Claudia Paladini

Unexpectedly large mass loss during the thermal pulse cycle of the red giant star R Sculptoris.

The asymptotic-giant-branch star R Sculptoris is surrounded by a detached shell of dust and gas. The shell originates from a thermal pulse during which the star underwent a brief period of increased mass loss. It has hitherto been impossible to constrain observationally the timescales and mass-loss properties during and after a thermal pulse—parameters that determine the lifetime of the asymptotic giant branch and the amount of elements returned by the star. Here we report observations of CO emission from the circumstellar envelope and shell around R Sculptoris with an angular resolution of 1.3″. What was previously thought to be only a thin, spherical shell with a clumpy structure is revealed to also contain a spiral structure. Spiral structures associated with circumstellar envelopes have been previously seen, leading to the conclusion that the systems must be binaries. Combining the observational data with hydrodynamic simulations, we conclude that R Sculptoris is a binary system that underwent a thermal pulse about 1,800 years ago, lasting approximately 200 years. About 3 × 10−3 solar masses of material were ejected at a velocity of 14.3 km s−1 and at a rate around 30 times higher than the pre-pulse mass-loss rate. This shows that about three times more mass was returned to the interstellar medium during and immediately after the pulse than previously thought.

Observing and modeling the dynamic atmosphere of the low mass-loss C-star R Sculptoris at high angular resolution

Context. We study the circumstellar environment of the carbon-rich star R Sculptoris using the near- and mid-infrared high spatial resolution observations from the ESO-VLTI focal instruments VINCI and MIDI, respectively. Aims. These observations aim at increasing our knowledge of the dynamic processes in play within the very close circumstellar environment where the mass loss of AGB stars is initiated. Methods. We first compare the spectro-interferometric measurements of the star at different epochs to detect the dynamic signatures of the circumstellar structures at different spatial and spectral scales. We then interpret these data using a self-consistent dynamic model atmosphere to discuss the dynamic picture deduced from the observations. Since the hydrodynamic computation needs stellar parameters as input, a considerable effort is first applied to determining these parameters. Results. Interferometric observations do not show any significant variability effect at the 16 m baseline between phases 0.17 and 0.23 in the K band, and for both the 15 m baseline between phases 0.66 and 0.97 and the 31 m baseline between phases 0.90 and 0.97 in the N band. We find fairly good agreement between the dynamic model and the spectrophotometric data from 0.4 to 25 μm. The model agrees well with the time-dependent flux data at 8.5 μm, whereas it is too faint at 11.3 and 12.5 μm. The VINCI visibility measurements are reproduced well, meaning that the extension of the model is suitable in the K-band. In the mid-infrared, the model has the proper extension to reveal molecular structures of C2H2 and HCN located above the stellar photosphere. However, the windless model used is not able to reproduce the more extended and dense dusty environment. Conclusions. Among the different explanations for the discrepancy between the model and the measurements, the strong nonequilibrium process of dust formation is one of the most probable. The transition from windless atmospheres to models with considerable mass-loss rates occurs in a very narrow range of stellar parameters, especially for the effective temperature, the C/O ratio, and the pulsation amplitude. A denser sampling of such critical regions of the parameter space with additional models might lead to a better representation of the extended structures of low mass-loss carbon stars like R Sculptoris. The complete dynamic coupling of gas and dust and the approximation of grain opacities with the small-particle limit in the dynamic calculation could also contribute to the difference between the model and the data.

Interferometric properties of pulsating C-rich AGB stars - Intensity profiles and uniform disc diameters of dynamic model atmospheres

Aims. On the basis of a set of dynamic model atmospheres of C-rich AGB stars, we present the first theoretical study of centre-to-limb variation (CLV) properties and relative radius interpretation on narrow and broad-band filters. We computed visibility profiles and the equivalent uniform disc (UD) radii to investigate the dependence of these quantities on the wavelength and pulsation phase. Methods. After an accurate morphological analysis of the visibility and intensity profiles determined in narrow and broad-band filters, we fitted our visibility profiles with a UD function simulating the observational approach. The UD-radii were computed using three different fitting-methods to investigate the influence of the visibility sampling profile: single point, two points and a least squares method. Results. The intensity and visibility profiles of models characterises by mass loss show a behaviour very different from a UD. We found that UD-radii are wavelength dependent and that this dependence is stronger if mass loss is present. Strong opacity contributions from C2H2 affect all radius measurements at 3 μm and in the N-band, resulting in higher values for the UD-radii. In the case of models with mass loss the predicted behaviour of UD-radii versus phase is complicated, while the radial changes are almost sinusoidal for models without mass loss. Compared to the M-type stars, for the C-stars no windows are available for measuring the pure continuum.

The PTI Carbon Star Angular Size Survey: Effective Temperatures and Non-sphericity

We report new interferometric angular diameter observations of 41 carbon stars observed with the Palomar Testbed Interferometer. Two of these stars are CH carbon stars and represent the first such measurements of this subtype. Of these, 39 have Yamashita spectral classes and are of sufficiently high quality that we can determine the dependence of effective temperature on spectral type. We find that there is a tendency for the effective temperature to increase with increasing temperature index by ~120 K per step, starting at T_(EFF) ≃ 2500 K for C3, y, although there is a large amount of scatter in this relationship. Overall, the median effective temperature of the carbon star sample is 2800 ± 270 K and the median linear radius is 360 ± 100 R_☉. We also find agreement, on average within 15 K, with the T_(EFF) determinations of Bergeat et al. and a refinement of the carbon star angular size prediction based on V & K magnitudes is presented that is good to an rms of 12%. A subsample of our stars have sufficient {u, v} coverage to permit non-spherical modeling of their photospheres, and a general tendency for detection of statistically significant departures from sphericity with increasing interferometric signal-to-noise is seen. The implications of most—and potentially all—carbon stars being non-spherical is considered in the context of surface inhomogeneities and a rotation-mass-loss connection.

Cannibals in the thick disk: the young

This work was partly supported by the European Union FP7 programme through ERC grant number 320360. P.J. acknowledges King’s College Cambridge for partially supporting this work. K.H. is supported by Marshall Scholarship and King’s College Cambridge Studenship. R.J.I. thanks the STFC for funding his Rutherford Fellowship. Based on observations made with the Mercator Telescope, operated on the island of La Palma by the Flemish Community, at the Spanish Observatorio del Roque de los Muchachos of the Instituto de Astrofi sica de Canarias. Based on observations obtained with the HERMES spectrograph, which is supported by the Research Foundation – Flanders (FWO), Belgium, the Research Council of KU Leuven, Belgium, the Fonds National de la Recherche Scientifique (F.R.S. – FNRS), Belgium, the Royal Observatory of Belgium, the Observatoire de Geneve, Switzerland and the Thuringer Landessternwarte Tautenburg, Germany.

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Aims. The circumstellar environment of L-2 Pup, an oxygen-rich semiregular variable, was observed to understand the evolution of mass loss and the shaping of ejecta in the late stages of stellar evolution. Methods. High-angular resolution observations from a single 8 m telescope were obtained using aperture masking in the near-infrared (1.64, 2.30 and 3.74 mu m) on the NACO/VLT, both in imaging and polarimetric modes. Results. The aperture-masking images of L-2 Pup at 2.30 mu m show a resolved structure that resembles a toroidal structure with a major axis of similar to 140 milliarcseconds (mas) and an east-west orientation. Two clumps can be seen on either side of the star, similar to 65 mas from the star, beyond the edge of the circumstellar envelope (estimated diameter is similar to 27 mas), while a faint, hook-like structure appear toward the northeast. The patterns are visible both in the imaging and polarimetric mode, although the latter was only used to measure the total intensity (Stokes I). The overall shape of the structure is similar at the 3.74 mu m pseudo-continuum (dust emission), where the clumps appear to be embedded within a dark, dusty lane. The faint, hook-like patterns are also seen at this wavelength, extending northeast and southwest with the central, dark lane being an apparent axis of symmetry. We interpret the structure as a circumstellar torus with inner radius of 4.2 au. With a rotation velocity of 10 kms(-1) as suggested by the SiO maser profile, we estimate a stellar mass of 0.7 M-circle dot.

-rich stars as evolved blue stragglers

Within the context of a large project aimed at studying early F-, A- and late B-type stars, we present the abundance analysis of the photospheres of 21 members of the open cluster NGC 5460, an inte ...

Dissecting the AGB star L-2 Puppis: a torus in the making

Context. SVPsc is an asymptotic giant branch (AGB) star surrounded by an oxygen-rich dust envelope. The mm-CO line profile of the objects outflow shows a clear double-component structure. Because of the high angular resolution, mid-IR interferometry may give strong constraints on the origin of this composite profile. Aims. The aim of this work is to investigate the morphology of the environment around SVPsc using high-angular resolution interferometry observations in the mid-IR with the Very Large Telescope MID-infrared Interferometric instrument (VLTI/MIDI). Methods. Interferometric data in the N-band taken at different baseline lengths (ranging from 32-64 m) and position angles (73-142 degrees) allow a study of the morphology of the circumstellar environment close to the star. The data are interpreted on the basis of 2-dimensional, chromatic geometrical models using the fitting software tool GEM-FIND developed for this purpose. Results. The results favor two scenarios: (i) the presence of a highly inclined, optically thin, dusty disk surrounding the central star; (ii) the presence of an unresolved binary companion at a separation of 13.7(-4.8)(+4.2) AU and a position angle of 121.8 degrees(-24.5)degrees(+15.4)degrees. NE. The derived orbital period of the binary is 38.1(-22.6)(+20.4) yr. This detection is in good agreement with hydrodynamic simulations showing that a close companion could be responsible for the entrainment of the gas and dust into a circumbinary structure.

A detailed spectroscopic analysis of the open cluster NGC 5460

Context. A new generation of powerful and efficient spectropolarimeters has recently been used to provide the first sample of magnetic Ap stars of accurately known ages. Modelling of these data offer the possibility of significant new insights into the physics and main sequence evolution of these remarkable stars. Aims. New spectra have been obtained with the ESPaDOnS spectropolarimeter, and are supplemented with unpolarised spectra from the ESO UVES, UVES-FLAMES, and HARPS spectrographs, of the very peculiar large-field magnetic Ap star HD 318107, a member of the open cluster NGC 6405 and thus a star with a well-determined age. The available data provide sufficient material with which to re-analyse the first-order model of the magnetic field geometry and to derive chemical abundances of Si, Ti, Fe, Nd, Pr, Mg, Cr, Mn, O, and Ca. Methods. The models were obtained using ZEEMAN, a program which synthesises spectral line profiles for stars that have magnetic fields. The magnetic field structure was modelled with a low-order colinear multipole expansion, using coefficients derived from the observed variations of the field strength with rotation phase. The abundances of several elements were determined using spectral synthesis. After experiments with a very simple model of uniform abundance on each of three rings of equal width in co-latitude and symmetric about the assumed magnetic axis, we decided to model the spectra assuming uniform abundances of each element over the stellar surface. Results. The new magnetic field measurements allow us to refine the rotation period of HD 318107 to P = 9.7088 ± 0.0007 days. Appropriate magnetic field model parameters were found that very coarsely describe the (apparently rather complex) field moment variations. Spectrum synthesis leads to the derivation of mean abundances for the elements Mg, Si, Ca, Ti, Cr, Fe, Nd, and Pr. All of these elements except for Mg and Ca are strongly overabundant compared to the solar abundance ratios. There is considerable ?? ??

The geometry of the close environment of SV Piscium as probed by VLTI/MIDI

Context. The Mass loss of Evolved StarS (MESS) sample observed with PACS on board the Herschel Space Observatory revealed that several asymptotic giant branch (AGB) stars are surrounded by an asymmetric circumstellar envelope (CSE) whose morphology is most likely caused by the interaction with a stellar companion. The evolution of AGB stars in binary systems plays a crucial role in understanding the formation of asymmetries in planetary nebulae (PNe), but at present, only a handful of cases are known where the interaction of a companion with the stellar AGB wind is observed. Aims. We probe the environment of the very evolved AGB star π 1 Gruis on large and small scales to identify the triggers of the observed asymmetries. Methods. Observations made with Herschel/PACS at 70 μm and 160 μm picture the large-scale environment of π 1 Gru. The close surroundings of the star are probed by interferometric observations from the VLTI/AMBER archive. An analysis of the proper motion data of Hipparcos and Tycho-2 together with the Hipparcos Intermediate Astrometric Data help identify the possible cause for the observed asymmetry. Results. The Herschel/PACS images of π 1 Gru show an elliptical CSE whose properties agree with those derived from a CO map published in the literature. In addition, an arc east of the star is visible at a distance of 38 �� from the primary. This arc is most likely part of an Archimedean spiral caused by an already known G0V companion that is orbiting the primary at a projected distance of 460 au with a period of more than 6200 yr. However, the presence of the elliptical CSE, proper motion variations, and geometric modelling of the VLTI/AMBER observations point towards a third component in the system, with an orbital period shorter than 10 yr, orbiting much closer to the primary than the G0V star.