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Téléphone : 04 92 00 19 68

Durée : 3 mois
Lieu : mont-gros
Financement : Oui (autre)
Domaine : Physique stellaire
Niveau d'études : Master 2

Désignation du projet : The circumstellar enveloppes of Cepheids and their impact on the period-luminosity relationship in the area of JWST and E-ELT.
Résumé : Cepheids are the backbone of the extragalactic distance ladder because their pulsation period, which is easily measured observationally, correlates directly with their luminosity, through the Period-Luminosity (PL) relation (Leavitt & Pickering 1912). As a recent example, the discovery of the accelerated expansion of the Universe (Riess et al. 1998; 2011 Nobel prize) is largely based on the Cepheid distance ladder. The ability to measure with a high accuracy extragalactic distances using Cepheids is an absolute requisite for the establishment of cosmological models on a firm observational ground. However, despite considerable efforts, the accuracy of the Cepheid scale is still unsatisfactory. The calibration of the PL zero point (ZP) is affected by systematics at a 5% level, and it is today the largest contributor to the error on the Hubble constant H0 (Riess et al. 2011, 2016). Within this context, an innovative approach in order to derive the distance of nearby Cepheids has been developed over the past decade: the Parallax of Pulsation (PoP) (Davis 1979, Kervella et al. 2004), based on long-baseline interferometry and spectroscopy. However, the derived distance is still highly sensitive to the physics of the Cepheid and its close environment (Nardetto et al. 2016a, 2016b). The overall goal of the project is to determine the impact of circumstellar enveloppes of Cepheids on the period-luminosity relation. We would like the M2 student to start from the Gaia release sample of Cepheids in our Milky Way, and also from extragalactic known Cepheids, in order to determine which of them are observable with current and futur instruments with High Angular Resolution and high sensitivity: VEGA/GRAVITY/MATISSE/SPHERE/ALMA/NOEMA/E-ELT/JWST. The student will also clarify which Cepheids have been observed in spectroscopy and photometry (and in which bands), and classify the different approaches and models available in order to study the circumst ellar environment of Cepheids. This work of classification will provide to the student a large view about the current state of Cepheids in the area of Gaia and large surveys, and an excellent basis for the PhD Thesis we propose, starting in September 2017 (see the broad view proposition on the SF2A website).

The funding for this M2 internship is already secured through an ANR grant obtained in 2015 (UnlockCepheids, PI: P. Kervella). For the 3 months of the stage, the student will be based at the Observatoire de la Côte d’Azur in Nice (Mt Gros). Members of the team are french speaking but english language is a requirement in order to interact with our international collaborators.