Thermal infrared properties of classical and type II Cepheids Diffraction limited 10 microns imaging with VLT/VISIR

TL;DR

This study uses diffraction-limited 10-micron imaging to detect and analyze circumstellar envelopes around classical and type II Cepheids, revealing widespread mass loss that could affect their use as standard candles.

Contribution

It provides new thermal IR photometry and spatially resolved imaging of Cepheids, expanding the known sample of stars with circumstellar envelopes and quantifying their properties.

Findings

IR excess detected in 10 of 11 stars at wavelengths >10μm

Extended emission observed around five Cepheids indicating circumstellar envelopes

Mass loss estimates range from 10^-10 to 10^-8 solar masses

Abstract

We present new thermal 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 {\mu}m. The SEDs show a IR excess at wavelengths longer than 10{\mu}m in ten of the eleven stars. We tentatively attribute these excesses to circumstellar emission created by mass loss from the Cepheids. With some hypotheses for the dust composition, we estimated a total mass of the envelope ranging from 10-10 to 10-8 M\odot. 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) for the other stars. The averaged circumstellar envelope brightnesses relative to the stellar photosphere are {\alpha}(AX Cir) = 13.8\pm2.5%, {\alpha}(X Sgr) = 7.9\pm1.4%, {\alpha}(W Sgr) = 3.8\pm0.6%, {\alpha}(Y Oph) = 15.1\pm1.4% and {\alpha}(U Car) = 16.3\pm1.4% at 8.59 {\mu}m. 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.