Aims. We aim to study the presence of both rotation and expansion in post-AGB nebulae, in particular around IW Car, a binary post-AGB star that was suspected to be surrounded by a Keplerian disk. ...Methods. We obtained high-quality ALMA observations of 12CO and 13CO J = 3−2 lines in IW Car. The maps were analyzed by means of a simplified model of CO emission, based on those used for similar objects. Results. Our observations clearly show the presence of gas components in rotation, in an equatorial disk, and expansion, which shows an hourglass-like structure with a symmetry axis perpendicular to the rotation plane and is probably formed of material extracted from the disk. Our modeling can reproduce the observations and shows moderate uncertainties. The rotation velocity corresponds to a central stellar mass of approximately 1 M⊙. We also derive the total mass of the molecule-rich nebula, found to be of ~4 × 10-3M⊙; the outflow is approximately eight times less massive than the disk. From the kinematical age of the outflow and the mass values derived for both components, we infer a (future) lifetime of the disk of approximately 5000–10 000 yr.
The asteroseismic and planetary studies, like all research related to stars, need precise and accurate stellar atmospheric parameters as input. We aim at deriving the effective temperature (T
eff), ...the surface gravity (log g), the metallicity (Fe/H), the projected rotational velocity (v sin i) and the MK type for 169 F-, G-, K- and M-type Kepler targets which were observed spectroscopically from the ground with five different instruments. We use two different spectroscopic methods to analyse 189 high-resolution, high-signal-to-noise spectra acquired for the 169 stars. For 67 stars, the spectroscopic atmospheric parameters are derived for the first time. KIC 9693187 and 11179629 are discovered to be double-lined spectroscopic binary systems. The results obtained for those stars for which independent determinations of the atmospheric parameters are available in the literature are used for a comparative analysis. As a result, we show that for solar-type stars the accuracy of present determinations of atmospheric parameters is ±150 K in T
eff, ±0.15 dex in Fe/H and ±0.3 dex in log g. Finally, we confirm that the curve-of-growth analysis and the method of spectral synthesis yield systematically different atmospheric parameters when they are applied to stars hotter than 6000 K.
Aims. We aim to study the rotating and expanding gas in the Red Rectangle, which is a well known bipolar nebula surrounding a double stellar system whose primary is a post-asymptotic giant branch ...(post-AGB) star. We analyze the properties of both components and the relation between them. Rotating disks have been very elusive in post-AGB nebulae, in which gas is almost always found to be in expansion. Methods. We present new high-quality ALMA observations of this source in C17O J = 6−5 and H13CN J = 4−3 line emission and results from a new reduction of already published 13CO J = 3−2 data. A detailed model fitting of all the molecular line data, also discussing previous maps and single-dish observations of lines of CO, CII, and CI, was performed using a sophisticated code that includes an accurate nonlocal treatment of radiative transfer in 2D (assuming axial symmetry). These observations (of low- and high-opacity lines requiring various degrees of excitation) and the corresponding modeling allowed us to deepen the analysis of the nebular properties. We also stress the uncertainties, particularly in the determination of the boundaries of the CO-rich gas and some properties of the outflow. Results. We confirm the presence of a rotating equatorial disk and an outflow, which is mainly formed of gas leaving the disk. The mass of the disk is ~0.01 M⊙, and that of the CO-rich outflow is around ten times smaller. High temperatures of ≳100 K are derived for most components. From comparison of the mass values, we roughly estimate the lifetime of the rotating disk, which is found to be of about 10 000 yr. Taking data of a few other post-AGB composite nebulae into account, we find that the lifetimes of disks around post-AGB stars typically range between about 5000 and more than 20 000 yr. The angular momentum of the disk is found to be high, ~9 M⊙ AU km s-1, which is comparable to that of the stellar system at present. Our observations of H13CN show a particularly wide velocity dispersion and indicate that this molecule is only abundant in the inner Keplerian disk, at ≲60 AU from the stellar system. We suggest that HCN is formed in a dense photodissociation region (PDR) due to the UV excess known to be produced by the stellar system, following chemical mechanisms that are well established for interstellar medium PDRs and disks orbiting young stars. We further suggest that this UV excess could lead to an efficient formation and excitation of PAHs and other C-bearing macromolecules, whose emission is very intense in the optical counterpart.
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.
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 an A-type (Teff = 8250 250 K) luminous (8200 700 L ) metal-poor (Fe/H = −1.18 0.10) low-mass (Minitial 1.5-2.0 M ) 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 an intriguing photospheric composition with no confirmed carbon-enhancement (upper limit of C/Fe < 0.50) nor enrichment of s-process elements. We derived an oxygen abundance of O/Fe = 0.29 0.1. For Fe and O, we took the effects of nonlocal thermodynamic equilibrium into account. We could not derive an upper limit for the nitrogen abundance as there are no useful nitrogen lines within our spectral coverage. The chemical pattern displayed by this object has not been observed in single or binary post-AGBs. Based on its derived stellar parameters and inferred evolutionary state, single-star nucleosynthesis models predict that this star should have undergone TDU episodes while on the AGB, and it should be carbon enriched. However, our observations are in contrast with these predictions. We identify two possible Galactic analogs that are likely to be post-AGB stars, but the lack of accurate distances (hence luminosities) to these objects does not allow us to confirm their post-AGB status. If they have low luminosities, then they are likely to be dusty post-RGB stars. The discovery of J005252.87-722842.9 reveals a new stellar evolutionary channel whereby a star evolves without any TDU episodes.
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.
Context. Classical novae are the product of thermonuclear runaway-initiated explosions occurring on accreting white dwarfs. Aims. V339 Del (Nova Delphinus 2013) was one of the brightest classical ...novae of the last hundred years. Spectroscopy and photometry are available from γ-rays through infrared at stages that have frequently not been observed well. The complete data set is intended to provide a benchmark for comparison with modeling and for understanding more sparsely monitored historical classical and recurrent novae. This paper is the first in the series of reports on the development of the nova. We report here on the early stages of the outburst, through the X-ray active stage. Methods. A time sequence of optical, flux calibrated high resolution spectra was obtained with the Nordic Optical Telescope (NOT) using FIES simultaneously, or contemporaneously, with the Space Telescope Imaging Spectrograph (STIS) aboard the Hubble Space Telescope during the early stages of the outburst. These were supplemented with Mercator/HERMES optical spectra. High resolution IUE ultraviolet spectra of OS And 1986, taken during the Fe curtain phase, served as a template for the distance determination. We used standard plasma diagnostics (e.g., O III and N II line ratios, and the Hβ line flux) to constrain electron densities and temperatures of the ejecta. Using Monte Carlo modeling of the ejecta, we derived the structure, filling factor, and mass from comparisons of the optical and ultraviolet line profiles. Results. We derive an extinction of E(B − V) = 0.23 ± 0.05 from the spectral energy distribution, the interstellar absorption, and H I emission lines. The distance, about 4−4.5 kpc, is in agreement with the inferred distance from near infrared interferometry. The maximum velocity was about 2500 km s-1, measured from the UV resonance and optical profiles. The ejecta showed considerable fine structure in all transitions, much of which persisted as emission knots. The line profiles were modeled using a bipolar conical structure for the ejecta within a relatively restricted range of parameters. For V339 Del, we find that an inclination to the line of sight of about 35°−55°, an opening angle of 60°−80°, and an inner radius ΔR/R(t) ≈ 0.3 based on vrad,max matches the permitted and intercombination lines. The filling factor is f ≈ 0.1, and the derived range in the ejecta mass is (2−3) × 10-5M⊙.
Barium (Ba), CH, and extrinsic or Tc-poor S-type stars are evolved low- and intermediate-mass stars that show enhancement of slow-neutron-capture-process elements on their surface, an indication of ...mass accretion from a former asymptotic giant branch companion, which is now a white dwarf (WD). Ba and CH stars can be found in the main-sequence (MS), the sub-giant, and the giant phase, while extrinsic S-type stars populate the giant branches only. As these polluted stars evolve, they might be involved in a second phase of interaction with their now white dwarf companion. In this paper, we consider systems composed of a main-sequence Ba star and a WD companion when the former evolves along the red giant branch (RGB). We want to determine if the orbital properties of the known population of Ba, CH, and S giants can be inferred from the evolution of their suspected dwarf progenitors. For this purpose, we used the BINSTAR binary evolution code and model MS+WD binary systems, considering different binary interaction mechanisms, such as a tidally enhanced wind mass loss, and a reduced circularisation efficiency. To explore their impact on the second RGB ascent, we compared the modelled orbits with the observed period and eccentricity distributions of Ba and related giants. We show that, independently of the considered mechanism, there is a strong period cut-off below which core-He burning stars should not be found in binary systems with a WD companion. This limit is shorter for more massive RGB stars and for more metal-poor systems. However, we still find a few low-mass short-period giant systems that are difficult to explain with our models, as well as two systems with very high eccentricities.
Aims. We present a mid-IR interferometric survey of the circumstellar environment of a specific class of post-asymptotic giant branch (post-AGB) binaries. For this class the presence of a compact ...dusty disk has been postulated on the basis of various spatially unresolved measurements. The aim is to determine the angular extent of the N-band emission directly and to resolve the compact circumstellar structures. Methods. Our interferometric survey was performed with the MIDI instrument on the VLTI. In total 19 different systems were observed using variable baseline configurations. Combining all the visibilities at a single wavelength at 10.7 μm, we fitted two parametric models to the data: a uniform disk and a ring model mimicking a temperature gradient. We compared our observables of the whole sample, with synthetic data computed from a grid of radiative transfer models of passively irradiated disks in hydrostatic equilibrium. These models are computed with a Monte Carlo code that has been widely applied to describe the structure of protoplanetary disks around young stellar objects (YSO). Results. The spatially resolved observations show that the majority of our targets cluster closely together in the distance-independent size-colour diagram, and have extremely compact N-band emission regions. The typical uniform disk diameter of the N-band emission region is ~40 mas, which corresponds to a typical brightness temperature of 400–600 K. The resolved objects display very similar characteristics in the interferometric observables and in the spectral energy distributions. Therefore, the physical properties of the disks around our targets must be similar. Our results are discussed in the light of recently published sample studies of YSOs to compare quantitatively the secondary discs around post-AGB stars to the ones around YSOs. Conclusions. Our high-angular-resolution survey further confirms the disk nature of the circumstellar structures present around wide post-AGB binaries. The grid of protoplanetary disk models covers very well the observed objects. Much like for young stars, the spatially resolved N-band emission region is determined by the hot inner rim of the disk. Continued comparisons between post-AGB and protoplanetary disks will help to understand grain growth and disk evolution processes, and to constrain planet formation theories. These second-generation disks are an important missing ingredient in binary evolution theory of intermediate-mass stars.
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 M⊙ 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. Conclusions. The disk in the AC Her system is in a very evolved state, as shown by its small gas/dust ratio and large inner hole. 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 AC Her, but we find strong similarities with the protoplanetary disk HD 100546.
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