Examinando por Autor "Gratton, R."
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Ítem Revealing asymmetrical dust distribution in the inner regions of HD141569(European Southern Observatory (ESO), 2021) Singh, G.; Bhowmik, T.; Boccaletti, A.; Thébault, P.; Kral, Q.; Milli, J.; Mazoyer, J.; Pantin, E.; Van Holstein, R. G.; Olofsson, Johan; Boukrouche, R.; Di Folco, E.; Janson, M.; Langlois, M.; Maire, A.-L.; Vigan, A.; Benisty, M.; Augereau, J.-C.; Perrot, C.; Gratton, R.; Henning, T.; Ménard, F.; Rickman, E.; Wahhaj, Z.; Zurlo, A.; Biller, B.; Bonnefoy, M.; Chauvin, G.; Delorme, P.; Desidera, S.; D’Orazi, V.; Feldt, M.; Hagelberg, J.; Keppler, M.; Kopytova, T.; Lagadec, E.; Lagrange, A.-M.; Mesa, D.; Meyer, M.; Rouan, D.; Sissa, E.; Schmidt, T. O. B.; Jaquet, M.; Fusco, T.; Pavlov, A.; Rabou, P.Context. The combination of high-contrast imaging with spectroscopy and polarimetry offers a pathway to studying the grain distribution and properties of debris disks in exquisite detail. Here, we focus on the case of a gas-rich debris disk around HD 141569A, which features a multiple-ring morphology first identified with SPHERE in the near-infrared. Aims. We obtained polarimetric differential imaging with SPHERE in the H-band to compare the scattering properties of the innermost ring at 44 au with former observations in total intensity with the same instrument. In polarimetric imaging, we observed that the intensity of the ring peaks in the south-east, mostly in the forward direction, whereas in total intensity imaging, the ring is detected only at the south. This noticeable characteristic suggests a non-uniform dust density in the ring. With these two sets of images, we aim to study the distribution of the dust to solve for the actual dust distribution. Methods. We implemented a density function varying azimuthally along the ring and generated synthetic images both in polarimetry and in total intensity, which are then compared to the actual data. The search for the best-fit model was performed both with a grid-based and an MCMC approach. Using the outcome of this modelization, we further measured the polarized scattering phase function for the observed scattering angle between 33° and 147° as well as the spectral reflectance of the southern part of the ring between 0.98 and 2.1 μm. We tentatively derived the grain properties by comparing these quantities with MCFOST models and assuming Mie scattering. Results. We find that the dust density peaks in the south-west at an azimuthal angle of 220°~238° with a rather broad width of 61°~127°. The difference in the intensity distributions observed in polarimetry and total intensity is the result of this particular morphology. Although there are still uncertainties that remain in the determination of the anisotropic scattering factor, the implementation of an azimuthal density variation to fit the data proved to be robust. Upon elaborating on the origin of this dust density distribution, we conclude that it could be the result of a massive collision when we account for the effect of the high gas mass that is present in the system on the dynamics of grains. In terms of grain composition, our preliminary interpretation indicates a mixture of porous sub-micron sized astro-silicate and carbonaceous grains. Conclusions. The SPHERE observations have allowed, for the first time, for meaningful constraints to be placed on the dust distribution beyond the standard picture of a uniform ring-like debris disk. However, future studies with a multiwavelength approach and additional detailed modeling would be required to better characterize the grain properties in the HD 141569 system.Ítem The Gaia-ESO Survey: a new approach to chemically characterising young open clusters(European Southern Observatory (ESO), 2021) Baratella, M.; D’Orazi, V.; Sheminova, V.; Spina, L.; Carraro, G.; Gratton, R.; Magrini, L.; Randich, L.; Lugaro, M.; Pignatari, M.; Romano, D.; Biazzo, K.; Bragaglia, A.; Casali, G.; Desidera, S.; Frasca, A.; De Silva, G.; Melo, C.; Van Der Swaelmen, M.; Tautvaišienė, G.; Jiménez-Esteban, F. M.; Gilmore, G.; Bensby, T.; Smiljanic, R.; Bayo, Amelia; Franciosini, E.; Gonneau, A.; Hourihane, A.; Jofré, P.; Monaco, L.; Morbidelli, L.; Sacco, G.; Sbordone, L.; Worley, C.; Zaggia, S.Context. Young open clusters (ages of less than 200 Myr) have been observed to exhibit several peculiarities in their chemical compositions. These anomalies include a slightly sub-solar iron content, super-solar abundances of some atomic species (e.g. ionised chromium), and atypical enhancements of [Ba/Fe], with values up to ~0.7 dex. Regarding the behaviour of the other s-process elements like yttrium, zirconium, lanthanum, and cerium, there is general disagreement in the literature: some authors claim that they follow the same trend as barium, while others find solar abundances at all ages. Aims. In this work we expand upon our previous analysis of a sample of five young open clusters (IC 2391, IC 2602, IC 4665, NGC 2516, and NGC 2547) and one star-forming region (NGC 2264), with the aim of determining abundances of different neutron-capture elements, mainly Cu I, Sr I, Sr II, Y II, Zr II, Ba II, La II, and Ce II. For NGC 2264 and NGC 2547 we present the measurements of these elements for the first time. Methods. We analysed high-resolution, high signal-to-noise spectra of 23 solar-type stars observed within the Gaia-ESO survey. After a careful selection, we derived abundances of isolated and clean lines via spectral synthesis computations and in a strictly differential way with respect to the Sun. Results. We find that our clusters have solar [Cu/Fe] within the uncertainties, while we confirm that [Ba/Fe] is super-solar, with values ranging from +0.22 to +0.64 dex. Our analysis also points to a mild enhancement of Y, with [Y/Fe] ratios covering values between 0 and +0.3 dex. For the other s-process elements we find that [X/Fe] ratios are solar at all ages. Conclusions. It is not possible to reconcile the anomalous behaviour of Ba and Y at young ages with standard stellar yields and Galactic chemical evolution model predictions. We explore different possible scenarios related to the behaviour of spectral lines, from the dependence on the different ionisation stages and the sensitivity to the presence of magnetic fields (through the Landé factor) to the first ionisation potential effect. We also investigate the possibility that they may arise from alterations of the structure of the stellar photosphere due to the increased levels of stellar activity that affect the spectral line formation, and consequently the derived abundances. These effects seem to be stronger in stars at ages of less than ~ 100 Myr. However, we are still unable to explain these enhancements, and the Ba puzzle remains unsolved. With the present study we suggest that other elements, for example Sr, Zr, La, and Ce, might be more reliable tracer of the s-process at young ages, and we strongly encourage further critical observations.