A thin shell of ionized gas explaining the IR excess of classical Cepheids

Abstract

Despite observational evidences, InfraRed (IR) excess of classical Cepheids are seldom studied and poorly understood, but probably induces systematics on the Period-Luminosity (PL) relation used in the calibration of the extragalactic distance scale. This study aims to understand the physical origin of the IR excess found in the spectral energy distribution (SED) of 5 Cepheids : RS Pup (P=41.46d), zeta Gem (P=10.15d), eta Aql (P=7.18d), V Cen (P=5.49d) and SU Cyg (P=3.85d). A time series of atmospheric models along the pulsation cycle are fitted to a compilation of data, including optical and near-IR photometry, Spitzer spectra (secured at a specific phase), interferometric angular diameters, effective temperature and radial velocity measurements. Herschel images in two bands are also analyzed qualitatively. In this fitting process, based on the SPIPS algorithm, a residual is found in the SED, whatever the pulsation phase, and for wavelengths larger than about $1.2\mu$m, which corresponds to the so-determined infrared excess of Cepheids. This IR excess is then corrected from interstellar medium absorption in order to infer or not the presence of dust shells, and is finally used in order to fit a model of a shell of ionized gas. For all Cepheids, we find a continuum IR excess increasing up to about -0.1 magnitudes at 30$\mu$m, which cannot be explained by a hot or cold dust model of CircumStellar Environment (CSE). We show, for the first time, that the IR excess of Cepheids can be explained by free-free emission from a thin shell of ionized gas, with a thickness of about 15% of the star radius, a mass of $10^{-9}-10^{-7}$ Msol and a temperature ranging from 3500 to 4500K. This result has to be tested with interferometers operating in visible, in the mid-IR or in the radio domain. The impact of such CSEs of ionized gas on the PL relation of Cepheids needs also more investigations.

Accepted in A&A, 21 pages, 17 figures

References in corpus (12)

• Large Magellanic Cloud Cepheid Standards Provide a 1% Foundation for the Determination of the Hubble Constant and Stronger Evidence for Physics Beyond LambdaCDM
• A distance to the Large Magellanic Cloud that is precise to one per cent
• Three-dimensional mapping of the local interstellar medium with composite data
• The Luminosities and Distance Scales of Type II Cepheid and RR Lyrae variables
• The Relationship between the Optical Depth of the 9.7 micron Silicate Absorption Feature and Infrared Differential Extinction in Dense Clouds
• On the Enhancement of Mass Loss in Cepheids Due to Radial Pulsation
• Extended envelopes around Galactic Cepheids III. Y Oph and alpha Per from near-infrared interferometry with CHARA/FLUOR
• Tuning in on Cepheids: Radial velocity amplitude modulations. A source of systematic uncertainty for Baade-Wesselink distances
• The Secret Lives of Cepheids: $\delta$ Cep -- the Prototype of a New Class of Pulsating X-ray Variable Stars
• Spectral Calibration in the Mid-Infrared: Challenges and Solutions
• VEGA/CHARA interferometric observations of Cepheids. I. A resolved structure around the prototype classical Cepheid delta Cep in the visible spectral range
• The long-period Galactic Cepheid RS Puppis - II. 3D structure and mass of the nebula from VLT/FORS polarimetry

Cited by in corpus (14)

• Inspecting the Cepheid distance ladder: The Hubble Space Telescope distance to the SNIa host galaxy NGC 5584
• Analysing the spectral energy distributions of Galactic classical Cepheids
• Extended envelopes around Galactic Cepheids. V. Multi-wavelength and time-dependent analysis of IR excess
• HARPS-N high spectral resolution observations of Cepheids II. The impact of the surface-brightness color relation on the Baade-Wesselink projection factor of eta Aql
• Theoretical analysis of surface brightness-colour relations for late-type stars using MARCS model atmospheres
• Metallicity estimation of MW, SMC and LMC classical Cepheids from the shape of the $V$- and $I$-band light curves
• Atmospheric parameters of Cepheids from flux ratios with ATHOS: I. The temperature scale
• Multiplicity of Galactic Cepheids from long-baseline interferometry V. High-accuracy orbital parallax and mass of SU Cygni
• X-rays in Cepheids: XMM-Newton Observations of $\eta$ Aql
• Non-thermal $X$-rays from pulsation-driven shocks in Cepheids
• First Detection of Radio Emission Associated with a Classical Cepheid
• Discovery of ionized circumstellar gas emission around the long-period Cepheid $\ell$ Carinae with ALMA
• RR Lyrae stars with variable mean magnitudes
• Reconstruction of Cepheid Radial Velocity Curves from the shape of the V-band Light Curves