The circumstellar envelopes of Cepheids and their impact on the period-luminosity relationship in the JWST and ELT era.

Cepheids are the keystone of the extragalactic distance ladder since their pulsation periods correlate directly with their luminosity, through the Period-Luminosity (PL) relation (Leavitt & Pickering 1912). The discovery of the accelerated expansion of the Universe (Riess et al. 1998; 2011 Nobel prize) is largely based on the Cepheid distance ladder. However, the calibration of the PL relation is still suffering from systematics errors of at least 2% and it is the largest contributor on the Hubble constant H0 (Riess et al. 2016). These systematics could be partly due to the CircumStellar Envelopes (CSEs) of Cepheids discovered in the last decade by interferometry. Using Spitzer Space Telescope observations, we reconstruct the spectral energy distribution of 5 Cepheids and we report the observation of an infrared (IR) excess continuum. We show for the first time that the IR excess can be modeled by a free-free emission due to a thin circumstellar shell of ionized gas in the chromospheric region.