Thermal infrared properties of classical and type II Cepheids

Diffraction limited 10 μm imaging with VLT/VISIR^⋆,⋆⋆

¹ LESIA, Observatoire de Paris, CNRS UMR 8109, UPMC, Université Paris Diderot, 5 place Jules Janssen, 92195 Meudon, France
e-mail: alexandre.gallenne@obspm.fr
² European Southern Observatory, Alonso de Córdova 3107, Casilla 19001, Santiago 19, Chile

Received: 20 May 2011
Accepted: 24 November 2011

Abstract

We present new thermal infrared (IR) photometry and spectral energy distributions (SEDs) of eight classical Cepheids (type I) and three type II Cepheids, using VISIR thermal IR photometric measurements, supplemented with literature data. We used the BURST mode of the instrument to get diffraction-limited images at 8.59, 11.25, and 11.85 μm. The SEDs show a IR excess at wavelengths longer than 10   μm in ten of the eleven stars. We tentatively attribute these excesses to circumstellar emission created by mass loss from the Cepheids. On the basis of some hypotheses for the dust composition, we estimate a total mass of the envelope ranging from 10^-10 to 10^-8   M_⊙. We also detect a spatially extended emission around AX Cir, X Sgr, W Sgr, Y Oph, and U Car, while we do not resolve the circumstellar envelope (CSE) of the other stars. The averaged circumstellar envelope brightnesses relative to the stellar photosphere are α(AX   Cir) = 13.8 ± 2.5%,α(X   Sgr) = 7.9 ± 1.4%,α(W   Sgr) = 3.8 ± 0.6%,α(Y   Oph) = 15.1 ± 1.4%, and α(U   Car) = 16.3 ± 1.4% at 8.59 μm. With this study, we extend the number of classical Cepheids with detected CSEs from 9 to 14, confirming that at least a large fraction of all Cepheids are experiencing significant mass loss. The presence of these CSEs may also impact the future use of Cepheids as standard candles at near and thermal infrared wavelengths.