post-AGB Stars Research Articles

Exploring the possibility of polycyclic aromatic hydrocarbons and fulleranes as the carrier of the 21 micron emission feature

IntroductionThe 21 μm emission feature discovered in a small sample of carbon-rich protoplanetary nebulae has remained unidentified for over 30 years. A dozen of different molecular species (both organics and inorganics) have been proposed as the carrier candidates of this important feature, among which polycyclic aromatic hydrocarbons (PAHs) and fulleranes (hydrogenated fullerenes) have yet to be sufficiently examined.MethodIn this study, we attempt to fit the 21 μm features in observed spectra of the above-mentioned astronomical sources via theoretically simulated spectra of various PAHs and fulleranes, aiming to investigate whether the two hydrocarbon families can reproduce the 21 μm feature.Results and DiscussionBased on the fitting outcomes we conclude that fulleranes can provide a more plausible explanation for the origin of 21 μm feature than PAHs.

Exploring the Ionized Core of the Proto-Planetary Nebula CRL 618 and Its Vicinity with ALMA

Exploring the Ionized Core of the Proto-Planetary Nebula CRL 618 and Its Vicinity with ALMA

A Dust-Scattering Model for M1-92: A Revised Estimate of the Mass Distribution and Inclination

Preplanetary nebulae (PPNe) are formed from mass-ejecting late-stage AGB stars. Much of the light from the star gets scattered or absorbed by dust particles, giving rise to the observed reflection nebula seen at visible and near-IR wavelengths. Precursors to planetary nebulae (PNe), PPNe generally have not yet undergone any ionization by UV radiation from the still-buried stellar core. Bipolar PPNe are a common form of observed PPNe. This study lays the groundwork for future dynamical studies by reconstructing the dust density distribution of a particularly symmetric bipolar PPN, M1-92 (Minkowski’s Footprint, IRAS 19343+2926). For this purpose, we develop an efficient single-scattering radiative transfer model with corrections for double-scattering. Using a V-band image from the Hubble Space Telescope (HST), we infer the dust density profile and orientation of M1-92. These results indicate that M1-92’s slowly expanding equatorial torus exhibits an outer radial cutoff in its density, which implicates the influence of a binary companion during the formation of the nebula.

Planetary Nebulae Research: Past, Present, and Future

We review the evolution of our understanding of the planetary nebulae phenomenon and their place in the scheme of stellar evolution. The historical steps leading to our current understanding of central star evolution and nebular formation are discussed. Recent optical imaging, X-ray, ultraviolet, infrared, millimeter wave, and radio observations have led to a much more complex picture of the structure of planetary nebulae. The optically bright regions have multiple shell structures (rims, shells, crowns, and haloes), which can be understood within the interacting winds framework. However, the physical mechanism responsible for bipolar and multipolar structures that emerged during the proto-planetary nebulae phase is yet to be identified. Our morphological classifications of planetary nebulae are hampered by the effects of sensitivity, orientation, and field-of-view coverage, and the fraction of bipolar or multipolar nebulae may be much higher than commonly assumed. The optically bright bipolar lobes may represent low-density, ionization-bounded cavities carved out of a neutral envelope by collimated fast winds. Planetary nebulae are sites of active synthesis of complex organic compounds, suggesting that planetary nebulae play a major role in the chemical enrichment of the Galaxy. Possible avenues of future advancement are discussed.

A comprehensive analysis towards the Fermi-LAT source 4FGL J1846.9–0227: jets of a protoplanetary nebula producing γ-rays?

ABSTRACT Most of the $\gamma$-ray sources in the Fermi-LAT 14-yr source catalogue are associated with pulsars and blazars. However, unveiling the nature of the still unassociated $\gamma$-ray sources is important for the understanding of high energy emission mechanisms in astrophysical objects. This work presents a comprehensive study towards the region covered by the Fermi source 4FGL J1846.9$-$0227, previously suggested to be a blazar and a massive protostar. Using multiwavelength observations, we analysed several astrophysical objects in the region as possible counterparts of the Fermi-LAT source. Having discarded most of them after a detailed and comprehensive analysis, we suggest that the most likely candidate to be such a counterpart is IRAS 18443$-$0231, a likely protoplanetary nebula. We found that the radio continuum emission at 3 GHz of IRAS 18443$-$0231 shows a compact source related to faint emission with jet-like morphology. Additionally, we identified an associated redshifted CO molecular outflow. Using data from several catalogues, we obtained radio spectral index values ranging from $-$0.57 to $-$0.39 for IRAS 18443$-$0231, indicating synchrotron emission due to particles accelerated by the jets. We point out that these jets could explain the $\gamma$-ray emission through mechanisms such as proton–proton collisions and relativistic bremsstrahlung. IRAS 18443$-$0231, lying almost at the centre of the Fermi confidence ellipse and related to the hard X-ray source 4XMM J184700.4$-$022752, would be the first association between a protoplanetary nebula and $\gamma$-ray emission.

Analysis of Gaia DR3 Data of Selected High Galactic Latitude Hα Emission Stars

Gaia DR3 data of 52 high galactic latitude Hα emission stars selected from the paper of Stephenson are analyzed. Ten new variable stars and eight new high velocity stars have been found. Details of six post-AGB stars and several other interesting stars are discussed. Many of the stars presented in this paper have Gaia DR3 light curves and spectra. A detailed study of these 52 stars is needed to derive their chemical composition and to further understand their binarity, circumstellar matter and evolutionary status. The astrophysical parameters from the Gaia DR3 data reveal several hot (OBA) and several F and G-type stars in this sample. Some of them are metal-poor stars.

A search for Galactic post-asymptotic giant branch stars in Gaia DR3

When low- and intermediate-mass stars leave the asymptotic giant branch (AGB) phase, and before they reach the planetary nebula stage, they enter a very brief and rather puzzling stellar evolutionary stage called post-AGB stage. The post-AGB phase lasts very briefly, about a few thousand years at most. The number of objects that are confirmed in this phase therefore is really small, and our understanding of this elusive stellar evolutionary stage is accordingly very limited. We provide a reliable catalogue of Galactic post-AGB stars together with their physical and evolutionary properties obtained through Gaia DR3 astrometry and photometry. As an added product, we provide information for a sample of other types of stellar objects, whose observational properties mimic those of post-AGB stars. Post-AGB stars are characterised by their infrared excesses and high luminosities. The publication of precise parallaxes in Gaia DR3 made it possible to calculate accurate distances and to revise the derivation of luminosities for post-AGB candidates, so that objects outside the expected luminosity range can be discarded. We started by identifying post-AGB stars or possible candidates from the bibliography, and we then searched for their Gaia DR3 counterpart sources. Using the available photometry, interstellar extinction, spectroscopically derived temperatures or spectral types and parallax-derived distances from the literature, we fitted their spectral energy distributions and estimated their luminosities and circumstellar extinctions. By a comparison to models, the luminosity values allowed us to determine which objects are likely post-AGB stars from other target types. Their position in the Hertzsprung-Russell diagram allows a direct comparison with updated post-AGB evolutionary tracks and an estimation of their masses and evolutionary ages. We obtained a sample of 69 reliable post-AGB candidates that meet our classification criteria, which provide their coordinates, distances, effective temperature, interstellar and circumstellar extinction, luminosity, mass, and evolutionary age. In addition, we provide similar data for other stellar objects in our initial compilation, such as supergiant stars and young stellar objects. Our identifications and parameters are compared with others found in the recent literature for the subject. We selected the data with the best precision in parallax and distance to obtain more accurate luminosities, which allowed us to confidently classify the objects of the sample in different stellar phases. In turn, this allowed us to provide a small but reliable sample of post-AGB objects. The derived mean evolutionary time and average mass values agree with theoretical expectations and with the mean mass value obtained in a previous work for the subsequent evolutionary stage, the planetary nebula stage.

A deep search for C2H in the oxygen-rich post-AGB star OH 231.8+4.2

In carbon-rich environment, C2H emission is seen to be associated with HC3N, but whether it can trace the HC3N molecule in oxygen-rich environment is unknown. Here, we have searched for rotational emission from ethynyl radical (C2H) in the oxygen-rich circumstellar envelope (CSE) of the post-asymptotic giant branch (post-AGB) star OH 231.8+4.2, a renowned galactic proto-planetary nebula (PPN) known to display cyanoacetylene (HC3N) emission. Our observations were conducted using the James Clerk Maxwell Telescope (JCMT), and the total on-source time is 11.9 h. Base on local thermodynamic equilibrium (LTE) excitation analysis, we have calculated the column density and the abundance relative to H2 for C2H. The calculated column density for C2H is less than 1.4 × 1013 cm-2, which corresponds to a fractional abundance, f (C2H), less than 4.5 × 10–9. This inference suggests that the typical transformation pathway from C2H and C2H2 to HC3N may not play a significant role in O-rich environments. It indicates the presence of alternative, unidentified pathways that contribute to the formation of HC3N in O-rich circumstellar envelopes.

Gaia DR3 Data of Very Young Open Cluster NGC 6530: Distance and Evolutionary Stage of Selected Stars

Gaia EDR3 and Gaia DR3 data of selected stars in the very young (1.5 million years old) open cluster NGC 6530 are presented. All the selected stars have very accurate Gaia DR3 distances. Most of the selected stars are very young B stars and several of them are young stellar objects. From the analysis of selected stars, the distance to NGC 6530 is found to be 1318 pc. The two very red stars VAJ 304 and 338 have little bit less proper motion than other members of the NGC 6530 and their Mv values are found to be −3.57 and −3.68 respectively. They are much brighter than K5 III RGB stars and less luminous than massive K5 red supergiants. It is likely VAJ 304 and VAJ 338 are not members of NGC 6530 they may be field K5 type AGB or post-AGB stars. Further study of these two stars is needed.

CO emission survey of asymptotic giant branch stars with ultraviolet excesses

Context. The transition from the spherically symmetric envelopes around asymptotic giant branch (AGB) stars to the asymmetric morphologies observed in planetary nebulae is still not well understood, and the shaping mechanisms are a subject of debate. Even though binarity is widely accepted as a promising option, it is limited by the complication of identifying binary AGB stars observationally. Recently, the presence of ultraviolet excesses in AGB stars has been suggested as a potential indicator of binarity. Aims. Our main goals are to characterise the properties of the circumstellar envelopes (CSEs) around candidate AGB binary stars, specifically those selected based on their UV excess emission, and to compare these properties with those derived from previous CO-based studies of AGB stars. Methods. We observed the 12CO (J=1–0) and 12CO (J=2–1) millimetre-wavelength emission in a sample of 29 AGB binary candidates with the IRAM-30 m antenna. We measured the systemic velocities and the terminal expansion velocities from their line profiles. Population diagrams were used to interpret the results, enabling the estimation of excitation temperatures (Tex), mass-loss rates (Ṁ), and the characteristic sizes of the envelope layers where the CO millimetre emission originates (Rs). We explored different trends between the envelope parameters deduced, multiwavelength flux measurements, and other properties of our sample, and compared them with those previously derived from larger samples of AGB stars found in the literature. Results. We detected 12CO emission in 15 sources, of which 5 are first detections. We found relatively low expansion velocities (3 km s−1 ≲ Vexp ≲ 20 km s−1) in our sample. We derived the average excitation temperature and column density of the CO-emitting layers, which we used to estimate self-consistently the average mass-loss rate (10−8 M⊙ yr−1 ≲ Ṁ ≲ 10−5 M⊙ yr−1) and the CO pho-todissociation radius (5 × 1015 cm ≲ RCO ≲ 2 × 1017 cm) of our targets. We find a correlation between CO intensity and IRAS 60 µm fluxes, revealing a CO-to-IRAS 60 µm ratio lower than for AGB stars and closer to that found for pre-planetary nebulae (pPNe). An anti-correlation is observed between 12CO (and IRAS 60 µm) and the near-ultraviolet (NUV), but no such correlation is observed with the far-ultraviolet (FUV). It is also worth noting that there is no correlation between bolometric luminosity and NUV or FUV. Conclusions. For the first time we have studied the mass-loss properties of UV-excess AGB binary candidates and estimated their main CSE parameters. Our sample of uvAGB stars shows similarities with the broader category of AGB stars, except for a distinct CO-to-IRAS 60 µm trend suggesting enhanced CO photodissociation. Our findings, based on single-dish low-J CO line emission observations, support the dust-driven wind scenario and indicate that alternative mass-loss mechanisms are not necessary (in principle) to explain the ~200–2000 yr old mass-loss ejecta in uvAGBs. The different relationships between 12CO and IRAS 60 µm, with NUV and FUV are consistent with an intrinsic origin of NUV emission, but potential dominance of an extrinsic process (e.g. presence of a binary companion) in FUV emission.

The first measurements of carbon isotopic ratios in post-RGB stars: SZ Mon and DF Cyg

ABSTRACT Dusty post-red giant branch (post-RGB) stars are low- and intermediate-mass stars where the RGB evolution was prematurely terminated by a poorly understood binary interaction. These binary stars are considered to be low-luminosity analogues of post-asymptotic giant branch (post-AGB) binary stars. In this study, we investigated the chemical composition of two dusty post-RGB binary stars, SZ Mon and DF Cyg, using multiwavelength spectroscopic data from HERMES/Mercator (optical) and the APOGEE survey (near-infrared). Owing to challenges posed by existing spectral analysis tools for the study of evolved stars with complex atmospheres, we developed E-iSpec: a dedicated spectral analysis tool for evolved stars, to consistently determine atmospheric parameters, elemental abundances, and carbon isotopic ratios. Our abundance analysis revealed that observed depletion patterns and estimated depletion efficiencies resemble those found in post-AGB binary stars. However, the onset of chemical depletion in post-RGB targets occurs at higher condensation temperatures ($T_{\rm turn-off,~post-RGB}\approx 1\, 400$ K), than in most post-AGB stars ($T_{\rm turn-off,~post-AGB}\approx 1\, 100$ K). Additionally, our study resulted in the first estimates of carbon isotopic ratios for post-RGB stars (12C/13CSZ Mon = 8 ± 4, 12C/13CDF Cyg = 12 ± 3). We found that the observationally derived CNO abundances and the carbon isotopic ratios of our post-RGB binary targets are in good agreement with theoretical predictions from the ATON single star evolutionary models involving first dredge-up and moderately deep extra mixing. This agreement emphasizes that in post-RGB binary targets, the observed CNO abundances reflect the chemical composition expected from single star nucleosynthesis (i.e. convective and non-convective mixing processes) occurring during the RGB phase before it is terminated.

The distance to CRL 618 through its radio expansion parallax

ABSTRACT CRL 618 is a post-Asymptotic Giant Branch star that has started to ionize its ejecta. Its central H ii region has been observed over the last 40 yr and has steadily increased in flux density at radio wavelengths. In this paper, we present data that we obtained with the Very Large Array in its highest frequency band (43 GHz) in 2011 and compare these with archival data in the same frequency band from 1998. By applying the so-called expansion-parallax method, we are able to estimate an expansion rate of 4.0 ± 0.4 mas yr−1 along the major axis of the nebula and derive a distance of 1.1 ± 0.2 kpc. Within errors, this distance estimation is in good agreement with the value of ∼900 pc derived from the expansion of the optical lobes.

Imaging Spectropolarimetry—A New Observing Mode on the Hubble Space Telescope’s Advanced Camera for Surveys

Imaging spectropolarimetry is a new observing mode on the Advanced Camera for Surveys (ACS) aboard the Hubble Space Telescope (HST) that was commissioned in Cycle 30 and is available to HST observers starting in Cycle 31 (i.e., from 2023). It is a technique that is accessible from ground-based observatories, but the superb spatial resolution afforded by HST/ACS combined with the slitless nature of HST/ACS grism spectroscopy opens up the possibility of studying polarized extended emission in a way that is not currently possible even with Adaptive Optics facilities on the ground. This mode could help to study interesting targets including (but not limited to) QSOs, active galactic nucleus and Radio Galaxies, ISM Dust Properties, Pre-Planetary Nebulae, Proto-Planetary and Debris Disks, Supernovae/Supernova Remnants, and solar system objects. This research note presents the preliminary results from the calibration programs used to calibrate imaging spectropolarimetry on HST/ACS.

Gas-phase Molecules in Protoplanetary Nebulae with the 21 μm Emission Feature

It has been more than 30 years since the enigmatic 21 μm emission feature was first discovered in protoplanetary nebulae (PPNs). Although dozens of different dust carrier candidates have been proposed, there is as yet no widely accepted one. We present the results of molecular observations toward 21 μm objects using the 10 m Submillimeter Telescope of Arizona Radio Observatory in the 1.3 mm band and the 13.7 m telescope of Purple Mountain Observatory in the 3 mm band, aiming to investigate whether the gas-phase environments of these unusual sources have some peculiarities compared to normal PPNs. We detect 31 emission lines belonging to seven different molecular species, most of which are the first detection in 21 μm PPNs. The observations provide clues to the identification of the 21 μm feature. We report a correlation study between the fractional abundance of gas-phase molecules and the strengths of the 21 μm emission. Our study shows that, given the small sample size, the 21 μm feature has weak or no correlations with the gas-phase molecules. Future radio observations of high spatial and spectral resolution toward a large sample are desirable to elucidate the 21 μm emission phenomena.

A radical transition in the post-main-sequence system U Equulei

Context. U Equ is an unusual maser-hosting infrared source discovered in the 1990s. It was tentatively classified as a post-asymptotic giant branch (post-AGB) star with a unique optical spectrum displaying rare emission and absorption features from molecular gas at a temperature of about 500 K. In 2022, we serendipitously discovered that its optical spectrum had drastically changed since the last observations in the 1990s. Aims. We aim to characterize the drastic change in the spectrum and analyze the photometric behavior of the object since 1989. Methods. Optical high-resolution spectra of U Equ from the Southern African Large Telescope were supplemented by archival data and near-infrared photometry from the Nordic Optical Telescope. New spectral line observations with the Effelsberg 100 m radio telescope and Atacam Large Millimeter Array are presented. Radiative transfer modeling of multiple epoch spectral energy distributions was performed. Results. No circumstellar molecular features are present in the contemporary optical spectra of U Equ. Nonphotospheric absorption and emission from neutral and ionized species dominate the current spectrum. Some of the observed features indicate an outflow with a projected terminal velocity of 215 km s−1. Broad H&K lines of [Ca II] indicate a photosphere of spectral type F or similar. For the first time, we find SiO J = 1−0 υ=1 maser emission in U Equ. Our collected photometric measurements show that the source has monotonically increased its optical and near-infrared fluxes since about the beginning of this century and continues to do so. The current rise in the optical regime is about 1 mag. Spectral energy distributions at different epochs show dusty circumstellar material that is very likely arranged in a highly inclined disk. Adopting a distance of 4 kpc, informed by the Gaia parallax of U Equ, we find that the source luminosity is about 104 L⊙. This luminosity has likely increased by a factor of a few in the last decades, which is most probably related to the drastic change in the optical circumstellar spectrum of the object. Conclusions. The object has changed considerably in the past three decades, either due to geometrical reconfiguration of the circumstellar medium, evolutionary changes in the central star, or owing to an accretion event that started very recently in the system. Observationally, U Equ appears to resemble category 0 of disk-hosting post-AGB stars reported previously, especially the post-common-envelope binary HD 101584. It is uncertain whether the drastic spectral change and the associated optical and mid-infrared rise in brightness witnessed in U Equ are common in post-AGB stars, but this radical change may be related to the real-time onset of the evolution of the system into a planetary nebula. We find that the post-AGB star V576 Car has undergone a similar transformation as U Equ in the past few decades, which means that the phenomenon is not extremely rare.

A dynamic view of V Hydrae

Context. The well studied carbon star V Hydrae is known to exhibit a complex asymmetric environment made of a dense equatorial wind and high-velocity outflows, hinting at its transition from the AGB phase to the asymmetric planetary nebula phase. In addition, V Hydrae also exhibits a long secondary period of 17 yr in its light curve, suggesting the presence of a binary companion that could shape the circumstellar environment. Aims. In this paper, we aim to confirm the binary nature of V Hydrae by deriving its orbital parameters and investigating the effect of the orbital motion on the circumbinary environment. Methods. In a first step, we used a radial-velocity monitoring performed with the HERMES spectrograph to disentangle the pulsation signal of the AGB from its orbital motion and to obtain the spectroscopic orbit. We combined the spectroscopic results with astrometric information to get the complete set of orbital parameters, including the system inclination. Next, we reported the time variations of the sodium and potassium resonance doublets. Finally, following the methods used for post-AGB stars, we carried out spatio-kinematic modelling of a conical jet to reproduce the observed spectral-line modulation. Results. We found the orbital solution of V Hydrae for a period of 17 yr. We correlated the companion passage across the line of sight with the obscuration event and the blue-shifted absorption of alkaline resonant lines. Those variations were modelled by a conical jet emitted from the companion, whose opening angle is wide and whose sky-projected orientation is found to be consistent with the axis of the large-scale bipolar outflow previously detected in the radio-emission lines of CO. Conclusions. We show that the periodic variation seen for V Hydrae is likely to be due to orbital motion. The presence of a conical jet offers a coherent model to explain the various features of V Hydrae environment.

Tungsten in barium stars

Abstract Classical barium stars are red giants that received from their evolved binary companions material exposed to the slow neutron-capture nucleosynthesis, i.e., the s-process. Such a mechanism is expected to have taken place in the interiors of Thermally-Pulsing Asymptotic Giant Branch (TP-AGB) stars. As post-interacting binaries, barium stars figure as powerful tracers of the s-process nucleosynthesis, evolution of binary systems, and mechanisms of mass transfer. The present study is the fourth in a series of high-resolution spectroscopic analyses on a sample of 180 barium stars, for which we report tungsten (W, Z = 74) abundances. The abundances were derived from synthetic spectrum computations of the W i absorption features at 4843.8 Å and 5224.7 Å. We were able to extract abundances for 94 stars; the measured [W/Fe] ratios range from ∼0.0 to 2.0 dex, increasing with decreasing of metallicity. We noticed that in the plane [W/Fe] versus [s/Fe] barium stars follow the same trend observed in post-AGB stars. The observational data were also compared with predictions of the fruity and Monash AGB nucleosynthesis models. These expect values between −0.20 and +0.10 dex for the [W/hs] ratios, whereas a larger spread is observed in the program stars, with [W/hs] ranging from −0.40 to +0.60 dex. The stars with high [W/hs] ratios may represent evidence for the operation of the intermediate neuron-capture process at metallicities close to solar.

Evidence of Third Dredge-up in Post-AGB Stars in Galactic Globular Clusters

To better understand the mixing and mass loss experienced by low-mass stars as they ascend the asymptotic giant branch (AGB), I have gathered from the literature the abundances of CNO and s-process elements in post-AGB stars in Galactic globular clusters. These species are mixed to the surface during third dredge-up (3DU) events, so their abundance should increase as the star ascends the AGB. Of the 17 stars in this sample, CNO abundances are available for 11. Of these, four are enhanced in CNO relative to the red giant branch stars from which they descended, which I take as evidence of 3DU on the AGB. The enhancement is mainly in the form of carbon. Of the six stars for which only heavy-element abundances are available, one shows s-process enhancements that previous authors have interpreted as evidence of 3DU. Combining these 17 stars with other recent samples reveals that most globular-cluster post-AGB stars have luminosities log(L/L⊙)∼3.25 . They are the progeny of blue horizontal-branch (HB) stars in clusters with intermediate metallicity ([Fe/H] ∼ −1.5). A second group consists of sub-luminous stars associated with high-metallicity clusters ([Fe/H] ∼ −1.0) with red HBs. They may be burning helium, rather than hydrogen. A third group of hot, super-luminous stars is evolving quickly across the Hertzsprung–Russell diagram. Some of them may be merger remnants.

S-Process Enriched Evolved Binaries in the Galaxy and the Magellanic Clouds

Abstract Post-asymptotic giant branch stars (post-AGB) in binary systems, with typical orbital periods between $\sim\!100$ to $\sim$ 1 000 days, result from a poorly understood interaction that terminates their precursory AGB phase. The majority of these binaries display a photospheric anomaly called ‘chemical depletion’, thought to arise from an interaction between the circumbinary disc and the post-AGB star, leading to the reaccretion of pure gas onto the star, devoid of refractory elements due to dust formation. In this paper, we focus on a subset of chemically peculiar binary post-AGBs in the Galaxy and the Magellanic Clouds (MCs). Our detailed stellar parameter and chemical abundance analysis utilising high-resolution optical spectra from VLT+UVES revealed that our targets span a $T_{\rm eff}$ of 4 900–7 250 K and [Fe/H] of −0.5 - −1.57 dex. Interestingly, these targets exhibit a carbon ([C/Fe] ranging from 0.5 - 1.0 dex, dependant on metallicity) and s-process enrichment ( $\textrm{[s/Fe]}\,\geq\!1$ dex) contrary to the commonly observed chemical depletion pattern. Using spectral energy distribution (SED) fitting and period–luminosity–colour (PLC) relation methods, we determine the luminosity of the targets (2 700–8 300 $\rm L_{\odot}$ ), which enables confirmation of their evolutionary phase and estimation of initial masses (as a function of metallicity) (1–2.5 $\textrm{M}_{\odot}$ ). In conjunction with predictions from dedicated ATON stellar evolutionary models, our results indicate a predominant intrinsic enrichment of carbon and s-process elements in our binary post-AGB targets. We qualitatively rule out extrinsic enrichment and inherited s-process enrichment from the host galaxy as plausible explanations for the observed overabundances. Our chemically peculiar subset of intrinsic carbon and s-process enriched binary post-AGBs also hints at potential variation in the efficiency of chemical depletion between stars with C-rich and O-rich circumbinary disc chemistries. However, critical observational studies of circumbinary disc chemistry, along with specific condensation temperature estimates in C-rich environments, are necessary to address gaps in our current understanding of disc-binary interactions inducing chemical depletion in binary post-AGB systems.

Systematic Search for Water Fountain Candidates Using the Databases of Circumstellar Maser Sources

Water fountains (WFs) are thought to be objects in the morphological evolution of the circumstellar envelopes of low- and intermediate-mass evolved stars, transitioning from spherically symmetric to asymmetric shapes. We used databases of circumstellar 1612 MHz OH and 22.235 GHz H2O maser sources to search for new WF candidates using the criterion of a larger velocity range of the H2O maser emission compared to that of the OH maser emission. Thus, it is in principle possible to identify WFs with H2O velocity ranges smaller than those for the previously known WFs. For the OH maser line, we analyzed database entries of 8474 observations from 2195 sources, and 6085 observations from 3642 sources for the H2O maser line. After a close examination of the velocity ranges and line profiles, we identified 11 sources that meet the criterion mentioned above. We examined the IRAS colors of the selected sources and found that two of them (IRAS 19069+0916 and IRAS 19319+2214) are in the color region for post-AGB stars. We find that the maser velocity criterion can discover other astrophysically interesting objects than just WFs. Such objects may include peculiar planetary nebulae with maser emissions and stellar merger remnants.