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Theses

GRAVITY in the galactic center : exploring the central parsec through optical interferometry

Abstract : The central parsec is one of the most interesting regions of our Galaxy. It is populated by a nuclear stellar cluster where massive, energetic and young stars coexist with colder evolved stars, hot plasma and streams of interstellar matter. A compact object called SgrA* with a mass of several million solar masses lies in its center becoming the closest supermassive black hole candidate, but being fainter in all wavelengths than other galactic nuclei. One of the closest stars to SgrA*, S2, completes an orbit around it in just 16 years and is bright enough to be used as a robust probe the gravitational potential of the central source by orbit tracking. In the infrared, SgrA* presents a quiescent emission with random short episodes called flares where its brightness sharply increases up to a factor 4, lasting several hours before dimming. The origin of these flares is likely due to the presence of accretion processes in the close environment of the black hole and their study can provide essential information about the exotic nature of SgrA*. Unfortunately, these events are random and require a sufficiently sampled monitoring over time.The GRAVITY instrument, part of the second generation of VLTI, enables the use of optical interferometry to study the Galactic Center. It is able to track the orbit of the star S2 with unprecedented accuracy, up to 10 micro-arcseconds. This thesis work is focused on the data analysis of the first products of the GRAVITY instrument in the Galactic Center.In the first part of the thesis, the astrometry of the star S2 is obtained by the use of the first two years of GRAVITY observations, in which I have actively participated. For that purpose, abinary star model is used to reproduce the interferometric data. Right before and after the pericenter passage, who took place in 2018, I have obtained the positions of S2 with respect to SgrA* reaching an astrometric accuracy of 30 micro-arcseconds, comparable with the expected size of the shadow of the supermassive black hole. In addition, I have obtained a new light curve of SgrA* which complements the data already published and confirms the quiescent-flare scenario. The results obtained are part of a larger study involving orbit fitting where several tests of General Relativity have been successfully performed, as well as interpretations of the flares as a product of accretion processes in the near SgrA* orbit.The second part of the thesis is focused on an evolved star called GCIRS 7, which is also located in the central parsec and serves as a reference for GRAVITY observations. This star, which has a large variability in the infrared, is known to contribute to the interstellar medium of the Galactic Center. By complementing GRAVITY archival data from 2017 with observations I have taken in 2019, I have implemented an atmosphere model, widely used for the study of other advanced stars, to explain the visibility curves of GCIRS 7 obtained by GRAVITY. The model consists of a photosphere and a thin molecular shell. The results show that the data can be interpreted as a photosphere with the same diameter for both epochs, but with a shell being colder and larger in 2019 compared to 2017. An estimation of the density of the thin shell reveals a saturation of the model for 2017 due to a high density, but 2019 data is partially reproduced. The results can be explained by a layer expansion and cooling likely due to an episode of mass loss.
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Gustavo Rodríguez Coira de La Peña. GRAVITY in the galactic center : exploring the central parsec through optical interferometry. Astrophysics [astro-ph]. Université Paris sciences et lettres, 2020. English. ⟨NNT : 2020UPSLO013⟩. ⟨tel-03278335⟩

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