Examinando por Autor "Monaco, L"

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    Daily variability of Ceres’ albedo detected by means of radial velocities changes of the reflected sunlight
    (Oxford Univ Press, 2016-05) Molaro, P; Lanza, AF; Monaco, L; Tosi, F; Lo Curto, G; Fulle, M; Pasquini, L
    Bright features have been recently discovered by Dawn on Ceres, which extend previous photometric and Space Telescope observations. These features should produce distortions of the line profiles of the reflected solar spectrum and therefore an apparent radial velocity variation modulated by the rotation of the dwarf planet. Here we report on two sequences of observations of Ceres performed in the nights of 2015 July 31, August 26 and 27 by means of the high-precision High Accuracy Radial Velocity Planet Searcher (HARPS) spectrograph at the 3.6 m La Silla European Southern Observatory (ESO) telescope. The observations revealed a quite complex behaviour which likely combines a radial velocity modulation due to the rotation with an amplitude of ≈±6 m s−1 and an unexpected diurnal effect. The latter changes imply changes in the albedo of Occator's bright features due to the blaze produced by the exposure to solar radiation. The short-term variability of Ceres’ albedo is on time-scales ranging from hours to months and can both be confirmed and followed by means of dedicated radial velocity observations.
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    The Gaia-ESO Survey: lithium depletion in the Gamma Velorum cluster and inflated radii in low-mass pre-main-sequence stars
    (OXFORD UNIV PRESS, 2017-01) Jeffries, RD; Jackson, RJ; Franciosini, E; Randich, S; Barrado, D; Frasca, A; Klutsch, A; Lanzafame, AC; Prisinzano, L; Sacco, GG; Gilmore, G; Vallenari, A; Alfaro, EJ; Koposov, SE; Pancino, E; Bayo, A; Casey, AR; Costado, MT; Damiani, F; Hourihane, A; Lewis, J; Jofre, P; Magrini, L; Monaco, L; Morbidelli, L; Worley, CC; Zaggia, S; Zwitter, T
    We show that non-magnetic models for the evolution of pre-main-sequence (PMS) stars cannot simultaneously describe the colour-magnitude diagram (CMD) and the pattern of lithium depletion seen in the cluster of young, low-mass stars surrounding gamma(2) Velorum. The age of 7.5 +/- 1 Myr inferred from the CMD is much younger than that implied by the strong Li depletion seen in the cluster M-dwarfs, and the Li depletion occurs at much redder colours than predicted. The epoch at which a star of a given mass depletes its Li and the surface temperature of that star are both dependent on its radius. We demonstrate that if the low-mass stars have radii similar to 10 per cent larger at a given mass and age, then both the CMD and the Li-depletion pattern of the Gamma Velorum cluster are explained at a common age of similar or equal to 18-21 Myr. This radius inflation could be produced by some combination of magnetic suppression of convection and extensive cool starspots. Models that incorporate radius inflation suggest that PMS stars, similar to those in the Gamma Velorum cluster, in the range 0.2 < M/M-circle dot < 0.7, are at least a factor of 2 older and similar to 7 per cent cooler than previously thought and that their masses are much larger (by > 30 per cent) than inferred from conventional, non-magnetic models in the Hertzsprung-Russell diagram. Systematic changes of this size may be of great importance in understanding the evolution of young stars, disc lifetimes and the formation of planetary systems.
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    TOPoS: VI. The metal-weak tail of the metallicity distribution functions of the Milky Way and the Gaia -Sausage-Enceladus structure
    (EDP Sciences, 2021-07) Bonifacio, P.; Monaco, L; Salvadori, S; Caffau, E.; Spite, M; Sbordone, L.; Spite, F.; Ludwig, H.-G; DI Matteo, P.; Haywood, M.; François, P.; Koch-Hansen, A.J
    Context. The goal of the Turn-Off Primordial Stars survey (TOPoS) project is to find and analyse turn-off (TO) stars of extremely low metallicity. To select the targets for spectroscopic follow-up at high spectral resolution, we relied on low-resolution spectra from the Sloan Digital Sky Survey (SDSS). Aims. In this paper, we use the metallicity estimates we obtained from our analysis of the SDSS spectra to construct the metallicity distribution function (MDF) of the Milky Way, with special emphasis on its metal-weak tail. The goal is to provide the underlying distribution out of which the TOPoS sample was extracted. Methods. We made use of SDSS photometry, Gaia photometry, and distance estimates derived from the Gaia parallaxes to derive a metallicity estimate for a large sample of over 24 million TO stars. This sample was used to derive the metallicity bias of the sample for which SDSS spectra are available. Results. We determined that the spectroscopic sample is strongly biased in favour of metal-poor stars, as intended. A comparison with the unbiased photometric sample allows us to correct for the selection bias. We selected a sub-sample of stars with reliable parallaxes for which we combined the SDSS radial velocities with Gaia proper motions and parallaxes to compute actions and orbital parameters in the Galactic potential. This allowed us to characterise the stars dynamically, and in particular to select a sub-sample that belongs to the Gaia-Sausage-Enceladus (GSE) accretion event. We are thus also able to provide the MDF of GSE. Conclusions. The metal-weak tail derived in our study is very similar to that derived in the H3 survey and in the Hamburg/ESO Survey. This allows us to average the three MDFs and provide an error bar for each metallicity bin. Inasmuch as the GSE structure is representative of the progenitor galaxy that collided with the Milky Way, that galaxy appears to be strongly deficient in metal-poor stars compared to the Milky Way, suggesting that the metal-weak tail of the latter has been largely formed by accretion of low-mass galaxies rather than massive galaxies, such as the GSE progenitor. © P. Bonifacio et al. 2021.
  • Miniatura
    Ítem
    TOPoS: VI. The metal-weak tail of the metallicity distribution functions of the Milky Way and the Gaia -Sausage-Enceladus structure
    (EDP Sciences, 2021-05) Spite, P.; Bonifacio, P.; Monaco, L; Salvadori, S; Caffau, E.; Sbordone, M
    Context. The goal of the Turn-Off Primordial Stars survey (TOPoS) project is to find and analyse turn-off (TO) stars of extremely low metallicity. To select the targets for spectroscopic follow-up at high spectral resolution, we relied on low-resolution spectra from the Sloan Digital Sky Survey (SDSS). Aims. In this paper, we use the metallicity estimates we obtained from our analysis of the SDSS spectra to construct the metallicity distribution function (MDF) of the Milky Way, with special emphasis on its metal-weak tail. The goal is to provide the underlying distribution out of which the TOPoS sample was extracted. Methods. We made use of SDSS photometry, Gaia photometry, and distance estimates derived from the Gaia parallaxes to derive a metallicity estimate for a large sample of over 24 million TO stars. This sample was used to derive the metallicity bias of the sample for which SDSS spectra are available. Results. We determined that the spectroscopic sample is strongly biased in favour of metal-poor stars, as intended. A comparison with the unbiased photometric sample allows us to correct for the selection bias. We selected a sub-sample of stars with reliable parallaxes for which we combined the SDSS radial velocities with Gaia proper motions and parallaxes to compute actions and orbital parameters in the Galactic potential. This allowed us to characterise the stars dynamically, and in particular to select a sub-sample that belongs to the Gaia-Sausage-Enceladus (GSE) accretion event. We are thus also able to provide the MDF of GSE. Conclusions. The metal-weak tail derived in our study is very similar to that derived in the H3 survey and in the Hamburg/ESO Survey. This allows us to average the three MDFs and provide an error bar for each metallicity bin. Inasmuch as the GSE structure is representative of the progenitor galaxy that collided with the Milky Way, that galaxy appears to be strongly deficient in metal-poor stars compared to the Milky Way, suggesting that the metal-weak tail of the latter has been largely formed by accretion of low-mass galaxies rather than massive galaxies, such as the GSE progenitor.