IR Excess of Classical Cepheids Modeled by a Thin Shell of Ionized Gas

Description

Cepheids are the keystone of the extragalactic distance ladder because 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 PL relation. However, the calibration of the PL relation still suffers from systematics errors of at least 2% and it is the largest contributor on the Hubble constant H 0 uncertainty (Riess et al. 2016). These systematics could be partly attributed to the circumstellar envelopes (CSEs) of Cepheids discovered in the last decade using interferometry. Using Spitzer Space Telescope observations, we reconstruct the spectral energy distribution of 5 Cepheids and find an infrared (IR) excess continuum for all of them. 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.

Abstract

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