Stavros Akras

Context. The properties of NGC 7009 knots are investigated by employing UV-to-near-infrared data from the X-shooter spectrograph. NGC 7009 is a widely studied elliptical planetary nebula (PN) that exhibits a pair of knots. These structures belong to the category of low-ionization structures (LISs), which are characterized by enhanced emission from low-ionization species and whose origin remains enigmatic. Aims. Our main goal is to conduct the first dedicated spectroscopic study of LISs in planetary nebulae that covers a wide spectral range from the UV to the near-IR, simultaneously. As a pilot study, we focus on NGC 7009 to investigate its ionized, atomic, and molecular gas, aiming to better characterize the physical properties of its LISs and explore their largely unexplored molecular content. Methods. We obtained spectroscopic data with the X-shooter spectrograph, covering simultaneously and under the same conditions a spectral range from 300 nm to 2.5 μm. The spectrum was processed through the ESO Reflex pipeline, and the physical properties of LISs were derived with the latest version of P Y N EB . Results. More than 250 emission lines were detected in the spectra of NGC 7009 LISs, including rare, forbidden transitions from low-ionization species such as Fe ⁺ and Ni ⁺ , which have only very recently been identified in the spectra of LISs, as well as two transitions from P ⁺ , identified here for the first time in LISs. The detection of multiple shock tracers, together with the shock excitation of [Fe II ] and [O I ], as revealed by emission-line diagnostics, supports the presence of shocks at LISs. The richness of the spectrum allowed us to derive electron temperatures and densities at the LISs using both collisionally excited and optical recombination lines. In the near-IR spectrum of the LISs, nine ro-vibrational H ₂ emission lines were detected, seven of which were reported here for the first time in NGC 7009. These lines were used to construct the H ₂ excitation diagram, providing new insights into the physical conditions of the nebular gas. The excitation of molecular hydrogen is most likely dominated by collisional processes through shocks. Finally, the estimated lifetime of H ₂ at LISs is approximately 5.0 × 10 ⁴ years for K1a (East LIS) and 6.6 × 10 ⁴ years for K4 (West LIS).

We present the fifth data release (DR5) of the Southern Photometric Local Universe Survey (S-PLUS), covering 4592 deg ² across 2491 fields. Observations were conducted with the T80-South, a Brazilian robotic telescope equipped with the Javalambre 12-filter system, containing five broadband and seven narrowband filters. Data products feature FITS images and extensive catalogs containing fluxes, magnitudes, and shape parameters for over 113 million detections. In addition, several value-added catalogs are provided, offering photometric redshifts (photo- zs ), object classifications, masks, and extinction coefficients. For the first time, this release includes coverage of 110 deg ² along the Galactic disk, facilitating new research into Galactic structure and stellar populations. The release also provides full coverage of the Hydra Supercluster and numerous other nearby clusters with improved data reduction and calibration, enhancing photo- z accuracy, which is vital for large-scale structure studies. A preliminary analysis of the Hydra and Antlia galaxy clusters up to 5 × R ₂₀₀ yields an updated catalog of 1706 cluster members based on both spectroscopic and high-quality photo- zs . Our photometric data shows that both clusters have a similar proportion of galaxies with an H α excess relative to their clustercentric distance, though Hydra has a higher fraction near its center. Additionally, the spatial distribution of all objects in our sample highlights a bridge connecting both clusters. We verify that S-PLUS DR5 provides a solid foundation for future scientific investigations, ranging from solar system studies to cosmology.

Context . The object K4-47 is a young planetary nebula that exhibits low-ionisation structures in the form of two ‘lobes’. The unusual chemistry of the nebula has raised questions about whether K4-47 is a true planetary nebula or if the origins are more exotic in nature. Aims . We aim to investigate the spatially resolved structure of the nebula, including in the sub-millimetre, for the first time. We examined the kinematics, chemistry, and mass-loss history of the nebula and probed the stellar properties of the central system. Methods . We used a combination of optical imaging and spectroscopy and sub-millimetre interferometry as well as archival radio interferometric data to study the kinematics and morphology of the nebula and the differences between the atomic and molecular gas phases. Results . We found extended CO (2–1) emission towards the northern optical lobe, which shows a clear velocity gradient along the same position angle as the optical bipolar nebula. Comparing the elemental and isotopic abundances to model predictions, we estimate an upper limit of 6 kpc for the distance to K4-47. The source hosts a fast (∼350 km s ⁻¹ ) atomic jet and a slower (∼50–60 km s ⁻¹ ) molecular outflow that are spatially coincident. The outflow velocity implies an age of 336 ± 119 yr. Using atomic line diagnostics, we found that the core has an electron temperature and density of ≈20 kK and 2800 cm ⁻³ , respectively. We derived a Zanstra effective temperature of 81 ± 2 kK for the central star. We also observed evidence of a significant circumstellar component in the line-of-sight extinction to the source. The progenitor mass of K4-47 is estimated to be 4–6 M _⊙ , based on the comparisons of measured abundances with model predictions. The central white dwarf of K4-47 may have an approximate mass of 0.9–1.1 M _⊙ , following the initial-final mass relation of white dwarfs. The released material in the nebula is dominated by neutral atomic gas. Conclusions . The resolved molecular environment of K4-47 indicates that it was shaped by the asymmetric outflow now seen primarily in optical emission. The progenitor of K4-47 was likely an asymptotic giant branch (AGB) star, possibly a J-type carbon star. We also observed that shocks, possibly from the bipolar lobes passing through the circumstellar environment, may play a non-negligible role in the excitation of gas in the core instead of just in photoionisation. The analysis indicates that K4-47 is indeed a true, albeit peculiar, planetary nebula.

Symbiotic stars (SySts) are considered potential progenitors of Type Ia supernovae (SNe Ia). However, theoretical studies suggest that their contribution to the total number of SNe Ia rate is very small. The number of known SySts remains significantly lower, by orders of magnitude, than predicted by theory. Continued searches for new members are therefore essential to better constrain population synthesis models. In this study, we conduct a search for new potential SySts captured in Gaia’s observations. Optical photometric data from IPHAS and VPHAS+, infrared data from 2MASS and WISE, and ultraviolet data from GALEX are combined to uncover hidden members in the Gaia catalogue of 12.4 million variable sources. For sources lacking optical data, synthetic optical data are computed using the low-resolution Gaia BP/RP spectra in order to identify the Hα emitters. By applying infrared and optical selection criteria, we identified 43 S-type and 11 D-type SySts that meet these requirements. Of these, 10 S-type and 3 D-type are considered highly probable candidates, while the remainder are already known member. Analysis of the Gaia BP/RP spectroscopic data confirms the presence of the Hα emission line as well as molecular bands in all thirteen systems, validating their classification as bona-fide SySts. Systematic searches for SySts and other rare stellar groups, integrating data from multiple publicly available catalogues, are essential to uncover previously hidden members.

Context . Low-ionization structures (LISs), found in all morphological types of planetary nebulae (PNe), are small-scale features that are prominent in emission from low-ionization species such as [N II], [S II], [O II], and [O I]. Observational and theoretical efforts aim to better understand their origin and nature. The recent detection of molecular hydrogen (H ₂ ) emission associated with LISs in a few PNe has added a new piece to the puzzle of understanding these nebular structures. Aims . Although observational studies indicate that LISs are characterized by lower electron densities than their host PNe, model predictions suggest that these structures have higher total densities. The detection of H ₂ emission from LISs in more PNe could help reconcile the observations with model predictions. Methods . We conducted observations of five PNe with already known LISs using the Near InfraRed Imager and Spectrometer (NIRI) mounted on the 8 m Gemini North telescope. A narrowband filter, centered on the H ₂ 1–0 2.122 µm emission line, was used along with a continuum filter to ensure continuum subtraction. Results . We present a deep, high-angular-resolution near-IR narrowband H ₂ 1–0 S(1) imaging survey of five Galactic PNe with LISs. We nearly double the sample of LISs detected in the H ₂ 1–0 2.122 µm emission line, as well as the number of host PNe. These findings allow us to demonstrate that the systematically lower electron density in LISs – relative to the rims and shells of their host nebulae – is linked to the presence of H ₂ molecular gas. Furthermore, we provide the first estimate of the excited H ₂ molecular mass in LISs, which is between 200 and 5000 times lower than the corresponding ionized gas mass.

Integral field spectroscopy (IFS) offers a distinct advantage for studying extended sources by enabling spatially resolved emission maps for several emission lines without the need for specific filters. This study conducts a detailed analysis of iron and nickel emission lines in 12 planetary nebulae (PNe) using integral field unit (IFU) data from MUSE to provide insights into their formation and evolution mechanisms. New diagnostic line ratios, combined with machine-learning algorithms, were used to distinguish excitation mechanisms such as shock and photoionization. Electron densities and elemental abundances were estimated using different atomic data through the PyNeb package. The contribution of fluorescent excitation of nickel lines was also examined. A total of 16 iron- and nickel-rich clumps are detected in seven out of 12 PNe. New clumps are discovered in NGC 3132 and IC 4406. The most prominent lines are [Fe II] 8617 Angstrom and [Ni II] 7378 Angstrom. Both emission lines are observed emanating directly from the low-ionization structures (LIS) of NGC 3242, NGC 7009, and NGC 6153, as well as from clumps in NGC 6369 and Tc 1. Their abundances are found to be below solar values, indicating that a fraction of Fe and Ni remains depleted in dust grains. The depletion factors exhibit a strong correlation over a wide range. A machine-learning approach allows us to classify ten out of 16 clumps as shock-excited and to establish a new shock/photoionization selection criterion: log([Ni II] 7378 Angstrom / H-alpha) and log([Fe II] 8617 Angstrom / H-alpha) greater than -2.20.

Context. We investigate the physico-chemical properties of the planetary nebula (PN) NGC 3242 in both 1D and 2D, using integral field unit (IFU) data. This PN has a complex morphology with multiple shells and contains a pair of structures with a lower degree of ionization compared to the main nebular components. The origin of these so-called low-ionization structures (LISs) remains a mystery. Aims. With the capabilities offered by IFU spectroscopy, we aim to gain a better understanding of the behavior of nebular properties in the LISs and examine the spatial distribution of physico-chemical parameters in NGC 3242. Methods. Data from the Multi Unit Spectroscopic Explorer (MUSE) at the Very Large Telescope (VLT) were used in order to perform a spatially resolved physico-chemical analysis of NGC 3242. The analysis was performed both in 2D, based on emission line maps, as well as in 1D, via simulations of long-slit spectroscopy, with pseudo-slits. Results. Through the deeper investigation of MUSE data, we detected new structures perpendicular to the pair of LISs of NGC 3242, which are mainly seen in the light of [S III] and [N II]. In addition, two arc-like structures were revealed. Moreover, an inner jet-like structure was found through its [Fe III] emission. The interaction of the jet with the rim may be related to the formation of knots and blobs. The higher value of T_e, is estimated from the [S III] diagnostic lines, followed by T_e [N II]), T_e (H I) and, finally, T_e (He I). In all cases, T_e is higher at the inner nebular structures. Regarding electron density, n_e, is lower at the LISs, while an increase is observed at the nebular rim. Diagnostic diagrams confirm that NGC 3242 is a highly ionized nebula. Moreover, for the first time, the MUSE data have unveiled the atomic line [C I] λ8727 in this PN, which is primarily emitted from the LISs. This finding suggests that these structures may consist of a molecular core surrounded by neutral and ionized gas.