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L’enveloppe circumstellaire des Céphéides et l’étalonnage des distances dans l’Univers

Abstract : Cepheids are essential to calibrate the distances in the Universe. However, these stars exhibit an infrared excess which could originate in their circumstellar envelope, and thus lead to photometric biases in the calculation of distances. However, the nature and origin of these envelopes, as well as the physical mechanisms responsible for the infrared excess remain unknown. The objective of this thesis is to determine the physical mechanisms at the origin of the infrared excess of Cepheids coming from their circumstellar envelope, and to evaluate its effects on the calibration of distances in the Universe.In the first part, I reconstruct the infrared excess of 5 Galactic Cepheids by comparing spectroscopic observations of the Spitzer space telescope with the photosphere model of these Cepheids during the pulsation cycle. I show that most of the excess infrared cannot be explained by a dust envelope. I then consider an analytical model of an ionized hydrogen gas shell. Adjusting the physical parameters of this model allows me to reproduce the excess infrared satisfactorily. In particular, I determine envelopes whose radius is about 1.15 Rstar, that is in a region which could be assimilated to the lower chromosphere of the Cepheids. To better understand the origin of the ionized gas, I then study high spectral resolution profiles of Halpha and calcium infrared triplet lines obtained by the UVES spectroscope at VLTI, from a sample of 24 Cepheids, during the pulse cycle. I show that the chromosphere is activated in each cycle by the propagation of the main supersonic shock, which heats up and ionizes the gas in the atmosphere. The ballistic fallout from the higher layers can also accelerate to supersonic speed and ionize the gas. In the case of Cepheids of long periods, the de-synchronization between the propagation of the shock and the fallout of the atmosphere causes a double absorption profile which allows me to estimate the radius of the chromosphere at 1.5 Rstar. In addition, absorption lines of Halpha centered on the star's frame of reference suggest the existence of gas envelopes at a distance of at least 2 Rstar. Finally, thanks to the interferometric observations that I carried out using the MATISSE/VLTI instrument within the framework of the consortium, I bring new constraints in the infrared bands L, M and N in studying the Cepheid l Car. I show for the first time the existence of a centro-symmetric environment resolved in L band, whose radius represents about 1.9 Rstar, in agreement with previous interferometric observations in K band. In addition, the analysis of the observed spectra show the absence of significant dust signature, once again reinforcing the hypothesis of a gaseous envelope. However, the radius of the envelope thus determined requires to improve the model of an envelope of ionized gas, which cannot simultaneously reproduce the IR excess and the size determined by MATISSE. A theoretical perspective of improving the model consists in studying negative hydrogen as a source of gas opacity. New observations to come with the MATISSE instrument will make it possible to study the properties of the envelopes as a function of the position of the Cepheids in the HR diagram. The objective of these observations is to establish a grid of photometric corrections to be applied when determining the distance of Cepheids.
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Submitted on : Tuesday, March 30, 2021 - 12:38:21 PM
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Vincent Hocdé. L’enveloppe circumstellaire des Céphéides et l’étalonnage des distances dans l’Univers. Astrophysique stellaire et solaire [astro-ph.SR]. Université Côte d'Azur, 2020. Français. ⟨NNT : 2020COAZ4077⟩. ⟨tel-03185461⟩

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