Ester Pasquato

Chemically tagging the Hyades stream: does it partly originate from the Hyades cluster?

The Hyades stream has long been thought to be a dispersed vestige of the Hyades cluster. However, recent analyses of the parallax distribution, of the mass function, and of the actionspace distribution of stream stars have shown it to be rather composed of orbits trapped at a resonance of a density disturbance. This resonant scenario should leave a clearly different signature in the element abundances of stream stars than the dispersed cluster scenario, since the Hyades cluster is chemically homogeneous. Here, we study the metallicity as well as the element abundances of Li, Na, Mg, Fe, Zr, Ba, La, Ce, Nd and Eu for a random sample of stars belonging to the Hyades stream, and compare them with those of stars from the Hyades cluster. From this analysis: (i) we independently confirm that the Hyades stream cannot be solely composed of stars originating in the Hyades cluster; (ii) we show that some stars (namely 2/21) from the Hyades stream nevertheless have abundances compatible with an origin in the cluster; (iii) we emphasize that the use of Li as a chemical tag of the cluster origin of mainsequence stars is very efficient in the range 5500 K ≤ T eff ≤ 6200 K, since the Li sequence in the Hyades cluster is very tight, while at the same time spanning a large abundance range; (iv) we show that, while this evaporated population has a metallicity excess of ∼0.2 dex with respect to the local thin-disc population, identical to that of the Hyades cluster, the remainder of the Hyades stream population has still a metallicity excess of ∼0.06–0.15 dex, consistent with an origin in the inner Galaxy and (v) we show that the Hyades stream can be interpreted as an inner 4:1 resonance of the spiral pattern: this then also reproduces an orbital family compatible with the Sirius stream, and places the origin of the Hyades stream up to 1 kpc inwards from the solar radius, which might explain the observed metallicity excess of the stream population.

Fundamental parameters of 16 late-type stars derived from their angular diameter measured with VLTI/AMBER

Thanks to their large angular dimension and brightness, red giants and supergiants are privileged targets for optical long-baseline interferometers. 16 red giants and supergiants have been observed with the VLTI/AMBER facility over a 2-year period, at medium spectral resolution (R = 1500) in the K band. The limb-darkened angular diameters are derived from fits of stellar atmospheric models on the visibility and the triple product data. The angular diameters do not show any significant temporal variation, except for one target: TX Psc, which shows a variation of 4 per cent using visibility data. For the eight targets previously measured by long-baseline interferometry (LBI) in the same spectral range, the difference between our diameters and the literature values is less than 5 per cent, except for TX Psc, which shows a difference of 11 per cent. For the eight other targets, the present angular diameters are the first measured from LBI. Angular diameters are then used to determine several fundamental stellar parameters, and to locate these targets in the Hertzsprung-Russell diagram (HRD). Except for the enigmatic Tc-poor low-mass carbon star W Ori, the location of Tc-rich stars in the HRD matches remarkably well the thermally-pulsating asymptotic giant branch, as it is predicted by the stellar evolution models. For pulsating stars with periods available, we compute the pulsation constant and locate the stars along the various sequences in the period-luminosity diagram. We confirm the increase in mass along the pulsation sequences, as predicted by theory, except for W Ori which, despite being less massive, appears to have a longer period than T Cet along the first-overtone sequence.

SPIDAST: a new modular software to process spectrointerferometric measurements

Extracting stellar fundamental parameters from Spectrointerferometric (SPI) data requires reliable estimates of observables and with robust uncertainties (visibility, triple product, phase closure). A number of fine calibration procedures are necessary throughout the reduction process. Testing departures from centrosymmetry of brightness distributions is a useful complement. Developing a set of automatic routines called SPIDAST (made available to the community) to reduce, calibrate and interpret raw data sets of instantaneous spectrointerferograms at the spectral channel level, we complement (and in some respects improve) the ones contained in the AMDLIB Data Reduction Software. Our new software SPIDAST is designed to work in an automatic mode, free from subjective choices, while being versatile enough to suit various processing strategies. SPIDAST performs the following automated operations: weighting of nonaberrant SPI data (visibility, triple product), fine spectral calibration (subpixel level), accurate and robust determinations of stellar diameters for calibrator sources (and their uncertainties as well), correction for the degradations of the interferometer response in visibility and triple product, calculation of the centrosymmetry parameter from the calibrated triple product, fit of parametric chromatic models on SPI observables, to extract model parameters. SPIDAST is currently applied to the scientific study of 18 cool giant and supergiant stars, observed with the VLTI/AMBER facility at medium resolution in the K band. Because part of their calibrators have no diameter in the current catalogues, SPIDAST provides new determinations of the angular diameters of all calibrators. Comparison of SPIDAST final calibrated observables with AMDLIB determinations shows good agreement, under good and poor seeing conditions.

Limits in astrometric accuracy induced by surface brightness asymmetries in red supergiant stars

Aims. Surface brightness asymmetries are a very common feature of stars. Among other effects they cause a difference between the projected barycentre and photocentre. The evolution of those surface features makes this difference time-dependent. In some cases, e.g. for supergiant stars, the displacement can be a non-negligible fraction of the star radius R, and if R > 1 AU, of the parallax. Methods. We investigate the impact of surface brightness asymmetries on both the Gaia astrometric solution and the data processing flow with a theoretical approach. Results. We show that when the amplitude of the displacement is comparable to the epoch astrometric precision, the resulting astrometric solution of a genuine single star may be, in some cases, of low quality (with some parameters up to 10σ off). In this case, we provide an analytical prediction of the impact of the photocentre motion on both χ 2 and the uncertainty in the astrometric parameters. Non-single star solutions are found, if allowed for the closest stars. A closer look at the parameters of the orbital solutions reveals however that they are spurious (since the semi-major axis is smaller than either its error or the stellar radius). It is thus possible to filter out those spurious orbital solutions. Interestingly, for the stocastic solutions, the stochastic noise appears to be a good estimate of the photocentric noise.