We report a spectropolarimetric study of the post-AGB binary system 89 Herculis, based on data acquired with the high-resolution Catania Astrophysical Observatory Spectropolarimeter, the HArps-North POlarimeter and the Echelle SpectroPolarimetric Device for the Observation of Stars.
The linear polarization clearly detected across single atmospheric lines in absorption is characterized by complex Q and U morphologies that are variable with the orbital period of the system. Gauss-level magnetic fields, continuum depolarization due to pulsations, hot spots and scattering in the circumstellar environment were excluded as possible origin of the observed polarization.
In the context of the optical pumping mechanism, we suggest that the anisotropy of the stellar radiation field, fuelled by the close binary companion, can be responsible of the observed periodic properties of the spectral line polarization.
We conclude that high resolution linear spectropolarimetry could be an important diagnostic tool in the study of aspherical envelopes of cool and evolved stars.
There is a class of binary post-AGB stars with a remarkable near-infrared excess that are surrounded by Keplerian or quasi-Keplerian disks and extended outflows composed of gas escaping from the disk. The Keplerian dynamics had been well identified in four cases, namely the Red Rectangle, AC Her, IW Car, and IRAS 08544-4431. In these objects, the mass of the outflow represents ~ 10 % of the nebular mass, the disk being the dominant component of the nebula. We present interferometric NOEMA maps of 12CO and 13CO J=2-1 in 89 Her and 12CO J=2-1 in AC Her, IRAS 19125+0343, and R Sct. Several proper…
We studied the polarised spectrum of the post-AGB binary system 89\,Herculis on the basis of data collected with the high resolution \emph{Catania Astrophysical Observatory Spectropolarimeter}, \emph{HArps-North POlarimeter} and \emph{Echelle SpectroPolarimetric Device for the Observation of Stars}. We find the existence of linear polarisation in the strongest metal lines in absorption and with low excitation potentials. Signals are characterized by complex Q and U morphologies varying with the orbital period. As possible origin of this "Second Solar Spectrum"-like behaviour, we rule out magne…
We present a long-term optical spectroscopic study of the post-AGB binary system 89 Herculis, with the aim to characterize the relationship between photospheric instabilities and dynamics in the close circumstellar environment of the system. This study is based on spectra acquired with the high-resolution Catania Astrophysical Observatory Spectropolarimeter and archive data, covering a time interval between 1978 and 2018. We find long-term changes in the radial velocity curve of the system, occurring mostly in amplitude, which correlate with the variability observed in the blue-shifted absorpt…
Binary post-AGB stars are interesting laboratories to study both the evolution of binaries as well as the structure of circumstellar disks. A multiwavelength high angular resolution study of the prototypical object 89 Herculis is performed with the aim of identifying and locating the different emission components seen in the SED. A large interferometric data set, collected over the past decade and covering optical and near-IR wavelengths, is analyzed with simple geometric models. Combining the interferometric constraints with the photometry and the optical spectra, we reassess the energy budge…
The presence of disks and outflows is widespread among post-AGB binaries. In the first paper of this series, a surprisingly large fraction of optical light was found to be resolved in the 89 Her post-AGB system. The data showed this flux to arise from close to the central binary. Scattering off the inner rim of the circumbinary disk, or in a dusty outflow were suggested as two possible origins. With detailed dust radiative transfer models of the disk we aim to discriminate between these two configurations. By including Herschel/SPIRE photometry, we extend the SED such that it now fully covers …
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. We aim to properly study the hourglass-like structure that surrounds the Keplerian disk around 89 Her. We present total-power on-the-fly maps of $^{12}$CO and $^{13}$CO $J$=2-1 emission lines in 89…
Binaries with circumbinary disks are commonly found among optically bright post-AGB stars. Although clearly linked to binary interaction processes, the formation, evolution and fate of these disks are still badly understood. Due to their compactness, interferometric techniques are required to resolve them. Here, we discuss our high-quality multiwavelength interferometric data of two prototypical yet very different post-AGB binaries, AC and 89 Herculis, as well as the modeling thereof with radiative transfer models. A detailed account of the data and models of both objects is published in three…
There is a class of binary post-AGB stars (binary system including a post-AGB star) that are surrounded by Keplerian disks and outflows resulting from gas escaping from the disk. To date, there are seven sources that have been studied in detail through interferometric millimeter-wave maps of CO lines (ALMA/NOEMA). For the cases of the Red Rectangle, IW Carinae, IRAS 08544-4431, and AC Herculis, it is found that around greater than 85% of the total nebular mass is located in the disk with Keplerian dynamics. The remainder of the nebular mass is located in an expanding component. This outflow is…
We present various infrared two-color diagrams (2CDs) for AGB stars, post-AGB stars, and Planetary Nebulae (PNe) and investigate possible evolutionary tracks. We use catalogs from the available literature for the sample of 4903 AGB stars (3373 O-rich; 1168 C-rich; 362 S-type), 660 post-AGB stars (326 post-AGB; 334 pre-PNe), and 1510 PNe in our Galaxy. For each object in the catalog, we cross-identify the IRAS, AKARI, MSX, and 2MASS counterparts. The IR 2CDs can provide useful information about the structure and evolution of the dust envelopes as well as the central stars. To find possible evol…
We present the results of our search for low- and intermediate mass evolved stars in the outer Galaxy using AllWISE catalogue photometry. We show that the [3.4]-[12] versus [4.6]-[22] colour-colour diagram is most suitable for separating C-rich/O-rich AGB and post- AGB star candidates. We are able to select 2,510 AGB and 24,821 post-AGB star candidates. However, the latter are severely mixed with the known young stellar objects in this diagram.
This review presents the latest advances in the nebular studies of post-AGB objects. Post-AGB stars are great tools to test nucleosynthesis and evolution models for stars of low and intermediate masses, and the evolution of dust in harsh environment. I will present the newly discovered class of post-RGB stars, formed via binary interaction on the RGB. Binary systems can also lead to the formation of two class of aspherical post-AGB, the Proto-Planetary Nebulae and the naked post-AGBs (dusty RV Taus , a.k.a. Van Winckel's stars).
There is ample evidence for strong magnetic fields in the envelopes of (Post-)Asymptotic Giant Branch (AGB) stars as well as supergiant stars. The origin and role of these fields are still unclear. This paper updates the current status of magnetic field observations around AGB, post-AGB stars and describes their possible role during these stages of evolution. The discovery of magnetically aligned dust around a supergiant star is also highlighted. In our search for the origin of the magnetic fields, recent observations show the signatures of possible magnetic activity and rotation, indicating t…
We report HI 21-cm line observations of the AGB star X Her obtained with the Green Bank Telescope (GBT) and the Very Large Array (VLA). We have detected HI emission totaling M_HI=2.1e-03 M_sun associated with the circumstellar envelope of the star. The HI distribution exhibits a head-tail morphology, similar to those previously observed around Mira and RS Cnc. The tail extends ~6.0' (0.24 pc) in the plane of the sky, along the direction of the star's space motion. We also detect a velocity gradient of ~6.5 km/s across the envelope, consistent with the HI tracing a turbulent wake that arises fr…
A sample of 27 sources, catalogued as pre-main sequence stars by the Pico dos Dias Survey (PDS), is analyzed to investigate a possible contamination by post-AGB stars. The far-infrared excess, due to dust present in the circumstellar envelope, is typical for both categories: young stars and objects that have already left the main sequence and are suffering a severe mass-loss. The presence of two known post-AGB stars in our sample inspired us to seek for other very likely or possible post-AGB objects among PDS sources previously suggested to be Herbig Ae/Be stars, by revisiting the observationa…
Post-AGB stars evolve on a very fast track and hence not many are known. Their spectral properties make them, in principle, ideal objects to test our theories on the late phases of stellar evolution. This has, however, proven much more difficult than anticipated, mainly because the morphological, dynamical and chemical diversity in Galactic post-AGB stars is very large indeed. Here I focus on recent results and touch upon the bright near future of post-AGB research.
Obscured by their circumstellar dusty envelopes post-AGB stars emit a large fraction of their energy in the infrared and thus, infrared sky surveys like IRAS were essential for discoveries of post-AGBs in the past. Now, with the AKARI infrared sky survey we can extend our knowledge about the late stages of stellar evolution. The long-term goal of our work is to define new photometric criteria to distinguish new post-AGB candidates from the AKARI data.
The investigation of post-AGB objects (proto-planetary nebulae) is very important from the standpoint of physical and chemical changes occurring during the late stages of stellar evolution. The Toruń catalogue of Galactic post-AGB and related objects is an evolutive catalogue containing astrometric, photometric and spectroscopic data as well as HST images for all known post-AGB objects and candidates in our Galaxy. This free-access catalogue can serve as an ideal tool to study different groups of post-AGB objects, especially due to the fact that all information is gathered in one place. The se…
We present AKARI/IRC observations of a sample of six extremely red IRAS sources, of which three are variable OH/IR stars and the rest are early post-AGB stars. The OH/IR stars show a red continuum with the expected strong 10 micron silicate absorption feature, while the post-AGB stars show an even redder continuum accompanied with a comparably weak silicate absorption. We modelled the spectral energy distributions with DUSTY. While for the OH/IR stars a reasonable fit can be obtained with almost pure silicate dust, the post-AGB stars require a mixture of silicate and carbon-rich dust. We assum…
We intended to study the incidence and characteristics of water masers in the envelopes of stars in the post-AGB and PN evolutionary stages. We have used the 64-m antenna in Parkes (Australia) to search for water maser emission at 22 GHz, towards a sample of 74 sources with IRAS colours characteristic of post-AGB stars and PNe, at declination $< -32 °$. In our sample, 39% of the sources are PNe or PNe candidates, and 50% are post-AGB stars or post-AGB candidates. We have detected four new water masers, all of them in optically obscured sources: three in PNe candidates (IRAS 12405-6219, IRAS 15…
We present results of a survey of post-asymptotic giant branch stars (post-AGBs) at high galactic latitude. To date, few post-AGB stars are known throughout the Galaxy and the number of known members of the older populations like the galactic halo is even smaller. This study looks at the number of post-AGB stars which are produced using different synthetic population methods and compare the results with observations. The resulting synthetic populations are compared to observational results from a complete and studied subsample from the photographic Palomar-Green (PG) survey (with high resoluti…
During the last years, many observational studies have revealed that binaries play an active role in the shaping of non spherical planetary nebulae. We review the different works that lead to the direct or indirect evidence for the presence of binary companions during the Asymptotic Giant Branch, proto-Planetary Nebula and Planetary Nebula phases. We also discuss how these binaries can influence the stellar evolution and possible future directions in the field.
The AKARI and Spitzer satellites provided an unique opportunity to observe a variety of stars, which are considered as departing from the Asymptotic Giant Branch (AGB) and have started their post-AGB evolution recently. Most of these stars are absent optically and are bright in the mid-IR wavelength range. Spectra of close to 200 objects have been obtained. For all of them the 1-60 micron spectral energy distribution has been constructed using photometric data from various surveys. We report here on the results of Spitzer observations of 88 IRAS selected post-AGB candidates and discuss them in…
Hot (OB) post-AGB stars are immediate progenitors of planetary nebulae (PNe). Very few hot post-AGB stars are known. Detecting new hot post-AGB candidates and follow-up multiwavelength studies will enable us to further understand the processes during the post-AGB evolution that lead to the formation of PNe. Case-Hamburg OB star surveys and their extension (LS, LSS, and LSE catalogues) and IRAS (point source) catalogues are good sources for detecting new hot post-AGB candidates from low resolution spectroscopy. Spectral types are determined from low resolution optical spectra of 44 stars select…
Circumbinary discs are commonly observed around post-asymptotic giant branch (AGB) systems and are known to play an important role in their evolution. Several studies have pointed out that a circumbinary disc interacts through resonances with the central binary and leads to angular momentum transfer from the central binary orbit to the disc. This interaction may be responsible for a substantial increase in the binary eccentricity. We investigate whether this disc eccentricity-pumping mechanism can be responsible for the high eccentricities commonly found in post-AGB binary systems.
Many disc-type post-asymptotic giant branch (post-AGB) stars are chemically peculiar, showing underabundances of refractory elements in their photospheres that correlate with condensation temperature. The aim of this paper is to investigate how accretion from a circumbinary disc can cause this phenomenon of depletion and how this impacts the evolution of post-AGB stars. We used the \texttt{MESA} code to evolve stars in the post-AGB phase, while including accretion of metal-poor gas. We compared the models to a sample of 58 observed disc-type post-AGB stars with chemical abundance data. For eac…
We present AKARI/IRC and Spitzer/IRS observations of a selected sample of galactic IRAS sources considered to be heavily obscured AGB/post-AGB stars based on their characteristic IRAS colours. All of them are completely invisible in the optical range but extremely bright in the infrared. Based on AKARI and Spitzer spectroscopy and using DUSTY we are able to determine the dominant chemistry of their circumstellar shells as well as the properties of the dust grains contained in these shells. Most of the sources are found to be C-rich (being the reddest C-rich stars observed so far). We find only…
We present a complete study of the morphology of post-Asymptotic Giant Branch (AGB) stars. Post-AGB is a very short evolutionary phase between the end of the AGB and the beginning of the Planetary Nebula (PN) stage (between 100 and 10,000 yrs). We have defined the end of the post-AGB phase and the beginning of the PN phase when the star is hot enough to fully ionize the hydrogen envelope. Post-AGB stars have a circumstellar shell that is illuminated by the central stars or partially ionized. However, this circumstellar shell is too small to be resolved from ground-based observations. Thus, we …
Dynamical stellar-evolution modeling through the AGB phase reveals that radial pulsations with very fast-growing amplitudes develop if the luminosity to mass ratio of stars with tenuous envelopes exceeds a critical limit. An instability going nonlinear already after a few pulsation cycles might qualify as a source of the superwind - postulated to shed a substantial part of a star's envelope over a very short time - of hitherto persistently mysterious nature.
We explore modifications to the current scenario for the slow neutron capture process in asymptotic giant branch (AGB) stars to account for the Pb deficiency observed in post-AGB stars of low metallicity ([Fe/H] ~ -1.2) and low initial mass (~ 1 - 1.5 Msun) in the Large and Small Magellanic Clouds. We calculated the stellar evolution and nucleosynthesis for a 1.3 Msun star with [Fe/H]=-1.3 and tested different amounts and distributions of protons leading to the production of the main neutron source within the 13C-pocket and proton ingestion scenarios. No s-process models can fully reproduce th…
An abundance analysis based on a high-resolution spectrum is presented for a newly discovered post-AGB star in the globular cluster M79. The surprising result is that the iron abundance of the star is apparently about 0.6 dex less than that of the cluster's red giants as reported by published studies including a recent high-resolution spectroscopic analysis by Carretta and colleagues. Abundances relative to iron appear to be the same for the post-AGB star and the red giants for the 15 common elements. It is suggested that the explanation for the lower abundances of the post-AGB star may be tha…
Post-AGB stars are the final stage of evolution of low-intermediate mass stars (M<8 Solar Masses). Those in binary systems have stable circumbinary discs. Using data from Herschel (PACS/SPIRE), we extend the SEDs of 50 galactic post-AGB binary systems to sub-millimetre wavelengths and use the slope of the SED as a diagnostic tool to probe the presence of large grains. Using a Monte Carlo radiative transfer code (MCMax), we create a large grid of models to quantify the observed spectral indices and use the presence of large grains in the disc as a proxy for evolution.
This paper is part of a larger project in which we systematically study the chemical abundances of Galactic and extragalactic post-asymptotic giant branch (post-AGB) stars. Lead (Pb) is the final product of the s-process nucleosynthesis and is predicted to have large overabundances with respect to other s-process elements in AGB stars of low metallicities. However, Pb abundance studies of enriched post-AGB stars in the Magellanic Clouds show the Pb overabundance is not observed. We used high-resolution UVES and HERMES spectra for detailed spectral abundance studies of our sample of 14 Galactic…
So far, only one rotating disk has been clearly identified and studied in AGB or post-AGB objects (in the Red Rectangle), by means of observations with high spectral and spatial resolution. However, disks are thought to play a key role in the late stellar evolution and are suspected to surround many evolved stars. We aim to extend our knowledge on these structures. We present interferometric observations of CO J=2-1 emission from the nebula surrounding the post-AGB star AC Her, a source belonging to a class of objects that share properties with the Red Rectangle and show hints of Keplerian dis…
Post-asymptotic giant branch (post-AGB) stars with discs are all binaries. Many of these binaries have orbital periods between 100 and 1000 days so cannot have avoided mass transfer between the AGB star and its companion, likely through a common-envelope type interaction. We report on preliminary results of our project to model circumbinary discs around post-AGB stars using our binary population synthesis code binary_c. We combine a simple analytic thin-disc model with binary stellar evolution to estimate the impact of the disc on the binary, and vice versa, fast enough that we can model stell…
We have carried out a search for optically visible post-AGB candidates in the Small Magellanic Cloud (SMC). We used mid-IR observations from the Spitzer Space Telescope to select optically visible candidates with a mid-IR excess. We obtained low-resolution optical spectra for 801 candidates. After removing contaminants and poor quality spectra, the final sample comprised of 63 post-AGB/RGB candidates of A - F spectral type. Using the spectra, we estimated the stellar parameters: effective temperature, surface gravity and [Fe/H]. We also estimated the reddening and deduced the luminosity using …
In this chapter the focus is on the properties of post-Asymptotic Giant Branch (post-AGB) stars in binary systems. Their Spectral Energy Distributions (SEDs) are very characteristic: they show a near-infrared excess, indicative of the presence of warm dust, while the central stars are too hot to be in a dust-production evolutionary phase. This allows for an efficient detection of binary post-AGB candidates. It is now well established that the near-infrared excess is produced by the inner rim of a stable dusty disc that surrounds the binary system. These discs are scaled-up versions of protopla…
Recently Kamath et al. (2021) analysed Gaia EDR3 data of eighteen post-AGB supergiants with overabundance of s-process elements. For some of these stars in their sample they find unusually very low absolute luminosities which is not in agreement with the s-process nucleosynthesis on the AGB stage of evolution. It appears that they have not taken into account the RUWE values of these stars. I have taken a look at the RUWE values of these eighteen post-AGB stars with over abundance of s-process elements. Of the eighteen stars thirteen have RUWE values much greater than 1.4 and hence their distan…
Context: Our understanding of the s-process nucleosynthesis in asymptotic giant branch (AGB) stars is incomplete. AGB models predict, for example, large overabundances of lead (Pb) compared to other s-process elements in metal-poor low-mass AGB stars. This is indeed observed in some extrinsically enhanced metal-poor stars, but not in all. An extensive study of intrinsically s-process enriched objects is essential for improving our knowledge of the AGB third dredge-up and associated s-process nucleosynthesis. Aims: We compare the spectral abundance analysis of the SMC post-AGB star J004441.04-7…
A detailed chemical composition analysis based on a high-resolution (R=35,000) CCD spectrum is presented for a newly discovered post-AGB star in the globular cluster M79 for the first time. The elemental abundance results of M79 Post-AGB star are found to be [C/Fe]=-0.7, [O/Fe]=+1.4, [alpha- process/Fe]=0.5, and [s-process/Fe]=-0.1. The surprising result is that the iron abundance of the star is apparently about 0.6 dex less than that of the cluster's red giants as reported by published studies including a recent high-resolution spectroscopic analysis by Carretta and colleagues.
We present medium resolution (1000) K-band spectra of 12 post-AGB candidates and related stars. For several objects in our sample, these spectra were obtained for the first time. The Br$γ$ line in emission is detected in seven objects indicating the onset of photo-ionization in these objects. Four objects show the presence of He i line. We detect H2 emission line in the spectra of IRAS 06556+1623, IRAS 22023+5249, IRAS 18062+2410 and IRAS 20462+3416. H2 emission line ratio 1-0 S(1)/2-1 S(1) indicate that H2 is radiatively excited due to the UV radiation of hot post-AGB central stars. When comp…
Post-asymptotic giant branch (post-AGB) stars are known to be chemically diverse. In this paper we present the first observational evidence of a star that has failed the third dredge-up (TDU). J005252.87-722842.9 is a A-type ($T_{\rm eff}$ = 8250 $\pm$ 250K) luminous (8200 $\pm$ 700 $\rm L_{\odot}$), metal-poor ($\textrm{[Fe/H]}$ = $- 1.18 \pm$ 0.10), low-mass (M$_{\rm initial}$ $\approx$ 1.5 $-$ 2.0 $\rm M_{\odot}$) post-AGB star in the Small Magellanic Cloud. Through a systematic abundance study, using high-resolution optical spectra from UVES, we found that this likely post-AGB object shows…
Binary post-asymptotic giant branch (post-AGB) stars are thought to be the products of a strong but poorly-understood interaction during the AGB phase. The aim of this contribution is to update the orbital elements of a sample of galactic post-AGB binaries observed in a long-term radial-velocity monitoring campaign. Radial velocities are computed from high signal-to-noise spectra by use of a cross-correlation method. The radial-velocity curves are fitted by using both a least-squares algorithm and a Nelder-Mead simplex algorithm. We use a Monte Carlo method to compute uncertainties on the orbi…
An abundance analysis has been conducted for a sample of nine post-AGB candidate stars; eight of them have not been explored before. We find four very promising objects like HD 105262, HD 53300 and CpD$-62^o5428$ among them. We find strong evidence of dust-gas separation through selective depletion of refractive elements in HD 105262. The same effect is also observed in HD 53300, CpD$-62^o5428$ and HD 114855 although abundance peculiarities are relatively smaller for the last two stars. We find strong enrichment of nitrogen for HD 725, HD 842, HD 1457, HD 9233 and HD 61227 but no further evide…
Evolutionary calculations of population I stars with initial masses $M_0=1M_\odot$, $1.5M_\odot$ and $2M_\odot$ were carried out up to the stage of the proto--planetary nebula. Selected models of post--AGB evolutionary sequences with effective temperatures $3.6\times 10^3\,\mathrm{K}\lesssim T_\mathrm{eff}\lesssim 2\times 10^4\,\mathrm{K}$ were used as initial conditions in calculations of self--escited stellar oscillations. For the first time the sequences of hydrodynamic models of radially pulsating post--AGB stars were computed using the self--consistent solution of the equations of radiati…
The circumstellar environments of objects on the asymptotic giant branch and beyond are rich in molecular species. Nevertheless, methanol has never been detected in such an object, and is therefore often taken as a clear signpost for a young stellar object. However, we report the first detection of CH3OH in a post-AGB object, HD101584, using ALMA. Its emission, together with emissions from CO, SiO, SO, CS, and H2CO, comes from two extreme velocity spots on either side of the object where a high-velocity outflow appears to interact with the surrounding medium. We have derived molecular abundanc…
We observed two proto-planetary nebulae, HD 56126 representing a source with an elliptical circumstellar shell and IRAS 16594-4656 representing a source with a bipolar circumstellar shell, with ALMA in the 12CO and 13CO J=3-2 lines and neighboring continuum to see how the morpho-kinematics of CO gas and dust emission properties in their circumstellar environments differ.
Aims. We aim to determine the geometry, density gradient, and velocity structure of jets in post-asymptotic giant branch (post-AGB) binaries. Methods. Our high cadence time series of high-resolution optical spectra of jet-creating post-AGB binary systems provide us with a unique tomography of the jet. We determine the spatio-kinematic structure of the jets based on these data by fitting the synthetic spectral line profiles created by our model to the observed, orbital phase-resolved, Hα-line profiles of these systems. The fitting routine is provided with an initial spectrum and is allowed to t…
The detected variety in chemistry and circumstellar shell morphology of the limited sample of Galactic post-AGB stars is so large that there is no consensus yet on how the different objects are linked by evolutionary channels. The evaluation is complicated by the fact that their distances and hence luminosities remain largely unknown. Via cross-correlation of the Spitzer SAGE catalogue with optical catalogues we selected a sample of LMC post-AGB candidates based on their [8]-[24] colour index and estimated luminosity. We determined the fundamental properties of the central stars of 105 of thes…
We report for the first time the stellar atmospheric parameters for a a set of post-AGB stars classified by Suarez et al. (2006). The stellar spectra were taken from optical region, with low-resolution and have different spectral ranges. We select a sample of 70 objects with A-K spectral types and luminosities I and Ie. The large majority of these objects have been scarcely studied and are located toward the galactic south pole region. We employ a set of empirical relationships that use pseudo-equivalent widths like spectral feature to estimate effective temperature, surface gravity and metall…
Stellar photospheres of post-asymptotic giant branch stars bear witness to the internal chemical enrichment processes, integrated over their entire stellar evolution. Here we study post-AGB stars in the Large Magellanic Cloud (LMC). With their known distances, these rare objects are ideal tracers of AGB nucleosynthesis and dredge-up phenomena. We used the UVES spectrograph mounted on the Very Large Telescope (VLT) at the European Southern Observatory (ESO), to obtain high-resolution spectra with high signal-to-noise (S/N) of a sample of four post-AGB stars. The objects display a spectral energ…
A number of empirical correlations that allows us to calculate the effective temperature and surface gravity for a set evolved post-AGB and RV Tauri stars are determined using as calibrators the intrinsic colours of 2MASS (Two Micron All Sky Survey) photometry. We have analyzed a total sample of 36 stars where 25 are post-AGB stars and 11 are RV Tauri stars, respectively. A group of 11 stars with parallaxes measures were used as calibrators of the absolute magnitude. The result for T$_{\rm eff}$ and log g from intrinsic colours (J-H)$_{0}$ and (H-K$_{s}$)$_{0}$ at the near-infrared pass bands …
The first high-resolution (R~50,000) optical spectrum of the B-type star, LS III +52 24, identified as the optical counterpart of the hot post-AGB candidate IRAS 22023+5249 (I22023) is presented. We report the detailed identifications of the observed absorption and emission features in the full wavelength range (4290-9015 A) as well as the atmospheric parameters and photospheric abundances (under the Local Thermodinamic Equilibrium approximation) for the first time. The nebular parameters (Te, Ne) are also derived. We estimate Teff=24,000 K, log g=3.0, xi=7 kms-1 and the derived abundances ind…
We present GADRAD, a Python module that adopts heuristic search techniques in the form of genetic algorithms, to efficiently model post-asymptotic giant branch (post- AGB) disc environments. GADRAD systematically constructs the multi-dimensional pa- parameter probability density functions that arise from the fitting of radiative transfer and geometric models to optical interferometric data products. The result provides unbiased descriptions of the object's potential morphology, component luminosities and temperatures, dust composition, disc density profiles and mass. Correlation in the estimat…
A new type of peculiarity -- a splitting or asymmetry of strong absorption lines, is found in the optical spectra of selected post-AGB stars with C-rich circumstellar envelopes. The effect is maximal in BaII lines whose profile is split into two-three components. The particular components of the split absorption lines are shown to be formed in a structured circumstellar envelope, suggesting an efficient dredge-up of the heavy metals produced during the preceding evolution of this star into the envelope. We suspect that the splitting (or asymmetry) of the profiles of strongest absorptions with …
By tracing the distribution of cool dust in the extended envelopes of post-AGB stars and (proto)-planetary nebulae ((P)PNe) we aim to recover, or constrain, the mass loss history experienced by these stars in their recent past. The Far-Infrared Surveyor (FIS) instrument on board the AKARI satellite was used to obtain far-infrared maps for a selected sample of post-AGB stars and (P)PNe. We derived flux densities (aperture photometry) for 13 post-AGB stars and (P)PNe at four far-infrared wavelengths (60, 90, 140, and 160 um). Radial (azimuthally averaged) profiles are used to investigate the pre…
