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Issue A&A
Volume 448, Number 2, March III 2006
Page(s) 623 - 631
Section Interstellar and circumstellar matter
DOI http://dx.doi.org/10.1051/0004-6361:20053603

A&A 448, 623-631 (2006)
DOI: 10.1051/0004-6361:20053603

Extended envelopes around Galactic Cepheids

I. $\ell$ Carinae from near and mid-infrared interferometry with the VLTI
P. Kervella, A. Mérand, G. Perrin and V. Coudé du Foresto

LESIA, UMR 8109, Observatoire de Paris-Meudon, 5 place Jules Janssen, 92195 Meudon Cedex, France
    e-mail: Pierre.Kervella@obspm.fr

(Received 9 June 2005 / Accepted 25 October 2005)

Abstract
We present the results of long-baseline interferometric observations of the bright southern Cepheid $\ell$ Carinae in the infrared N ( $8{-}13\,\mu$m) and K ( $2.0{-}2.4\,\mu$m) 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 $3\,R_\star$ ($\approx$ $500\,R_\odot \approx 2{-}3$ 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  $2\,R_\star$, 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 $\ell$ 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.


Key words: Cepheids -- techniques: interferometric -- stars: circumstellar matter -- stars: individual: $\ell$ Car

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