Extended envelopes around Galactic Cepheids. I. l Carinae from near and mid-infrared interferometry with the VLTI

International audienceWe present the results of long-baseline interferometric observations of the bright southern Cepheid l Carinae in the infrared N (8-13 mum) and K (2.0-2.4 mum) bands, using the MIDI and VINCI instruments of the VLT Interferometer. We resolve in the N band a large circumstellar envelope (CSE) that we model with a Gaussian of 3 Rstar (≈500 R&sun; ≈ 2-3 AU) half width at half maximum. The signature of this envelope is also detected in our K band data as a deviation from a single limb darkened disk visibility function. The superimposition of a Gaussian CSE on the limb darkened disk model of the Cepheid star results in a significantly better fit of our VINCI data. The extracted CSE parameters in the K band are a half width at half maximum of 2 Rstar, comparable to the N band model, and a total brightness of 4% of the stellar photosphere. A possibility is that this CSE is linked to the relatively large mass loss rate of l Car. Though its physical nature cannot be determined from our data, we discuss an analogy with the molecular envelopes of RV Tauri, red supergiants and Miras