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The quest for quiet or dormant black holes has been ongoing since several decades. Ellipsoidal variables possibly indicate the existence of a very high-mass invisible companion and are thought to be one of the best ways to find such dormant black holes. This, however, is not a panacea as we show here with one example. We indeed report the discovery of a new semidetached interacting binary, V1315 Cas, discovered as an ellipsoidal variable. Using data from photometric surveys (ASAS-SN, TESS) and high-resolution spectroscopy, we derived a nearly circular orbit with an orbital period of P orb = 34.54 d. The binary system consists of an evolved F-type star primary that is likely still filling its Roche lobe and a B-type star secondary. Using phoebe 2, we derived the following masses and radii: for the primary, |$M_p =0.84 \pm 0.03 \, \rm {M}_{\odot}$| and |$R_p =18.51^{+0.12}_{-0.07} \, \rm {R}_{\odot}$|⁠ ; for the secondary, |$M_s =7.3 \pm 0.3 \, \rm {M}_{\odot}$|  and |$R_s =4.02^{+2.3}_{-2.0}\, \rm {R}_{\odot}$|⁠. Modelling the evolution of the system with mesa , we found an age of ∼7.7 × 107 yr. The system is at the end of a period of rapid non-conservative mass transfer that reversed its mass ratio, while significantly widening its orbit. The primary shows carbon depletion and nitrogen overabundance, indicative of CNO-processed material being exposed due to mass transfer. An infrared excess and stationary H α emission suggest the presence of a circumstellar or circumbinary disc. V1315 Cas will likely become a detached stripped star binary. [ABSTRACT FROM AUTHOR]

Aims. We aim to study the dust distribution in the central regions of the Keplerian disk of the Red Rectangle, the prototype of binary post-AGB stars with rotating circumbinary disks, and to compare it with the distribution of relevant molecular gas tracers. Methods. We present new high-resolution (20 milliarcseconds, mas) ALMA observations of continuum and line emissions at 0.9 mm. The maps have been analyzed by means of a simple model of dust and free–free emission that is able to reproduce the continuum data. Results. (i) We find that most of the dust emission in the Red Rectangle is concentrated in the inner disk regions, with a typical size of 250 AU in diameter and 50 AU in width. (ii) The settlement of dust grains onto inner equatorial regions is remarkable when compared with the relatively widespread gas distribution. (iii) This region is basically coincident with the warm PDR (photo-dominated region) where CI, CII, and certain molecules such as HCN are presumably formed, as well as probably PAHs (polycyclic aromatic hydrocarbons, whose emission is very strong in this source). (iv) We confirm the large size of the grains, with a typical radius ∼150 μm. The opacity of dust at 0.9 mm is deduced to be relatively large, ∼0.5. (v) We also confirm the existence of a very compact HII region in the center, for which we measure an extent of 10–15 mas (∼10 AU) and a total flux of 7–8 mJy at 0.9 mm. [ABSTRACT FROM AUTHOR]

Context. A sample of 895 s-process-rich candidates has been found among the 454 180 giant stars surveyed by LAMOST at low spectral resolution (R ~ 1800). In a previous study, taking advantage of the higher resolution (R ~ 86 000) offered by the HERMES-Mercator spectrograph, we performed a reanalysis of 15 of the brightest stars of this sample. Among these 15 programme stars, which have close-to-solar metallicities, 11 showed mild to strong heavy-element overabundances. However, we did not investigate the nucleosynthetic process(es) at the origin of these overabundances in our former study. Aims. We derive the abundances in s- and r-process elements of the 15 targets in order to investigate whether some stars also show an i-process signature, as sometimes found in their lower-metallicity counterparts (namely, the carbon-enhanced metal-poor (CEMP)-rs stars). Methods. Abundances are derived from the high-resolution HERMES spectra for Pr, Nd, Sm, and Eu using the TURBOSPECTRUM radiative-transfer local thermodynamic equilibrium code with MARCS model atmospheres. Results. Using the new classification scheme proposed in our recent study, we find that two stars show overabundances in both sand r-process elements that are well above the level expected from Galactic chemical evolution (GCE), an analogous situation to that of CEMP-rs stars at lower metallicities. We compare the abundances of the most enriched stars with the nucleosynthetic predictions from the STAREVOL stellar evolutionary code and find abundances compatible with an i-process occurring in asymptotic giant branch stars. Conclusions. Despite there being a larger number of heavy elements with which to characterize the enrichment pattern, the limit between CEMP-s and CEMP-rs stars remains indistinct. Nevertheless, it is interesting to note that an increasing number of extrinsic stars are found to have abundances better reproduced by an i-process pattern even at close-to-solar metallicities. [ABSTRACT FROM AUTHOR]

Context. Stable circumbinary discs around evolved post-asymptotic giant branch (post-AGB) binary systems composed of gas and dust show many similarities with protoplanetary discs around young stellar objects. These discs can provide constraints on both binary evolution and the formation of macrostructures within circumstellar discs. Here we focus on one post-AGB binary system: IRAS 08544-4431. Aims. We aim to refine the physical model of IRAS 08544-4431 with a radiative transfer treatment and continue the near-infrared and mid-infrared interferometric analysis covering the H, K, L, and N bands. Results from geometric modelling of these data in our previous study constrain the shape of the inner rim of the disc and its radial dust structure. We aim to capture the previously detected amount of over-resolved flux and the radial intensity profile at and beyond the inner dust disc rim to put constraints on the physical processes in the inner disc regions. Methods. We used a three-dimensional Monte Carlo radiative transfer code to investigate the physical structure of the disc by reproducing both the photometry and the multi-wavelength infrared interferometric dataset. We first performed a parametric study to explore the effect of the individual parameters and selected the most important parameters, which were then used in a thorough grid search to fit the structural characteristics. We developed a strategy to identify the models that were best able to reproduce our extensive multi-wavelength dataset. Results. We find a family of models that successfully fit the infrared photometric and interferometric data in all bands. These models show a flaring geometry with efficient settling. Larger grains are present in the inner disc as probed by our infrared interferometric observations. Some over-resolved flux component was recovered in all bands, but the optimised models still fall short in explaining all the over-resolved flux. This suggests that another dusty structure within the system that is not included in our models plays a role. The structure of this over-resolved component is unclear, but it has a colour temperature between 1400 and 3600 K. Conclusions. Multi-wavelength infrared interferometric observations of circumstellar discs allow the inner disc regions to be studied in unprecedented detail. The refined physical models can reproduce most of the investigated features, including the photometric characteristics, the radial extent, and the overall shape of the visibility curves. Our multi-wavelength interferometric observations combined with photometry show that the disc around IRAS 08544-4431 is similar to protoplanetary discs around young stars with similar dust masses and efficient dust growth. The resulting disc geometry is capable of reproducing part of the over-resolved flux, but to fully reproduce the over-resolved flux component, an additional component is needed. Multi-scale high-angular-resolution analysis combining VLTI, VLT/SPHERE, and ALMA data is needed to fully define the structure of the system. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Context. Determining ages of stars for reconstructing the history of the Milky Way remains one of the most difficult tasks in astrophysics. This involves knowing when it is possible to relate the stellar mass with its age and when it is not. The young α-rich (YAR) stars present such a case in which we are still not sure about their ages because they are relatively massive, implying young ages, but their abundances are α-enhanced, which implies old ages. Aims. We report the results from new observations from a long-term radial-velocity-monitoring campaign complemented with high-resolution spectroscopy, as well as new astrometry and seismology of a sample of 41 red giants from the third version of APOKASC, which includes YAR stars. The aim is to better characterize the YAR stars in terms of binarity, mass, abundance trends, and kinematic properties. Methods. The radial velocities of HERMES, APOGEE, and Gaia were combined to determine the binary fraction among YAR stars. In combination with their mass estimate, evolutionary status, chemical composition, and kinematic properties, it allowed us to better constrain the nature of these objects. Results. We found that stars with M < 1 M⊙ were all single, whereas stars with M > 1 M⊙ could be either single or binary. This is in agreement with theoretical predictions of population synthesis models. Studying their [C/N], [C/Fe], and [N/Fe], trends with mass, it became clear that many YAR stars do not follow the APOKASC stars, favoring the scenario that most of them are the product of mass transfer. Our sample further includes two likely undermassive stars, that is to say of such as low mass that they cannot have reached the red clump within the age of the Universe, unless their low mass is the signature of mass loss in previous evolutionary phases. These stars do not show signatures of currently being binaries. Both YAR and undermassive stars might show some anomalous APOGEE abundances for the elements N, Na, P, K, and Cr; although, higher-resolution optical spectroscopy might be needed to confirm these findings. Conclusions. Considering the significant fraction of stars that are formed in pairs and the variety of ways that makes mass transfer possible, the diversity in properties in terms of binarity, and chemistry of the YAR and undermassive stars studied here implies that most of these objects are likely not young. [ABSTRACT FROM AUTHOR]

We present new orbital elements for two symbiotic systems involving a giant of spectral type S, namely V420 Hya and ER Del. These orbital elements are compared with existing elements for S-type binaries and for symbiotic binaries with M-giant primaries. It is shown that among the S-type binaries, most of the short-period systems (with P in the range 300 - 1000 d) exhibit some kind of symbiotic activity (Hα emission, UV continuum, hard X-rays), but symbiotic systems are not restricted to the short-period systems. the complex and varying Hα profile of V420 Hya has been decomposed to several components (broad emission with σ ~ 140 km/s, narrow emission with σ ~ 60 km/s, and narrow absorption components). Their orbital modulation reveals that the broad emission is located close to the companion (under the hypothesis of a system with a mass ratio Mg/Mc = 2), and that this broad emission is mutilated by absorption from matter located along the line of sight and flowing towards the observer (faster than the giant) at all orbital phases.

We present a time series of high-resolution spectra for two poorly studied candidate post-AGB stars surrounded by dusty disks, BD+46 442 and IRAS 19135+3937. We find that both stars show radial velocity variations with periods of 130-140 days. IRAS 19135+3937 has been previously classified as a semi-regular variable based on the light variations with a similar period, but our spectra reveal an evolved star which is too warm to be on the AGB. the variability of BD+46 442 has not been studied till now. the stars reveal a number of spectroscopic peculiarities that resemble features in the interactive binaries including the S-type Symbiotics. Namely, H profile alternates between a double-peak emission and a P Cyg profile, which correlates with the radial velocity phase. A weak asymmetry is tentatively detected in the cross-correlation function that moves in anti-phase with the main component. In addition, strong photospheric lines reveal a narrow central absorption. We conclude that these peculiarities are consistent with a picture where the observed post-AGB stars orbit a much fainter companion at a sub-AU separation and transfer mass on it. the companion is surrounded by an accretion disk, possibly with some kind of an outflow. the SED shows that whole system is surrounded by stable dusty disk as well. It remains to be seen with more observations how much of the line asymmetry could be due to the contribution of low-amplitude pulsations.

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. [ABSTRACT FROM AUTHOR]

The Red Rectangle is a prototype of carbon-rich reflection nebulae around low-mass stars in the final phases of evolution. Infrared observations of of the Red Rectangle reveal that oxygen-rich material is present in the circumbinary disk. The oxygen rich dust has properties similar to those of dusty disks around young stars.

Context. The observed orbital characteristics of post-asymptotic giant branch and post-red giant branch (post-RGB) binaries are not understood. We suspect that the missing ingredients needed to explain them probably lie in the continuous interaction of the central binary with its circumstellar environment. Aims. We aim at studying the circumbinary material in these complex systems by investigating the connection between the innermost structures and large-scale structures. Methods. We perform high-angular resolution observations of HD 101584 in the near-infrared continuum. HD 101584 has a complex structure as seen at millimeter wavelengths, with a disk-like morphology and a bipolar outflow due to an episode of a strong binary interaction. To account for the complexity of the target, we first perform an image reconstruction and use this result to fit a geometrical model to extract the morphological and thermal features of the environment. Results. The image reveals an unexpected double ring structure. We interpret the inner ring as having been produced by emission from dust located in the plane of the disk, and the outer ring having been produced by emission from dust that is located 1.6 [D/1kpc] au above the disk plane. The inner ring diameter (3.94 [D/1kpc] au) and temperature (T = 1540 ± 10 K) are compatible with the dust sublimation front of the disk. The origin of the out-of-plane ring (with a diameter of 7.39 [D/1kpc] au and a temperature of 1014 ± 10 K) could be episodic ejection or a dust condensation front in the outflow. Conclusions. The observed outer ring is possibly linked with the blue-shifted side of the large-scale outflow seen by the Atacama Large Millimeter/submillimeter Array and may trace its launching location to the central star. Such observations place morphological constraints on the ejection mechanism. Additional observations are needed to constrain the origin of the out-of-plane structure. [ABSTRACT FROM AUTHOR]

Context. S stars are late-type giants with overabundances of s-process elements. They come in two flavors depending on the presence or lack of presence of technetium (Tc), an element without stable isotopes. Intrinsic S stars are Tc-rich and genuine asymptotic giant branch (AGB) stars, while extrinsic S stars owe their s-process over abundances to the pollution from a former AGB companion, which is now a white dwarf (WD). In addition to Tc, another distinctive feature between intrinsic and extrinsic S stars is the overabundance of niobium (Nb) in the latter class. Indeed, since the mass transfer occurred long ago, 93Zr had time to decay into the only stable isotope of Nb, 93Nb, causing its overabundance. Aims. We discuss the case of the S stars BD+79° 156 and o1 Ori, whose specificity lies in sharing the distinctive features of both intrinsic and extrinsic S stars, namely the presence of Tc along with a Nb overabundance. Methods. We used high-resolution HERMES optical spectra, MARCS model atmospheres of S stars, Gaia DR2 parallaxes, and STAREVOL evolutionary tracks to determine the stellar parameters and chemical abundances of the two S stars, and to locate them in the Hertzsprung-Russell (HR) diagram. Results. BD+79° 156 is the first clear case of a bitrinsic star, that is, a doubly s-process-enriched object, first through mass transfer in a binary system and then through internal nucleosynthesis that is responsible for the Tc-enrichment in BD° 156, which must, therefore, have reached the AGB phase of its evolution. This hybrid nature of the s-process pattern in BD+79° 156 is supported by its binary nature and its location in the HR diagram that is just beyond the onset of the third dredge-up on the AGB. The Tc-rich, binary S-star o1 Ori with a WD companion was another long-standing candidate for a similar hybrid s-process enrichment. However, the marginal overabundance of Nb derived in o1 Ori does not allow one to trace evidence of large amounts of pollution coming from the AGB progenitor of its current WD companion unambiguously. As a side product, the current study offers a new way of detecting binary AGB stars with WD companions by identifying their Tc-rich nature along with a Nb overabundance. [ABSTRACT FROM AUTHOR]

Context. The fate of a massive star during the latest stages of its evolution is highly dependent on its mass-loss rate and geometry and therefore knowing the geometry of the circumstellar material close to the star and its surroundings is crucial. Aims. We aim to provide insight into the nature (i.e. geometry, rates) of mass-loss episodes, and in particular, the connection between the observed asymmetries due to the mass lost in a fast wind or during a previous, prodigious mass-losing phase. In this context, yellow hypergiants offer a good opportunity to study mass-loss events. Methods. We analysed a large set of optical and near-infrared data in spectroscopic and photometric, spectropolarimetric, and interferometric (GRAVITY/VLTI) modes, towards the yellow hypergiant IRAS 17163-3907. We used X-shooter optical observations to determine the spectral type of this yellow hypergiant and we present the first model-independent, reconstructed images of IRAS 17163-3907 at these wavelengths tracing milli-arcsecond scales. Lastly, we applied a 2D radiative transfer model to fit the dereddened photometry and the radial profiles of published diffraction-limited VISIR images at 8.59 μm, 11.85 μm, and 12.81 μm simultaneously, adopting a revised distance determination using Gaia Data Release 2 measurements. Results. We constrain the spectral type of IRAS 17163-3907 to be slightly earlier than A6Ia (Teff 8500 K). The interferometric observables around the 2 μm window towards IRAS 17163-3907 show that the Br emission appears to be more extended and asymmetric than the Na i and the continuum emission. Interestingly, the spectrum of IRAS 17163-3907 around 2 μm shows Mgii emission that is not previously seen in other objects of its class. In addition, Br shows variability in a time interval of four months that is not seen towards Na i. Lastly, in addition to the two known shells surrounding IRAS 17163-3907, we report on the existence of a third hot inner shell with a maximum dynamical age of only 30 yr. Conclusions. The 2 μm continuum originates directly from the star and not from hot dust surrounding the stellar object. The observed spectroscopic variability of Br could be a result of variability in the mass-loss rate. The interpretation of the presence of Na i emission at closer distances to the star compared to Br has been a challenge in various studies. To address this, we examine several scenarios. We argue that the presence of a pseudo-photosphere, which was traditionally considered to be the prominent explanation, is not needed and that it is rather an optical depth effect. The three observed distinct mass-loss episodes are characterised by different mass-loss rates and can inform theories of mass-loss mechanisms, which is a topic still under debate both in theory and observations. We discuss these in the context of photospheric pulsations and wind bi-stability mechanisms. [ABSTRACT FROM AUTHOR]

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 test three configurations, derived from three specific jet launching models: a stellar jet launched by the star, an X-wind, and a disk wind configuration. We apply a Markov-chain Monte Carlo routine in order to fit our model to the observations. Our fitting code is tested on the post-AGB binary IRAS 19135+3937. Results. We find that a model using the stellar jet configuration gives a marginally better fit to our observations. The jet has a wide half-opening angle of about 76° and reaches velocities up to 870 km s−1. Conclusions. Our methodology is successful in determining some parameters for jets in post-AGB binaries. The model for IRAS 19135+3937 includes a transparent, low density inner region (for a half-opening angle < 40°). The source feeding the accretion disk around the companion is most likely the circumbinary disk. We will apply this jet fitting routine to other jet-creating post-AGB stars in order to provide a more complete description of these objects. [ABSTRACT FROM AUTHOR]

We performed a photometric and spectroscopic analysis of two RV Tauri stars: GK Car and GZ Nor. Both objects are surrounded by hot circumstellar dust. Their pulsation periods, derived from ASAS (All Sky Automated Survey) photometric time-series, have been used to derive their luminosities and distances via the period–luminosity–distance (PLC) relation. In addition, for both objects, Gaia distances are available. The Gaia distances and luminosities are consistent with the values obtained from the PLC relationship. GK Car is at distance of 4.5 ± 1.3 kpc and has a luminosity of 1520 ± 840 L⊙, while GZ Nor is at distance of 8.4 ± 2.3 kpc and has a luminosity of 1240 ± 690 L⊙. Our abundance analysis reveals that both stars show depletion of refractory elements with [Fe/H] = −1.3 and [Zn/Ti] = +1.2 for GK Car and [Fe/H] = −2.0 and [Zn/Ti] = +0.8 for GZ Nor. In the WISE (Wide-Field Infrared Survey Explorer) colour–colour diagram, GK Car is located in the RV Tauri box as originally defined by Lloyd Evans and updated by Gezer et al. while GZ Nor is not. Despite this, we argue that both objects are surrounded by a gravitationally bound disc. As depletion is observed in binaries, we postulate that both stars are binaries as well. RV Tauri stars are generally acknowledged to be post-asymptotic giant branch (post-AGB) stars. Recent studies show that they might be either indeed post-AGB or post-red giant branch (post-RGB) objects depending on their luminosity. For both objects, the derived luminosities are relatively low for post-AGB objects, however, the uncertainties are quite large. We conclude that they could be either post-RGB or post-AGB objects. [ABSTRACT FROM AUTHOR]

Barium (Ba) dwarfs and CH subgiants are the less evolved analogues of Ba and CH giants. They are F- to G-type main-sequence stars polluted with heavy elements by their binary companions when the companion was on the asymptotic giant branch (AGB). This companion is now a white dwarf that in most cases cannot be directly detected. We present a large systematic study of 60 objects classified as Ba dwarfs or CH subgiants. Combining radial-velocity measurements from HERMES and SALT high-resolution spectra with radial-velocity data from CORAVEL and CORALIE, we determine the orbital parameters of 27 systems. We also derive their masses by comparing their location in the Hertzsprung–Russell diagram with evolutionary models. We confirm that Ba dwarfs and CH subgiants are not at different evolutionary stages, and that they have similar metallicities, despite their different names. Additionally, Ba giants appear significantly more massive than their main-sequence analogues. This is likely due to observational biases against the detection of hotter main-sequence post-mass-transfer objects. Combining our spectroscopic orbits with the HIPPARCOS astrometric data, we derive the orbital inclination and the mass of the WD companion for four systems. Since this cannot be done for all systems in our sample yet (but should be possible with upcoming Gaia data releases), we also analyse the mass-function distribution of our binaries. We can model this distribution with very narrow mass distributions for the two components and random orbital orientations on the sky. Finally, based on BINSTAR evolutionary models, we suggest that the orbital evolution of low-mass Ba systems can be affected by a second phase of interactions along the red giant branch of the Ba star, which impact the eccentricities and periods of the giants. [ABSTRACT FROM AUTHOR]

Context. Barium and S stars without technetium are red giants and are suspected of being members of binary systems due to their overabundances in heavy elements. These elements are produced by the s-process of nucleosynthesis, despite the stars not being evolved enough to be able to activate the s-process in their interiors. A companion formerly on the asymptotic giant branch (now a white dwarf) is supposed to be responsible for the barium- and S-star enrichment in s-process elements through mass transfer. Aims. This paper provides both long-period and revised orbits for barium and S stars, adding to previously published orbits. The sample of barium stars with strong anomalies (i.e., those classified as Ba3, Ba4, or Ba5 in the Warner scale) comprises all known stars of that kind, and in that sense forms a complete sample that allows us to investigate several orbital properties of these post-mass-transfer binaries in an unbiased way. Methods. Orbital elements are derived from radial velocities collected from a long-term radial-velocity monitoring campaign performed with the HERMES spectrograph mounted on the Mercator 1.2 m telescope. These new measurements were combined with older, CORAVEL measurements. With the aim of investigating possible correlations between orbital properties and abundances, we also collected a set of abundances for barium stars with orbital elements that is as homogeneous as possible. When unavailable in the literature, abundances were derived from high-resolution HERMES spectra. Results. We find orbital motion for all barium and extrinsic S stars monitored (except for the mild barium star HD 95345). We obtain the longest period known so far for a spectroscopic binary involving an S star, namely 57 Peg with a period of the order of 100−500 yr. We present the mass distribution for the barium stars, which ranges from 1 to 3 M⊙, with a tail extending up to 5 M⊙ in the case of mild barium stars. This high-mass tail is mostly comprised of high-metallicity objects ([Fe/H] ≥ −0.1). The distribution of the companion masses was extracted from the barium-star mass distribution combined with the finding that Q ≡ f(MBa,MWD)/sin3i = MWD3/(MBa + MWD)2 Q ≡ f (M Ba , M WD) / sin 3 i = M WD 3 / (M Ba + M WD) 2 $ Q \equiv f(M_{\rm Ba},M_{\rm WD}) / \sin^3i = M_{\rm WD}^3/(M_{\rm Ba}+M_{\rm WD})^2 $ is peaked at 0.057 ± 0.009 and 0.036 ± 0.027 M⊙ for strong and mild barium stars, respectively (f(MBa, MWD) is the mass function obtained from the orbital elements of spectroscopic binaries with one observable spectrum). Mass functions are compatible with WD companions whose masses range from 0.5 to 1 M⊙. Strong barium stars have a tendency to be found in systems with shorter periods than mild barium stars, although this correlation is rather lose, with metallicity and WD mass also playing a role. Using the initial–final mass relationship established for field WDs, we derived the distribution of the mass ratio q′=MAGB, ini/MBa (where MAGB, ini is the WD progenitor initial mass, i.e., the mass of the former primary component of the system) which is a proxy for the initial mass ratio (the less mass the barium star has accreted, the better the proxy). It appears that the distribution of q′ is highly nonuniform, and significantly different for mild and strong barium stars, the latter being characterized by values mostly in excess of 1.4, whereas mild barium stars occupy the range 1−1.4. Conclusions. The orbital properties presented in this paper pave the way for a comparison with binary-evolution and nucleosynthesis models, which should account for the various significant correlations found between abundances and dynamical parameters (e.g. between MBa on one hand and MWD, [Fe/H], and [s/Fe] on the other hand, between q′ and [s/Fe], between P and e, and between P and [s/Fe] altogether). [ABSTRACT FROM AUTHOR]

S-type stars are late-type giants enhanced with s -process elements originating either from nucleosynthesis during the Asymptotic Giant Branch (AGB) or from a pollution by a binary companion. The former are called intrinsic S stars, and the latter extrinsic S stars. The intrinsic S stars are on the AGB and have undergone third dredge-up events. The atmospheric parameters of S stars are more numerous than those of M-type giants (C/O ratio and s -process abundances affect the thermal structure and spectral synthesis), and hence they are more difficult to derive. These atmospheric parameters are also entangled within each other. Nevertheless, high-resolution spectroscopic data of S stars combined with the Gaia Data Release 2 (GDR2) parallaxes and with the MARCS model atmospheres for S-type stars were used to derive effective temperatures, surface gravities, and luminosities. These parameters not only allow to locate the intrinsic and extrinsic S stars in the Hertzsprung-Russell (HR) diagram but also allow the accurate abundance analysis of the s -process elements. [ABSTRACT FROM AUTHOR]

LoTr 5 is a planetary nebula with an unusual long-period binary central star. As far as we know, the pair consists of a rapidly rotating G-type star and a hot star, which is responsible for the ionization of the nebula. The rotation period of the G-type star is 5.95 d and the orbital period of the binary is now known to be ~2700 d, one of the longest in central star of planetary nebulae. The spectrum of the G central star shows a complex Ha double-peaked profile which varies with very short time-scales, also reported in other central stars of planetary nebulae and whose origin is still unknown. We present new radial velocity observations of the central star which allow us to confirm the orbital period for the long-period binary and discuss the possibility of a third component in the system at ~129 d to the G star. This is complemented with the analysis of archival light curves from Super Wide Angle Search for Planets, All Sky Automated Survey, and Optical Monitoring Camera. From the spectral fitting of the Gtype star, we obtain an effective temperature of Teff = 5410 ± 250K and surface gravity of log g = 2.7 ± 0.5, consistent with both giant and subgiant stars. We also present a detailed analysis of the Ha double-peaked profile and conclude that it does not present correlation with the rotation period and that the presence of an accretion disc via Roche lobe overflow is unlikely. [ABSTRACT FROM AUTHOR]

Whether planetary nebulae (PNe) are predominantly the product of binary stellar evolution as some population synthesis models (PSM) suggest remains an open question. Around 50 short-period binary central stars (P ∼ 1 d) are known, but with only four with measured orbital periods over 10 d, our knowledge is severely incomplete. Here we report on the first discovery from a systematic Southern African Large Telescope (SALT) High Resolution Spectrograph (HRS) survey for long-period binary central stars. We find a 142 d orbital period from radial velocities of the central star of NGC1360, HIP 16566. NGC1360 appears to be the product of common-envelope (CE) evolution, with nebula features similar to post-CE PNe, albeit with an orbital period considerably longer than expected to be typical of post-CE PSM. The most striking feature is a newly identified ring of candidate low-ionization structures. Previous spatiokinematic modelling of the nebula gives a nebula inclination of 30° ± 10°, and assuming the binary nucleus is coplanar with the nebula, multiwavelength observations best fit a more massive, evolved white dwarf (WD) companion. A WD companion in a 142 d orbit is not the focus of many PSM, making NGC1360 a valuable system with which to improve future PSM work. HIP 16566 is amongst many central stars in which large radial velocity variability was found by low-resolution surveys. The discovery of its binary nature may indicate long-period binaries may be more common than PSM models predict. [ABSTRACT FROM AUTHOR]

Context. The predicted orbital-period distribution of the subdwarf-B (sdB) population is bi-modal with peaks at short (<10 days) and long (>500 days) periods. Observationally, many short-period sdB systems are known, but only few wide sdB binaries have been studied in detail. Based on a long-term monitoring programme the wide sdB sample has been increased, discovering an unexpected positive correlation between the eccentricity and orbital period. Aims. In this article we present the orbital solution and spectral analysis of four new systems, BD-7°5977, EC 11031-1348, TYC 2084-448-1 and TYC3871-835-1, and update the orbital solution of PG 1104+243. Using the whole sample of wide sdB binaries, we aim to find possible correlations between orbital and spectral properties. The ultimate goal is to improve theoretical models of Roche-lobe overflow. Methods. High-resolution spectroscopic time series were obtained to determine the RVs of both the sdB and main sequence (MS) components. Literature photometry was used to construct the spectral-energy distributions, which were fitted with atmosphere models to determine the surface gravities and temperatures of both components in all systems. Spectral parameters of the cool companion were verified using the GSSP code. Furthermore the amount of accreted mass was estimated. Results. Orbital parameters matching the earlier observed period-eccentricity relation were found for three systems, while TYC 2084-448-1 is found to have a lower eccentricity than expected from the period-eccentricity trend indicated by the other systems. Based on new observations, the orbit of PG 1104+243 has a small but significant eccentricity of 0.04 &3177; 0.02, matching that of the other systems with similar periods. Furthermore, a positive correlation between accreted mass and orbital period was found, as well as a possible relation between the initial mass-ratio and the final period-eccentricity. Conclusions. The wide sdB-binary sample shows interesting possible correlations between orbital and spectral properties. However, a larger sample is necessary to statistically validate them. [ABSTRACT FROM AUTHOR]

Context. Lithium abundance A(Li) and surface rotation are good diagnostic tools to probe the internal mixing and angular momentum transfer in stars. Aims. We explore the relation between surface rotation, A(Li), and age in a sample of seismic solar-analogue stars, and we study their possible binary nature. Methods. We selected a sample of 18 solar-analogue stars observed by the NASA Kepler satellite for an in-depth analysis. Their seismic properties and surface rotation rates are well constrained from previous studies. About 53 h of high-resolution spectroscopy were obtained to derive fundamental parameters from spectroscopy and A(Li). These values were combined and compared with seismic masses, radii, and ages, as well as with surface rotation periods measured from Kepler photometry. Results. Based on radial velocities, we identify and confirm a total of six binary star systems. For each star, a signal-to-noise ratio of 80 ≲ S=N ≲ 210 was typically achieved in the final spectrum around the lithium line. We report fundamental parameters and A(Li). Using the surface rotation period derived from Kepler photometry, we obtained a well-defined relation between A(Li) and rotation. The seismic radius translates the surface rotation period into surface velocity. With models constrained by the characterisation of the individual mode frequencies for single stars, we identify a sequence of three solar analogues with similar mass (~1.1 M⊙) and stellar ages ranging between 1 to 9 Gyr. Within the realistic estimate of ~7% for the mass uncertainty, we find a good agreement between the measured A(Li) and the predicted A(Li) evolution from a grid of models calculated with the Toulouse-Geneva stellar evolution code, which includes rotational internal mixing, calibrated to reproduce solar chemical properties. We found a scatter in ages inferred from the global seismic parameters that is too large when compared with A(Li). Conclusions. We present the Li-abundance for a consistent spectroscopic survey of solar-analogue stars with a mass of 1:00 ± 0:15 M⊙ that are characterised through asteroseismology and surface rotation rates based on Kepler observations. The correlation between A(Li) and Prot supports the gyrochronological concept for stars younger than the Sun and becomes clearer when the confirmed binaries are excluded. The consensus between measured A(Li) for solar analogues with model grids, calibrated on the Sun's chemical properties, suggests that these targets share the same internal physics. In this light, the solar Li and rotation rate appear to be normal for a star like the Sun. [ABSTRACT FROM AUTHOR]

The importance of long-period binaries for the formation and evolution of planetary nebulae is still rather poorly understood, which in part is due to the lack of central star systems that are known to comprise such long-period binaries. Here, we report on the latest results from the on-going Mercator-HERMES survey for variability in the central stars of planetary nebulae. We present a study of the central stars of NGC 1514, BD+30°623, the spectrum of which shows features associated with a hot nebular progenitor as well as a possible A-type companion. Cross-correlation of high-resolution HERMES spectra against synthetic spectra shows the system to be a highly eccentric (e ~ 0.5) double-lined binary with a period of ~3300 days. Previous studies indicated that the cool component might be a horizontal branch star of mass ~0.55 M⊙ but the observed radial velocity amplitudes rule out such a low mass. If we assume that the nebular symmetry axis and binary orbital plane are perpendicular, then the data are more consistent with a post-main-sequence star ascending towards the giant branch. We also present the continued monitoring of the central star of LoTr 5, HD 112313, which has now completed one full cycle, allowing the orbital period (P~2700 days) and eccentricity (e ~ 0.3) to be derived. To date, the orbital periods of BD+30°623 and HD 112313 are the longest to have been measured spectroscopically in the central stars of planetary nebulae. Furthermore, these systems, along with BD+33ffi2642, comprise the only spectroscopic wide-binary central stars currently known. [ABSTRACT FROM AUTHOR]

We report on the discovery of a planetary companion candidate with a minimum mass M sin i = 4.6 ± 1.0 MJupiter orbiting the K2 III giant star HD 175370 (KIC 007940959). This star was a target in our programme to search for planets around a sample of 95 giant stars observed with Kepler. This detection was made possible using precise stellar radial velocity measurements of HD 175370 taken over five years and four months using the coudé echelle spectrograph of the 2-m Alfred Jensch Telescope and the fibre-fed echelle spectrograph High Efficiency and Resolution Mercator Echelle Spectrograph of the 1.2-m Mercator Telescope. Our radial velocity measurements reveal a periodic (349.5 ± 4.5 d) variation with a semi-amplitude K = 133 ± 25 m s-1, superimposed on a long-term trend. A low-mass stellar companion with an orbital period of ~88 yr in a highly eccentric orbit and a planet in a Keplerian orbit with an eccentricity e = 0.22 are the most plausible explanation of the radial velocity variations. However, we cannot exclude the existence of stellar envelope pulsations as a cause for the low-amplitude radial velocity variations and only future continued monitoring of this system may answer this uncertainty. From Kepler photometry, we find that HD 175370 is most likely a low-mass red giant branch or asymptotic giant branch star. [ABSTRACT FROM AUTHOR]

Aims. We present the first near-IR milli-arcsecond-scale image of a post-AGB binary that is surrounded by hot circumbinary dust. Methods. A very rich interferometric data set in six spectral channels was acquired of IRAS 08544-4431 with the new RAPID camera on the PIONIER beam combiner at the Very Large Telescope Interferometer (VLTI). A broadband image in the H-band was reconstructed by combining the data of all spectral channels using the SPARCO method. Results. We spatially separate all the building blocks of the IRAS 08544-4431 system in our milliarcsecond-resolution image. Our dissection reveals a dust sublimation front that is strikingly similar to that expected in early-stage protoplanetary disks, as well as an unexpected flux signal of ~4% from the secondary star. The energy output from this companion indicates the presence of a compact circum-companion accretion disk, which is likely the origin of the fast outflow detected in Hff. Conclusions. Our image provides the most detailed view into the heart of a dusty circumstellar disk to date. Our results demonstrate that binary evolution processes and circumstellar disk evolution can be studied in detail in space and over time. [ABSTRACT FROM AUTHOR]

Context. 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. The goal at large is to provide improved observational constraints to the models of the complex interplay between the AGB s-process nucleosynthesis and the associated mixing processes. Aims. 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 s-process enriched post-AGB stars in the Magellanic Clouds show a discrepancy between observed and predicted Pb abundances. The determined upper limits based on spectral studies are much lower than what is predicted. In this paper, we focus specifically on the Pb abundance of 14 Galactic s-process enhanced post-AGB stars to check whether the same discrepancy is present in the Galaxy as well. Among these 14 objects, two were not yet subject to a detailed abundance study in the literature. We apply the same method to obtain accurate abundances for the 12 others. Our homogeneous abundance results provide the input of detailed spectral synthesis computations in the spectral regions where Pb lines are located. Methods. We used high-resolution UVES and HERMES spectra for detailed spectral abundance studies of our sample of Galactic post-AGB stars. None of the sample stars display clear Pb lines, and we only deduced upper limits of the Pb abundance by using spectrum synthesis in the spectral ranges of the strongest Pb lines. Results. We do not find any clear evidence of Pb overabundances in our sample. The derived upper limits are strongly correlated with the effective temperature of the stars with increasing upper limits for increasing effective temperatures. We obtain stronger Pb constraints on the cooler objects. Moreover, we confirm the s-process enrichment and carbon enhancement of two unstudied 21 µm sources IRAS 13245-6428 and IRAS 14429-4539. The mildly s-process enhanced post-AGB star IRAS 17279-1119 is part of a binary system and may be the long sought precursor of extrinsic Ba stars. Conclusions. Stars with Teff > 7500 K do not provide strong constraints on the Pb abundance as the strongest line in the optical spectrum is only detectable at unrealistically high Pb atmospheric abundances. Combining the Pb abundance results from this study with abundances from our previous studies, we conclude that the discrepancy between theory and observation increases towards lower metallicities. The model predictions are consistent with the deduced upper limits on the Pb abundances for all stars with [Fe/H] > –0:7 dex. For stars with [Fe/H] < –0:7 dex, however, the model predictions overestimate the Pb abundances with respect to the other s-process elements. All objects, except IRAS 17279-1119, confirm the relation between neutron exposure [hs/ls] and third dredge-up efficiency [s/Fe], whereas no relation between metallicity and neutron exposure is detected within the metallicity range of our total sample (–1.4 < [Fe/H] < –0.2). The mild enrichment of IRAS 17279-1119 can probably be attributed to a cut-off of the AGB evolution due to binary interactions. To our knowledge, IRAS 17279-1119 is the first s-process enhanced Galactic post-AGB star known in a binary system and is a possible precursor of the extrinsic Ba dwarf stars. We corroborate the finding that the variety in abundance profiles shows that a large spread of neutron irradiation is needed for a given metallicity. Lead-rich stars are yet to be found among post-AGB stars. [ABSTRACT FROM AUTHOR]

Context. This paper is part of a larger project in which we systematically study the chemical abundances of extra-galactic post-asymptotic giant branch (post-AGB) stars. The aim of our programme is to derive chemical abundances of stars covering a large range in luminosity and metallicity with the ultimate goal of testing, constraining, and improving our knowledge of the poorly understood AGB phase, especially the third dredge-up mixing processes and associated s-process nucleosynthesis. Aims. Post-AGB photospheres are dominated by atomic lines and indicate the effects of internal chemical enrichment processes over the entire stellar lifetime. In this paper, we study two carefully selected post-AGB stars: J051213.81-693537.1 and J051848.86-700246.9 in the Large Magellanic Cloud (LMC). Both objects show signs of s-process enhancement. The combination of favourable atmospheric parameters for detailed abundance studies and their known distances (and hence luminosities and initial masses) make these objects ideal probes of the AGB third dredge-up and s-process nucleosynthesis in that they provide observational constraints for theoretical AGB models. Methods. We use high-resolution optical UVES spectra to determine accurate stellar parameters and subsequently perform detailed elemental abundance studies of post-AGB stars. Additionally, we use available photometric data covering optical and IR bands to construct spectral energy distributions for reddening and luminosity determinations. We then estimate initial masses from theoretical post-AGB tracks. Results. We obtained accurate atmospheric parameters for J051213.81-693537.1 (Teff = 5875 ± 125 K, log g = 1.00 ± 0.25 dex, [Fe/H] = -0.56 ± 0.16 dex) and J051848.86-700246.9 (Teff = 6000 ± 125 K, log g = 0.50 ± 0.25 dex, [Fe/H] = -1.06 ± 0.17 dex). Both stars show extreme s-process enrichment associated with relatively low C/O ratios of 1.26 ± 0.40 and 1.29 ± 0.30 for J051213-693537.1 and J051848-700246.9, respectively. We could only derive upper limits of the lead (Pb) abundance. These upper limits show a possible very slight Pb overabundance with respect to heavy s-elements for J051213-693537.1, while J051848-700246.9 shows an upper limit of the Pb abundance similar to [hs/Fe]. A comparison with theoretical post-AGB evolutionary tracks in the HR-diagram reveals that both stars have low initial masses between 1.0 and 1.5 M⊙. Conclusions. This study adds to the results obtained so far on a very limited number of s-process enriched stars in the Magellanic Clouds. With the addition of the two stars in this study, we find an increasing discrepancy between observed and predicted Pb abundances towards lower metallicities of the studied s-process rich post-AGB stars in the Magellanic Clouds. The more metal-rich J051213-693537.1 fits the theoretical Pb abundance predictions well, while the five other objects with [Fe/H] < 1, including J051848-700246.9, have much lower Pb overabundances than predicted. In all objects found so far, including the objects in this study, the C/O ratio is very moderate because of the enhancement of O as well as C. We find that all s-process rich stars in the LMC and SMC studied so far, cluster in the same region of the HR-diagram and are associated with low-mass stars with a low metallicity on average. We corroborate the published lack of correlation between the metallicity and the neutron irradiation, while the neutron exposure ([hs/ls]) is strongly correlated with the third dredge-up efficiency ([s/Fe]). These correlations seem to hold in our Galaxy as well as in the Magellanic Clouds. [ABSTRACT FROM AUTHOR]

We present high-precision photometric light curves of five O-type stars observed with the refurbished Kepler satellite during its Campaign 0. For one of the stars, we also assembled high-resolution ground-based spectroscopy with the HERMES spectrograph attached to the 1.2 m Mercator telescope. The stars EPIC 202060097 (O9.5V) and EPIC 202060098 (O7V) exhibit monoperiodic variability due to rotational modulation with an amplitude of 5.6 and 9.3 mmag and a rotation period of 2.63 and 5.03 d, respectively. EPIC 202060091 (O9V) and EPIC 202060093 (O9V:pe) reveal variability at low frequency but the cause is unclear. EPIC 202060092 (O9V:p) is discovered to be a spectroscopic binary with at least one multiperiodic γ Cep-type pulsator whose detected mode frequencies occur in the range [0.11, 6.99] d-1 and have amplitudes between 0.8 and 2.0 mmag. Its pulsation spectrum is shown to be fully compatible with the ones predicted by core-hydrogen burning O-star models. Despite the short duration of some 33 d and the limited data quality with a precision near 100 |imag of these first K2 data, the diversity of possible causes for O-star variability already revealed from campaigns of similar duration by the MOST and CoRoT satellites is confirmed with Kepler. We provide an overview of O-star space photometry and give arguments why future K2 monitoring during Campaigns 11 and 13 at short cadence, accompanied by time-resolved high-precision high-resolution spectroscopy, opens up the possibility of in-depth O-star seismology. [ABSTRACT FROM AUTHOR]

The HERMES/Mercator combination provides a niche in observational astrophysics as it allows to build up yearlong time-series of high-resolution optical spectra with appropriate sampling. In name of the HERMES consortium, I introduce the main characteristics of our spectrograph and operational model. Rather than reviewing all results, I highlight with a few example only some of the results obtained during the first six years of operations. [ABSTRACT FROM AUTHOR]

Sco X-1 has been the subject of many multiwavelength studies in the past, being the brightest persistent extrasolar X-ray source ever observed. Here, we revisit Sco X-1 with simultaneous short cadence Kepler optical photometry and Monitor of All-sky X-ray Image X-ray photometry over a 78 d period, as well as optical spectroscopy obtained with High Efficiency and ResolutionMercator Echelle Spectrograph (HERMES).We find Sco X-1 to be highly variable in all our data sets. The optical fluxes are clearly bimodal, implying the system can be found in two distinct optical states. These states are generally associated with the known flaring/normal branch X-ray states, although the flux distributions associated with these states overlap. Furthermore, we find that the optical power spectrum of Sco X-1 differs substantially between optical luminosity states. Additionally we find rms.flux relations in both optical states, but only find a linear relation during periods of low optical luminosity. The full optical/X-ray discrete correlation function displays a broad .12.5 h optical lag. However, during the normal branch phase, the X-ray and optical fluxes are anticorrelated, whilst being correlated during the flaring branch. We also performed a Cepstrum analysis on the full Kepler light curve to determine the presence of any echoes within the optical light curve alone. We find significant echo signals, consistent with the optical lags found using the discrete cross-correlation. We speculate that whilst some of the driving X-ray emission is reflected by the disc, some is absorbed and re-processed on the thermal time-scale, giving rise to both the observed optical lags and optical echoes. [ABSTRACT FROM AUTHOR]

Context. Many post-asymptotic giant branch (post-AGB) stars in binary systems have an infrared (IR) excess arising from a dusty circumbinary disk. The disk formation, current structure, and further evolution are, however, poorly understood. Aims. We aim to constrain the structure of the circumstellar material around the post-AGB binary and RV Tauri pulsator AC Her. We want to constrain the spatial distribution of the amorphous and of the crystalline dust. Methods. We present very high-quality mid-IR interferometric data that were obtained with the MIDI/VLTI instrument. We analyze the MIDI visibilities and differential phases in combination with the full spectral energy distribution, using the MCMax radiative transfer code, to find a good structure model of AC Her's circumbinary disk. We include a grain size distribution and midplane settling of dust self-consistently in our models. The spatial distribution of crystalline forsterite in the disk is investigated with the mid-IR features, the 69 μm band and the 11.3 μm signatures in the interferometric data. Results. All the data are well fitted by our best model. The inclination and position angle of the disk are precisely determined at i = 50 ± 8° and PA = 305 ± 10°. We firmly establish that the inner disk radius is about an order of magnitude larger than the dust sublimation radius. The best-fit dust grain size distribution shows that significant grain growth has occurred, with a significant amount of mm-sized grains now being settled to the midplane of the disk. A large total dust mass ≥10-3 MΘ is needed to fit the mm fluxes. By assuming αturb = 0:01, a good fit is obtained with a small grain size power law index of 3.25, combined with a small gas/dust ratio ≤10. The resulting gas mass is compatible with recent estimates employing direct gas diagnostics. The spatial distribution of the forsterite is different from the amorphous dust, as more warm forsterite is needed in the surface layers of the inner disk. Mid-IR interferometry offers unique constraints, complementary to mid-IR features, for studying the mineralogy in disks. A better uv coverage is needed to constrain in detail the distribution of the crystalline forsterite in the disk of ACHer, but we find strong similarities with the protoplanetary disk HD 100546. [ABSTRACT FROM AUTHOR]

Aims. 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. Methods. We present interferometric observations of 12CO 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 disks. Results. We clearly detect the Keplerian dynamics of a second disk orbiting an evolved star. Its main properties (size, temperature, central mass) are derived from direct interpretation of the data and model fitting. With this we confirm that there are disks orbiting the stars of this relatively wide class of post-AGB objects. [ABSTRACT FROM AUTHOR]

Aims. In our series of papers presenting the Herschel imaging of evolved planetary nebulae, we present images of the dust distribution in the Helix nebula (NGC 7293). Methods. Images at 70, 160, 250, 350, and 500 μm were obtained with the PACS and SPIRE instruments on board the Herschel satellite. Results. The broadband maps show the dust distribution over the main Helix nebula to be clumpy and predominantly present in the barrel wall. We determined the spectral energy distribution of the main nebula in a consistent way using Herschel, IRAS, and Planck flux values. The emissivity index of β = 0:99-0:09, in combination with the carbon rich molecular chemistry of the nebula, indicates that the dust consists mainly of amorphous carbon. The dust excess emission from the central star disk is detected at 70 m and the flux measurement agrees with previous measurement. We present the temperature and dust column density maps. The total dust mass across the Helix nebula (without its halo) is determined to be 3:5 × 10-3 M⊙ at a distance of 216 pc. The temperature map shows dust temperatures between 22 K and 42 K, which is similar to the kinetic temperature of the molecular gas, confirming that the dust and gas co-exist in high density clumps. Archived images are used to compare the location of the dust emission in the far infrared (Herschel) with the ionized (GALEX and Hβ) and molecular (H2) component. The different emission components are consistent with the Helix consisting of a thick walled barrel-like structure inclined to the line of sight. The radiation field decreases rapidly through the barrel wall. [ABSTRACT FROM AUTHOR]

Context. The analysis of stellar oscillations is one of the most reliable ways to probe stellar interiors. Recent space missions such as Kepler have provided us with an opportunity to study these oscillations with unprecedented detail. For many multi-periodic pulsators such as γ Doradus stars, this led to the detection of dozens to hundreds of oscillation frequencies that could not be found from ground based observations. Aims. We aim to detect non-uniform period spacings in the Fourier spectra of a sample of γ Doradus stars observed by Kepler. Such detection is complicated by both the large number of significant frequencies in the space photometry and by overlapping nonequidistant rotationally split multiplets. Methods. Guided by theoretical properties of gravity-mode oscillation of γ Doradus stars, we developed a period-spacing detection method and applied it to Kepler observations of a few stars, after having tested the performance from simulations. Results. The application of the technique resulted in the clear detection of non-uniform period spacing series for three out of the five treated Kepler targets. Disadvantages of the technique are also discussed, and include the disability to distinguish between different values of the spherical degree and azimuthal order of the oscillation modes without additional theoretical modelling. Conclusions. Despite the shortcomings, the method is shown to allow solid detections of period spacings for γ Doradus stars, which will allow future asteroseismic analyses of these stars. [ABSTRACT FROM AUTHOR]

Through CO mm-line and optical spectroscopy, we investigate the properties of the Fried Egg nebula IRAS 17163-3907, which has recently been proposed to be one of the rare members of the yellow hypergiant class. The CO J = 2-1 and J = 3-2 emission arises from a region within 20" of the star and is clearly associated with the circumstellar material. The CO lines show a multi-component asymmetrical profile, and an unexpected velocity gradient is resolved in the east-west direction, suggesting a bipolar outflow. This is in contrast with the apparent symmetry of the dust envelope as observed in the infrared. The optical spectrum of IRAS 17163-3907 between 5100 and 9000 Å was compared with that of the archetypal yellow hypergiant IRC+10420 and was found to be very similar. These results build on previous evidence that IRAS 17163-3907 is a yellow hypergiant [ABSTRACT FROM AUTHOR]

Context. The presence of stable disks around post-asymptotic giant branch (post-AGB) binaries is a widespread phenomenon. Also, the presence of (molecular) outflows is now commonly inferred in these systems. Aims. 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 binary system. The data showed that this flux arises from close to the central binary. Scattering off the inner rim of the circumbinary disk, or scattering in a dusty outflow were suggested as two possible origins. With detailed dust radiative transfer models of the circumbinary disk, we aim to discriminate between the two proposed configurations. Methods. By including Herschel/SPIRE photometry, we extend the spectral energy distribution (SED) such that it now fully covers UV to sub-mm wavelengths. The MCMax Monte Carlo radiative transfer code is used to create a large grid of disk models. Our models include a self-consistent treatment of dust settling as well as of scattering. A Si-rich composition with two additional opacity sources, metallic Fe or amorphous C, are tested. The SED is fit together with archival mid-IR (MIDI) visibilities, and the optical and near-IR visibilities of Paper I. In this way we constrain the structure of the disk, with a focus on its inner rim. Results. The near-IR visibility data require a smooth inner rim, here obtained with a double power-law parameterization of the radial surface density distribution. A model can be found that fits all of the IR photometric and interferometric data well, with either of the two continuum opacity sources. Our best-fit passive models are characterized by a significant amount of ~mm-sized grains, which are settled to the midplane of the disk. Not a single disk model fits our data at optical wavelengths because of the opposing constraints imposed by the optical and near-IR interferometric data. Conclusions. A geometry in which a passive, dusty, and puffed-up circumbinary disk is present, can reproduce all of the IR, but not the optical observations of 89 Her. Another dusty component (an outflow or halo) therefore needs to be added to the system. [ABSTRACT FROM AUTHOR]

We determine the orbital elements for the K0 IV + white dwarf (WD) system IP Eri, which appears to have a surprisingly long period of 1071 d and a significant eccentricity of 0.25. Previous spectroscopic analyses of the WD, based on a distance of 101 pc inferred from its Hipparcos parallax, yielded a mass of only 0.43 M⊙, implying it to be a helium-core WD. The orbital properties of IP Eri are similar to those of the newly discovered long-period subdwarf B star (sdB) binaries, which involve stars with Heburning cores surrounded by extremely thin H envelopes, and are therefore close relatives to He WDs. We performed a spectroscopic analysis of high-resolution spectra from the HERMES/Mercator spectrograph and concluded that the atmospheric parameters of the K0 component are Teff = 4960 K, log g = 3.3, [FE/7;H] = 0.09 and ξ = 1.5 km s-1. The detailed abundance analysis focuses on C, N, O abundances, carbon isotopic ratio, light (Na, Mg, Al, Si, Ca, Ti) and s-process (Sr, Y, Zr, Ba, La, Ce, Nd) elements. We conclude that IP Eri abundances agree with those of normal field stars of the same metallicity. The long period and non-null eccentricity indicate that this system cannot be the end product of a common-envelope phase; it calls instead for another less catastrophic binary-evolution channel presented in detail in a companion paper. [ABSTRACT FROM AUTHOR]

Context. We present 48 Herschel/PACS spectra of evolved stars in the wavelength range of 67–72 μm. This wavelength range covers the 69 μm band of crystalline olivine (Mg2–2xFe(2x)SiO4). The width and wavelength position of this band are sensitive to the temperature and composition of the crystalline olivine. Our sample covers a wide range of objects: from high mass-loss rate AGB stars (OH/IR stars, ˙M ≥ 10–5 M⊙/yr), through post-AGB stars with and without circumbinary disks, to planetary nebulae and even a few massive evolved stars. Aims. The goal of this study is to exploit the spectral properties of the 69 μm band to determine the composition and temperature of the crystalline olivine. Since the objects cover a range of evolutionary phases, we study the physical and chemical properties in this range of physical environments. Methods. We fit the 69 μm band and use its width and position to probe the composition and temperature of the crystalline olivine. Results. For 27 sources in the sample, we detected the 69 μm band of crystalline olivine (Mg(2–2x)Fe(2x)SiO4). The 69 μm band shows that all the sources produce pure forsterite grains containing no iron in their lattice structure. The temperature of the crystalline olivine as indicated by the 69 μm band, shows that on average the temperature of the crystalline olivine is highest in the group of OH/IR stars and the post-AGB stars with confirmed Keplerian disks. The temperature is lower for the other post-AGB stars and lowest for the planetary nebulae. A couple of the detected 69 μm bands are broader than those of pure magnesium-rich crystalline olivine, which we show can be due to a temperature gradient in the circumstellar environment of these stars. The disk sources in our sample with crystalline olivine are very diverse. They show either no 69 μm band, a moderately strong band, or a very strong band, together with a temperature for the crystalline olivine in their disk that is either very warm (~600 K), moderately warm (~200 K), or cold (~120 K), respectively. [ABSTRACT FROM AUTHOR]

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 J004441.04-732136.4 with state-of-the-art AGB model predictions with a main focus on Pb. The low signal-to-noise (S/N) in the Pb line region made the result of our previous study inconclusive. We acquired additional data covering the region of the strongest Pb line. Methods. By carefully complementing re-reduced previous data, with newly acquired UVES optical spectra, we improve the S/N of the spectrum around the strongest Pb line. Therefore, an upper limit for the Pb abundance is estimated from a merged weighted mean spectrum using synthetic spectral modeling. We then compare the abundance results from the combined spectra to predictions of tailored AGB evolutionary models from two independent evolution codes. In addition, we determine upper limits for Pb abundances for three previously studied LMC post-AGB objects. Results. Although theoretical predictions for J004441.04-732136.4 match the s-process distribution up to tungsten (W), the predicted very high Pb abundance is clearly not detected. The three additional LMC post-AGB stars show a similar lack of a very high Pb abundance. Conclusions. From our study, we conclude that none of these low-mass, low-metallicity post-AGB stars of the LMC and SMC are strong Pb producers. This conflicts with current theoretical predictions. [ABSTRACT FROM AUTHOR]

HERMES, a fibre-fed high-resolution ( R = 85000) échelle spectrograph with good stability and excellent throughput, is the work-horse instrument of the 1.2-m Mercator telescope on La Palma. HERMES targets building up time series of high-quality data of variable stellar phenomena, mainly for asteroseismology and binary-evolution research. In this paper we present the HERMES project and discuss the instrument design, performance, and a future upgrade. We also present some results of the first four years of HERMES observations. We illustrate the value of small telescopes, equipped with efficient instrumentation, for high-resolution spectroscopy. (© 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [ABSTRACT FROM AUTHOR]

Aims. Spectra of OH/IR stars show prominent spectral bands of crystalline olivine (Mg(2-2x)Fe(2x)SiO4). To learn more about the timescale of the outflows of OH/IR stars, we study the spectral band of crystalline olivine at 69 μm. Methods. The 69 μm band is of interest because its width and peak wavelength position are sensitive to the grain temperature and to the exact composition of the crystalline olivine. With Herschel/PACS, we observed the 69 µm band in the outflow of 14 OH/IR stars. By comparing the crystalline olivine features of our sample with those of model spectra, we determined the size of the outflow and its crystalline olivine abundance. Results. The temperature indicated by the observed 69 μm bands can only be reproduced by models with a geometrically compact superwind (RSW ≤ 2500 AU = 1400 R⋆). This means that the superwind started less than 1200 years ago (assuming an outflow velocity of 10 km/s). The small amount of mass lost in one superwind and the high progenitor mass of the OH/IR stars introduce a mass loss and thus evolutionary problem for these objects, which has not yet been understood. [ABSTRACT FROM AUTHOR]