the Hα, [N II]], [S II]], and [OIII]emission lines of the NW and SE lobes. The full width at 10% of the peak intensity (FW0.1) is shown as a horizontal dotted line.

4 Discussion

4.1 Shock-excited emission from IRAS 22568

As described above, the electron temperatures obtained in 2021 are incompatible with photoionisation and suggest shock excitation. In order to understand the origin of the line emission, we made use of the Mexican Million Models Database (3MdB; Morisset et al. 2015), which allowed us to analyse whether the line intensities are consistent with shocks or photoionisation. In Appendix C we explain the setup for the shock and photoionisation models employed below.

In Fig. 6, we show the location of shock and photoionisation models in a Te−Te diagram (namely, an [O III]4363/5007 vs. [N II]]5755/6584 diagram) and the location of IRAS 22568 in 2021 (red star). We did not search for the exact models that reproduce the observed line ratio, but used the maximum number of models to define the whole region in which they are located in the Te (O III)−Te(N II) diagram relative to the type of ionisation. While the [N II]] 5755/6584 line intensity ratio can easily be reproduced by photoionisation and shock models, the high value of the [O III]4363/5007 line intensity ratio can only be reproduced by shock models. The highest ratio obtained by photoionisation models is still 0.25 dex below the observed value.

If shocks were present in 2021, Te could not be determined in the classical manner, and the values listed in Table 1 should be considered with caution. The determination of Te requires detailed models involving shocks (see e.g., Montoro−Molina et al. 2022), and we defer this to a future paper. We also note that if shock excitation exists, the Hα/Hβ theoretical ratio for determining c(Hβ) could be higher than the assumed value of 2.85. Thus, we calculated c(Hβ) and intrinsic emission line intensities for an Hα/Hβ ratio of 3.1, but we obtained values that are very similar to and within the errors of those in Table 1.

The shocks observed in 2021 are most probably the mechanism that causes the large velocity widths observed in the PV maps from 2023 (Fig. 4). Emission lines with large velocity widths are well known in Herbig−Haro objects that are associated with young stars (e.g. Strom et al. 1974; Dopita 1978; Raga & Böhm 1986; Hartigan et al. 1987). It is well established today that these types of emission features trace bow shocks that are associated with high−velocity jets or bullets and their interaction with the surrounding medium.

Based on the radiative bow−shock models by Hartigan et al. (1987), the shock velocity and viewing angle of the NW and SE lobes (Fig. 4) can be determined in a very simple way. We concentrated on the Hα and [N II]] data, which have a better quality, and we note that the [S II]], and [O III] data provide compatible results. The shock velocity is the difference between the observed Vmin and Vmax in Table 3. In the NW lobe, the bow-shock velocity and viewing angle with respect to the observer are ≃195 km s−1 and ≃106±2° with respect to the line of sight, respectively; in the SE lobe, the corresponding values are ≃170 km s−1 and ≃64±3°. Using the radial velocity of the intensity peaks (with respect to the systemic velocity) as representative of the velocity of the bow shocks, we obtained velocities of ≃70 and ≃45 km s−1 in the NW and SE lobe, respectively.

We tried to identify the nebular features in the HST image (Fig. 1) that might be associated with the bow shocks, but IRAS 22568 shows many knots and curved filaments, and, given the small angular size of the lobes, which is comparable to the spatial resolution of the MES spectra, it is difficult to clearly determine an association. On the other hand, we cannot rule out that the spectral features are an overlapping of several bow shocks that are associated with different knots and filaments in the bipolar lobes. If this were so, the viewing angle and expansion velocities obtained above might represent averaged values for each lobe, weighted by the contribution of each individual bow shock. This may explain the differences in expansion velocity and tilt angle of each lobe, without implying that the lobes have a different orientation.

The comparison of the Hα and [N II]]PV maps shown in Fig. 4 with those by GL+91 (their Fig. 4) is revealing. Despite the differences in spatial and velocity resolution between the two sets of PV maps, those by GL+91 show the emission peaks of each lobe, a secondary peak in the SE lobe, and a velocity width of ≃350–380 km s−1 at the spatial position of the intensity peaks, as measured at the weakest contour level. These results agree very well with ours, suggesting that bow-shock excitation might have been present in 1988. Moreover, our high-resolution spectra include a recombination (Hα), a high-excitation ([O III]), and two low-excitation ([N II]]and [S II]]) emission lines, and in all four cases, the emission features are compatible with bow-shock excitation. Therefore, it is possible that the optical spectrum of IRAS 22568 in the three epochs is dominated by shocks. This casts doubt on the classification of IRAS 22568 as a low-ionisation PN. Nevertheless, it might be premature to conclude that IRAS 22568 is a post-AGB nebula because photoionised gas could be masked by shocks, which are the dominant excitation mechanism. Furthermore, as an additional result, the existence of shocks might be intimately related with the non-thermal radio continuum emission from IRAS 22568. Finally, it is worth to emphasize that in addition to the Te-sensitive line ratios in 2021 (Fig. 6), high-resolution spatially resolved spectroscopy has been crucial to determining the excitation mechanism in 2023: without this type of data, post-AGB nebula with shock-excited emission lines could be confused with low-ionisation young PNe when observed at low or intermediate spectral resolution.

IRAS 22568 belongs to a small group of post-AGB nebulae and PNe that present emission features with large velocity widths (typically ≥ 150km s−1) that are characteristic of bow-shock excitation. By means of high-resolution optical spectroscopy, large velocity widths have been detected in the PNe Hen2-111 (Meaburn & Walsh 1989), Ml-16 (Schwarz 1992; Gómez-Muñoz et al. 2023), KjPn8 (López et al. 1995), M2-48 (Vázquez et al. 2000; López-Martín et al. 2002), and IRAS 18061–2505 (Miranda et al. 2021). Other PNe present bow-shaped structures that were successfully reproduced using bow-shock models, although in these cases, the structures did not exhibit such large velocity widths (typically 40–90 km s−1), and they were irradiated by the central star; examples include K4-47, IC4634, and Hul-2 (Gonçalvez et al. 2004; Guerrero et al. 2008; Miranda et al. 2012; Fang et al. 2015, see also Mari et al. 2023 for more similar PNe). Similar emission features were observed in the post-AGB nebulae M 1-92 (Solf 1994), OH 231.8+04.2 (Sánchez-Contreras et al. 2000), Hen 3-1475 (Riera et al. 2003), M2-56 (Sánchez-Contreras et al. 2010). The reason for the scarcity of large velocity widths associated with jets in PNe is unclear, although it could be related to photoionisation being the dominant excitation mechanism, which (partially) hides the shock-excited emission, and/or to a weakness of the shock contribution with time.

Table 4

Properties of the Hα, [N II]], [S II]], and [O II]] emission lines in the high−resolution spectra.

thumbnail Fig. 6

Location of models from the 3MdB database (see text for details) in an [O III]4363/5007 vs. [N II]]5755/6584 diagram. The top panel shows models of the PNe, and the bottom panel shows shock and shock+precursor models. The colour code for the models corresponds to the Hα/Hβ line intensity ratio. The red star shows the location of the 2021 observations of IRAS 22568.

4.2 Origin of the variability

The r light curve (Fig. 3) shows that IRAS 22568 underwent an energetic event in around 1990, which increased its brightness. The consequences of this event are perceptible until today. The radio continuum emission from the nebula has been fading since at least 2005 (Cerrigone et al. 2017), and according to our spectra, IRAS 22568 recombined and cooled between 2021 and 2023. Moreover, we consulted the Spitzer Heritage Archive and found spectra of IRAS 22568 from 2005.95, which are shown in Fig. 7. These spectra display, among others, the [Ne III]15.6µm emission line, which indicates that the nebular excitation was higher in 2006 than in later epochs (2021 and 2023), when the [O III] emission lines were relatively weak. Consequently, it is quite possible that the [O III] lines were stronger in 2006 than in 2021, and that IRAS 22568 recombined and cooled since 2006 and even earlier. Finally, the spectral and kinematical properties provide clear evidence that the nebular emission was dominated by shocks in 2021 and 2023 and, perhaps, in 1988.

The variability of IRAS 22568 could be due to changes of the physical conditions in the shocks. New ejections could have reached and interacted with the lobes by 1990. Similarly, already ejected material could encounter high-density regions in the lobes and be decelerated by that date. In both cases, a rapid increase in Te and Ne is expected, resulting in an increase in the emission line fluxes and excitation. After the interaction weakens or ceases, recombination and cooling in the post-shock region would lead to a decrease in the excitation and brightness of the nebula, as observed since 2005–2006. Numerical simulations are necessary to confirm whether this scenario can account for the detailed observed variability and the involved timescales and if the physical conditions required for it to occur are reasonable for PNe. We note that if the variability is only due to changes in the shocks, IRAS 22568 would be a post-AGB nebula or proto-PN similar to M1-92, for example, rather than a PN.

Alternatively, a nova-like eruption might have caused the sudden increase in the brightness of IRAS 22568. This type of event is typical of binary systems hosting an AGB star and an accreting white dwarf. These symbiotic binaries are in a different evolutionary phase than post-AGB stars and PNe. To determine whether IRAS 22568 might be related to this type of binaries, we built its spectral energy distribution (SED; see Appendix D), which is shown in Fig. D.1. The SED presents an emission peak at λ ≃ 30 µm, which is typical of optically obscured post-AGB stars and young PNe (e.g. Ramos-Larios et al. 2009, 2012), but different from that of symbiotic binaries, where the peak of the SED usually lies at λ ≃ 1 µm (e.g. Rodríguez-Flores et al. 2014). Although more observations are necessary, the SED of IRAS 22568 does not seem to favour a nova scenario as the cause of its variability.

Finally, a third possibility is that a late thermal pulse (LTP) occurred around 1990, as is suggested by the remarkable similarities between the properties and temporal evolution of IRAS 22568 and those of the Stingray nebula (Hen 3-1357), whose central star, SAO 244567, was proposed to undergo an LTP (Reindl et al. 2014, 2017). The light curves of IRAS 22568 (Fig. 3) and Hen 3-1357 (Schaefer et al. 2015) are similar. In both objects, the spectral variability of the optical emission lines is consistent with cooling and recombination during decades (see Sect. 3.1 and Arkhipova et al. 2013; Peña et al. 2022; Balick et al. 2021). Their radio continuum emission was characterised by a fading of the flux density in the past decades and by negative spectral indices indicating non-thermal emission (Cerrigone et al. 2017; Harvey-Smith et al. 2018). As described above, non-thermal emission and shocks appear to be related to each other in IRAS 22568 (Sect. 4.1). The same may hold for Hen 3-1357, where shocks were suggested to occur along the nebularperime-ter (Balick et al. 2021). Similarly, a relation between shocks and non-thermal radio continuum emission was observed after the (very late) thermal pulse occurred in Sakurai’s object (Hajduk et al. 2024). Moreover, if an LTP in IRAS 22568 generated a shock wave that propagated into the previous material, all the knots and filaments would be shocked, increasing the nebular brightness, excitation, and the subsequent recombination and cooling. Nevertheless, in the LTP scenario, the situation may be more complicated. The increase in the effective temperature of the central star after the LTP (Lawlor 2023, and references therein) would provide an additional source of ionising photons, and the emission line spectra of IRAS 22568 would be produced in a combination of shocks and photoionisation, each presenting its own timescale of variability, depending on the evolution of the central star and nebula. It is interesting to note that the Spitzer spectra (Fig. 7) display broad emission features that can be attributed to dust made of polycyclic aromatic hydrocarbons (PAHs), which might be fresh byproducts of the LTP and indicate carbon-rich material in the nebula. Finally, if the LTP scenario is correct, IRAS 22568 would indeed be a nascent PN.

In contrast with SAO 244567, which is visible and whose properties and evolution have been studied for decades (Henize 1976; Parthasarathy et al. 1993, 1995; Reindl et al. 2014, 2017), the central star of IRAS 22568 has never been observed, and its current evolutionary stage is unknown. In particular, within the LTP scenario, we cannot assert whether the central star of IRAS 22568 increases its effective temperature or returns to the AGB (see Hajduk et al. 2020). Millimeter observations might elucidate whether the molecular abundances of IRAS 22568 are similar to those in other sources in which an LTP seems to have occurred (e.g., Schmidt et al. 2018). In addition, a photometric and spectroscopic monitoring in the coming years and modelling of the data are key to trace the evolution of IRAS 22568 and confirm the origin of the variability.

thumbnail Fig. 7

Spitzer spectra of IRAS 22568 obtained on 2005.95. The forbidden emission lines of ionised gas ([Ar II]]λ7.0µm, [Ar III]λ9.0µm, [Ne II]]λ2.8µm, and [Ne III]λ15.55µm) and broad emission of PAHs (6.4, 7.7, 8.6 and 11.3 µm) are indicated.

5 Conclusions

IRAS 22568 has been classified as a low-excitation PN and shows non-thermal radio continuum emission, suggesting that it is a nascent PN. We performed intermediate-resolution optical spectroscopy in 2021 and 2023 for this source and high-resolution spectroscopy in 2023, and we gathered a discrete r-filter light curve from 1953 to 2020.

It is clear that an energetic event occurred in IRAS 22568 around 1990 that manifested itself as a sudden and rapid increase in the nebular brightness. Afterwards, the brightness decreased much more slowly. Compared with a published spectrum from 1988, our spectrum of 2021 shows a noticeable increase in the emission line fluxes and the [O III] emission lines, which were not detected in 1988. Two years later, in 2023, the emission line fluxes decreased and the [O III] emission lines almost vanished. The observed spectral variability between 2021 and 2023 shows that IRAS 22568 is recombining and cooling, and these phenomena have probably occurred since at least 2005, as indicated by the reported fading of the radio continuum emission and archival mid-IR spectra around that epoch.

Some emission line ratios in 2021 reveal that the spectrum is excited by shocks. Moreover, the large velocity widths (~400 km s−1) observed in the high-resolution 2023 spectra are compatible with bow-shock excitation, which may also have been present in 1988. Our data suggest a connection between shock excitation and non-thermal radio continuum emission. Nevertheless, we cannot exclude that photoionisation has contributed to the spectra at least since 2006.

Changes in the physical conditions in the shocks might explain the observed variability, but modelling is necessary to confirm this scenario. A nova-like eruption is another possibility, although this is not favoured by the SED of IRAS 22568. Finally, a late thermal pulse in the central star of IRAS 22568 provides a simple and consistent explanation for the energetic event in 1990 and for the subsequent photometric and spectral evolution of the object. We found noticeable similarities between the variability of IRAS 22568 and that of the Stingray nebula, for which the variability was interpreted as caused by a late thermal pulse.

Acknowledgements

We are very grateful to our anonymous referee for his/her constructive comments that have improved the paper. We thank Calar Alto Observatory for allocation of director’s discretionary time to this programme. This research is based on observations collected at the Centro Astronómico Hispano en Andalucía (CAHA) at Calar Alto, operated jointly by Junta de Andalucía and Consejo Superior de Investigaciones Científicas (IAA-CSIC); upon observations at the Observatorio Astronómico Nacional on the Sierra San Pedro Mártir (OAN-SPM), Baja Califormia, México; on data from the program 9463 of the NASA/ESA Hubble Space Telescope obtained from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555; on data from the VLA and the National Radio Astronomy Observatory is a facility of the National Science Foundation operated under cooperative agreement by Associated Universities, Inc. IRAF used here is distributed by the National Optical Astronomy Observatories, which is operated by the Association of Universities for Research in Astronomy, Inc., under contract to the National Science Foundation. The POSS-II were produced at the Space Telescope Science Institute under U.S. Government grant NAG W-2166. The images of these surveys are based on photographic data obtained using the Oschin Schmidt Telescope on Palomar Mountain and the UK Schmidt Telescope. The plates were processed into the present compressed digital form with the permission of these insti-tutions.The Guide Star Catalogue-II is a joint project of the Space Telescope Science Institute and the Osservatorio Astronomico di Torino. Space Telescope Science Institute is operated by the Association of Universities for Research in Astronomy, for the National Aeronautics and Space Administration under contract NAS 5-26555. The participation of the Osservatorio Astronomico di Torino is supported by the Italian Council for Research in Astronomy. Additional support is provided by European Southern Observatory, Space Telescope European Coordinating Facility, the International GEMINI project and the European Space Agency Astrophysics Division. The PS1 and the PS1 public science archive have been made possible through contributions by the Institute for Astronomy, the University of Hawaii, the Pan-STARRS Project Office, the Max-Planck Society and its participating institutes, the Max Planck Institute for Astronomy, Heidelberg and the Max Planck Institute for Extraterrestrial Physics, Garching, The Johns Hopkins University, Durham University, the University of Edinburgh, the Queen’s University Belfast, the Harvard-Smithsonian Center for Astrophysics, the Las Cumbres Observatory Global Telescope Network Incorporated, the National Central University of Taiwan, the Space Telescope Science Institute, the National Aeronautics and Space Administration under Grant No. NNX08AR22G issued through the Planetary Science Division of the NASA Science Mission Directorate, the National Science Foundation Grant No. AST-1238877, the University of Maryland, Eotvos Lorand University (ELTE), the Los Alamos National Laboratory, and the Gordon and Betty Moore Foundation. Based on observations obtained with the Samuel Oschin 48-inch Telescope at the Palomar Observatory as part of the ZTF project. ZTF is supported by the National Science Foundation under Grant No. AST-1440341 and a collaboration including Caltech, IPAC, the Weizmann Institute for Science, the Oskar Klein Center at Stockholm University, the University of Maryland, the University of Washington, Deutsches Elektronen-Synchrotron and Humboldt University, Los Alamos National Laboratories, the TANGO Consortium of Taiwan, the University of Wisconsin at Milwaukee, and Lawrence Berkeley National Laboratories. Operations are conducted by COO, IPAC, and UW. This work is based [in part] on archival data (ID: 20590, PI: R. Sahai) obtained with the Spitzer Space Telescope, which was operated by the Jet Propulsion Laboratory, California Institute of Technology under a contract with NASA. Support for this work was provided by NASA. This work has made use of the SIM-BAD database, operated at the CDS, Strasbourg, France, and the NASA/IPAC Infrared Science Archive, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration. It also makes use of data products from 2MASS (a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by NASA and the NSF), AKARI (a JAXA project with the participation of ESA), HERSCHEL (Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator consortia and with important participation from NASA), IRAS (was a joint project of the US, UK and the Netherlands), MSX (funded by the Ballistic Missile Defense Organization with additional support from NASA Office of Space Science), and WISE (a joint project of the University of California, Los Angeles, and the Jet Propulsion Laboratory/California Institute of Technology, funded by the NASA). RAC, LFM, JFG are supported by grants PID2020-114461GB-I00, PID2023-146295NB-I00, and CEX2021-001131-S, funded by MCIN/AEI/10.13039/501100011033, and by grant P20-00880, funded by the Economic Transformation, Industry, Knowledge and Universities Council of the Regional Government of Andalusia and the European Regional Development Fund from the European Union. RC also acknowledges support by the predoctoral grant PRE2018-085518, funded by MCIN/AEI/ 10.13039/501100011033 and by ESF Investing in your Future. CM acknowledges the support of UNAM/DGAPA/PAPIIT grants IN101220 and IG101223.

Appendix A Atomic datasets used in PyNeb

Table A.1 shows the atomic datasets used for collisionally excited lines in the PyNeb calculations presented in this paper.

Table A.1

Atomic datasets used for collisionally excited lines in the PyNeb calculations.

Appendix B Emission line fluxes of IRAS22568 in the three epochs

In Table B.1 we report the emission line fluxes in 1988, 2021, and 2023. The emission line fluxes in 1988 are taken from GL+91 (their Table 2, column 4).

Table B.1

Absolute emission line fluxes F(λ) in units of 10−15 erg cm−2 s−1 on IRAS22568 in the three epochs.

Appendix C Parameters of the shock and photoionisation models in the 3MdB

C.1 Shock models

The shock models are taken from the work of Alarie & Moris-set (2019) who computed models using Mappings V (Sutherland & Dopita 2017) and store them in the 3MdB (Alarie & Moris-set 2019). These models use the same grid definition as Allen et al. (2008). We consider here the 15968 models that correspond to the “shock” and the “shock+precursor” results from the “Allen08” shock family, the precursor being the region pho-toionised by the photons emitted by the shock itself, for velocities higher than 100 km s−1 (at lower velocities no ionising photons are produced by the shock). This preionisation is treated by Mappings in a fully consistent way and is described in detail by Sutherland & Dopita (2017). The free parameters of the grid are the metallicities, the pre-shock density, the shock velocity and the magnetic field. The details of the parameter distribution are given in Alarie & Morisset (2019).

C.2 Photoionisation models

The PN photoionisation models have been computed by Delgado-Inglada et al. (2014) using the multi-purpose transfer code CLOUDY vl7 (Ferland et al. 2017). These models are taken from the 3MdB database, using the reference ref LIKE “PNe_202_”. We filter the models using the com6=1 AND MassFrac > 0.7 commands. The first one is to only consider the realistic PNe models as defined in Sect. 2.1 of Delgado-Inglada et al. (2014) and in the grid model webpage1. The second filter is used to only select the radiation-bonded models, as well as the matter-bounded models in which the mass of the nebula is greater than 70% of the corresponding radiation-bounded case. The input parameters of the grid are: the nebular density and metallicity, the luminosity and effective temperature of the ionising spectral energy distribution (which can be a Planck function or a stellar atmosphere model), the presence or absence of dust, the distance between the central ionising source and the inner radius of the nebula. This leads to 68041 models.

thumbnail Fig. D.1

SED of IRAS22568. The size of the circles is larger than the errors.

Appendix D Spectral energy distribution of IRAS22568

We have gathered available archival optical, infrared, millimeter, and centimeter data (~0.5–2×105 µm), and have built the SED of IRAS22568 (Fig. D.1). Infrared data were obtained from the Two Micron All Sky Survey (2MASS), AKARI, Herschel Space Observatory, Infrared Astronomical Satellite (IRAS), Midcourse Space Experiment (MSX), and Wide-field Infrared Survey Explorer (all-WISE survey). The flux density at 0.1 and 2.6 mm were taken from Marton et al. (2024) and Sanchez -Contreras et al. (2012), respectively. The flux density of the optical r-magnitude corresponds to the average value of those presented in the Table 3. Similarly, the radio flux density values are the average values reported by Cerrigone et al. (2017) at 4.8 and 8.6 GHz, while the value at 1.4 GHz was reported by Condon et al. (1998).

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All Tables

Table 1

Intrinsic line intensities (I(Hβ)=100) in IRAS 22568.

In the text
Table 2

Electron temperature and electron density of the ionised gas in IRAS 22568 in the three epochs.

In the text
Table 3

Multi-epoch broadband r-filter photometry on IRAS 22568.

In the text
Table 4

Properties of the Hα, [N II]], [S II]], and [O II]] emission lines in the high−resolution spectra.

In the text
Table A.1

Atomic datasets used for collisionally excited lines in the PyNeb calculations.

In the text
Table B.1

Absolute emission line fluxes F(λ) in units of 10−15 erg cm−2 s−1 on IRAS22568 in the three epochs.

In the text

All Figures

thumbnail Fig. 1

HST (F606W) image of IRAS 22568+6141 with the long-slit position used for our spectra represented in blue (width = 2″, PA −30°), and the spectra used by GL+91 represented in red (width =1″.5, PA −40°).
In the text
thumbnail Fig. 2

Intermediate-resolution optical spectra of IRAS 22568+6141 obtained at the Calar Alto Observatory in 2021 (blue) and 2023 (red), normalised to the peak flux of Hβ. The indicated [O III] λ4959,5007 emission lines were not detected in 1988 (see the text).
In the text
thumbnail Fig. 3

Light curve of IRAS 22568 in the r filter. The arrows represent lower or upper limits. See the text for details.
In the text
thumbnail Fig. 4

Position-velocity maps of the Hα, [N II]], [S II]], and [OIII]emission lines obtained at a position angle PA–30 in 2023.68. The increment step of the contours is 3σ ×2n, starting from n = 0 (white). The SE lobe and NW lobe are indicated.
In the text
thumbnail Fig. 5

Normalised line profiles of the Hα, [N II]], [S II]], and [OIII]emission lines of the NW and SE lobes. The full width at 10% of the peak intensity (FW0.1) is shown as a horizontal dotted line.
In the text
thumbnail Fig. 6

Location of models from the 3MdB database (see text for details) in an [O III]4363/5007 vs. [N II]]5755/6584 diagram. The top panel shows models of the PNe, and the bottom panel shows shock and shock+precursor models. The colour code for the models corresponds to the Hα/Hβ line intensity ratio. The red star shows the location of the 2021 observations of IRAS 22568.
In the text
thumbnail Fig. 7

Spitzer spectra of IRAS 22568 obtained on 2005.95. The forbidden emission lines of ionised gas ([Ar II]]λ7.0µm, [Ar III]λ9.0µm, [Ne II]]λ2.8µm, and [Ne III]λ15.55µm) and broad emission of PAHs (6.4, 7.7, 8.6 and 11.3 µm) are indicated.
In the text
thumbnail Fig. D.1

SED of IRAS22568. The size of the circles is larger than the errors.
In the text

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