Luis F. Miranda

We present full-polarization observations of the four ground-state transitions of OH, carried out with the Australia Telescope Compact Array toward two very young planetary nebulae (PNe), IRAS 16333−4807 and IRAS 17393−2727. We detect Zeeman splitting in the 1720 MHz transition in IRAS 16333−4807, with magnetic field strengths of ∼2 to 9 mG, consistent with previous measurements. We also identify a possible reversal in the line-of-sight magnetic field component at −55.7 km s ⁻¹ (previously oriented toward us, now away from us), although this may be affected by spectral blending or variability. For IRAS 17393−2727, we derive magnetic field strengths of ∼3 to 20 mG at 1612 MHz, in agreement with earlier estimates. The linear polarization properties suggest a possible change in the orientation of the sky-plane component of the magnetic field. The data are consistent with variations in the magnetic field configuration in both PNe on timescales of a few years, although higher angular resolution and multiepoch observations are required to confirm this interpretation.

We present new high-dispersion optical spectra of the planetary nebula NGC 2371 obtained with the Manchester Echelle Spectrometer at the OAN-SPM 2.1 m telescope, complemented with 3D morpho-kinematic modelling using ShapeX. The data reveal that the present-day morphology of NGC 2371 is the outcome of multiple episodic mass-loss events rather than a single outflow. Our best-fitting model simultaneously reproduces the direct images and the Position–Velocity (PV) diagrams, and consists of a barrel-shaped shell with younger polar caps, extended bipolar lobes, and a pair of misaligned low-excitation [N ii] knots interpreted as jet-like ejections. The derived kinematical ages of the main structures, spanning ≃1600 to ≃4400 yr, indicate successive episodes of mass loss with different geometries and timescales. The nearly perpendicular bipolar lobes, the absence of a pronounced waist, and the surface distortions of the large-scale structures cannot be explained solely by standard axisymmetric wind interactions. Instead, our results point to a combination of shaping agents, including a late thermal pulse (born-again scenario) possibly related to the H-deficient [WR]-type nature of the central star, binary-driven interactions, and episodic jet activity. NGC 2371 thus provides a particularly instructive case where multiple shaping agents may operate, and where some of the relevant physical processes remain only marginally explored in current models of PN formation and evolution.

We present new high-dispersion optical spectra of the planetary nebula NGC 2371 obtained with the Manchester Echelle Spectrometer at the OAN-SPM 2.1-m telescope, complemented with 3D morpho-kinematic modelling using ShapeX. The data reveal that the present-day morphology of NGC 2371 is the outcome of multiple episodic mass-loss events rather than a single outflow. Our best-fitting model simultaneously reproduces the direct images and the Position–Velocity (PV) diagrams, and consists of a barrel-shaped shell with younger polar caps, extended bipolar lobes, and a pair of misaligned low-excitation [N ii] knots interpreted as jet-like ejections. The derived kinematical ages of the main structures, spanning ≃1600 to ≃4400 yr, indicate successive episodes of mass loss with different geometries and timescales. The nearly perpendicular bipolar lobes, the absence of a pronounced waist, and the surface distortions of the large-scale structures cannot be explained solely by standard axisymmetric wind interactions. Instead, our results point to a combination of shaping agents, including a late thermal pulse that produced the H-deficient [WR] central star, binary-driven interactions, and episodic jet activity. NGC 2371 thus emerges as a highly unusual planetary nebula, possibly involving physical processes that remain poorly explored in current models of PN formation and evolution.

Stars such as the Sun expel their outer layers and form planetary nebulae (PNe) as they evolve into white dwarfs. PNe exhibit diverse morphologies, the origins of which are not fully understood. PNe with OH (OHPNe) and H ₂ O (H ₂ OPNe) masers are thought to be nascent PNe. However, the number of known OHPNe and H ₂ OPNe remains small, and only in eight cases has the position of the maser emission been found to coincide with the PN, using the high astrometric accuracy of interferometric observations. In order to identify more OHPNe and H ₂ OPNe, we used public databases and our own ATCA/VLA observations to match the positions of OH and H ₂ O masers with known PNe and radio continuum emitters, considering radio continuum emission as a possible tracer of the photoionized gas that characterizes PNe. Here we report the confirmation of positional coincidence of maser emission with one more PN, and 12 PN candidates. Moreover, we have confirmed three evolved stars as ‘water fountains’ (WFs) hosting H ₂ O masers. These WFs are associated with radio continuum emission, but their possible nature as PNe has not yet been confirmed. Although a final characterization of maser-emitting PNe as a group still requires confirmation of more objects, their distribution in the infrared color-color diagrams suggests that they are a heterogeneous group of PNe. In particular, the new OHPN IRAS 07027–7934 has been reported to contain a late [WC]-type central star, while the maser emission implies an O-rich envelope. This property is found in only one other known maser-emitting PN, although we found evidence that other confirmed and candidate OHPNe may also have mixed chemistry, since they show emission from polycyclic aromatic hydrocarbons. The new WF IRAS 18443–0231 shows radio continuum that is dominated by strong and variable non-thermal emission, as in magnetized outflows.

“Water fountains” (WFs) are optically obscured evolved stars. Most of them are thought to be in the post-asymptotic giant branch (post-AGB) phase, and they are characterized by H ₂ O maser emission tracing molecular jets. Interestingly, four WFs (IRAS 15445−5449, IRAS 18019−2216, IRAS 18443−0231, and IRAS 18464−0140) and one WF candidate (IRAS 18480+0008) are potentially planetary nebulae (PNe) because they exhibit radio continuum emission, suggesting the presence of a photoionized region characteristic of PNe. To classify these objects, we obtained K -band (2.0–2.3 μm) spectra of these WFs, including the only WF PN known (IRAS 15103−5754) for comparison. Our spectra reveal two groups of sources: (i) “low-velocity” WFs with an H ₂ O maser velocity spread of ≲50 km s ⁻¹ (IRAS 18019−2216, IRAS 18464−0140, and IRAS 18480+0008) showing the CO band at 2.29 μm in absorption, typical of cool giant stars, and no emission lines, and (ii) “high-velocity” WFs, velocity spread of ≳50 km s ⁻¹ (IRAS 15103−5754, IRAS 15445−5449, and IRAS 18443−0231), exhibiting emission lines of Br γ , He I , and H ₂ , consistent with hotter central stars and/or shock-excited emission. The emission line ratios of these lines in IRAS 18443−0231 indicate that it may be a nascent PN. The spectrum of IRAS 15445−5449 also shows a CO band and Na I doublet in emission, suggesting the presence of a compact circumstellar disk and/or active mass loss. These results favor the previously suggested notion that the difference between low- and high-velocity WFs is not simply a projection effect but a reflection of intrinsically different evolutionary stages. Moreover, the results are also consistent with the idea of an increase in the jet ejection velocity as the post-AGB evolution proceeds.

While NGC 1514 is an elliptical, but complex, planetary nebula at optical wavelengths, it was discovered to have a pair of infrared-bright, axisymmetric rings contained within its faint outer shell during the course of the Wide-field Infrared Survey Explorer all-sky survey. We have obtained JWST mid-infrared imaging and spectroscopy of the nebula through the use of simultaneous observations with the MIRI Imager and Medium Resolution Spectrometer, selecting the F770W, F1280W, and F2550W filters to match each of the medium-resolution spectrometer’s three grating positions. These observations show that the rings are clearly resolved and relatively distinct structures, with both filamentary and clumpy detail throughout. There is also cloud-like material that has a turbulent appearance in the interior of the rings, particularly at the longest wavelengths, and faint ejecta-like structures just outside the ring boundaries. Despite their brightness, the emission from the rings within the three imager passbands is shown to be dominated by thermal emission from very small grains, not line emission from atomic hydrogen or forbidden atomic lines, shocked molecular hydrogen, or polycyclic aromatic hydrocarbons. The Doppler velocities derived from the two brightest emission lines in the rings, however, suggest that the material from which the rings were formed was ejected during an early period of very heavy mass loss from the planetary nebula progenitor, then shaped by asymmetrical fast winds from the central binary pair.

We examined dust polarization within the planetary nebula (PN) K 3-35 using the Atacama Large Millimeter/Submillimeter Array (ALMA). This investigation aimed to identify and trace the magnetic field within the PN, as it potentially plays a crucial role in shaping this bipolar nebula. Our findings include a marginal detection of the polarized region and low fractional polarization (peaking at 1.4 per cent). Assuming a certain level of validity, we observed well-organized dust grains aligned along the equatorial plane of the PN, indicating a magnetic field alignment with the outflows. The limited detection of polarization at submillimeter wavelengths in this PN and others may be attributed to a pronounced optical depth. However, our analysis of K 3-35 with the code dusty does not seem to support this idea. We also modelled the Spectral Energy Distribution (SED) of K3-35, and our best-fitting models included a mixture of silicates and amorphous carbon. The grains of amorphous carbon are less susceptible to alignment with the magnetic field, which could, at least partially, explain the observed low polarization. The models presented in this article should be considered preliminary, and a more advanced approach is needed for a more complete interpretation of the results.

Context. Maser emission of SiO, H₂O, and OH is widespread in asymptotic giant branch (AGB) stars with oxygen- (O-) rich envelopes. This emission quickly disappears during the post-AGB phase and is extremely rare in planetary nebulae (PNe). So far, only eight PNe have been confirmed to show OH and/or H₂O maser emission, and none have ever been found to show SiO maser emission. Aims. We intend to obtain the first detection of a SiO maser from a PN. Such a detection would provide us with a useful tool to probe mass loss in PNe at a scales of a few AU from the central star, much shorter than the scales traced by H₂O or OH masers. Methods. We compiled two different samples. The first one comprises all known PNe with confirmed OH and/or H₂O maser emission, as well as two candidate PNe showing OH masers. For the second sample, we compiled single-dish SiO maser detections in the literature, and compared them with catalogs of PNe and radio continuum emission (which could trace photoionized gas in PNe). We identified five targets (either PN or radio continuum sources) within the beam of the single-dish SiO maser observations. We then carried out interferometric observations of both samples with the Australia Telescope Compact Array, to confirm the spatial association between continuum and SiO maser emission. Results. We find no SiO maser emission associated with any confirmed or candidate PN. In all targets, except IRAS 17390–3014, there is no spatial coincidence between SiO masers and radio continuum emission. While in IRAS 17390–3014 we cannot completely rule out a possible association, it is unlikely that the radio continuum emission arises from a PN. Conclusions. The absence of SiO maser emission in PNe showing OH or H₂O masers is of special interest, since thermal SiO emission has been reported in at least one of these targets, indicating that SiO molecules can be present in the gas phase. Since some maser-emitting PNe show evidence of having O-rich outer envelopes, and carbon- (C-) rich central stars and inner envelopes, we speculate that SiO abundance could be very low in the central regions where physical conditions are optimal for maser pumping, and C-bearing molecules may be dominant in the gas phase at those locations.

IRAS 22568+6141 has been classified as a low-excitation planetary nebula (PN) and presents non-thermal radio continuum emission, which could be a signature of a nascent PNe. We present intermediate-resolution long-slit spectra obtained in 2021 and 2023, high-resolution long-slit spectra taken in 2023, and a light curve at the r filter between 1953 and 2019. They all reveal changes in IRAS 22568+6141 with timescales of decades and a few years. The object underwent an energetic event around 1990 that suddenly increased its brightness, which has been fading since then. A comparison with a published spectrum from 1988 shows an increase in the Hβ flux in 2021 by factor of ≃6 and the [O III] emission lines, which were absent in 1988. Between 2021 and 2023, the Hβ flux decreased by a factor of ≃1.7, and the [O III] emission lines almost vanished. These results and the variability observed in other emission lines indicate that IRAS 22568+6141 was recombining and cooling between 2021 and 2023, and probably since 2005, as suggested by archival radio continuum and mid-IR observations. The intermediate- and high-resolution spectra show that the excitation of the emission lines was dominated by shocks in 2021 and 2023, and probably also in 1988, which may be related to the non-thermal radio continuum emission from the object. Although the variability might be due to changes in the physical conditions in the shocks or in a nova-like eruption, it better accommodates that expected from a late thermal pulse, which is further suggested by a comparison with other similar objects. New observations and monitoring in the coming years are crucial to corroborate the origin of the variability.

Me 2-1 is a high-excitation planetary nebula whose morphology and physical structure have not yet been investigated. We present narrow-band images in several emission lines, and high- and intermediate-resolution long-slit spectra aimed at investigating its morphology and 3D structure, and its physical parameters and chemical abundances. By applying deconvolution techniques to the images, we identified in Me 2-1: an elliptical ring; two elongated, curved structures (caps) that contain three pairs of bright point-symmetric (PS) knots; a shell interior of the ring; and a faint halo or attached shell. The caps are observed in all images, while the PS knots are only observed in the low-excitation emission line ones. These structures are also identified in the high-resolution long-slit spectra, allowing us to study their morphokinematics. The 3D reconstruction shows that Me 2-1 consists of a ring seen almost pole-on, and a virtually spherical shell, to which the caps and PS knots are attached. Caps and PS knots most probably trace the sites where high-velocity collimated bipolar outflows, ejected along a wobbling axis, collide with the spherical shell, are slowed down, and remain attached to it. Although the main excitation mechanism in Me 2-1 is found to be photoionization, a contribution of shocks in the PS knots is suggested by their emission line ratios. The combination of collimated outflows and a ring with a spherical shell is unusual among planetary nebulae. We speculate that two planets, each with less than one Jupiter mass, could be involved in the formation of Me 2-1 if both enter a common envelope evolution during the asymptotic giant branch phase of the progenitor. One planet is tidally disrupted, forming an accretion disk around the central star, from which collimated bipolar outflows are ejected; the other planet survives, causing wobbling of the accretion disk. The physical parameters and chemical abundances obtained from our intermediate-resolution spectrum are similar to those obtained in previous analyses, with the abundances also pointing to a low-mass progenitor of Me 2-1.