Post-asymptotic Giant Branch Stars Research Articles

Discovery of a hot post-AGB star in Galactic globular cluster E3

We report a new hot post-asymptotic giant branch (PAGB) star in the Galactic globular cluster (GC) E3, which is one of the first of the identified PAGB stars in a GC to show a binary signature. The star stands out as the brightest source in E3 in the Astrosat/UVIT images. We confirmed its membership with the cluster E3 using Gaia DR3 kinematics and parallax measurements. We supplemented the photometric observations with radial velocities (RVs) from high-resolution spectroscopic observations at two epochs and with ground- and space-based photometric observations from 0.13 μm to 22 μm. We find that the RVs vary over ∼6 km s−1 between the two epochs. This is an indication of the star being in a binary orbit. A simulation of possible binary systems with the observed RVs suggests a binary period of either 39.12 days or 17.83 days with mass ratio q ≥ 1.0. The [Fe/H] derived using the high-resolution spectra is ∼−0.7 dex, which closely matches the cluster metallicity. The spectroscopic and photometric measurements suggest Teff and logg of the star as 17 500 ± 1000 K and 2.37 ± 0.20 dex, respectively. Various PAGB evolutionary tracks on the Hertzsprung–Russell (H–R) diagram suggest a current mass of the star in the range 0.51–0.55 M⊙. The star is enriched with C and O abundances, showing similar CNO abundances compared to the other PAGB stars in GCs with the evidence of the third dredge-up on the AGB phase.

Chemical abundances of LINER galaxies – nitrogen abundance estimations

ABSTRACT In this work, we investigated the nitrogen and oxygen abundances in a sample of galaxies with Low Ionization Nuclear Emission Regions (LINERs) in their nucleus. Optical spectroscopic data (3600 – 10 000 Å) of 40 LINERs from the Mapping Nearby Galaxies (MaNGAs) survey were considered. Only objects classified as retired galaxies, that is, whose main ionization sources are post-Asymptotic Giant Branch (pAGB) stars, were selected. The abundance estimates were obtained through detailed photoionization models built with the cloudy code to reproduce a set of observational emission line intensities ratios of the sample. Our results show that LINERs have oxygen and nitrogen abundances in the ranges of $\rm 8.0 \: \lesssim \: 12+\log (O/H) \: \lesssim \: 9.0$ (mean value 8.74 ± 0.27) and $\rm 7.6 \: \lesssim \: 12+\log (N/H) \: \lesssim \: 8.5$ (mean value 8.05 ± 0.25), respectively. About 70 per cent of the sample have oversolar O/H and N/H abundances. Our abundance estimates are in consonance with those for Seyfert 2 nuclei and H ii regions with the highest metallicity, indicating that these distinct object classes show similar enrichment of the interstellar medium (ISM). The LINERs in our sample are located in the higher N/O region of the N/O versus O/H diagram, showing an unexpected negative correlation between these two parameters. These results suggest that these LINERs mainly exhibit a secondary nitrogen production and could be acting some other mechanisms that deviate them from the usual theoretical secondary nitrogen production curve and the H ii regions observations. However, we did not find any evidence in our data able to support the literature suggested mechanisms. Alternatively, our results show that LINERs do not present any correlation between the N/O abundances and the stellar masses of the hosting galaxies.

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.

Analysis of the 3.2–3.3 μm Interstellar Absorption Feature on Three Milky Way Sightlines

We report new analyses of spectra of the 3.2–3.3 μm absorption feature observed in the diffuse interstellar medium toward three Milky Way sources: 2MASS J17470898 − 2829561 (2M1747) and the Quintuplet Cluster, both located in the Galactic center, and Cygnus OB2-12. The 3.2–3.3 μm interval coincides with the CH-stretching region for compact polycyclic aromatic hydrocarbons (PAHs). We focus on the 2M1747 spectrum. Its published optical depth spectrum contains residual telluric transmission features, which arise from the 0.06 difference in mean airmasses between the observations of the source and its telluric standard star. We corrected the published spectrum by adding the airmass residual optical depth spectrum. The corrected spectrum is well fit by a superposition of four Gaussians. The absorption spectra of the other two sources were also fit by four Gaussians, with similar central wavelengths, widths, and relative peak opacities. We associate the three longer wavelength Gaussians covering the 3.23–3.31 μm interval with compact PAHs in positive, neutral, and negative charge states. We identify the shortest-wavelength Gaussian, near 3.21 μm, with irregularly shaped PAHs. Constraints imposed by spectral smoothness on the corrected 2M1747 spectrum, augmented by a PAH cluster formation model for post-asymptotic giant branch stars, suggests that >99% of the PAHs in the diffuse interstellar medium reside in small clusters. This study supports the PAH hypothesis, and it suggests that a family of primarily compact PAHs with a C66H20 (circumvalene) parent is consistent with the observed mid-infrared and ultraviolet interstellar absorption spectrum.

Detection of Tm ii Lines in the Spectra of Two s-process Rich Stars

Abstract Thulium abundance is unknown in the atmospheres of low and medium-mass evolved stars. The reason is the weakness of lines and the crowded spectra. We examined the spectra of two late-type stars in a broad wavelength region to search for thulium lines. Thulium abundance was estimated in the atmospheres of barium star HD 204075 and post-asymptotic giant branch star HD 235858 for the first time, log ϵ ( Tm ) ≃ 0.1 and ≲1.3, respectively.

LAMOST J040901.83+329355.6 – a new Galactic star with Wolf–Rayet characteristics in the transitional stage from post-asymptotic giant branch to central star of a planetary nebula

ABSTRACT The similarity in physical conditions in the winds of low-mass post-asymptotic giant branch stars and evolved massive stars leads to the appearance of an interesting phenomenon of spectral mimicry. Because of this, the discovery of every new star with a Wolf–Rayet (WR) spectrum requires a special study of its evolutionary status before it can be included in the list of Galactic WR stars. A couple of years ago, LAMOST J040901.83+323955.6 (hereafter J0409+3239) was selected as a WR star in the LAMOST spectroscopic data base by machine-learning methods. In this work, we investigate its evolutionary status. After analysing the spatial location of J0409+3239 in the Galaxy and its position in the colour–magnitude diagram, we conclude that J0409+3239 is instead a low-mass object displayng the WR phenomenon. Its luminosity is $L*=1000~\rm L_\odot$ and its effective temperature is Teff = 40 000 K. Using new and archival photometric data, we detected irregular variability on time-scales from hours to tens of days with an amplitude of up to ~0.2 mag. A comparison of the spectrum obtained in 2022 with that from 2014 also shows evidence of spectral variability. The absence of a clearly detected circumstellar nebula prevents the classification of J0409+3239 as [WR], namely as the central star of a planetary nebula (CSPN). However, the position of J0409+3239 on the Hertzsprung–Russell diagram suggests that this object is a low-mass star caught in a rare transitional phase to CSPN. Estimation of the J0409+3239 mass based on evolutionary tracks shows that it is less than $0.9~\rm M_\odot$, and thus that the age of the Galaxy is barely sufficient for the star to have evolved to its current stage.

Distance to Leo A Based on a Post-asymptotic-giant-branch Star

A yellow post-asymptotic-giant-branch (PAGB) star was recently identified in the dwarf galaxy Leo A. Using this yellow PAGB star, I derived a distance of 794.3 ± 10.0 kpc to Leo A, illustrating the yellow PAGB star can be used as a distance indicator.

Emission-line properties of IllustrisTNG galaxies: from local diagnostic diagrams to high-redshift predictions for JWST

ABSTRACT We compute synthetic, rest-frame optical and ultraviolet (UV) emission-line properties of galaxy populations at redshifts from z ≈ 0 to = 8 in a full cosmological framework. We achieve this by coupling, in post-processing, the cosmological IllustrisTNG simulations with new-generation nebular-emission models, accounting for line emission from young stars, post-asymptotic giant branch (PAGB) stars, accreting black holes (BHs) and, for the first time, fast radiative shocks. The optical emission-line properties of simulated galaxies dominated by different ionizing sources in our models are largely consistent with those expected from classical diagnostic diagrams and reflect the observed increase in [O iii]/Hβ at fixed [N ii]/Hα and the evolution of the Hα, [O iii]λ5007, and [O ii] λ3727 luminosity functions from z ≈ 0 to ∼ 2. At higher redshift, we find that the emission-line galaxy population is dominated by star-forming and active galaxies, with negligible fractions of shock- and PAGB-dominated galaxies. We highlight 10 UV-diagnostic diagrams able to robustly identify the dominant ionizing sources in high-redshift galaxies. We also compute the evolution of several optical- and UV-line-luminosity functions from z = 4 to 7, and the number of galaxies expected to be detectable per field of view in deep, medium-resolution spectroscopic observations with the NIRSpec instrument on board the JWST. We find that 2-h-long exposures are sufficient to achieve unbiased censuses of Hα and [O iii]λ5007 emitters, while at least 5 h are required for Hβ, and even 10 h will detect only progressively smaller fractions of [O ii] λ3727, O iii] λ1663, C iii] λ1908, C iv λ1550, [N ii]λ6584, Si iii] λ1888, and He ii λ1640 emitters, especially in the presence of dust.

PM 1-322: New variable planetary nebula

Context. Spectra of planetary nebulae (PNe) are characterised by strong forbidden emission lines and often also by an infrared (IR) excess. A few PNe show dust obscuration events and/or harbour long-period binaries. Some post-asymptotic giant branch stars, symbiotic stars, or B[e] stars may feature similar characteristics. Recently, dust clouds eclipsing white dwarfs were also detected. Aims. We report the discovery of an object with a very peculiar variability pattern that bears signatures compatible with the above-mentioned classes of objects. The object is ZTFJ201451.59+120353.4 and identifies with PM 1-322. Methods. The object was discovered in Zwicky Transient Facility archival data and investigated with historical and newly obtained photometric and spectroscopic observations. Results. The ZTF r and g data show a one magnitude deep, eclipse-like event with a duration of about half a year that occurred in 2022. The variability pattern of the star is further characterised by several dimming events in the optical region that are accompanied by simultaneous brightenings in the red and IR regions. Apart from that, two fast eruption-like events were recorded in ZTF r data. Archival data from WISE indicate long-term variability with a possible period of 6 or 12 yr. Our follow-up time series photometry reveals a stochastic short-term variability with an amplitude of about 0.1 mag on a timescale of about one hour. The spectral energy distribution is dominated by IR radiation. Our high-resolution spectroscopy shows strong forbidden emission lines from highly ionised species and symmetric double-peaked emission in Hα, which is very different from what is seen in earlier spectra obtained in 2007. Conclusions. Several explanatory scenarios are presented. Our most likely interpretation is that our target object involves a hot central star surrounded by gaseous and dusty disks, an extended nebula, and a possible companion star. Further observations are required to shed more light on the true nature of this enigmatic object.

Second-generation protoplanetary discs around evolved binaries: a high-resolution polarimetric view with SPHERE/IRDIS

ABSTRACT Binary post-asymptotic giant branch (post-AGB) stars are products of a poorly understood binary interaction process that occurs during the AGB phase. These systems comprise a post-AGB primary star, a main-sequence secondary companion and a stable circumbinary disc. Studying the structure and properties of these circumbinary discs is crucial for gaining insight into the binary interaction process that governs post-AGB binaries as well as comprehending the disc’s creation, evolution, and its interaction with the post-AGB binary system. We aim to use near-infrared polarimetric imaging to investigate the morphology and potential substructures of circumbinary discs around eight representative post-AGB binary stars. To achieve this, we performed polarimetric differential imaging in H and Y bands using the high-angular resolution capabilities of the European Southern Observatory-Very Large Telescope/SPHERE-Infra-Red Dual-beam Imaging and Spectroscopy instrument. We resolved the extended circumbinary disc structure for a diverse sample of eight post-AGB binary systems. Our analysis provided the first estimates of the disc scale height for two of the systems: IW Car and IRAS 15469–5311. We also investigated the morphological differences between the full discs (with the inner rim at the dust sublimation radius) and transition discs (which are expected to have larger inner cavities), as well as similarities to protoplanetary discs around young stellar objects. We found that the transition discs displayed a more intricate and asymmetric configuration. Surprisingly, no correlation was found between the overresolved flux in near-infrared interferometric data and the polarimetric observations, suggesting that scattering of light on the disc surface may not be the primary cause of the observed overresolved flux component.

Episodes of Molecular Emission in the Optical Spectrum of IRAS 22272+5435

Emission of carbon-bearing molecular lines in multiple optical spectra of IRAS 22272+5435 from 2015 to 2017 is detected, and, for the first time in post–asymptotic giant branch stars, real-time evolution of these features is observed. Specifically, lines of various vibrational transitions of the CN Red system and C2 Swan system are identified to be in emission. Contrary to the narrow and blueshifted AGB ejecta absorption lines, the emission profiles are broad and redshifted or blueshifted by no more than a few kilometers per second. Molecular emissions appear to be exclusive to spectra corresponding to pulsation phases closer to the star’s light maxima than minima, but there are also such spectra without emissions. The time required for emissions to reach the largest observed intensity or decrease to absorption is around 2–3 weeks. The C2 Phillips system lines are observed to have emission components as well, but no evidence for their short-term variability is found. It is proposed that the observed emissions originate within few stellar radii above the photosphere.

A study of carbon-rich post-AGB stars in the Milky Way to understand the production of carbonaceous dust from evolved stars

Context. Knowledge of the Gaia, DR3 parallaxes of Galactic post-asymptotic giant branch (AGB) stars makes it possible to exploit these objects as tracers of AGB evolution, nucleosynthesis, and dust production as well as to use them to shed new light on still poorly known physical processes experienced by AGB stars. Aims. The goal of this study is to reconstruct the evolution and the dust formation processes during the final AGB phases of a sample of carbon-rich, post-AGB Galactic stars, with particular attention to the determination of the past mass-loss history. Methods. We study the IR excess of Galactic sources classified as post-AGB single stars by means of dust formation modelling where dust grains form and grow in a static wind and expand from the surface of the star. The method is applied to various evolutionary stages of the final AGB phase of stars with different masses and metallicities. The results from a spectral energy distribution (SED) fitting are used to infer information on mass loss, efficiency of dust formation, and wind dynamics. Results. The detailed analysis of the SED of the sources investigated, which included the derivation of the luminosities and the dust properties, allows us to confirm previous results, mostly based on the surface chemical composition, that most of the investigated sources descend from low-mass (M < 1.5 M⊙) progenitors that reached the C-star stage. Metal-poor carbon stars are characterised by higher IR excesses with respect to their more metal-rich counterparts of similar luminosity due to a higher surface carbon-to-oxygen excess. This work confirms previous conclusions based on a limited sample of carbon-rich post-AGB objects in the Magellanic Clouds, namely that more luminous stars descending from higher-mass progenitors are generally more opaque due to shorter evolutionary timescales that place the dust shell closer to the central object. Through the study of the dynamics of the outflow and results from stellar evolution modelling, we find that the mass-loss rate at the tip of the AGB phase of metal-rich low-mass carbon stars is approximately 1#x2212;1.5 × 10−5 M⊙ yr−1, whereas in the metal-poor domain Ṁ ∼ 4 − 5 × 10−5 M⊙ yr−1 is required. These results indicate the need for an upwards revision of the theoretical mass-loss rates of low-mass carbon stars in the available literature, which in turn require a revised determination of carbon dust yields by AGB stars.

Polarisation of molecular lines in the circumstellar envelope of the post-asymptotic giant branch star OH 17.7–2.0

Context. The role of magnetic field in shaping planetary nebulae (PNe), either directly or indirectly after being enhanced by binary interaction, has long been a topic of debate. Large-scale magnetic fields around pre-PNe have been inferred from polarisation observations of masers. However, because masers probe very specific regions, it is still unclear if the maser results are representative of the intrinsic magnetic field in the circumstellar envelope (CSE). Aims. Molecular line polarisation of non-maser lines can provide important information about the magnetic field. A comparison between the magnetic field morphology determined from maser observations and that observed in the more diffuse CO gas can reveal if the two tracers probe the same magnetic field. Methods. We compared observations taken with the Atacama Large Millimeter/submillimeter Array (ALMA) of molecular line polarisation around the post-asymptotic giant branch (post-AGB) or pre-PN star OH 17.7−2.0 with previous observations of polarisation in the 1612 MHz OH maser region. Earlier mid-infrared observations indicate that OH 17.7−2.0 is a young bipolar pre-PN, with both a torus and bipolar outflow cavities embedded in a remnant AGB envelope. Results. We detect CO J = 2 − 1 molecular line polarisation at a level of ∼4% that displays an ordered linear polarisation structure. We find that, correcting for Faraday rotation of the OH maser linear polarisation vectors, the OH and CO linearly polarised emission trace the same large-scale magnetic field. A structure function analysis of the CO linear polarisation reveals a plane-of-the-sky magnetic field strength of B⊥ ∼ 1 mG in the CO region, consistent with previous OH Zeeman observations. Conclusions. The consistency of the ALMA CO molecular line polarisation observation with maser observations indicate that both can be used to determine the magnetic field strength and morphology in CSEs. The new observations indicate that the magnetic field has a strong toroidal field component projected on the torus structure and a poloidal field component along the outflow cavity. The existence of a strong, ordered, magnetic-field around OH 17.7−2.0 indicates that the magnetic field is likely involved in the formation of this bipolar pre-PN.

The nebula around the binary post-AGB star 89 Herculis

Context. There is a class of binary post-asymptotic giant branch (post-AGB) stars that exhibit remarkable near-infrared (NIR) excess. These stars are surrounded by disks with Keplerian or quasi-Keplerian dynamics and outflows composed of gas escaping from the rotating disk. Depending on the dominance of these components, there are two subclasses of binary post-AGB stars: disk-dominated and outflow-dominated. Aims. We aim to properly study the hourglass-like structure that surrounds the Keplerian disk around 89 Her. Methods. We present total-power on-the-fly maps of 12CO and 13CO J = 2 − 1 emission lines in 89 Her. Previous studies are known to suffer from flux losses in the most extended components. We merge these total-power maps with previous NOEMA maps. The resulting combined maps are expected to detect the whole nebula extent of the source. Results. Our new combined maps contain the entirety of the detectable flux of the source and at the same time are of high spatial resolution thanks to the interferometric observations. We find that the hourglass-like extended outflow around the rotating disk is larger and more massive than suggested by previous works. The total nebular mass of this very extended nebula is 1.8 × 10−2 M⊙, of which ∼65% comes from the outflow. The observational data and model results lead us to classify the envelope around 89 Her as an outflow-dominated nebula, together with R Sct and IRAS 19125+0343 (and very probably AI CMi, IRAS 20056+1834, and IRAS 18123+0511). The updated statistics on the masses of the two post-AGB main components reveal that there are two distinct subclasses of nebulae around binary post-AGB stars depending on which component is the dominant one. We speculate that the absence of an intermediate subclass of sources is due to the different initial conditions of the stellar system and not because both subclasses are in different stages of the post-AGB evolution.

Study of oxygen-rich post-AGB stars in the Milky Way as a means to explain the production of silicates among evolved stars

Context. The study of post-asymptotic giant branch (post-AGB) stars is a valuable tool in improving our understanding of poorly known aspects of the evolution of the stars throughout the asymptotic giant branch (AGB). This can be done thanks to the availability of more accurate determinations of their surface chemical composition and the peculiar shape of their spectral energy distribution (SED): the emission from the central star can be easily disentangled from the contribution from the dusty shell, which can then be characterized. Aims. The goal of the present study is to reconstruct the dust formation process and, more generally, the late phases of evolution for oxygen-rich stars across the AGB phase. This is performed by studying oxygen-rich, post-AGB stars and analyzing them in terms of their luminosity, effective temperature, and infrared excess. Methods. We studied sources classified as single, oxygen-rich, post-AGB stars in the Galaxy that exhibit a double-peaked (shell-type) SED. We used results from stellar evolution modeling, combined with dust formation and radiative transfer modeling, to reconstruct late AGB phases and the initial contraction to the post-AGB phase. We also determined the mass-loss and dust-formation rates for stars of different masses and chemical compositions. Results. The analysis of the IR excess of the post-AGB, oxygen-rich stars examined in this study outlines an interesting complexity with regard to the correlation between the dust in the surroundings of the stars, the evolutionary status, and the progenitor’s mass. The sources descending from massive AGBs (> 3 M⊙, depending on metallicity) are generally characterized by higher infrared excess than the lower mass counterparts, owing to the more intense dust formation taking place during the final AGB phases. From the determination of the location of the dusty regions, we can deduce that the expanding velocities of the outflow change significantly from star to star. We also discuss the possibility that radiation pressure is not able of accelerating the wind in the faintest objects.

Analysis of line intensity ratio for optical transitions of 3d6 levels and plasma screening effect on atomic structure of Fe III ion

Iron emission lines are observed from most classes of astronomical objects through a wide spectral range from infrared to X-ray. For a useful analysis of these lines, accurate atomic data for neutral iron and its ions is required. In this work, we have done calculations of energy levels and radiative data of Fe III by using the relativistic configuration interaction method with the implementation of the flexible atomic code (FAC). We have calculated emission-line intensity ratios for optical transitions among the fine-structure levels of the 3d6 configuration at temperature 104 K and density ne=104cm−3. We have also compared our results with the observed spectra of Herbig-Haro objects HH 202 of Orion Nebula, Planetary Nebula, and NGC 2392, post-asymptotic giant branch star (PAGB) HD341617. We have also discussed various discrepancies between the observed spectra from these nebulae and our calculations. The atomic structure of plasma-embedded Fe III is also analyzed using flexible atomic code (FAC). The graphical study shows that transition wavelengths, oscillator strengths, and transition rates are very sensitive to the plasma environment in the density range 1021cm−3 to 5×1022cm−3 and spectral lines associated to Δn = 0 transitions for Fe III ion are blueshifted. We have also studied the dependence of emission line intensity ratios on plasma density and temperature. We believe that our presented results are useful in spectroscopic analysis and examination of astrophysical and laboratory plasma.

Origins of B-type stars at high Galactic latitudes based on abundances and kinematics

ABSTRACT The spectral analysis and kinematic studies of faint blue stars of O and B types above the mid-plane of the Milky Way can lead to a better understanding of their populations and origins. In this paper, we present an analysis of the chemical composition and kinematics of 17 B-type stars. Our sample includes 13 B-type stars with |Z| > 0.5 kpc and |b| ≥ 20°, three B-type stars with |Z| ≤ 0.5 kpc and |b| < 20°, and a hot post-asymptotic giant branch (post-AGB) star without infrared excess, in the Milky Way. Our analysis rests on medium-resolution spectra and non-local thermodynamic equilibrium tlusty model atmospheres. The ages and masses of the stars are obtained using solar-metallicity PARSEC isochrones, and we calculate their velocities and Galactic trajectories using galpy and data from the Gaia EDR3. The larger projected rotational velocities and results of abundance analysis of 16 B-type stars indicate that they are likely to be B-type stars of normal Population I. The three B-type stars with |Z| ≤ 0.5 kpc and |b| < 20° can be formed in the Galactic disc. The origins of the 13 B-type stars with |Z| > 0.5 kpc and |b| ≥ 20° are as follows. (i) Analysis of the C and Mg abundances, kinematics and orbits show that 10 of them originate in the Galactic disc at different Galactocentric distances. They may be scattered from the disc via the binary ejection mechanism or the dynamical ejection mechanism. (ii) The three stars presenting abnormal C or Mg abundances and counter-rotating Vϕ might be post-AGB stars accreted from a disrupted satellite of the Milky Way.

Modelling of the post-asymptotic giant branch phase as a tool to understand asymptotic giant branch evolution and nucleosynthesis

ABSTRACT We study a sample of single Galactic post-asymptotic giant branch (post-AGB) stars with known surface chemical composition. Gaia EDR3 and Gaia DR3 parallaxes have enabled accurate determinations of the luminosities of these objects, thus making it possible to characterize them in terms of their initial masses, chemical compositions, and progenitor ages. We used extant evolutionary sequences of asymptotic giant branch (AGB) stars produced using the ATON stellar evolutionary code extended to the post-AGB phase, specifically for this study. The luminosity and surface carbon abundance together prove to be the most valuable indicator of the previous evolution and nucleosynthetic history of the star, particularly regarding the efficiency of mixing mechanisms and the growth of the core. This analysis allows us to place the post-AGB sample into classes based on their evolutionary history on the AGB. This includes low-mass objects that evolved as M-type stars without third dredge-up, carbon stars, and intermediate-mass AGB stars with hot bottom burning (HBB). Additionally, our analysis reveals that AGB nucleosynthesis is not homogeneous, even for stars with similar initial masses and metallicities. The approach of using observations of post-AGB stars to constrain AGB and post-AGB models shed new light on still debated issues related to the AGB evolution, such as the threshold mass required to become a C-star, the minimum mass for HBB and the amount of carbon that can be accumulated in the surface regions of the star during the AGB lifetime, as well as deep mixing experienced during the red giant branch phase.

Understanding dust production and mass loss in the AGB phase using post-AGB stars in the Magellanic Clouds

Context. The asymptotic giant branch (AGB) phase of evolution in low- and intermediate-mass stars is governed by poorly understood physical mechanisms, such as convection, mixing, dust production and mass loss, which play a crucial role in determining the internal structure and the evolution of these stars. The spectra of post-asymptotic giant branch (post-AGB) stars hold critical chemical fingerprints that serve as exquisite tracers of the evolution, nucleosynthesis, and dust production during the AGB phase. Aims. We aim to understand the variation in the surface chemistry that occurs during the AGB phase by analysing results from observations of single post-AGB stars in the Magellanic Clouds. We also aim to reconstruct dust-formation processes, which are active in the circumstellar envelope of AGB stars, occurring towards the end of the AGB phase and during the subsequent course of evolution when contraction to the post-AGB has begun. Methods. We study likely single post-AGB sources in the Magellanic Clouds that exhibit a double-peaked (shell-type) spectral energy distribution (SED). We interpret their SED by comparing with results from radiative transfer calculations to derive the luminosity and the dust content of the individual sources. Additionally, we compare the observationally derived stellar parameters and the photospheric chemical abundances of the target sample with results from stellar evolution modelling of AGB and post-AGB stars. This allows for the characterization of the individual sources in terms of the initial mass and formation epoch of the progenitors. The theoretically derived dust mineralogy and optical depth is used to assess when dust formation ceases and to determine the propagation velocity of the dust-gas system during post-AGB evolution. Results. We find that amongst our target sample of 13 likely single post-AGB stars with shell-type SED, eight objects are carbon stars descending from ∼1−2.5 M⊙ progenitors. Five of the 13 objects are of lower mass, descending from M < 1 M⊙ stars. Based on the dust mineralogy, we find that these five stars are surrounded by silicate dust, and thus failed to become carbon stars. The dust optical depth and the luminosity of the stars are correlated, owing to the faster evolutionary timescale of brighter stars, which makes the dusty layer closer to the central object. From our detailed analysis of the SEDs, we deduce that the dust currently observed around post-AGB stars was released after the onset of the central star contraction and an increase in the effective temperature to ∼3500−4000 K.

The 3.3 μm Infrared Emission Feature Profile in HD 44179

HD 44179 is a postasymptotic giant branch star with a biconical nebula known as the Red Rectangle. It shows the 3.3, 3.4, 6.2, 7.7, 8.6, and 11.3 μm infrared emission features (commonly known as the polycyclic aromatic hydrocarbon (PAH) emission bands and the Aromatic Infrared Bands). Previous observations of HD 44179 showed that the width of the 3.3 μm emission feature was narrower within 2″ of HD 44179 and wider further away from the star. This was interpreted as an indication that the 3.3 μm emission feature arises from two types of PAHs with spatially varying relative abundances. With new observations of HD 44179 taken with the NASA Infrared Telescope Facility we find that the width of the 3.3 μm emission feature profile is not narrower on HD 44179 and conclude that there is no evidence for two spatially varying components.