Extended envelopes around Galactic Cepheids
Among 45 Milky Way Cepheids, investigated in this study, 13 show near-infrared excess, which is interpreted as the presence of circumstellar envelopes.
Extended envelopes around Galactic Cepheids. V. Multi-wavelength and time-dependent analysis of IR excess
A. Gallenne, A. Mérand, P. Kervella, G. Pietrzyński, W. Gieren, V. Hocdé, L. Breuval, N. Nardetto, E. Lagadec
We performed multi-wavelength and time-dependent modelling of 45 Galactic Cepheids using the SPIPS algorithm in order to investigate the infrared (IR) excess coming from the circumstellar envelopes (CSEs). We used radial velocities, stellar effective temperatures, angular diameters, and photometry from 0.5 µm to 70 µm retrieved from the literature. We also present new mid-IR photometry with VLT/VISIR in the PAH1 (8.6 µm) and PAH2 (11.3 µm) filters.
The SPIPS analysis also allowed us to deduce average stellar parameters, such as the pulsation period stellar luminosity, effective temperature, and linear and angular radii.
We estimated an average IR excess in various photometric bands of ∼0.08 mag, ∼0.11 mag, ∼ 0.13 mag and ∼0.17 mag, respectively, at 2.2 µm, 5 µm, 10 µm, and 25 µm. We showed that Cepheid distance estimates from the SPIPS modelling are not biased by the presence of IR excess. The most affected parameters are the angular diameters and the colour excesses, which ‘compensate’ the presence of IR excess. However, we demonstrated that not fitting the colour excess can lead to biased distance estimates. The impact of these CSEs on the extragalactic P-L relations still need to be quantified; however it is likely mitigated as long-period Cepheids do not seem to exhibit more IR excess than short-period ones. We therefore expect short period Cepheids to be more biased due to their lower brightness. The presence of CSE does not seem to be linked to the evolution stage of the Cepheid either (i.e. first, second, or third crossing).
We see no correlation between the existence of the CSE and the CO molecular feature seen at 4.5 µm, which is therefore mostly related to the stellar photosphere.
