Examinando por Autor "Marchetti, Tommaso"

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    Ítem
    Astrometry in crowded fields towards the Galactic bulge
    (EDP Sciences, 2023-09) Luna, Alonso; Marchetti, Tommaso; Rejkuba, Marina; Minniti, Dante
    Context. The astrometry towards the Galactic bulge is hampered by high stellar crowding and patchy extinction. This effect is particularly severe for optical surveys such as the European Space Agency satellite Gala. Aims. In this study, we assess the consistency of proper motion measurements between optical (Gaia DR3) and near-infrared (VIRAC2) catalogues in comparison with proper motions measured with the Hubble Space Telescope (HST) observations in several crowded fields towards the Galactic bulge and in Galactic globular clusters. Methods. Assuming that the proper motion measurements are well characterised, the uncertainty-normalised proper motion differences between pairs of catalogues are expected to follow a normal distribution. A deviation from a normal distribution defines the inflation factor r. By multiplying the proper motion uncertainties with the appropriate inflation factor values, the Gaia (VIRAC2) proper motion measurements are brought into a 1ÏÃ Â agreement with the HST proper motions. Results. The inflation factor (r) depends on stellar surface density. For the brightest stars in our sample (G < 18), the dependence on G-band magnitude is strong, corresponding to the most precise Gaia DR3 proper motions. We used the number of observed Gaia DR3 sources as a proxy for the stellar surface density. Assuming that the HST proper motion measurements are well determined and free from systematic errors, we find that Gaia DR3 proper motion uncertainties are better characterised, having r < 1.5 in fields with a stellar number density with fewer than 200 Gaia DR3 sources per arcmin2, and are underestimated by up to a factor of 4 in fields with stellar densities higher than 300 sources per arcmin2. For the most crowded fields in VIRAC2, the proper motion uncertainties are underestimated by a factor of 1.1 up to 1.5, with a dependence on J-band magnitude. In all fields, the brighter sources have the higher r value. At the faint end (G > 19), the inflation factor is close to 1, meaning that the proper motions already fully agree with the HST measurements within 1. Conclusions. In the crowded fields common to both catalogues, VIRAC2 proper motions agree with HST proper motions and do not need an inflation factor for their uncertainties. Because of the depth and completeness of VIRAC2 in these fields, it is an ideal complement to Gaia DR3 for proper motion studies towards the Galactic bulge. © 2023 EDP Sciences. All rights reserved.
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    Ítem
    Search for hypervelocity stras in the central regions of the galaxy
    (Universidad Andrés Bello, 2023) Ruiz Fernández, Alonso Luna; Minniti, D.; Rejkuba, Marina; Marchetti, Tommaso; Alonso García, Javier; Leigh, Nathan; Facultad de Ciencias Exactas
    In this thesis, I propose a new approach to search for hypervelocity stars in the inner part of our Galaxy. Hypervelocity stars are moving at velocities around 1000 km s−1 , acquiring such velocity after an interaction with the supermassive black hole at the centre of the Milky Way: Sgr A*. Hence, these stars inform us about i) dynamical stellar interactions with Sgr A*, ii) phenomena in regions with high stellar density, iii) the stellar content in the inner part of the Galaxy, and iv) the presence of massive objects, such as intermediate and stellar-mass black holes. Hypervelocity stars are extremely rare, to identify them it was necessary to characterise the proper motions of the stars in the used catalogues: Gaia , VVV and VIRAC2, which contain hundreds of millions of sources. I explore also different approaches to identify and validate the reliable data in such catalogues, that result in being able to identify the unique objects in these catalogues, in this case, hypervelocity stars. The results of this work are the characterisation of the astrometric data of Gaia DR3 and VIRAC2 with respect to data from the Hubble Space Telescope (HST) in observations towards the Galactic bulge. On one hand, the uncertainties in the proper motions of Gaia DR3 are underestimated with respect to the HST proper motions, thus the need for an inflation factor to bring them into a 1σ consistency. On the other hand, VIRAC2 proper motions are in agreement with HST proper motions. For this reason, I used VIRAC2 proper motions to search for hypervelocity stars. With the developed method, I identified 139 candidate hypervelocity stars in the inner 60 sqdeg. around the Galactic centre. These stars appear to be ejected after an interaction with Sgr A *. Besides that, we identified more than 1000 high-velocity stars (> 600 km s−1 ), probably produced in the Galactic disc, although their origin is uncertain.